| Author |
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Andre Jute
Guest
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 Posted:
Sun Oct 16, 2005 9:17 pm Post subject:
Battery bias directly to grid |
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I've posted a circuit for discussion at:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
What are the implications of this circuit?
What, theoretically, makes it better than the standard battery bias
application in the cathode circuit?
Will the batteries charge as efficiently in this position as in the
cathode circuit; what are the effects of the resistors topping and
tailing the battery set?
What are this circuit's dangers?
Are there alternatives that work better or are safer?
Andre Jute
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Patrick Turner
Guest
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| Posted:
Sun Oct 16, 2005 10:40 pm Post subject:
Re: Battery bias directly to grid |
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Andre Jute wrote:
| Quote: | I've posted a circuit for discussion at:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
What are the implications of this circuit?
What, theoretically, makes it better than the standard battery bias
application in the cathode circuit?
Will the batteries charge as efficiently in this position as in the
cathode circuit; what are the effects of the resistors topping and
tailing the battery set?
What are this circuit's dangers?
Are there alternatives that work better or are safer?
Andre Jute
|
Unfortunately the images won't appear in the page in either Netscape
or IE.
I have to say I have yet to use a battery in a tube amp for biasing,
but they can work well and unlike even large value electrolytic caps
used for bypassing an Rk the battery remains a low impedance as F falls.
So the battery acts to render the amp a fixed bias amp, with possibly
better bass extension.
But for me the use of batteries isn't warranted since i can easily
achieve a bandwidth down to 5Hz without much phase shift
so any improvement due to batteries might be an illusion.
And the idea of a battery leaking junk liquid all over the internals
isn't nice...
If I could see more of the details I might be able to comment
more appropriately.
Patrick Turner. |
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Andre Jute
Guest
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| Posted:
Sun Oct 16, 2005 11:18 pm Post subject:
Re: Battery bias directly to grid |
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Circuit sent to you by e-mail. Anyone else unable to see it? It works
on my Mac in Internet Explorer but I see that in Safari the gremlins
render the URL copied from the newsgroup and pasted into the search box
as
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htmpage/
from which you have to knock off the "page/" before it works. This is
the right URL again...
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
Andre Jute
Patrick Turner wrote:
| Quote: | Andre Jute wrote:
I've posted a circuit for discussion at:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
What are the implications of this circuit?
What, theoretically, makes it better than the standard battery bias
application in the cathode circuit?
Will the batteries charge as efficiently in this position as in the
cathode circuit; what are the effects of the resistors topping and
tailing the battery set?
What are this circuit's dangers?
Are there alternatives that work better or are safer?
Andre Jute
Unfortunately the images won't appear in the page in either Netscape
or IE.
I have to say I have yet to use a battery in a tube amp for biasing,
but they can work well and unlike even large value electrolytic caps
used for bypassing an Rk the battery remains a low impedance as F falls.
So the battery acts to render the amp a fixed bias amp, with possibly
better bass extension.
But for me the use of batteries isn't warranted since i can easily
achieve a bandwidth down to 5Hz without much phase shift
so any improvement due to batteries might be an illusion.
And the idea of a battery leaking junk liquid all over the internals
isn't nice...
If I could see more of the details I might be able to comment
more appropriately.
Patrick Turner. |
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Sander deWaal
Guest
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| Posted:
Sun Oct 16, 2005 11:25 pm Post subject:
Re: Battery bias directly to grid |
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"Andre Jute" <fiultra@yahoo.com> said:
Both Mozilla and IE in XP won't show the circuits......
Anyway, we've had this discussion before, IIRC?
A rechargeable battery in the cathode circuit will be kept charged,
while a battery in the grid circuit will draw minimal current as to
last several years. But what happens after that?
And what is the audible difference between the 2 possibilities?
Have you tried them, Andre?
