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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod

My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control. Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.

Thank you.

Jon

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Andre Jute Andre Jute is offline
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Default Eico Tone Control Mod

Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo. A more creative act would be to throw it
in the skip. A complete and accurate description of your big project
would be "snip-snip a coupla wires". The rest is shrinkwrapped waffle.
-- AJ

On Nov 26, 11:02 pm, Jon Yaeger wrote:
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control. Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.

Thank you.

Jon


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Peter Wieck Peter Wieck is offline
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Default Eico Tone Control Mod

On Nov 26, 7:47 pm, Andre Jute wrote:
Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo. A more creative act would be to throw it
in the skip. A complete and accurate description of your big project
would be "snip-snip a coupla wires". rest of the jealous crap snipped


Save for the single, irrefutable fact that whatever Jon did to
whatever he did it to actually does and will work. This would be
unlike any design you have submitted to this NG, past or present.

Oh, and we are still waiting for the Lundahl designs - even one (1) of
them.

That you took the iron (advance against royalties to put it in terms
you *might* understand) and did not produce the promised designs makes
you a simple thief. Even if you committed theft-by-deception, a likely
more venial sin in your dark little mind.

It is hardly the habits of an honorable man to criticize the small
works of another based on comparison to his own non-existent great
works.

Peter Wieck
Wyncote, PA
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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod



On Nov 26, 7:47 pm, Andre Jute wrote:
Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo. A more creative act would be to throw it
in the skip. A complete and accurate description of your big project
would be "snip-snip a coupla wires". rest of the jealous crap snipped


Ah, I see you folded. Couldn't rise to the challenge . . . really didn't
think that you would or could.

The Eico stuff . . . being "lowest-common-denominator" as you say . . . is
in fact perfect for experimenting. For as much as I like Mikkel's clever
implementation, I wouldn't put it on, say, a McIntosh amplifier.

You do reveal your ignorance. Some Eico iron, such as the HF-87 outputs,
hold their own among the best U.S. iron ever wound. A properly modified
Eico ST-70 rivals gear costing 10X as much. It offers an excellent bang for
the buck.

I'm glad that you are relaxing in the full comfort that a healthy
inheritance / trust fund can bring. Enjoy the evening!

Jon

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Ian Iveson Ian Iveson is offline
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Default Eico Tone Control Mod

Jon Yaeger wrote:

My latest project is to complete some modifications to an
Eico 2080
integrated amplifier, which uses 7591s in PP
configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very
nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of
controls.

In order to achieve goal #1, I used one of the tone
control tube positions
as a cathode follower, to drive the 50K motorized volume
control. Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed
under the chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted
on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the
proposed design.

Thank you.


I would be interested to know how you make your decisions.
Such as:

Choice of valve, values of coupling caps, valve loading and
configuration.

That's everything, I suppose.

And why wouldn't you put Mikkel's remote in a Mac? Is this a
quality or a conservation issue? How is the remote powered,
BTW?

cheers, Ian







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Peter Wieck Peter Wieck is offline
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Default Eico Tone Control Mod

On Nov 26, 8:35 pm, Jon Yaeger wrote:

I'm glad that you are relaxing in the full comfort that a healthy
inheritance / trust fund can bring. Enjoy the evening!


Wait a minute!!?? Are you calling Andre a Remittance Man? My, but what
a deadly insult. That would be akin to calling westiepoo his batty-
boy...

Yikes, Jon, you are getting vicious in your old age!

Peter Wieck
Wyncote, PA
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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod

in article , Ian Iveson at
wrote on 11/26/07 9:42 PM:

Jon Yaeger wrote:

My latest project is to complete some modifications to an
Eico 2080
integrated amplifier, which uses 7591s in PP
configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very
nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of
controls.

In order to achieve goal #1, I used one of the tone
control tube positions
as a cathode follower, to drive the 50K motorized volume
control. Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed
under the chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted
on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the
proposed design.

Thank you.


I would be interested to know how you make your decisions.
Such as:

Choice of valve, values of coupling caps, valve loading and
configuration.

That's everything, I suppose.

And why wouldn't you put Mikkel's remote in a Mac? Is this a
quality or a conservation issue? How is the remote powered,
BTW?

cheers, Ian


Ian,

Good questions, all.

