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Jon Yaeger
Guest

Posted: Tue Sep 06, 2005 4:07 am Post subject: Potentiometer question

I'm in the process of reconfiguring an Eico 2080 integrated tube amplifier (essentially a glorified ST-70). I've got a motorized ALPS 50K dual pot that I'd like to use for the volume control. I am planning to use a 1 Meg balance control. I'm going to remove the tone control circuit and leave the 6EU7 phono circuit in place. The existing level control is 500K, and the anode resistor on the 2nd stage of the phono circuit is 40K. The level control feeds the grid of 12AX7 without a grid stopper resistor. I've posted the relevant schematic on alt.binaries.schematics.electronic. I'd imagine the 50K load is a bit low for the phono section. It will no doubt change the frequency poles and response of the amplifier. It is hard to find a higher value ALPS control . . . What will be the effects of the lower potentiometer and what options and suggestions do you recommend? Thanks Jon

Ian Iveson
Guest

Posted: Tue Sep 06, 2005 4:45 am Post subject: Re: Potentiometer question

"Jon Yaeger" wrote

Quote:

I'm in the process of reconfiguring an Eico 2080 integrated tube
amplifier
(essentially a glorified ST-70).

I've got a motorized ALPS 50K dual pot that I'd like to use for
the volume
control. I am planning to use a 1 Meg balance control.

I'm going to remove the tone control circuit and leave the 6EU7
phono
circuit in place.

The existing level control is 500K, and the anode resistor on the
2nd stage
of the phono circuit is 40K. The level control feeds the grid of
12AX7
without a grid stopper resistor. I've posted the relevant
schematic on
alt.binaries.schematics.electronic.

I'd imagine the 50K load is a bit low for the phono section. It
will no
doubt change the frequency poles and response of the amplifier.
It is hard
to find a higher value ALPS control . . .

What will be the effects of the lower potentiometer and what
options and
suggestions do you recommend?

Jon Yaeger
Guest

Posted: Tue Sep 06, 2005 4:45 am Post subject: Re: Potentiometer question

Quote:

Ian,

Thanks for your reply and for the links.

I've got other ALPS pots, including detented 250 MB. And tons of Noble
controls, too.

I want to use a MOTORIZED control, and I gather the choices are very
limited. I planned to put Mikell Simonsen's remote volume control kit to
work, and he supplies a 50K pot. I robbed another motorized pot from an
Onkyo receiver and this was also 50K. Hence my predicament . . . .

Best,

jon

Ian Iveson
Guest

Posted: Tue Sep 06, 2005 4:45 am Post subject: Re: Potentiometer question

"Jon Yaeger" wrote

Quote:

...I want to use a MOTORIZED control, and I gather the choices are
very
limited. I planned to put Mikell Simonsen's remote volume control
kit to
work, and he supplies a 50K pot. I robbed another motorized pot
from an
Onkyo receiver and this was also 50K. Hence my predicament . . .
.

OK...in my simple mind I imagined a separate motor. Sorry. I can't
find rotary motorised Panasonics anymore and I guess neither can
Mikkel. I hope he's charging proper money for his gizmos these days,
incidentally.

Different mind set...I buy only what I use so I have almost nothing
lying about.

You could use a buffer stage, but I suppose you really want to know
about the equalisation part of problem. I don't know what an RIAA
curve looks like, but I can show you the response of your circuit
and try out any ideas you may have about how to modify it.

cheers, Ian

Patrick Turner
Guest

Posted: Tue Sep 06, 2005 7:21 pm Post subject: Re: Potentiometer question

Jon Yaeger wrote:

Quote:

Just a quick comment

6EU7 appears to be a quiet 12AX7? If so the circuit is easy to
simulate, but for the moment:

100k alps blue are quite common I thought...here for example

http://www.partsconnexion.com/catalog/controlsswitches.html

also note a couple of 250k Noble on same page.

For 500k and others there is here:

http://www.bella-audioparts.com/volume%20pots.htm

Just happened to be wandering about suppliers to Audio Note...and
thinking I can get shitloads of dollars for a Pound or a Euro...

cheers, Ian

Ian,

Thanks for your reply and for the links.

I've got other ALPS pots, including detented 250 MB. And tons of Noble
controls, too.

I want to use a MOTORIZED control, and I gather the choices are very
limited. I planned to put Mikell Simonsen's remote volume control kit to
work, and he supplies a 50K pot. I robbed another motorized pot from an
Onkyo receiver and this was also 50K. Hence my predicament . . . .