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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Andre Jute
Guest
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| Posted:
Sun Oct 16, 2005 11:37 pm Post subject:
Re: Battery bias directly to grid |
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For those who cannot see it, the circuit consists of a DACT 20K ladder
attenuator to ground, then two AA 1.2V batteries in series from the
wiper of the attenuator to the grid of a 417A tube. The cathode of the
tube is connected directly to ground. That is the entire circuit under
discussion. -- AJ
Andre Jute wrote:
| Quote: | I've posted a circuit for discussion at:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
What are the implications of this circuit?
What, theoretically, makes it better than the standard battery bias
application in the cathode circuit?
Will the batteries charge as efficiently in this position as in the
cathode circuit; what are the effects of the resistors topping and
tailing the battery set?
What are this circuit's dangers?
Are there alternatives that work better or are safer?
Andre Jute |
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Andre Jute
Guest
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| Posted:
Sun Oct 16, 2005 11:52 pm Post subject:
Re: Battery bias directly to grid |
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Sander deWaal wrote:
| Quote: | ...we've had this discussion before, IIRC?
A rechargeable battery in the cathode circuit will be kept charged,
while a battery in the grid circuit will draw minimal current as to
last several years. But what happens after that?
And what is the audible difference between the 2 possibilities?
Have you tried them, Andre?
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005
|
I just got bored reading the opinions of socially inept anoraks about
women who would leave the room if they entered and decided to throw in
something for discussion. I have no problem with stuff we discussed
before. There is always a little something extra to learn, and bias
methods are a lot less boring than most of the crap on the board right
now.
I constantly use battery bias in the cathode circuit. It gives you
better bass, as all kinds of fixed bias do. But John Byrns, the last
time we discussed the sound shaping of capacitors, said that the
battery is itself a soundshaping element and should be removed from the
cathode into the grid circuit.
I used direct grid battery bias briefly about fifteen years ago but it
involved me in screaming match with the Magnequest Scum. That was
before I learned that whatever I did they turned into a screaming
match, and that they and their hangers-would lie about electronics to
keep the screaming match going. I can't even remember what it sounded
like.
But questions remain to be answered before I entrust valuable tubes to
the topology. It is much more useful to work them out in RAT than to
put my mind in gear and hug my findings to me, wouldn't you say? (Or
has everyone now given up on the ARRL dream of sharing information,
which motivated the founders and early members of RAT?)
Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak. What would
happen in case of a loose connection? In fact, what would happen if
your suggestion of a battery running out of juice came true?
Are there any other questions to be answered with this circuit?
Andre Jute |
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Sander deWaal
Guest
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| Posted:
Mon Oct 17, 2005 12:14 am Post subject:
Re: Battery bias directly to grid |
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"Andre Jute" <fiultra@yahoo.com> said:
| Quote: | I constantly use battery bias in the cathode circuit. It gives you
better bass, as all kinds of fixed bias do. But John Byrns, the last
time we discussed the sound shaping of capacitors, said that the
battery is itself a soundshaping element and should be removed from the
cathode into the grid circuit.
|
My instincts tell me to keep the battery or rechargeable battery in
the cathode circuit.
A battery in the grid circuit via a potmeter would mean DC on the pot,
which introduces noise.
Elsewhere you mention the use of a stepped DACT attenuator. Good.
That ekiminates pot noise.
But from the way you describe it, it seems that the wiper of the
attenuator is connected via two AA cells to the grid.
This means you can't adjust -Vg, you just vary the resistance to
ground. -Vg is determined by the 2 cells, which is 3 volts.
A battery in the grid circuit means excellent frequency response in
the very low frequency region, but is that a wise thing in an amp
where I assume there is no global feedback?
This means the only limiting factor is the output transformer, which
now has to deal with very low frequencies, and that will pose some
problems by itself.
As a rule, I let the first driver stage of an amp cutoff the lowest
frequencies, by deliberately choosing a small cathode cap.