I did the layout quite some time ago so I'll do my best to reconstruct from
memory.

In the original 2080 circuit, section 2 of a 7247 (the 1/2 that is more like
a 12AU7 than a 12AX7) is configured as a cathode follower, driving the tone
control network.

I chose to use a 6CG7 as a CF in lieu of the 7247 for a number of reasons
including the fact that the 7247 is an odd and somewhat rare tube, and the
6CG7 is noted for its linearity. The 6CG7 uses a 9 in socket.

The Eico had a 0.25 uF coupling cap after the CF, whereas I use 0.47 I kept
the same values of the bias and feedback resistors that were used for the
6CG7. The 165 volt supply to the plates of the 7247/6CG7s are derived from
a common source - i.e. a 10K 2W resistor connected to a 230V supply -
except that two of the plates (for the high gain section of the tubes,
following the tone control network) also have individual 82K resistors in
series with the 10K resistor. I know that is hard to follow and it would be
better if I had a schematic to post.

My rationale -- perhaps flawed -- is that the low gain section of the 7247
and the 6CG7 are similar enough perhaps to be interchangeable. I looked at
load lines for each tube, and using a ruler, noted that with a plate current
of 4 ma and a plate voltage of 165, the intersecting point (grid bias) for
the 6CG7 was about -6.0 volts; for the 7247 about -7.0 volts.

Another reader kindly pointed out that the 6CG7 was far from optimally
biased in my configuration. I'll have to pull out the load lines and do the
work, and also plug in various values in TubeCad to see what changes what
and to try to determine optimal values.

Looking again at the drawing again with fresh eyes, I see that grounding the
grid of the following tube with the mute switch is a big mistake!

So, what other issues do you see? Do you have any suggestions? I am
clearly a novice at design.

Thanks.

Jon




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Ham Ham is offline
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Default Eico Tone Control Mod


"Andre Jute" wrote in message ...
Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo. A more creative act would be to throw it
in the skip.


A more useful act would be to throw you in the skip.


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robert casey robert casey is offline
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Default Eico Tone Control Mod

Jon Yaeger wrote:

My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by


Take a look at my pages of electronic volume controls using tubes
http://pw2.netcom.com/~wa2ise/radios/6be6vol.html
and
http://pw2.netcom.com/~wa2ise/radios/elvctube.jpg


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robert casey robert casey is offline
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Default Eico Tone Control Mod


Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo.


Save for the single, irrefutable fact that whatever Jon did to
whatever he did it to actually does and will work.


At least there'd be no serious criticism that he is hacking up a rare
and great piece of equipment... If in fact that it is "el cheapo", and
that if he can improve it, then more power to him.


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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod



And why wouldn't you put Mikkel's remote in a Mac? Is this a
quality or a conservation issue? How is the remote powered,
BTW?

cheers, Ian



Ian,

The Mac is considered by many to be a classic piece. There is no chance
that my mods will do anything but decrease it's value to almost anyone other
than myself. And maybe to me too . . .

The Eico is not considered a classic; the one I have is in rough cosmetic
shape, and the improvements are solely intended for my ears and convenience.

The remote gets its power from the filament supply in the amplifier; the
control from a 9V battery.

Jon

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Iain Churches[_2_] Iain Churches[_2_] is offline
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Default Eico Tone Control Mod


"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control.
Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the
chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.



Hello Jon. Nice project!

Some pics would be of interest, and also some info about the remote
gain control.

With a CF I like to elevate the heater DC so that the cathode
to heater differential does not exceed 100V.

Nice to see a real project on RAT.

Iain



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Alex Alex is offline
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Default Eico Tone Control Mod

Hello, John.

1. Current through the valve 6CG7 will be lower than 4mA, since a lot will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand hence
lower output impedance on "tape" output?

2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For
that reason also why not reduce the cathode resistor.

3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.

4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because your
interstage capacitor 0.47uF will not be sufficient. Why not then increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to your
"tape" output.

5. [Warning!] Connect a 100R...1K resistor in series with the grid close to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.

6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe, ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.

Regards,
Alex

"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control.

Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the

chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.

Thank you.

Jon



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Alex Alex is offline
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Default Eico Tone Control Mod

Hello, John.