Best,

jon

Why don't you add another twin triode to make a cathode follower buffer
using a 12AU7? then that would power the 50k pot which is a nice value for a
pot
because with a low source R the maximum Rout of the pot is 12.5kohms
at the -6dB point.
This minimisers noise and HF losses.

I can't get the ABSE postings; the headers I get are over a week old
and don't include the binaries posts.

The 6EU7 will easily power a 33k load, ( 100k DC RL + cap coupled 50k pot ).
If the Iaq of the 6EU7 is 0.7mA, then 10Vrms into 33k is only a 0.303 mA Ia
swing.

Work out your voltages used in actual normal use, or measure the thd.
If thd is not seriously increased by the lower load, she'll be right on that
score,
but you were worried about the F response changing due to loading.

I can't see the schematic, but maybe things are not as bad as you think;
6EU7 gain may still be about enough with a load drop from say 100k to 33k.

I routinely use 50k pots in preamp but with 6CG7 anodes which are fed with
a bjt CCS, thus the load seen by the anode is *only* 50k, a nice value to get

low thd with a medium µ triode.

If you don't mind a 6 dB reduction in circuit sensitivity, you can also
use a dual 100k linear balance pot so that each 1/2 of the pot is
in *series* with the gain pot.
In the centre position, the gain pot = 50k, so balance circuit insertion loss

is 6dB, and when swinging the balance you get a maximum +6dB boost to one
channel and
-3 dB cut in the other.
Normally the balance is set centrally, so RL seen by the source is
50k balance pot + 50k gain pot = 100k. See my old schematic at
http://www.turneraudio.com.au/htmlwebpgs02/schem10tubepreamp.htm

Perhaps you could email me your EICO schematic.
Try to keep it under 250kB please...

Patrick Turner.

Patrick Turner
Guest

Posted: Tue Sep 06, 2005 7:26 pm Post subject: Re: Potentiometer question

Jon Yaeger wrote:

Quote:

Just another idea, perhaps you could place a mosfet source follower direct
coupled off the
6EU7 anode circuit.
ARC does this routinely in their hybrid SP11 preamps.

Fitting a mosfet would be a lot easier than fitting a another tube to make a
cathode follower
or to use a µ follower like I have in my april2000 preamp circuit.

Patrick Turner.

Jon Yaeger
Guest

Posted: Tue Sep 06, 2005 7:32 pm Post subject: Re: Potentiometer question

in article 431DA628.BCED4192@turneraudio.com.au, Patrick Turner at
info@turneraudio.com.au wrote on 9/6/05 10:26 AM:

Quote:

Jon Yaeger wrote:

Just a quick comment

6EU7 appears to be a quiet 12AX7? If so the circuit is easy to
simulate, but for the moment:

100k alps blue are quite common I thought...here for example

http://www.partsconnexion.com/catalog/controlsswitches.html

also note a couple of 250k Noble on same page.

For 500k and others there is here:

http://www.bella-audioparts.com/volume%20pots.htm

Just happened to be wandering about suppliers to Audio Note...and
thinking I can get shitloads of dollars for a Pound or a Euro...

cheers, Ian

Ian,

Thanks for your reply and for the links.

I've got other ALPS pots, including detented 250 MB. And tons of Noble
controls, too.

I want to use a MOTORIZED control, and I gather the choices are very
limited. I planned to put Mikell Simonsen's remote volume control kit to
work, and he supplies a 50K pot. I robbed another motorized pot from an
Onkyo receiver and this was also 50K. Hence my predicament . . . .

Best,

jon

Just another idea, perhaps you could place a mosfet source follower direct
coupled off the
6EU7 anode circuit.
ARC does this routinely in their hybrid SP11 preamps.

Fitting a mosfet would be a lot easier than fitting a another tube to make a
cathode follower
or to use a µ follower like I have in my april2000 preamp circuit.

Patrick Turner.

That's a good solution, but I want to keep the amp all glass (including the
rectifier!) if possible . . . .

Ian Iveson
Guest

Posted: Tue Sep 06, 2005 8:42 pm Post subject: Re: Potentiometer question

"Jon Yaeger" wrote

Quote:

below

The response as the circuit stands is pretty close to the ideal
RIAA, complete with rumble filter.

You will lose bass with a 50k pot. Compensate by reducing R10, the
resistor between the cathodes of the 6EU7, to around 56k to get it
back. Response is then very close to what you had.

Rumble filter will be more pronounced. You can compensate by
increasing C8 by lots...up to 10 times to give the same proportion
to the 50k.

