| Author |
Message |
Patrick Turner
Guest
|
 Posted:
Tue Oct 18, 2005 12:03 am Post subject:
Re: Battery bias directly to grid |
|
|
bruce seifried wrote:
| Quote: | In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
|
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
The only chance of transients being generated by gain level change is if the
input
grid draws dc current and a small dc voltage exists across the resistances
of the DACT, so that when switching, you hear the switch steps.
I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
Patrick Turner.
|
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|
 |
Patrick Turner
Guest
|
| Posted:
Tue Oct 18, 2005 12:11 am Post subject:
Re: Battery bias directly to grid |
|
|
Sander deWaal wrote:
| Quote: | Patrick Turner <info@turneraudio.com.au> said:
But 6J6 and others tend to be a bit oscillatory.
Once you get past that stage, it's a fine audio tube.
|
Nope, its also hopelessly microphonic.
EC97 is another crook tube for MC amps.
| Quote: |
Anything which can oscillate, does oscillate, unless something to stop it
is used.
Murphy's Law # 113: oscillators won't start, amplifiers will
oscillate.
Amendment: the latter will oscillate in frequencies just beyond the
range of your test gear.
|
Its easy to tell if a phono amp is oscillating at 300MHz.
Just turn up the gain and touch parts of the 0V rail or bypassed points near
the devices with a screw driver.
Usually this stops or alters the level at which oscillation is occuring, and
the
sudden RF change causes a click to be heard where none should be heard.
RF can't run and hide from me.
Patrick Turner.
| Quote: |
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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Patrick Turner
Guest
|
| Posted:
Tue Oct 18, 2005 12:17 am Post subject:
Re: Battery bias directly to grid |
|
|
Ruud Broens wrote:
| Quote: | "Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:aer7l15emrfs1gaudpa0h88visfpcobarp@4ax.com...
: "Ruud Broens" <broensr@wanadoo.nl> said:
: >:
: >You know your Murphy's - got an irish pub in town, too ? ;-)
:
:
: Thank's sockpuupet Rewd for, admitting you have a drinking problem.
:
: --
it's called a drinking challenge :-)
but you know me, it's just coffee
B.
|
I quite like dining at a favourite cafe and reading the newspapers.
Once, after 10 years of regular visits, I finally managed to
tip a fine plate of curry into my lap while juggling
papers.
I poured it back onto the plate, and continued, with strange looks from
some young girls nearby.
I don't care what girls think, or worry that the news of the day
will pollute what i eat, but Murphy dined with me that day.
Patrick Turner. |
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Sander deWaal
Guest
|
| Posted:
Tue Oct 18, 2005 1:13 am Post subject:
Re: Battery bias directly to grid |
|
|
Patrick Turner <info@turneraudio.com.au> said:
| Quote: | But 6J6 and others tend to be a bit oscillatory.
Once you get past that stage, it's a fine audio tube.
Nope, its also hopelessly microphonic.
EC97 is another crook tube for MC amps.
|
Excuse me, I wasn't talking about MC preamps.
One ECC91/6J6 with the 2 sections in parallel make for a fine line
stage, or even as a driver for a MOSFET output stage.
Microphonics are not a worry at such high signal levels.
BTW the 6DJ8/ECC88 family is equally bad in that regard.
I recently found 4 NOS E288CC Philips at a hamfest, and decided to
make a phono stage around them.
After several attempts with rubber washers and even a trampoline-based
PCB with the tube sockets mounted on springs, I quit and decided to
look for something else for my phono stage.
I now use 6072s to the utmost satisfaction (they're EHs,
ferchristsake!).
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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Jon Yaeger
Guest
|
| Posted:
Tue Oct 18, 2005 2:27 am Post subject:
Re: Battery bias directly to grid |
|
|
in article 4353F7A2.D5932B00@turneraudio.com.au, Patrick Turner at
info@turneraudio.com.au wrote on 10/17/05 3:17 PM:
| Quote: |
Ruud Broens wrote:
"Sander deWaal" <nospam@wanadoo.nl> wrote in message
news:aer7l15emrfs1gaudpa0h88visfpcobarp@4ax.com...