Bear in mind that I always build PP amps though, the sonical effect
may well be very significant in a SE amplifier.
| Quote: | Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak. What would
happen in case of a loose connection? In fact, what would happen if
your suggestion of a battery running out of juice came true?
|
The Ri would increase, to the point where the grid may float a bit
free.
Reverse grid current would come into play, which usually isn't a good
thing.
Of course, this is all armchair engineering on my part :-)
I've never tried batteries in either the cathode or grid circuits, but
it seems to be a simple and elegant solution.
--
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005 |
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Andre Jute
Guest
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| Posted:
Mon Oct 17, 2005 1:00 am Post subject:
Re: Battery bias directly to grid |
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Sander deWaal wrote:
| Quote: | "Andre Jute" <fiultra@yahoo.com> said:
I constantly use battery bias in the cathode circuit. It gives you
better bass, as all kinds of fixed bias do. But John Byrns, the last
time we discussed the sound shaping of capacitors, said that the
battery is itself a soundshaping element and should be removed from the
cathode into the grid circuit.
My instincts tell me to keep the battery or rechargeable battery in
the cathode circuit.
A battery in the grid circuit via a potmeter would mean DC on the pot,
which introduces noise.
Elsewhere you mention the use of a stepped DACT attenuator. Good.
That ekiminates pot noise.
But from the way you describe it, it seems that the wiper of the
attenuator is connected via two AA cells to the grid.
This means you can't adjust -Vg, you just vary the resistance to
ground. -Vg is determined by the 2 cells, which is 3 volts.
|
There are basically three places you can inject battery bias into a
stage. One is into the cathode circuit, where the battery takes the
place of the resistor and the bypas cap; this is the standard way I
have always used, elegant and goodsounding. The second place is in the
input circuit directly between the wiper of the pot at the input and
the grid, as already described. The third is in the place of the grid
leak resistor. The third one is the only place where small multiturn
pot is really conveniently introduced to adjust the negative bias
voltage from a battery. My idea of simplicity has always been simply to
use the battery without adjustment. The voltage is around 2.4V for two
AAA batteries, not the nominal 3V you mention. That is ideal for
running my favourite 417A hot and hard (it doesn't come alive at
textbook dissipation but it is a sturdy little tube that one British
custom amp maker operated at 200V for amps he sold without a problem)
with the full 2V available from CD players.
| Quote: |
A battery in the grid circuit means excellent frequency response in
the very low frequency region, but is that a wise thing in an amp
where I assume there is no global feedback?
|
I'm talking about battery bias for an input tube. I take care of the
frequency choking in the driver tube's cathode arrangements.
| Quote: | This means the only limiting factor is the output transformer, which
now has to deal with very low frequencies, and that will pose some
problems by itself.
|
Not to mention, if this were true, what it would do to my modestly
sized horns!
| Quote: | As a rule, I let the first driver stage of an amp cutoff the lowest
frequencies, by deliberately choosing a small cathode cap.
Bear in mind that I always build PP amps though, the sonical effect
may well be very significant in a SE amplifier.
|
A good trick with simple SE amps is to decide your cutoff and tune all
stages, including the power, to the same time constant. It is worth a
little extra clarity in the sound.
| Quote: | Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak. What would
happen in case of a loose connection? In fact, what would happen if
your suggestion of a battery running out of juice came true?
The Ri would increase, to the point where the grid may float a bit
free.
Reverse grid current would come into play, which usually isn't a good
thing.
Of course, this is all armchair engineering on my part :-)
|
Well, no, I don't see that as a negative. I'd rather not build a
circuit that will give trouble of any kind. I'm not an experimenter who
never switches his soldering iron off. I like knowing how my amp will
respond before I build it. Of course, we are at the margins of audible
improvement here.
| Quote: | I've never tried batteries in either the cathode or grid circuits, but
it seems to be a simple and elegant solution.
|
Simple and elegant is always good for me. That is how I first started
on battery bias, looking for parts I could elimate. That gaincard guy
is a piker without the courage of his convictions.
| Quote: | --
"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005
|
Poor Howard might be into silicon, and quite unaware of coining an
awful, awful pun.
