Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

[Morris Slutsky]

There's been a lot of discussion on AGA lately about the Marshall 2204
"Cascaded" preamp circuit and whether or not it sucks and how much. A
lot of people really do like the high gain preamp. But what sucks
about the cascaded Marshall circuit is that there's no way to switch
the high and low gain besides physically moving a cable from one input
jack to the other. That circuit was a long time ago, though, these
days everyone likes channel switching amps which use relays or other
switching logic to push the signal through different tube stages and
recombine them at the power amp. Which is complicated and no fun to
homebrew.

Anyway I came up with this circuit, which allows easy footswitching
with no relays or anything between a 'clean' input stage and a 'high
gain' input stage, with appropriate frequency response voicing. It's
built and working, as part of a homemade amplifier, and I really do
like the sound so far.

I don't know if it's original or not, in the context of a guitar amp,
but I think it's pretty neat.

http://img718.imageshack.us/img718/8861/foldedfeedback.png

This circuit pretty much looks like a Marshall "cascaded" preamp
here. But with the footswitch open, the 47K resistor and 0.44 uF
capacitor form a feedback loop between the plate of the second triode
(half of a 12AX7) and the cathode of the first triode. The 0.44 uF
capacitor really doesn't do anything here but just pass the feedback
signal through - the DC is already blocked by the 0.022 uF coupling
capacitor between the second triode's plate and the output load, and
any high-pass filtering is going to be dominated by the 0.022 uF cap
as well. The result of the feedback loop is a stage with a moderate
amount of gain, pretty flat frequency response, and low distortion
unless driven to the limits where it'll hardclip, which a typical
guitar pickup won't be able to do. So it's a typical 'clean' input
stage.

Now close the footswitch, this grounds the middle of the feedback loop
and breaks it. The 0.44 uF capacitor now becomes a cathode bypass
capacitor for the first triode, bringing in a gain boost above about
300 Hz. The 47 K resistor becomes just a dummy load hanging off the
output. Without feedback, the stage distorts much more readily, gain
is much higher, and the frequency response is not as even. Lows are
rolled off by the interstage coupling RC network and the treble rolls
off due to Miller effect capacitance. It ends up being a typical
'high gain' input stage with appropriate frequency response and
distortion. Crunch!

Small-signal SPICE simulation of the frequency response and gain of
each stage is here:

http://img20.imageshack.us/img20/379/freqresponse.jpg

What makes this really amenable to footswitching is that although the
gain at the switching point is high, the impedance is very low, so
noise pickup isn't much of an issue. The feedback loop keeps the
impedance low to such noise signals, it's sort of a virtual ground.
So I just think this circuit is really neat.

There's a lot of talk on AGA about the differences in voicing a clean
and distorted channel. It's true that you can't totally tweak
everything here, but you can probably do quite a bit to change
frequency response and gain. The feedback resistor could probably
even include a variable pot.

[Stephen Cowell[_2_]]

"Morris Slutsky" > wrote
....
> I don't know if it's original or not, in the context of a guitar amp,
> but I think it's pretty neat.
>
> http://img718.imageshack.us/img718/8861/foldedfeedback.png

I like it! Two thoughts:

1.) If you replace one gain stage with this, you'll
invert the signal... this can cause problems sometimes.
2.) Switching the cathode cap out also rolls off lows.

I'd like to hear it in action.
__
Steve
..

[Engineer[_2_]]

On Jan 24, 10:24*am, "Stephen Cowell"
> wrote:
> "Morris Slutsky" > wrote
> ...
>
> > I don't know if it's original or not, in the context of a guitar amp,
> > but I think it's pretty neat.
>
> >http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
> I like it! *Two thoughts:
>
> 1.) *If you replace one gain stage with this, you'll
> * * invert the signal... this can cause problems sometimes.
> 2.) *Switching the cathode cap out also rolls off lows.
>
> I'd like to hear it in action.
> __
> Steve
> .

Looks good! More thoughts... (for a flat frequency response
application)
-- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode
decouplng (and feedback) over a much wider frequency range
-- plate load on the 2nd triode is 100K//47K (at least when switch is
closed), or about 32K; this seems a bit low.
-- bypass the 2nd triode cathode resistor (and make it smaller, e,g.
2.7K) for more gain, then increase the above 47K for the right amount
of feedback
This design might serve to switch a single audio input from microphone
gain to AUX gain (not tried)
Cheers,
Roger

[pimpom]