: "Ruud Broens" <broensr@wanadoo.nl> said:
: >:
: >You know your Murphy's - got an irish pub in town, too ? ;-)
:
:
: Thank's sockpuupet Rewd for, admitting you have a drinking problem.
:
: --
it's called a drinking challenge :-)
but you know me, it's just coffee
B.
I quite like dining at a favourite cafe and reading the newspapers.
Once, after 10 years of regular visits, I finally managed to
tip a fine plate of curry into my lap while juggling
papers.
I poured it back onto the plate, and continued, with strange looks from
some young girls nearby.
I don't care what girls think, or worry that the news of the day
will pollute what i eat, but Murphy dined with me that day.
Patrick Turner.
|
What?! That faux pas didn't curry their favor?? |
|
| Back to top |
|
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Patrick Turner
Guest
|
| Posted:
Tue Oct 18, 2005 3:45 pm Post subject:
Re: Battery bias directly to grid |
|
|
Sander deWaal wrote:
| Quote: | Patrick Turner <info@turneraudio.com.au> said:
But 6J6 and others tend to be a bit oscillatory.
Once you get past that stage, it's a fine audio tube.
Nope, its also hopelessly microphonic.
EC97 is another crook tube for MC amps.
Excuse me, I wasn't talking about MC preamps.
One ECC91/6J6 with the 2 sections in parallel make for a fine line
stage, or even as a driver for a MOSFET output stage.
Microphonics are not a worry at such high signal levels.
BTW the 6DJ8/ECC88 family is equally bad in that regard.
I recently found 4 NOS E288CC Philips at a hamfest, and decided to
make a phono stage around them.
After several attempts with rubber washers and even a trampoline-based
PCB with the tube sockets mounted on springs, I quit and decided to
look for something else for my phono stage.
|
I have found most 6DJ8 quite ok for phono stages.
| Quote: |
I now use 6072s to the utmost satisfaction (they're EHs,
ferchristsake!).
|
Is that a 12AY7?
But one should always sort through tubes for the least microphonic, least
noisy tubes for low signal inputs, even if they are NOS or mil spec.
of 35 12AU7 i tested 2 yrs ago I found about 10 had a noise voltage of
2uV or less at g1, and also were not micro.
But I chucked out a few because they were so bad in both respects,
and had low gain in the test ciruit, ( so must have had low emission ).
The worst of the noisy ones had up to 20uV of input noise and had grids
that were
slightly positive with respect to the bias applied.
And the rest are of variable quality.
When supplying preamps tp ppl I have to be sure to
test the tubes for low noise; one could never assume the input tubes won't
be noisy
or microphonic.
Using a j-fet for the first device makes it all so much easier.
Patrick Turner.
| Quote: |
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
|
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| Back to top |
|
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bruce seifried
Guest
|
| Posted:
Tue Oct 18, 2005 9:02 pm Post subject:
Re: Battery bias directly to grid |
|
|
In article <4353F476.257BE164@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
| Quote: | bruce seifried wrote:
In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
|
Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.
| Quote: | The only chance of transients being generated by gain level change is if the
input grid draws dc current and a small dc voltage exists across the resistances
of the DACT, so that when switching, you hear the switch steps.
|
Are we talking about the same circuit?
The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.
| Quote: | I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
|
Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.
This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.
cheers,
-bruce seifried
-bruce seifried |
|
| Back to top |
|
|
Patrick Turner
Guest
|
| Posted:
Tue Oct 18, 2005 9:16 pm Post subject:
Re: Battery bias directly to grid |
|
|
bruce seifried wrote:
| Quote: | In article <4353F476.257BE164@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.
The only chance of transients being generated by gain level change is if the
input grid draws dc current and a small dc voltage exists across the resistances
of the DACT, so that when switching, you hear the switch steps.
Are we talking about the same circuit?
|
There is no chance of transient problems with the schematic Andre sent me,
it has a 20k DACT attenuator, then the wiper goes to a battery, then
to the grid.