Andre Jute |
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bruce seifried
Guest
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| Posted:
Mon Oct 17, 2005 1:10 am Post subject:
Re: Battery bias directly to grid |
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In article <1129479453.815805.139670@z14g2000cwz.googlegroups.com>,
"Andre Jute" <fiultra@yahoo.com> wrote:
| Quote: | I've posted a circuit for discussion at:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm
What are the implications of this circuit?
What, theoretically, makes it better than the standard battery bias
application in the cathode circuit?
Will the batteries charge as efficiently in this position as in the
cathode circuit; what are the effects of the resistors topping and
tailing the battery set?
What are this circuit's dangers?
Are there alternatives that work better or are safer?
Andre Jute
|
I note that the circuit you're describing is quite similar to an amp
design you have published on your website, the Type 39, Series VI "The
Tradition":
http://members.lycos.co.uk/fiultra/T39-KISS-300B-Ultrafi-crct.jpg
I assume you've actually built this circuit and tried it, so you should
know that rather then the circuit *charging* the batteries, they will
eventually be *discharged* by the input attenuator, especially in low
gain settings. You are also sending dc out the input jack to the
preceding piece of gear, which, in the case of a transformered output
circuit, will discharge the batteries at *high* gain settings.
Changing the gain setting will also tend to send a transient pulse into
your speakers.
If you're going to use this topology, you'll need to bite the bullet and
use a cap between the pot and the batteries/grid.
-bruce seifried |
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Ruud Broens
Guest
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| Posted:
Mon Oct 17, 2005 2:33 am Post subject:
Re: Battery bias directly to grid |
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"Andre Jute" <fiultra@yahoo.com> wrote in message
news:1129492844.739919.248050@o13g2000cwo.googlegroups.com...
:
: Sander deWaal wrote:
: > "Andre Jute" <fiultra@yahoo.com> said:
: >
: > >I constantly use battery bias in the cathode circuit. It gives you
: > >better bass, as all kinds of fixed bias do. But John Byrns, the last
: > >time we discussed the sound shaping of capacitors, said that the
: > >battery is itself a soundshaping element and should be removed from the
: > >cathode into the grid circuit.
: >
: >
: > My instincts tell me to keep the battery or rechargeable battery in
: > the cathode circuit.
: > A battery in the grid circuit via a potmeter would mean DC on the pot,
: > which introduces noise.
: >
: > Elsewhere you mention the use of a stepped DACT attenuator. Good.
: > That ekiminates pot noise.
: > But from the way you describe it, it seems that the wiper of the
: > attenuator is connected via two AA cells to the grid.
: > This means you can't adjust -Vg, you just vary the resistance to
: > ground. -Vg is determined by the 2 cells, which is 3 volts.
:
: There are basically three places you can inject battery bias into a
: stage. One is into the cathode circuit, where the battery takes the
: place of the resistor and the bypas cap; this is the standard way I
: have always used, elegant and goodsounding. The second place is in the
: input circuit directly between the wiper of the pot at the input and
: the grid, as already described.
: The third is in the place of the grid leak resistor.
Did you try that (as drawn in the T39 mk6)?
Problem 1:
you constantly discharging the bias batteries through the DACT.
Problem 2:
While not perfect, a battery of AA type will still have quite a
low (voltage source) impedance (well below 1 Ohm)
- not much input signal remaining when it's shorted by a battery :-)
Problem 3:
when the source signal is not capacitor-coupled, then surely at
max volume you risk frying something there :(
Don't try this at home, kids :-)
Second version has it's problems, eg. when the batteries go flat,
your bias will only be 1V or less
In the cathode circuit seems best. As presumably the environmental
temperature is going to be high, a selfdischarge period of only a few
months wouldn't surprise me. A 20 mA cathode current will have to
do trickle charging - depending on how many hours you use the amp,
it would require a 3000 mAH cell type (heavy use of amp) up to an
AAA 450 mAH cell (occasional use). In the latter case, the bias will
change the first couple of hours as the cells are recharged after a long
inactive period. Hmm.