1. Current through the valve 6CG7 will be lower than 4mA, since a lot will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand hence
lower output impedance on "tape" output?

2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For
that reason also why not reduce the cathode resistor.

3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.

4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because your
interstage capacitor 0.47uF will not be sufficient. Why not then increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to your
"tape" output.

5. [Warning!] Connect a 100R...1K resistor in series with the grid close to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.

6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe, ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.

Regards,
Alex

"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control.

Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the

chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.

Thank you.

Jon





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Andre Jute Andre Jute is offline
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Default Working on Classic amps: butchery or necessary updating?

Robert Casey wrote about Yaeger's Eico wiresnips:

At least there'd be no serious criticism that he is hacking up a rare
and great piece of equipment... If in fact that it is "el cheapo", and
that if he can improve it, then more power to him.


I agree. He could learn something.

Up to fifteen years ago I thought nothing of rebuilding a QUAD II in
my own image but eventually I decided that Peter Walker & Co knew more
than I do and restored all mine to original, and over a period of
years called in those that had left here modded and did the same
restoration to original on them. In my defense I point out that
Patrick still blithely alters Quad Ii quite extensively.

So now I think how much work you can justifiably do on a "classic" is
relevant to the time period in which you do it, and to the original
quality of the "classic". It would be a crime to hack up a Quad II or
a Radford, but Dynaco and Eico and so on are pretty common and
therefore fair game.

The Quad II case is confused by the fact that there is a couple of
modern factory reissues that include many of the updates I did, and
that Patrick now does, so in fact throwing the sanction of official
approval over our humble endeavours.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review



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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod

in article , robert casey at
wrote on 11/27/07 12:24 AM:

Jon Yaeger wrote:

My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by


Take a look at my pages of electronic volume controls using tubes
http://pw2.netcom.com/~wa2ise/radios/6be6vol.html
and
http://pw2.netcom.com/~wa2ise/radios/elvctube.jpg




Robert,

Thanks for the interesting info.

I guess the pentode provides a nice degree of isolation. There is still the
"problem" that a motorized pot would have to be installed.

I'm trying to stay with triodes as much as I can in this project.

I appreciate your help.

Jon

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Jon Yaeger Jon Yaeger is offline
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Default Eico Tone Control Mod

Great advice! Thank you.

When I get some free time I'll incorporate many of these suggestions and
revise the design. I'll then post the circuit.

Jon








in article , Alex at
wrote on 11/27/07 6:48 AM:

Hello, John.

1. Current through the valve 6CG7 will be lower than 4mA, since a lot will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand hence
lower output impedance on "tape" output?

2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For
that reason also why not reduce the cathode resistor.

3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.

4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because your
interstage capacitor 0.47uF will not be sufficient. Why not then increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to your
"tape" output.

5. [Warning!] Connect a 100R...1K resistor in series with the grid close to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.

6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe, ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.

Regards,
Alex

"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube positions
as a cathode follower, to drive the 50K motorized volume control.

Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the

chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed design.

Thank you.

Jon






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robert casey robert casey is offline
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Posts: 340
Default Eico Tone Control Mod



Thanks for the interesting info.

I guess the pentode provides a nice degree of isolation. There is still the
"problem" that a motorized pot would have to be installed.

I'm trying to stay with triodes as much as I can in this project.


You could have a remote variable DC "power supply" to control the dual
control tubes. Or a pot that only handles DC and no audio signal.
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Alex Alex is offline
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Default Eico Tone Control Mod

This is how I would be designing that cathode follower:

1. Divide available supply voltage (165V) into three parts, allow 1/3 to
drop across the cathode load, and 2/3 -- across the tube itself. It is not a
strict rule, but a guideline. If you leave too little across the tube, you
will not be able to run a decent current through it and consequently will
lose transconductance, hence output impedance on "tape" will be high. If you
leave too little on the cathode resistor, it will shunt the output,
consequently, gain will drop (from say 0.9 to 0.7, t.h.d. will rise).
So let us choose 55V across the cathode rsistor, 110V across the tube.