So at all levels of gain the bias remains and there are no transients.
| Quote: |
The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
|
Huh?
I don't follow you at all.
You must be talking about some other circuit.
Patrick Turner.
| Quote: |
Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.
I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.
This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.
cheers,
-bruce seifried
-bruce seifried |
|
|
| Back to top |
|
|
bruce seifried
Guest
|
| Posted:
Tue Oct 18, 2005 10:15 pm Post subject:
Re: Battery bias directly to grid |
|
|
In article <43551ED2.2319BC54@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
| Quote: | bruce seifried wrote:
In article <4353F476.257BE164@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any
other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low
end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but
not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.
The only chance of transients being generated by gain level change is if
the
input grid draws dc current and a small dc voltage exists across the
resistances
of the DACT, so that when switching, you hear the switch steps.
Are we talking about the same circuit?
There is no chance of transient problems with the schematic Andre sent me,
it has a 20k DACT attenuator, then the wiper goes to a battery, then
to the grid.
So at all levels of gain the bias remains and there are no transients.
The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
Huh?
I don't follow you at all.
You must be talking about some other circuit.
Patrick Turner.
|
Aha... apparently we *are* talking about different cicuits.
Until it was changed several days ago, I had been referencing the T39
schematic on this page:
http://members.lycos.co.uk/fiultra/T39-KISS-300B-Ultrafi-crct.jpg
I was never able to view the other schematic due to unknown browser
demons, but its description sounded like that of the T39 input biasing
arrangement, which I must say was bizarre, but easily fixable (...and I
happen to believe that a good quality capacitor makes a better capacitor
than a battery).
Carry on...
-bruce seifried
| Quote: | Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.
I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.
This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.
cheers,
-bruce seifried |
|
|
| Back to top |
|
|
Andre Jute
Guest
|
| Posted:
Tue Oct 18, 2005 11:09 pm Post subject:
Re: Battery bias directly to grid |
|
|
Uh-uh, Bruce. What you're talking about is the battery in the place of
the grid leak resistor, between grid and ground. That's a low impedance
path to ground. I've tried that one with a DACT, a high precision Swiss
medical switch with SMD resistors and ultrashort paths, with the tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.
I republished the T39 KISS Ultrafi circuit with the batteries in the
third possible position, in the cathode circuit, simply because I could
hear no difference with the battery in the grid leak position, and the
cathode circuit position has the very great advantage of being more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.
One final note on the *discussion* circuit I published (battery between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and several
others. But I'm not interested in defending a circuit before
professional production engineers. I'm interested in simplifying the
circuit to what sounds best, regardless of expense or "professional
practice", and in this case, because I am publishing the circuit for
amateurs, in making it reasonably longlasting (there is a companion
circuit, called the T44 "Populaire", with autobias, recommended for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...
Thanks to all who contributed valuable insights (even if we were all
talking about different circuits!). I'm removing the discussion circuit
because it really doesn't add much to our wisdom; it may be a viable
alternative (in that it works) but it is a decidedly less desirable one
than the battery in the cathode circuit. That's the point of discussion
with one's peers, to choose the best course of action from among
possibilities.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"an unbelievably comprehensive web site" -- Hi-Fi News & Record Review
bruce seifried wrote:
| Quote: | In article <43551ED2.2319BC54@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353F476.257BE164@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any
other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low
end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but
not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.
The only chance of transients being generated by gain level change is if
the
input grid draws dc current and a small dc voltage exists across the
resistances
of the DACT, so that when switching, you hear the switch steps.
Are we talking about the same circuit?
There is no chance of transient problems with the schematic Andre sent me,
it has a 20k DACT attenuator, then the wiper goes to a battery, then
to the grid.
So at all levels of gain the bias remains and there are no transients.
The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
Huh?
I don't follow you at all.
You must be talking about some other circuit.
Patrick Turner.
Aha... apparently we *are* talking about different cicuits.