Rudy.
: Andre Jute
: |
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Adam Stouffer
Guest
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Chris Hornbeck
Guest
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| Posted:
Mon Oct 17, 2005 3:37 am Post subject:
Re: Battery bias directly to grid |
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On 16 Oct 2005 11:52:27 -0700, "Andre Jute" <fiultra@yahoo.com> wrote:
| Quote: | Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak.
|
It'll work just fine. If you're worried about signal pass
through the battery put a nice film cap across it. IME,
not necessary though.
Another variation that can be used with floating sources,
like phono inputs, is to put the battery between input
signal "grounds" and amplifier/chassis ground. A single
cell is perfect for 12AX7's. The phono cartridge floats
up at bias voltage.
| Quote: | What would
happen in case of a loose connection?
|
Exactly the same as if cathode resistor bias were used.
| Quote: | In fact, what would happen if
your suggestion of a battery running out of juice came true?
|
You're using a plate resistor, aren't you? Sorry,
didn't see the circuit.
Thanks, as always,
Chris Hornbeck
"and thus it's your EXPECTATIONS exceding the homeosonic design
limitations that you're misinterpreting..." -SSJV |
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Chris Hornbeck
Guest
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| Posted:
Mon Oct 17, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
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On Sun, 16 Oct 2005 23:07:51 -0400, Jon Yaeger <jono_1@bellsouth.net>
wrote:
| Quote: | ***** I didn't know there were four grids in that tube. Now it makes
sense.
|
He meant four grid connection pins. The 417 was originally
meant for grounded grid service.
Chris Hornbeck |
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Jon Yaeger
Guest
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| Posted:
Mon Oct 17, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
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in article 1129510760.869425.23050@g47g2000cwa.googlegroups.com, Andre Jute
at fiultra@yahoo.com wrote on 10/16/05 8:59 PM:
***** I'm responding to a couple of points, not to be argumentative, but
because I really don't understand.
| Quote: | Jon Yaeger wrote:
in article 1129488747.358162.79260@g44g2000cwa.googlegroups.com, Andre Jute
at fiultra@yahoo.com wrote on 10/16/05 2:52 PM:
I constantly use battery bias in the cathode circuit. It gives you
better bass, as all kinds of fixed bias do. But John Byrns, the last
time we discussed the sound shaping of capacitors, said that the
battery is itself a soundshaping element and should be removed from the
cathode into the grid circuit.
*** I'm not an engineer, so I don't have the benefit of backing up my
opinions with good quantitative data.
Nonetheless, if simplicity is your goal I can't imagine how you could
benefit from putting something as complex and as variable as a NiCad in the
signal path (could it be considered an "active" passive device?)
A nicad is less variable than a cap and less noisy than a resistor. The
question we're answering is, in which part of the signal circuit should
it be put? No part of a tube stage (or amp for that matter) is without
the signal path!
|
***** Granted, a power supply is a signal path, but intuitiveIy it seems
that a component directly in line with a signal would have more effect on
the sound than something "on the side."
***** I could accept on faith that a battery might have less noise than a
resistor, but how is it less variable than a cap, especially with respect to
time? Are you comparing it to a film or an electrolytic cap? An
unregenerated NiCad has a degradation cycle with a much longer duty cycle
than an (electrolytic) capacitor's discharge cycle, no?
| Quote: |
NiCads have a non-linear discharge curve and internal impedance increases as
charge decreases. This changes the setpoint of your input level control and
linearity overall. NiCads also have the potential to actually reverse
polarity if they are allowed to discharge fully. You can imagine what a
positive grid bias would do to the current drawn by the tube . . . .