2. Refer to the tube anode plots or anode-grid plots and find out what plate
current could have been with no grid bias for the given plate voltage. In
your case, for 6CG7 @Ua=110V and Ug=0 in theory the tube will have Ia=13mA
plate current.
Divide this value (13mA) by half and let it be your quiescent current. Thus
the working point will be in the middle providing maximum dynamic range.
So, in your case, let Ia=6.5mA.

3. Calculate the cathode resistor: Rk= 55V / 6.5mA = 8.5K ~ 8.2K. (Note: in
your original design you thought it would be 39K -- too much!).

4. Work out grid bias for your tube to run at Ia=6.5mA @ Ua=110V. From the
plots for 6CG7: Ug = -2V (approximately).

5. Calculate cathode self bias resistor: Rb= 2V / 6.5mA = 307R ~ 330R (Note:
in your design you assumed it to be 1.5K -- too much.)

6. Just for the circuit verification you can assess the output impedance of
your cathode follower. According to the plots, transconductance S=2.5mA/V @
6.5mA. So your output impedance will be around 1/S = 400 ohm. Not bad! Can
handle almost any (valve or transistorised) equipment connected to "tape"
output!

7. Now -- soldering iron in hand! (Do not forget about the anti-parasitic
resistor in series with the grid!)

Tube plots can be found on http://frank.pocnet.net/sheets/093/6/6CG7.pdf

Regards,
Alex

"Jon Yaeger" wrote in message
...
Great advice! Thank you.

When I get some free time I'll incorporate many of these suggestions and
revise the design. I'll then post the circuit.

Jon








in article , Alex at
wrote on 11/27/07 6:48 AM:

Hello, John.

1. Current through the valve 6CG7 will be lower than 4mA, since a lot

will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand

hence
lower output impedance on "tape" output?

2. Beware of the cathode-to-heater voltage. It shall be less than 100V.

For
that reason also why not reduce the cathode resistor.

3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu

of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.

4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because

your
interstage capacitor 0.47uF will not be sufficient. Why not then

increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to

your
"tape" output.

5. [Warning!] Connect a 100R...1K resistor in series with the grid close

to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.

6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe,

ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.

Regards,
Alex

"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is

similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube

positions
as a cathode follower, to drive the 50K motorized volume control.

Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the

chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed

design.

Thank you.

Jon








  #20   Report Post  
Posted to rec.audio.tubes
Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default Working on Classic amps: butchery or necessary updating?



Andre Jute wrote:

Robert Casey wrote about Yaeger's Eico wiresnips:

At least there'd be no serious criticism that he is hacking up a rare
and great piece of equipment... If in fact that it is "el cheapo", and
that if he can improve it, then more power to him.


I agree. He could learn something.

Up to fifteen years ago I thought nothing of rebuilding a QUAD II in
my own image but eventually I decided that Peter Walker & Co knew more
than I do and restored all mine to original, and over a period of
years called in those that had left here modded and did the same
restoration to original on them. In my defense I point out that
Patrick still blithely alters Quad Ii quite extensively.

So now I think how much work you can justifiably do on a "classic" is
relevant to the time period in which you do it, and to the original
quality of the "classic". It would be a crime to hack up a Quad II or
a Radford, but Dynaco and Eico and so on are pretty common and
therefore fair game.

The Quad II case is confused by the fact that there is a couple of
modern factory reissues that include many of the updates I did, and
that Patrick now does, so in fact throwing the sanction of official
approval over our humble endeavours.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review


I had an enthusiast from the UK send me pics of his efforts to upgrade
Quad-II,
and all he'd done was replace R&C and fit IEC and better wiring.

This does not address the tragic failings in the Walker design.

The guy said Quad-II fetch 700 BP which was a lot of money he thought.
I said I didn't care if they fetched 7,000 BP, they are still just old
junk.

If you had a Morris Oxford in mint condition, its still a lousy motor
car.

I have no idea what the schematic differences are between any Quad-II
re-issues,
or what the schematic is for Quad 40.

There are various ways to improve Quad-II listed at my website pages.

I recall about 100,000 Quad-II amps might have been made.
If 3 remain in 3 different museums around the world, I am happy
for those visiting to see them.

Nothing much stops anyone from seriously modding the other 99,997 amps
that could be lurking around.

Patrick Turner.