Until it was changed several days ago, I had been referencing the T39
schematic on this page:
http://members.lycos.co.uk/fiultra/T39-KISS-300B-Ultrafi-crct.jpg
I was never able to view the other schematic due to unknown browser
demons, but its description sounded like that of the T39 input biasing
arrangement, which I must say was bizarre, but easily fixable (...and I
happen to believe that a good quality capacitor makes a better capacitor
than a battery).
Carry on...
-bruce seifried
Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.
I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.
This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.
cheers,
-bruce seifried |
|
|
| Back to top |
|
|
Andre Jute
Guest
|
| Posted:
Wed Oct 19, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
|
|
Seems to me, my dear fellow, that your letter illustrates either that a
Marxist can make a meal out of very little or that you too can be an
"ultrafi fetishist". Those self-charging advantages of the battery in
the cathode circuit you are so keen to dismiss are huge benefits for no
cost. Your better ultrafidelista is not an audiophool. He is imbued
with the conviction that simplicity means better sound. The battery in
the cathode is the simplest safe implementation, requiring no other
components.
Once more, the three circuits considered in these discussions:
1. Battery in cathode circuit. Grid leak is taken care of by leg of
attenuator to ground. This is the common version. The entire circuit is
attenuator, tube, battery and a couple of pieces of wire and maybe a
grid stopper resistor.
2. Battery shunted in grid leak position between grid and ground,
parallel to leg of attenuator to ground.
3. Battery seriesed in line between wiper of attenuator and grid.
Your idea of a battery in series below the attenuator would be a fourth
version and has the disavantage that it would vary the bias with volume
setting. How the battery could be charged by the signal after the amp
is switched off, as you claim, beats me. Maybe you know some magic that
I don't, which doesn't quite seem likely.
My source in all these experiments was a Quad 67 CD player, which has a
capacitively coupled output. I don't have any sources without a cap on
the output. Very few audiophiles can buy such a thing, and no competent
DIYer would build such a thing.
It is true that I already know what the ultrafidelista think. Most of
them think what I think and change their when I change mine; it's like
a ballet. But I was wondering if there is something I overlooked about
battery bias that might be worth some margin of extra silence. Seems
not.
On the subject of noise, you're on a hiding to nothing. A battery is a
more silent component than anything else including a straight piece of
wire. What I was reaching for was a more, even a marginally more,
silent implementation of a battery. The truth is that I couldn't hear
that the battery in the grid circuit was superior to the battery in the
cathode circuit, so therefore the more convenient and conservatively
safer cathode circuit it is.
Of course, by the nature of tubes, where there is very little that is
not already known, most experiments do not lead to improvements. On the
other hand, it became to clear during the course of this discussion
that the vast majority of RATs don't know about battery bias and that
only a few have ever tried it. It was definitely a worthwhile
discussion for me, and I hope it has given those who have not tried
battery bias (in the cathode, gentlemen, in the cathode) something to
do this winter.
HTH.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"an unbelievably comprehensive web site" -- Hi-Fi News & Record Review
Ian Iveson wrote:
| Quote: | I can't speak for other heads, but the inside of mine finds itself
no further forward. It would help if you were to say what you are
using as a source in your listening test.
Using the battery *instead of* the grid leak gives you a 1ohm shunt
across the lower leg of the attenuator, which is not a good test of
the principle. Perhaps you mean *in series with* the grid leak?
All methods of grid bias in the absence of DC blocking will result
in biasing the source, obviously. Here again it is not clear whether
that in itself may have degraded the sound, so again the test is
inconclusive.
A battery at the foot of the attenuator would not discharge itself
in the absence of a grid leak, and the circuit would be just as
simple as the one you have chosen. The battery could also be charged
through the signal input with the amp off, so recharging could be a
switched function of a dedicated preamp. I dare say there would be
objections to this option, and I wonder what they would be.
What I would really like to know is would it be better, leaving
aside the issue of DC blocking and recharging, to bias via the grid
or the cathode? And would a series or a shunt connection to the grid
be more appropriate?
Do batteries have the same impedance in both directions? Are they
more noisy when discharging, or charging, or in equilibrium? What
are the mechanisms of distortion and will it be less in the
high-impedance low current input or the low impedance high current
cathode circuit. Will it be less by a factor greater than the
relative gain?