Feedback to the second stage gives an output impedance of just a few
kohm at 1k, but it increases at low frequencies, reaching 5k at
250Hz, and 10k at 120Hz. Distortion will result, mostly 2H.

I assume the input signal is just a few millivolts. I don't know
what the HT is.

cheers, Ian

in message news:BF426F57.337D5%jono_1@bellsouth.net...

Quote:

Just a quick comment

6EU7 appears to be a quiet 12AX7? If so the circuit is easy to
simulate, but for the moment:

100k alps blue are quite common I thought...here for example

http://www.partsconnexion.com/catalog/controlsswitches.html

also note a couple of 250k Noble on same page.

For 500k and others there is here:

http://www.bella-audioparts.com/volume%20pots.htm

Just happened to be wandering about suppliers to Audio Note...and
thinking I can get shitloads of dollars for a Pound or a Euro...

cheers, Ian

Ian,

Thanks for your reply and for the links.

I've got other ALPS pots, including detented 250 MB. And tons of
Noble
controls, too.

I want to use a MOTORIZED control, and I gather the choices are
very
limited. I planned to put Mikell Simonsen's remote volume control
kit to
work, and he supplies a 50K pot. I robbed another motorized pot
from an
Onkyo receiver and this was also 50K. Hence my predicament . . .
.

Best,

jon

Ian Iveson
Guest

Posted: Tue Sep 06, 2005 10:39 pm Post subject: Re: Potentiometer question

Like this

http://www.ivesonaudio.pwp.blueyonder.co.uk/2080s1phono.GIF

cheers, Ian

"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:3OiTe.678$fb.303@fe1.news.blueyonder.co.uk...

Quote:

"Jon Yaeger" wrote

below

The response as the circuit stands is pretty close to the ideal
RIAA, complete with rumble filter.

You will lose bass with a 50k pot. Compensate by reducing R10, the
resistor between the cathodes of the 6EU7, to around 56k to get it
back. Response is then very close to what you had.

Rumble filter will be more pronounced. You can compensate by
increasing C8 by lots...up to 10 times to give the same proportion
to the 50k.

Feedback to the second stage gives an output impedance of just a
few kohm at 1k, but it increases at low frequencies, reaching 5k
at 250Hz, and 10k at 120Hz. Distortion will result, mostly 2H.

I assume the input signal is just a few millivolts. I don't know
what the HT is.

cheers, Ian

Chris Hornbeck
Guest

Posted: Wed Sep 07, 2005 4:45 am Post subject: Re: Potentiometer question

On Tue, 06 Sep 2005 17:39:25 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:

Quote:

Like this

http://www.ivesonaudio.pwp.blueyonder.co.uk/2080s1phono.GIF

Kind of the post-modern extrapolation from the
Dynaco era; very interesting and complete, but...

Why not a third stage follower? Is it such a crime?

Oy!

Thanks, of course, as always,

Chris Hornbeck

Jon Yaeger
Guest

Posted: Wed Sep 07, 2005 4:45 am Post subject: Re: Potentiometer question

in article hvkTe.11658$2n6.4601@fe3.news.blueyonder.co.uk, Ian Iveson at
IanIveson.home@blueyonder.co.uk wrote on 9/6/05 1:39 PM:

Quote:

Like this

http://www.ivesonaudio.pwp.blueyonder.co.uk/2080s1phono.GIF

cheers, Ian

"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:3OiTe.678$fb.303@fe1.news.blueyonder.co.uk...
"Jon Yaeger" wrote

below

The response as the circuit stands is pretty close to the ideal
RIAA, complete with rumble filter.

You will lose bass with a 50k pot. Compensate by reducing R10, the
resistor between the cathodes of the 6EU7, to around 56k to get it
back. Response is then very close to what you had.

Rumble filter will be more pronounced. You can compensate by
increasing C8 by lots...up to 10 times to give the same proportion
to the 50k.

Feedback to the second stage gives an output impedance of just a
few kohm at 1k, but it increases at low frequencies, reaching 5k
at 250Hz, and 10k at 120Hz. Distortion will result, mostly 2H.

I assume the input signal is just a few millivolts. I don't know
what the HT is.

cheers, Ian

Ian,

Thank you for taking considerable time to analyze the circuit. Nice CAD
program! Good information, and very generous of you. I will digest your
thoughts on component changes, too.

I thought of another possible approach to the problem, and have posted it on
ABSE, entitled "A Dumb Idea?"