  #21   Report Post  
Posted to rec.audio.tubes
Jon Yaeger Jon Yaeger is offline
external usenet poster
 
Posts: 645
Default Eico Tone Control Mod

Alex,

I'm very much obliged for your time and excellent advice!

Thank you.

Jon





in article , Alex at
wrote on 11/28/07 6:31 AM:

This is how I would be designing that cathode follower:

1. Divide available supply voltage (165V) into three parts, allow 1/3 to
drop across the cathode load, and 2/3 -- across the tube itself. It is not a
strict rule, but a guideline. If you leave too little across the tube, you
will not be able to run a decent current through it and consequently will
lose transconductance, hence output impedance on "tape" will be high. If you
leave too little on the cathode resistor, it will shunt the output,
consequently, gain will drop (from say 0.9 to 0.7, t.h.d. will rise).
So let us choose 55V across the cathode rsistor, 110V across the tube.

2. Refer to the tube anode plots or anode-grid plots and find out what plate
current could have been with no grid bias for the given plate voltage. In
your case, for 6CG7 @Ua=110V and Ug=0 in theory the tube will have Ia=13mA
plate current.
Divide this value (13mA) by half and let it be your quiescent current. Thus
the working point will be in the middle providing maximum dynamic range.
So, in your case, let Ia=6.5mA.

3. Calculate the cathode resistor: Rk= 55V / 6.5mA = 8.5K ~ 8.2K. (Note: in
your original design you thought it would be 39K -- too much!).

4. Work out grid bias for your tube to run at Ia=6.5mA @ Ua=110V. From the
plots for 6CG7: Ug = -2V (approximately).

5. Calculate cathode self bias resistor: Rb= 2V / 6.5mA = 307R ~ 330R (Note:
in your design you assumed it to be 1.5K -- too much.)

6. Just for the circuit verification you can assess the output impedance of
your cathode follower. According to the plots, transconductance S=2.5mA/V @
6.5mA. So your output impedance will be around 1/S = 400 ohm. Not bad! Can
handle almost any (valve or transistorised) equipment connected to "tape"
output!

7. Now -- soldering iron in hand! (Do not forget about the anti-parasitic
resistor in series with the grid!)

Tube plots can be found on
http://frank.pocnet.net/sheets/093/6/6CG7.pdf

Regards,
Alex

"Jon Yaeger" wrote in message
...
Great advice! Thank you.

When I get some free time I'll incorporate many of these suggestions and
revise the design. I'll then post the circuit.

Jon








in article , Alex at
wrote on 11/27/07 6:48 AM:

Hello, John.

1. Current through the valve 6CG7 will be lower than 4mA, since a lot

will
drop across the cathode resistor 39K. Have you considered to reduce the
cathode resistor to, say, 10...15K to get higher transconductance oand

hence
lower output impedance on "tape" output?

2. Beware of the cathode-to-heater voltage. It shall be less than 100V.

For
that reason also why not reduce the cathode resistor.

3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu

of
this tube is low (about 20) and it will not give good power supply noise
rejection ratio.

4. If your taperecorder connected to "tape" output happens to have
relatively low input impedance (say, 10K) you will lose bass, because

your
interstage capacitor 0.47uF will not be sufficient. Why not then

increase it
to 2.2uF? (Otherwise make sure to connect only high impedance loads to

your
"tape" output.

5. [Warning!] Connect a 100R...1K resistor in series with the grid close

to
the socket. It will guarantee against parasitic oscillation (in Kolpitz
mode) on the Ls and Cs of the circuit, particularly when your input
"Balance" pot happens to go to ground.

6. Good thing is that your choice tube has a very common pinout. You can
plug almost any twin triode in there and compare results. I believe,

ECC85
is a better option. And if you decide to play with high transconductance
ones, like 6ES8 (to get very low output impedance on "tape"), you will
indeed appreciate advice #5.

Regards,
Alex

"Jon Yaeger" wrote in message
...
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is

similar
in many respects to the ST-70 circuit.

Among my goals we

1. To install a remote volume control, using the very nice kit sold by
Mikkel Simonsen, formerly of R.A.T.

2. Remove the tone control circuitry;

3. Design a new faceplate for the reduced number of controls.

In order to achieve goal #1, I used one of the tone control tube

positions
as a cathode follower, to drive the 50K motorized volume control.
Mikkel's
circuitry included a driver for a mute relay.