I would be surprised if these issues had not already been resolved,
considering you say that the cathode connection is so common. It
seems unlikely that the obvious alternatives were not explored, or
that the merely practical charging and DC blocking issues won the
day with the "ultra fi" fetishists.
And don't forget all of your famous guru friends, who I am sure
would be happy to advise you. I would be interested to hear their
views.
cheers, Ian
"Andre Jute" <fiultra@yahoo.com> wrote in message
news:1129658943.962078.103860@g14g2000cwa.googlegroups.com...
Uh-uh, Bruce. What you're talking about is the battery in the
place of
the grid leak resistor, between grid and ground. That's a low
impedance
path to ground. I've tried that one with a DACT, a high precision
Swiss
medical switch with SMD resistors and ultrashort paths, with the
tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.
I republished the T39 KISS Ultrafi circuit with the batteries in
the
third possible position, in the cathode circuit, simply because I
could
hear no difference with the battery in the grid leak position, and
the
cathode circuit position has the very great advantage of being
more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.
One final note on the *discussion* circuit I published (battery
between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and
several
others. But I'm not interested in defending a circuit before
professional production engineers. I'm interested in simplifying
the
circuit to what sounds best, regardless of expense or
"professional
practice", and in this case, because I am publishing the circuit
for
amateurs, in making it reasonably longlasting (there is a
companion
circuit, called the T44 "Populaire", with autobias, recommended
for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...
Thanks to all who contributed valuable insights (even if we were
all
talking about different circuits!). I'm removing the discussion
circuit
because it really doesn't add much to our wisdom; it may be a
viable
alternative (in that it works) but it is a decidedly less
desirable one
than the battery in the cathode circuit. That's the point of
discussion
with one's peers, to choose the best course of action from among
possibilities.
Andre Jute |
|
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| Back to top |
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|
bruce seifried
Guest
|
| Posted:
Wed Oct 19, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
|
|
In article <1129658943.962078.103860@g14g2000cwa.googlegroups.com>,
"Andre Jute" <fiultra@yahoo.com> wrote:
| Quote: | Uh-uh, Bruce. What you're talking about is the battery in the place of
the grid leak resistor, between grid and ground. That's a low impedance
path to ground.
|
Got it. Low impedance, quite! I was looking at the T39 mk VI schematic,
since the "discussion" schematic was refusing to appear.
| Quote: | I've tried that one with a DACT, a high precision Swiss
medical switch with SMD resistors and ultrashort paths, with the tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.
|
Well, good on ya, mate, but I'll stand by my observations.
| Quote: | I republished the T39 KISS Ultrafi circuit with the batteries in the
third possible position, in the cathode circuit, simply because I could
hear no difference with the battery in the grid leak position, and the
cathode circuit position has the very great advantage of being more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.
|
I'm glad that the discussion here helped you decide to change that
schematic, since it was, shall we say, a somewhat unhealthy arrangement
of parts. A newbe building an amp based on the T39 circuit might be a
little perturbed to find that his or her batteries would be shorted to
ground whenever the attenuator was turned off. An input attenuator
having a wonky curve because it was loaded down by a 1 ohm load would be
another "surprise".
| Quote: | One final note on the *discussion* circuit I published (battery between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and several
others.
|
Just for the record, again, I was making reference to the T39 mk VI.
I've made no comment on the "discussion" circuit.
A cap connected between the wiper and grid, with the batteries in series
with a 100k-500k resistor connected in the grid leak position, is what I
was proposing would work well. No charging, however. Always a compromise.
| Quote: | But I'm not interested in defending a circuit before
professional production engineers.
|
Which I am. Sorry, can't (and shan't) take the hat off...
| Quote: | I'm interested in simplifying the
circuit to what sounds best, regardless of expense or "professional
practice", and in this case, because I am publishing the circuit for
amateurs, in making it reasonably longlasting (there is a companion
circuit, called the T44 "Populaire", with autobias, recommended for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...
|
"Professional practice" sounds vaguely derogatory. You rebel, you.
| Quote: | Thanks to all who contributed valuable insights (even if we were all
talking about different circuits!). I'm removing the discussion circuit
because it really doesn't add much to our wisdom; it may be a viable
alternative (in that it works) but it is a decidedly less desirable one
than the battery in the cathode circuit. That's the point of discussion
with one's peers, to choose the best course of action from among
possibilities.
|
Best of luck.