Morris Slutsky wrote:
> There's been a lot of discussion on AGA lately about the
> Marshall 2204
> "Cascaded" preamp circuit and whether or not it sucks and how
> much. A
> lot of people really do like the high gain preamp. But what
> sucks
> about the cascaded Marshall circuit is that there's no way to
> switch
> the high and low gain besides physically moving a cable from
> one input
> jack to the other. That circuit was a long time ago, though,
> these
> days everyone likes channel switching amps which use relays or
> other
> switching logic to push the signal through different tube
> stages and
> recombine them at the power amp. Which is complicated and no
> fun to
> homebrew.
>
> Anyway I came up with this circuit, which allows easy
> footswitching
> with no relays or anything between a 'clean' input stage and a
> 'high
> gain' input stage, with appropriate frequency response voicing.
> It's
> built and working, as part of a homemade amplifier, and I
> really do
> like the sound so far.
>
> I don't know if it's original or not, in the context of a
> guitar amp,
> but I think it's pretty neat.
>
> http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
> This circuit pretty much looks like a Marshall "cascaded"
> preamp
> here. But with the footswitch open, the 47K resistor and 0.44
> uF
> capacitor form a feedback loop between the plate of the second
> triode
> (half of a 12AX7) and the cathode of the first triode. The
> 0.44 uF
> capacitor really doesn't do anything here but just pass the
> feedback
> signal through - the DC is already blocked by the 0.022 uF
> coupling
> capacitor between the second triode's plate and the output
> load, and
> any high-pass filtering is going to be dominated by the 0.022
> uF cap
> as well. The result of the feedback loop is a stage with a
> moderate
> amount of gain, pretty flat frequency response, and low
> distortion
> unless driven to the limits where it'll hardclip, which a
> typical
> guitar pickup won't be able to do. So it's a typical 'clean'
> input
> stage.
>
> Now close the footswitch, this grounds the middle of the
> feedback loop
> and breaks it. The 0.44 uF capacitor now becomes a cathode
> bypass
> capacitor for the first triode, bringing in a gain boost above
> about
> 300 Hz. The 47 K resistor becomes just a dummy load hanging
> off the
> output. Without feedback, the stage distorts much more
> readily, gain
> is much higher, and the frequency response is not as even.
> Lows are
> rolled off by the interstage coupling RC network and the treble
> rolls
> off due to Miller effect capacitance. It ends up being a
> typical
> 'high gain' input stage with appropriate frequency response and
> distortion. Crunch!
>
> Small-signal SPICE simulation of the frequency response and
> gain of
> each stage is here:
>
> http://img20.imageshack.us/img20/379/freqresponse.jpg
>
> What makes this really amenable to footswitching is that
> although the
> gain at the switching point is high, the impedance is very low,
> so
> noise pickup isn't much of an issue. The feedback loop keeps
> the
> impedance low to such noise signals, it's sort of a virtual
> ground.
> So I just think this circuit is really neat.
>
> There's a lot of talk on AGA about the differences in voicing a
> clean
> and distorted channel. It's true that you can't totally tweak
> everything here, but you can probably do quite a bit to change
> frequency response and gain. The feedback resistor could
> probably
> even include a variable pot.

Did you notice clicks when you press the foot switch, caused by
the 0.44uF cap being charged and discharged at the cathode bias
potential? You can avoid that by paralleling the foot switch with
a resistor. The value wouldn't be critical - say 47-100k. This
will also keep the output terminal at ground potential even when
the foot switch is open. This is desireable to avoid a 'thump' if
the unit is a standalone piece that may be hot-plugged to the
main amp.

[MooseFET]

On Jan 24, 8:51*am, Engineer > wrote:
> On Jan 24, 10:24*am, "Stephen Cowell"
>
>
>
> > wrote:
> > "Morris Slutsky" > wrote
> > ...
>
> > > I don't know if it's original or not, in the context of a guitar amp,
> > > but I think it's pretty neat.
>
> > >http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
> > I like it! *Two thoughts:
>
> > 1.) *If you replace one gain stage with this, you'll
> > * * invert the signal... this can cause problems sometimes.
> > 2.) *Switching the cathode cap out also rolls off lows.
>
> > I'd like to hear it in action.
> > __
> > Steve
> > .
>
> Looks good! *More thoughts... (for a flat frequency response
> application)
> -- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode
> decouplng (and feedback) over a much wider frequency range

Beware of leakage currents in the 100uF if you go this way. You will
hear the click as the leakage current gets sent to the output node.
Since the plate resistor of the second stage is 100K, this is the
impedance you need to assume for the short term effect. IIRC the
12AX7 has a higher plate impedance than this.

A two device amplifier with the feedback like shown is not a
novel idea. It has been used quite a bit.

The wiring of the switched to ground will add some capacitance etc
to that node when the switch is open. This will cause peaking of
the high frequencies. It is better to run wiring at the low
impedance points of the circuit.

With the switch closed, the first section is running the tube at
its full gain. This means that unit to unit variations and the
nonlinear effects of this part will show up in the results.







> -- plate load on the 2nd triode is 100K//47K (at least when switch is
> closed), or about 32K; this seems a bit low.
> -- bypass the 2nd triode cathode resistor (and make it smaller, e,g.
> 2.7K) for more gain, then increase the above 47K for the right amount
> of feedback
> This design might serve to switch a single audio input from microphone
> gain to AUX gain (not tried)
> Cheers,
> Roger

[Don Lancaster]

Stephen Cowell wrote:
>
> 2.) Switching the cathode cap out also rolls off lows.
>

But also provides improved sound staging, reduced
midrange granularity, and better bass speed.