I am housing his circuit inside of a metal box installed under the
chassis,
to reduce interference as much as possible.

A schematic of one channel the proposed design is posted on:

http://www.yaegeraudio.com/eico_mod.pdf

I welcome any comments / criticisms / suggestions on the proposed

design.

Thank you.

Jon









  #22   Report Post  
Posted to rec.audio.tubes
Ian Iveson Ian Iveson is offline
external usenet poster
 
Posts: 960
Default Eico Tone Control Mod

Thanks, Jon. Sorry for delay, my continuity manager has been
on holiday.

Points I would make have been made already. Except, AFAIK
(thread's gone from my server), for the issue of
bootstrapping.

A little discussion on whether to use cathode bias, or
fixed, might be interesting. I note that the output
impedance for the circuit you have shown increases tenfold
if you change the source impedance from a short to an open
circuit. This may not matter much, but OTOH you don't appear
to need the bootstrapping.

What do you think Alex?

cheers, Ian


  #23   Report Post  
Posted to rec.audio.tubes
RdM RdM is offline
external usenet poster
 
Posts: 49
Default Eico Tone Control Mod

"Ian Iveson" wrote in
eyonder.co.uk:

X-Newsreader: Microsoft Outlook Express 6.00.2900.2869


(thread's gone from my server)


Sure the previous articles will have gone from the server, but why don't you
have a threaded saved local version of them? I have messages going back to '02
on this humble PII233 PC, and back further on another PC's newsreader install.

They're only text messages;- they don'y take up a lot of room! [Well ... !]

But the defaults in many newsreaders (incl Agent which mine is an earlier
version of) are set to "purge" after definition which IMO is an insane idea.

Well, unnecessary, anyway, if based on threads you are reading or have read.

I have no idea how Outlook (Outbreak?) Express handles default purge settings.

But I suggest you look into it;- !
  #24   Report Post  
Posted to rec.audio.tubes
Ian Iveson Ian Iveson is offline
external usenet poster
 
Posts: 960
Default Eico Tone Control Mod

Ross wrote:

Sure the previous articles will have gone from the server,
but why don't you
have a threaded saved local version of them? I have
messages going back to '02
on this humble PII233 PC, and back further on another PC's
newsreader install.

They're only text messages;- they don'y take up a lot of
room! [Well ... !]

But the defaults in many newsreaders (incl Agent which
mine is an earlier
version of) are set to "purge" after definition which
IMO is an insane idea.

Well, unnecessary, anyway, if based on threads you are
reading or have read.

I have no idea how Outlook (Outbreak?) Express handles
default purge settings.

But I suggest you look into it;- !


Hi Ross,

But...

Maybe it was the MI5 posts, or perhaps somewhere there is a
buffer of varying capacity, whatever: the thread went away
with unexpected speed.

And, it will take months, someday, to tie up hundreds of
loose ends already in my library of loose ends.

Sometimes I think it doesn't matter. Sometimes I'm just lost
for words. Other times I'm just too slow to condense too
many words into a coherent posting.

It would be nice to discuss the cathode follower, and
especially this bootstrapping mullarky.

By the time I'd checked through my ideas, Alex had dealt
with them and the thread seemed to have degenerated into an
expert/novice type of exchange. Being neither, I felt
estranged.

Anyway, what are the ups and downs of cathode or grid bias
for a CF, I wonder. A potential divider between HT and
ground could be used for the grid. The input impedance could
then be 470k, which would be OK with the source of 47k,
IIRC.

I'm not keen on cathode followers at all, although they are
sometimes necessary. I don't see any advantage in
unnecessarily complicating their operation.

To address your point, ahem, I could buffer a heap of r.a.t
locally but then I'd just get slower. It's a motivation
thing.

I can't help thinking, a bit more each day, that there's
no-one here to talk to.

Ian


  #25   Report Post  
Posted to rec.audio.tubes
RdM RdM is offline
external usenet poster
 
Posts: 49
Default Eico Tone Control Mod

"Ian Iveson" wrote

Ross wrote:

Sure the previous articles will have gone from the server,
but why don't you
have a threaded saved local version of them? I have
messages going back to '02
on this humble PII233 PC, and back further on another PC's
newsreader install.