-bruce seifried
eclair engineering |
|
| Back to top |
|
|
Ian Iveson
Guest
|
| Posted:
Wed Oct 19, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
|
|
I can't speak for other heads, but the inside of mine finds itself
no further forward. It would help if you were to say what you are
using as a source in your listening test.
Using the battery *instead of* the grid leak gives you a 1ohm shunt
across the lower leg of the attenuator, which is not a good test of
the principle. Perhaps you mean *in series with* the grid leak?
All methods of grid bias in the absence of DC blocking will result
in biasing the source, obviously. Here again it is not clear whether
that in itself may have degraded the sound, so again the test is
inconclusive.
A battery at the foot of the attenuator would not discharge itself
in the absence of a grid leak, and the circuit would be just as
simple as the one you have chosen. The battery could also be charged
through the signal input with the amp off, so recharging could be a
switched function of a dedicated preamp. I dare say there would be
objections to this option, and I wonder what they would be.
What I would really like to know is would it be better, leaving
aside the issue of DC blocking and recharging, to bias via the grid
or the cathode? And would a series or a shunt connection to the grid
be more appropriate?
Do batteries have the same impedance in both directions? Are they
more noisy when discharging, or charging, or in equilibrium? What
are the mechanisms of distortion and will it be less in the
high-impedance low current input or the low impedance high current
cathode circuit. Will it be less by a factor greater than the
relative gain?
I would be surprised if these issues had not already been resolved,
considering you say that the cathode connection is so common. It
seems unlikely that the obvious alternatives were not explored, or
that the merely practical charging and DC blocking issues won the
day with the "ultra fi" fetishists.
And don't forget all of your famous guru friends, who I am sure
would be happy to advise you. I would be interested to hear their
views.
cheers, Ian
"Andre Jute" <fiultra@yahoo.com> wrote in message
news:1129658943.962078.103860@g14g2000cwa.googlegroups.com...
| Quote: | Uh-uh, Bruce. What you're talking about is the battery in the
place of
the grid leak resistor, between grid and ground. That's a low
impedance
path to ground. I've tried that one with a DACT, a high precision
Swiss
medical switch with SMD resistors and ultrashort paths, with the
tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.
I republished the T39 KISS Ultrafi circuit with the batteries in
the
third possible position, in the cathode circuit, simply because I
could
hear no difference with the battery in the grid leak position, and
the
cathode circuit position has the very great advantage of being
more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.
One final note on the *discussion* circuit I published (battery
between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and
several
others. But I'm not interested in defending a circuit before
professional production engineers. I'm interested in simplifying
the
circuit to what sounds best, regardless of expense or
"professional
practice", and in this case, because I am publishing the circuit
for
amateurs, in making it reasonably longlasting (there is a
companion
circuit, called the T44 "Populaire", with autobias, recommended
for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...
Thanks to all who contributed valuable insights (even if we were
all
talking about different circuits!). I'm removing the discussion
circuit
because it really doesn't add much to our wisdom; it may be a
viable
alternative (in that it works) but it is a decidedly less
desirable one
than the battery in the cathode circuit. That's the point of
discussion
with one's peers, to choose the best course of action from among
possibilities.
Andre Jute |
|
|
| Back to top |
|
|
Wessel Dirksen
Guest
|
| Posted:
Wed Oct 19, 2005 12:48 pm Post subject:
Re: Battery bias directly to grid |
|
|
Andre Jute schreef:
| Quote: | Seems to me, my dear fellow, that your letter illustrates either that a
Marxist can make a meal out of very little or that you too can be an
"ultrafi fetishist". Those self-charging advantages of the battery in
the cathode circuit you are so keen to dismiss are huge benefits for no
cost. Your better ultrafidelista is not an audiophool. He is imbued
with the conviction that simplicity means better sound. The battery in
the cathode is the simplest safe implementation, requiring no other
components.