That's merely an argument for putting the battery in the cathode
circuit where it will be charged.
|
***** OK. That changes the entire perspective. But how exactly does a
battery get charged in a cathode circuit? The charging EMF across the
battery's terminals must exceed the voltage generated by the chemical
reaction, for charging to occur.
| Quote: |
As pointed out by others, there is no mechanism for charging the cell.
Not in the direct signal path between pot wiper and grid. But in the
cathode circuit the battery is constantly charged.
NiCads are supposed to be charged with DC (pulsating DC is fine) but I
believe A/C is inadvisable.
There is no AC on two out of the three possibilities, and the AC signal
on the third possible placement of the battery, between pot wiper and
grid, has a very low current.
What kind of artifacts, distortions, or noise are introduced by a cell in
line with a signal? This ought to be observable with a scope and a signal
source. I would certainly know what the waveforms look like before I did
anything else (although I personally would not employ one in this manner).
The battery (in the cathode anyway) looks better on the scope than the
resistor and cap. I have no idea how it scopes in the other two
positions.
|
***** Easy to test if you've already prototyped the circuit.
| Quote: |
I wish someone would kindly explain the alleged benefit of using four grid
stopper resistors where one would do. On the face of things, it would seem
to add complexity and noise and little else.
You'd be surprised how many tubes are radiophonic, because, surprise,
surprise, they were designed to be radio frequency tubes. The tube I
use so often, the 417A, is an RF tube. It has four grids and RF has be
stopped right up close to the pin, so it is convenient to use four
resistors and to use the unconnected centre pin of the tube for the
other ends of these resistors and the input. The 417A is also tricky in
layout, because if the paths are fractions of an inch too long the tube
picks up rubbish from the air. Furthermore, you'd better not put hang
more than 10K on its grid. It is one of those tubes you just have to
know about before you start implementing it, or your life could be very
frustrating for a very long time as you debug the circuit and layout.
But the sonic qualities make the troubles worth the result.
|
***** I didn't know there were four grids in that tube. Now it makes
sense.
| Quote: |
To summarize, battery bias directly in the cathode circuit is proven
and preferred to autobias by many audiophiles, and the other two
possibilities for placing battery bias have serious downsides, as
listed in posts by others and summarized by me in a recent post.
|
***** I've recently been cautioned about the importance of layout and
topology in sonic "signature." Stuff from a fellow who has spent his life
building and testing both tube and SS amps. He has hundreds of thousands of
dollars in sophisticated test gear. After he makes his measurements he
listens. Some of his observations run against the grain of "accepted"
wisdom. Like a star ground is not optimum. And never ground the signal to
the chassis.
| Quote: |
Thanks for polite input.
|
***** Well, being an ass wasn't getting me anywhere . . . .
;-)
| Quote: |
Andre Jute
Jon
I used direct grid battery bias briefly about fifteen years ago but it
involved me in screaming match with the Magnequest Scum. That was
before I learned that whatever I did they turned into a screaming
match, and that they and their hangers-would lie about electronics to
keep the screaming match going. I can't even remember what it sounded
like.
But questions remain to be answered before I entrust valuable tubes to
the topology. It is much more useful to work them out in RAT than to
put my mind in gear and hug my findings to me, wouldn't you say? (Or
has everyone now given up on the ARRL dream of sharing information,
which motivated the founders and early members of RAT?)
Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak. What would
happen in case of a loose connection? In fact, what would happen if
your suggestion of a battery running out of juice came true?
Are there any other questions to be answered with this circuit?
Andre Jute
|
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Andre Jute
Guest
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| Posted:
Mon Oct 17, 2005 4:43 am Post subject:
Re: Battery bias directly to grid |
|
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Jon Yaeger wrote:
| Quote: | in article 1129488747.358162.79260@g44g2000cwa.googlegroups.com, Andre Jute
at fiultra@yahoo.com wrote on 10/16/05 2:52 PM:
I constantly use battery bias in the cathode circuit. It gives you
better bass, as all kinds of fixed bias do. But John Byrns, the last
time we discussed the sound shaping of capacitors, said that the
battery is itself a soundshaping element and should be removed from the
cathode into the grid circuit.