Since I want to use a balance control, this approach may or may not be
suitable. Guess I have to get the books and pencils out to see if I can
guestimate the results. Or else build it and sweep it.

Thanks & best regards,

Jon

Patrick Turner
Guest

Posted: Wed Sep 07, 2005 10:26 pm Post subject: Re: Potentiometer question

Jon Yaeger wrote:

Quote:

in article hvkTe.11658$2n6.4601@fe3.news.blueyonder.co.uk, Ian Iveson at
IanIveson.home@blueyonder.co.uk wrote on 9/6/05 1:39 PM:

Like this

http://www.ivesonaudio.pwp.blueyonder.co.uk/2080s1phono.GIF

cheers, Ian

"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:3OiTe.678$fb.303@fe1.news.blueyonder.co.uk...
"Jon Yaeger" wrote

below

The response as the circuit stands is pretty close to the ideal
RIAA, complete with rumble filter.

You will lose bass with a 50k pot. Compensate by reducing R10, the
resistor between the cathodes of the 6EU7, to around 56k to get it
back. Response is then very close to what you had.

Rumble filter will be more pronounced. You can compensate by
increasing C8 by lots...up to 10 times to give the same proportion
to the 50k.

Feedback to the second stage gives an output impedance of just a
few kohm at 1k, but it increases at low frequencies, reaching 5k
at 250Hz, and 10k at 120Hz. Distortion will result, mostly 2H.

I assume the input signal is just a few millivolts. I don't know
what the HT is.

cheers, Ian

Ian,

Thank you for taking considerable time to analyze the circuit. Nice CAD
program! Good information, and very generous of you. I will digest your
thoughts on component changes, too.

I thought of another possible approach to the problem, and have posted it on
ABSE, entitled "A Dumb Idea?"

Since I want to use a balance control, this approach may or may not be
suitable. Guess I have to get the books and pencils out to see if I can
guestimate the results. Or else build it and sweep it.

Thanks & best regards,

Jon

I have had a look at your schematic of the Eico.

The existing schematic for each channel
after V3B and V4B shows a 0.25uF, with a 0.02uF bass cut cap that can be shunted
with a switch
and then 1/2 a dual 250k pot for balance in series with 1/2 a dual 500k pot for
gain which you wish to
replace with 50k.

The V3B and V4B are triodes with the tone control included as the FB circuit
so the Rout from the anodes is very low, less than 10k, because the tone control
stage
looks like a unity gain type using a Baxandal tone network in a shunt FB
arrangement.
Its a good way to do a tone control stage.
For it to work properly, the source must be a low R so that's why V3A, V4A
are cathode follower buffers to give the low R needed.

So all you have to do is change the dual 250k balance pot to
a dual 100k pot, and use the 50k level pot as you wish.
There will be some reduction of the signal level since at present there is
an effective divider of 125k balance pot plus 500k gain pot, so the balance
control gives an insertion loss of 2dB in the centre position.
Using 100k balance pot plus 50k gain pot gives a divider of 50k plus 50k
in the centre position so the insertion loss is 6dB, so the sensitivity of the
whole
amp is going to be about 4 dB less than what it is now.

If the amp was designed to go to full power with 200mV applied to all the line
level inputs, then it will need
330mV with the new pot values.

You should know what input voltages are needed for clipping with the volume
turned up fully.

The load on the tone control triodes will then be 100k with the new pots.

The tone control twin triodes are now 6EU7/12AX7 but could be replaced with
12AT7 if R41 and R40 are replaced with 39k to increase the idle current flow in
the
tubes.

You could even use 12AU7 for the whole tone control tubes,
but then you'd have to double the value of all the cathode bias R and
also make sure the B+ was sufficiently high, since instead of 0.7mA
that probably now flows in each 1/2 12AX7 you would have
2 mA at least.

The 0.25 cap needs to be increased to 2uF, and the 0.02 uf
to about 0.1 uF to keep around the same time constants for the CR coupling
and to get the rumble filter cut effect occuring at the same F.

With luck the whole thing won't motorboat when you crank the volume and set the
tone control
to max bass boost; this sort of circuit is renowned for doing this especially
in Phono as it is aided by a tendency for LF acoustic FB as well.

You will probably find there is a maximum bass boost and bass cut of +/- 16dB at

30Hz, and similar +/- at 20kHz, and this is absurdly excessive.
Using 12AU7 for the tone amp will reduce this cut and boost by about 6dB, still
leaving more than you should ever need; max boost and cut of LF & HF should be
no more
than 8dB.