Once more, the three circuits considered in these discussions:
1. Battery in cathode circuit. Grid leak is taken care of by leg of
attenuator to ground. This is the common version. The entire circuit is
attenuator, tube, battery and a couple of pieces of wire and maybe a
grid stopper resistor.
|
A quick newbie question about this circuit #1 if I may:
This looks interesting and I would like to try it but I don't know how
for sure how to calculate/implement this. Is the DC bias voltage of the
batteries all there is to it in determining the idle current through
the tube? Or does the value of Ra (in the absence of setting a specific
value for Rk) need to be calculated differently than a normal bypassed
grounded cathode circuit?
Thanks, Wessel
| Quote: | 2. Battery shunted in grid leak position between grid and ground,
parallel to leg of attenuator to ground.
3. Battery seriesed in line between wiper of attenuator and grid.
Your idea of a battery in series below the attenuator would be a fourth
version and has the disavantage that it would vary the bias with volume
setting. How the battery could be charged by the signal after the amp
is switched off, as you claim, beats me. Maybe you know some magic that
I don't, which doesn't quite seem likely.
My source in all these experiments was a Quad 67 CD player, which has a
capacitively coupled output. I don't have any sources without a cap on
the output. Very few audiophiles can buy such a thing, and no competent
DIYer would build such a thing.
It is true that I already know what the ultrafidelista think. Most of
them think what I think and change their when I change mine; it's like
a ballet. But I was wondering if there is something I overlooked about
battery bias that might be worth some margin of extra silence. Seems
not.
On the subject of noise, you're on a hiding to nothing. A battery is a
more silent component than anything else including a straight piece of
wire. What I was reaching for was a more, even a marginally more,
silent implementation of a battery. The truth is that I couldn't hear
that the battery in the grid circuit was superior to the battery in the
cathode circuit, so therefore the more convenient and conservatively
safer cathode circuit it is.
Of course, by the nature of tubes, where there is very little that is
not already known, most experiments do not lead to improvements. On the
other hand, it became to clear during the course of this discussion
that the vast majority of RATs don't know about battery bias and that
only a few have ever tried it. It was definitely a worthwhile
discussion for me, and I hope it has given those who have not tried
battery bias (in the cathode, gentlemen, in the cathode) something to
do this winter.
HTH.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"an unbelievably comprehensive web site" -- Hi-Fi News & Record Review
Ian Iveson wrote:
I can't speak for other heads, but the inside of mine finds itself
no further forward. It would help if you were to say what you are
using as a source in your listening test.
Using the battery *instead of* the grid leak gives you a 1ohm shunt
across the lower leg of the attenuator, which is not a good test of
the principle. Perhaps you mean *in series with* the grid leak?
All methods of grid bias in the absence of DC blocking will result
in biasing the source, obviously. Here again it is not clear whether
that in itself may have degraded the sound, so again the test is
inconclusive.
A battery at the foot of the attenuator would not discharge itself
in the absence of a grid leak, and the circuit would be just as
simple as the one you have chosen. The battery could also be charged
through the signal input with the amp off, so recharging could be a
switched function of a dedicated preamp. I dare say there would be
objections to this option, and I wonder what they would be.
What I would really like to know is would it be better, leaving
aside the issue of DC blocking and recharging, to bias via the grid
or the cathode? And would a series or a shunt connection to the grid
be more appropriate?
Do batteries have the same impedance in both directions? Are they
more noisy when discharging, or charging, or in equilibrium? What
are the mechanisms of distortion and will it be less in the
high-impedance low current input or the low impedance high current
cathode circuit. Will it be less by a factor greater than the
relative gain?
I would be surprised if these issues had not already been resolved,
considering you say that the cathode connection is so common. It
seems unlikely that the obvious alternatives were not explored, or
that the merely practical charging and DC blocking issues won the
day with the "ultra fi" fetishists.