*** I'm not an engineer, so I don't have the benefit of backing up my
opinions with good quantitative data.
Nonetheless, if simplicity is your goal I can't imagine how you could
benefit from putting something as complex and as variable as a NiCad in the
signal path (could it be considered an "active" passive device?)
|
A nicad is less variable than a cap and less noisy than a resistor. The
question we're answering is, in which part of the signal circuit should
it be put? No part of a tube stage (or amp for that matter) is without
the signal path!
| Quote: | NiCads have a non-linear discharge curve and internal impedance increases as
charge decreases. This changes the setpoint of your input level control and
linearity overall. NiCads also have the potential to actually reverse
polarity if they are allowed to discharge fully. You can imagine what a
positive grid bias would do to the current drawn by the tube . . . .
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That's merely an argument for putting the battery in the cathode
circuit where it will be charged.
| Quote: | As pointed out by others, there is no mechanism for charging the cell.
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Not in the direct signal path between pot wiper and grid. But in the
cathode circuit the battery is constantly charged.
| Quote: | NiCads are supposed to be charged with DC (pulsating DC is fine) but I
believe A/C is inadvisable.
|
There is no AC on two out of the three possibilities, and the AC signal
on the third possible placement of the battery, between pot wiper and
grid, has a very low current.
| Quote: |
What kind of artifacts, distortions, or noise are introduced by a cell in
line with a signal? This ought to be observable with a scope and a signal
source. I would certainly know what the waveforms look like before I did
anything else (although I personally would not employ one in this manner).
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The battery (in the cathode anyway) looks better on the scope than the
resistor and cap. I have no idea how it scopes in the other two
positions.
| Quote: | I wish someone would kindly explain the alleged benefit of using four grid
stopper resistors where one would do. On the face of things, it would seem
to add complexity and noise and little else.
|
You'd be surprised how many tubes are radiophonic, because, surprise,
surprise, they were designed to be radio frequency tubes. The tube I
use so often, the 417A, is an RF tube. It has four grids and RF has be
stopped right up close to the pin, so it is convenient to use four
resistors and to use the unconnected centre pin of the tube for the
other ends of these resistors and the input. The 417A is also tricky in
layout, because if the paths are fractions of an inch too long the tube
picks up rubbish from the air. Furthermore, you'd better not put hang
more than 10K on its grid. It is one of those tubes you just have to
know about before you start implementing it, or your life could be very
frustrating for a very long time as you debug the circuit and layout.
But the sonic qualities make the troubles worth the result.
To summarize, battery bias directly in the cathode circuit is proven
and preferred to autobias by many audiophiles, and the other two
possibilities for placing battery bias have serious downsides, as
listed in posts by others and summarized by me in a recent post.
Thanks for polite input.
Andre Jute
| Quote: | Jon
I used direct grid battery bias briefly about fifteen years ago but it
involved me in screaming match with the Magnequest Scum. That was
before I learned that whatever I did they turned into a screaming
match, and that they and their hangers-would lie about electronics to
keep the screaming match going. I can't even remember what it sounded
like.
But questions remain to be answered before I entrust valuable tubes to
the topology. It is much more useful to work them out in RAT than to
put my mind in gear and hug my findings to me, wouldn't you say? (Or
has everyone now given up on the ARRL dream of sharing information,
which motivated the founders and early members of RAT?)
Battery bias directly in the grid circuit between pot wiper and the
tube grid uses the downwards leg of a pot as the grid leak. What would
happen in case of a loose connection? In fact, what would happen if
your suggestion of a battery running out of juice came true?
Are there any other questions to be answered with this circuit?
Andre Jute
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