So be prepared to abolish the silly little low values for the rail supply bypass

caps for each stage to much larger values, say 100uF for all,
with perhaps a string of zeners to shunt reg the C22 which would be 100uF.
R93 would need to be reduced to about 3.3k to allow for the current in the
zeners which
needs to be about the same as the current in the two 6EU7 twin triodes used for
the
two channels of the phono stages.

Patrick Turner.

Ian Iveson
Guest

Posted: Fri Sep 09, 2005 2:33 pm Post subject: Re: Potentiometer question

"Chris Hornbeck" wrote

Quote:

Why not a third stage follower? Is it such a crime?

Not at all (not in the UK, anyway AFAIK, yet...could slip in with
all this anti-terrorist stuff...caught in possession, third stage
follower, five years solitary without charge). A buffer stage has
already been suggested. This is an illustration of the best that can
be achieved by minimal modification of the existing circuit.

It's on the edge of adequacy, but for small signals may be
indistinguishable from the original. It would be interesting to
listen.

Quick and easy to simulate, anyway.

Anyone up to showing how to calculate the four time constants in the
circuit?

cheers, Ian

Patrick Turner
Guest

Posted: Fri Sep 09, 2005 11:48 pm Post subject: Re: Potentiometer question

Ian Iveson wrote:

Quote:

"Chris Hornbeck" wrote

Why not a third stage follower? Is it such a crime?

Not at all (not in the UK, anyway AFAIK, yet...could slip in with
all this anti-terrorist stuff...caught in possession, third stage
follower, five years solitary without charge). A buffer stage has
already been suggested. This is an illustration of the best that can
be achieved by minimal modification of the existing circuit.

It's on the edge of adequacy, but for small signals may be
indistinguishable from the original. It would be interesting to
listen.

Quick and easy to simulate, anyway.

Anyone up to showing how to calculate the four time constants in the
circuit?

cheers, Ian

It appears that Our Illustrious Colleague has taken a chain saw to his
dear lil Eico
and expunged the whole tone control stage from the chassis.
I have refrained from dobbing the said chain saw massacarist in to the
GOT,
who needs a break before another weekend of sins against triodes,
and because we can feel compassion for such brazen sinners in the case
of
tone control expungements.

One humble suggestion available to OIC is that he fill the gaping holes
in the chassis where the tone amp used to be with
a pair of usable 9 pin bases, and so he can add two new twin triodes
which would allow the wholesome
construction of at least a half decent phono stage such as as can be
seen amoungst the scribbled out
schematic at
http://www.turneraudio.com.au/htmlwebpgs02/schem10tubepreamp.htm

I would recommend this phono amp to Queen Liz II if she liked building
amps
suitable for moving magnet carts, or high output Ortophon MCs, of which
there is a large variety.

Possibly he can then use the idea of balance controls in series with 50k
gain controls also shown on the
above rough schematic.

And then if he had an uncle, his name would be Bob.

Patrick Turner.

Chris Hornbeck
Guest

Posted: Sun Sep 11, 2005 4:44 am Post subject: Re: Potentiometer question

On Fri, 09 Sep 2005 09:33:43 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:

Quote:

Why not a third stage follower? Is it such a crime?

Not at all (not in the UK, anyway AFAIK, yet...could slip in with
all this anti-terrorist stuff...caught in possession, third stage
follower, five years solitary without charge).

Things are much worse in the US. You really *don't*
want to know.

Quote:

Anyone up to showing how to calculate the four time constants in the
circuit?

Quick and easy to simulate, anyway.
The conflicting pairs (500Hz and 2122Hz) and (C1-R8 and
C2-R11) are the interesting bits.

Many strategies have been worked out over the years to
rule-of thumb the 500/2122 pair, but IMO the only good
approach to the subsonic pole pair (not even counting
the power supply's non-zero impedance) is to make a
single dominant pole.

In this case, maybe move the loop feedback to the
upstream side of C2 and choose a value of C1 the makes
a pole at some ridiculously high number, maybe 30 Hz.
Then C2 can be large enough to couple to modern
loads.

And while I'm up on this soapbox, let me just preach
for a moment on the value of a decent load for the poor
unbuffered second stage. Can I get an Amen? Even
Dynaco, heartless and Godless though they may have
been, provided 100K. Can we do less? 'S all i'm sayin'.

Arf. Thanks as always, and No, nobody wants to ASCII in
the complete calculations. It's the computer age, Mon.
You just look it up on the Internet!

"Trailing clouds of Glory, I'm down" -Firesign Theater

Chris Hornbeck

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