And don't forget all of your famous guru friends, who I am sure
would be happy to advise you. I would be interested to hear their
views.
cheers, Ian
"Andre Jute" <fiultra@yahoo.com> wrote in message
news:1129658943.962078.103860@g14g2000cwa.googlegroups.com...
Uh-uh, Bruce. What you're talking about is the battery in the
place of
the grid leak resistor, between grid and ground. That's a low
impedance
path to ground. I've tried that one with a DACT, a high precision
Swiss
medical switch with SMD resistors and ultrashort paths, with the
tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.
I republished the T39 KISS Ultrafi circuit with the batteries in
the
third possible position, in the cathode circuit, simply because I
could
hear no difference with the battery in the grid leak position, and
the
cathode circuit position has the very great advantage of being
more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.
One final note on the *discussion* circuit I published (battery
between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and
several
others. But I'm not interested in defending a circuit before
professional production engineers. I'm interested in simplifying
the
circuit to what sounds best, regardless of expense or
"professional
practice", and in this case, because I am publishing the circuit
for
amateurs, in making it reasonably longlasting (there is a
companion
circuit, called the T44 "Populaire", with autobias, recommended
for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...
Thanks to all who contributed valuable insights (even if we were
all
talking about different circuits!). I'm removing the discussion
circuit
because it really doesn't add much to our wisdom; it may be a
viable
alternative (in that it works) but it is a decidedly less
desirable one
than the battery in the cathode circuit. That's the point of
discussion
with one's peers, to choose the best course of action from among
possibilities.
Andre Jute |
|
|
| Back to top |
|
|
Patrick Turner
Guest
|
| Posted:
Wed Oct 19, 2005 1:54 pm Post subject:
Re: Battery bias directly to grid |
|
|
bruce seifried wrote:
| Quote: | In article <43551ED2.2319BC54@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353F476.257BE164@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
bruce seifried wrote:
In article <4353775E.5B755A29@turneraudio.com.au>,
Patrick Turner <info@turneraudio.com.au> wrote:
But the transients you speak of don't occur any more than with any
other
biasing method.
Patrick Turner.
With the Dact 20K input attenuator, and a +2.4 volt dc source connected
to its wiper, you will see something on the order of a few tens of
millivolts being generated when changing the gain setting at the low
end
of the attenuator. Feeding this to the input of a dc-coupled power amp
will give you low level, audible transients. Not a deal breaker, but
not
good design practice.
Switching the attenuator from its lowest gain setting to off, or vice
versa, will give you a 2.4 volt step signal into your input. I would
call this a rather large transient, and not especially good for the
health of any speaker attached.
-bruce seifried
I don't understand how switching gain settings changes the tube bias;
there is always -2.4V at the grid regardless of the gain setting.
Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.
The only chance of transients being generated by gain level change is if
the
input grid draws dc current and a small dc voltage exists across the
resistances
of the DACT, so that when switching, you hear the switch steps.
Are we talking about the same circuit?
There is no chance of transient problems with the schematic Andre sent me,
it has a 20k DACT attenuator, then the wiper goes to a battery, then
to the grid.
So at all levels of gain the bias remains and there are no transients.
The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
Huh?
I don't follow you at all.
You must be talking about some other circuit.
Patrick Turner.
Aha... apparently we *are* talking about different cicuits.
Until it was changed several days ago, I had been referencing the T39
schematic on this page:
http://members.lycos.co.uk/fiultra/T39-KISS-300B-Ultrafi-crct.jpg
I was never able to view the other schematic due to unknown browser
demons, but its description sounded like that of the T39 input biasing
arrangement, which I must say was bizarre, but easily fixable (...and I
happen to believe that a good quality capacitor makes a better capacitor
than a battery).
Carry on...
-bruce seifried
|
Both of Andre's circuits which use battery bias either in the grid or cathode
circuit
are NOT prone to switching transients when the gain is changed.
Patrick Turner.
| Quote: |
Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.
I have heard this in amps with a DACT. But its a tiny effect compared to
music transients.
Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.
This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.
cheers,
-bruce seifried |
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