They're only text messages;- they don'y take up a lot of
room! [Well ... !]

But the defaults in many newsreaders (incl Agent which
mine is an earlier
version of) are set to "purge" after definition which
IMO is an insane idea.

Well, unnecessary, anyway, if based on threads you are
reading or have read.

I have no idea how Outlook (Outbreak?) Express handles
default purge settings.

But I suggest you look into it;- !


Hi Ross,

But...

Maybe it was the MI5 posts, or perhaps somewhere there is a
buffer of varying capacity, whatever: the thread went away
with unexpected speed.


Yes;- recently my own isp appears to have much less retention.

And, it will take months, someday, to tie up hundreds of
loose ends already in my library of loose ends.


Well put! I fear I am in a very similiar position ... maybe worse!

Sometimes I think it doesn't matter. Sometimes I'm just lost
for words. Other times I'm just too slow to condense too
many words into a coherent posting.


Can relate to that ...

It would be nice to discuss the cathode follower, and
especially this bootstrapping mullarky.


Well, you could start a new thread?

By the time I'd checked through my ideas, Alex had dealt
with them and the thread seemed to have degenerated into an
expert/novice type of exchange. Being neither, I felt
estranged.


Feelings (and thoughts!) are temporal;-

Anyway, what are the ups and downs of cathode or grid bias
for a CF, I wonder. A potential divider between HT and
ground could be used for the grid. The input impedance could
then be 470k, which would be OK with the source of 47k,
IIRC.


I can't comment, I'm sorry. At this moment!

I'm not keen on cathode followers at all, although they are
sometimes necessary. I don't see any advantage in
unnecessarily complicating their operation.


Of course not unnecessarily, but what of any refinement possible?

To address your point, ahem, I could buffer a heap of r.a.t
locally but then I'd just get slower. It's a motivation
thing.


I'm already slowed right up. Even so, I mark threads that are or look
interesting & download bodies, mark read the non-interesting etc.

Sometimes I can change my mind about the latter.

Thus at least I have some reading for later when I might have time.
This across several groups. I hardly ever have time to post ...

(Just as well;- anyway, I am still merely hoarding, pre-constructional;-)

I can't help thinking, a bit more each day, that there's
no-one here to talk to.


Sounds like depression too. Well, maybe that's just me projecting!

OTOH perhaps it's not quite intended to be utterly utile for talk as such!

You mean 'talk with', of course, anyway ...
--
Ian


It's a tough life being a bachelor at times, yes;-?
Ross
--
"Truckin - like the doodah man
once told me you got to play your hand
sometime - the cards ain't worth a dime
if you don't lay em down

Sometimes the light's all shining on me
Other times I can barely see
Lately it occurs to me
What a long strange trip it's been"

The Grateful Dead, American Beauty, "Truckin".


  #26   Report Post  
Posted to rec.audio.tubes
Alex Alex is offline
external usenet poster
 
Posts: 111
Default Eico Tone Control Mod

I agree, Ian, a fixed bias is better.

Use (low noise resistors) 1M from grid to ground and 2.2M from grid to
supply voltage (165V) and you are in business.

One disadvantage is some cold emission field stress while the tube is
warming up with the positive grid bias.

Another disadvantage -- the circuit will not work if you plug in a low
current (high-mu) triode. It might not be able to pass 6.5mA at all, will go
into positive grid voltage "saturation". The cathode resistor needs to be
recalculated and changed for every tube type. In a bootstrap configuration
literally any tube will work (better or worse) without the circuit change.

Regards,
Alex

"Ian Iveson" wrote in message
. uk...
Thanks, Jon. Sorry for delay, my continuity manager has been
on holiday.

Points I would make have been made already. Except, AFAIK
(thread's gone from my server), for the issue of
bootstrapping.

A little discussion on whether to use cathode bias, or
fixed, might be interesting. I note that the output
impedance for the circuit you have shown increases tenfold
if you change the source impedance from a short to an open
circuit. This may not matter much, but OTOH you don't appear
to need the bootstrapping.

What do you think Alex?

cheers, Ian




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