Full details at http://www.tinaja.com/glib/marcia.pdf


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

[WB]

Morris Slutsky wrote:
>
> Anyway I came up with this circuit, which allows easy footswitching
> with no relays or anything between a 'clean' input stage and a 'high
> gain' input stage, with appropriate frequency response voicing. It's
> built and working, as part of a homemade amplifier, and I really do
> like the sound so far.
>

Did you try to simply by-pass the second tube ?

[Stephen Cowell[_2_]]

"Don Lancaster" > wrote in message
...
> Stephen Cowell wrote:
>>
>> 2.) Switching the cathode cap out also rolls off lows.
>>
>
> But also provides improved sound staging, reduced
> midrange granularity, and better bass speed.
>
> Full details at http://www.tinaja.com/glib/marcia.pdf

I can smell the cork from here!
__
Steve
..

[Stephen Cowell[_2_]]

"WB" > wrote
> Morris Slutsky wrote:
>>
>> Anyway I came up with this circuit, which allows easy footswitching
>> with no relays or anything between a 'clean' input stage and a 'high
>> gain' input stage, with appropriate frequency response voicing. It's
>> built and working, as part of a homemade amplifier, and I really do
>> like the sound so far.
>>
>
> Did you try to simply by-pass the second tube ?

1.) You need a voicing change for guitar 'crunch'... lifting
the cathode cap gives this (necessary) change.

2.) 'Bypassing' a stage is not easy... pops, thumps,
DC states, inversion of phase... all apply.

Your suggestion would result in 180degree phase change
when you stomp... not a good idea, in and of itself.
__
Steve
..

[Lord Valve]

Stephen Cowell wrote:

> "WB" > wrote
> > Morris Slutsky wrote:
> >>
> >> Anyway I came up with this circuit, which allows easy footswitching
> >> with no relays or anything between a 'clean' input stage and a 'high
> >> gain' input stage, with appropriate frequency response voicing. It's
> >> built and working, as part of a homemade amplifier, and I really do
> >> like the sound so far.
> >>
> >
> > Did you try to simply by-pass the second tube ?
>
> 1.) You need a voicing change for guitar 'crunch'... lifting
> the cathode cap gives this (necessary) change.
>
> 2.) 'Bypassing' a stage is not easy... pops, thumps,
> DC states, inversion of phase... all apply.
>
> Your suggestion would result in 180degree phase change
> when you stomp... not a good idea, in and of itself.
> __
> Steve
> .

Since this is crossposted to rec.audio.tubes, there'll be a disconnect
between the dudes who handle their glass with white gloves and keep their
gear on the shelf and the dudes who pound the crap out of it and pour it
fulla beer...

LV

[Morris Slutsky]

On Jan 24, 12:54*pm, Don Lancaster > wrote:
> Stephen Cowell wrote:
>
> > 2.) *Switching the cathode cap out also rolls off lows.
>
> But also provides improved sound staging, reduced
> midrange granularity, and better bass speed.
>
> Full details athttp://www.tinaja.com/glib/marcia.pdf
>
> --
> Many thanks,
>
> Don Lancaster * * * * * * * * * * * * *voice phone: (928)428-4073
> Synergetics * 3860 West First Street * Box 809 Thatcher, AZ 85552
> rss:http://www.tinaja.com/whtnu.xml* email:
>
> Please visit my GURU's LAIR web site athttp://www.tinaja.com

I think I get better bass speed in the high-gain mode, actually.
Because it's much easier to play Megadeth that way.

It's an honor to have you here, by the way, your writing does in fact
rule.

[Tim Williams]

"Lord Valve" > wrote in message
...
>> Spam. Tacky spam at that. Audio tends to be that way.
>
> Don't read it, asshole. ...

Ah, I see RAT is still just as ****ty as ever...

The smart posters left just in time. All that remained since then are
trolls and trollfeeders. Sad, really.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

[Too Long in The Wasteland]

Lord Valve wrote:
> Since this is crossposted to rec.audio.tubes, there'll be a disconnect
> between the dudes who handle their glass with white gloves and keep their
> gear on the shelf and the dudes who pound the crap out of it and pour it
> fulla beer...
>

I doubt if they drink beer with their pinkies extended.

[Morris Slutsky]

> A two device amplifier with the feedback like shown is not a
> novel idea. *It has been used quite a bit.
>
> The wiring of the switched to ground will add some capacitance etc
> to that node when the switch is open. *This will cause peaking of
> the high frequencies. *It is better to run wiring at the low
> impedance points of the circuit.
>
> With the switch closed, the first section is running the tube at
> its full gain. *This means that unit to unit variations and the
> nonlinear effects of this part will show up in the results.

Moose,

I just checked SPICE about capacitance to ground at the footswitch
point. There is a small peak at about 100 KHz from a 100 pF
capacitance there, and a really huge one from 1000 pF. I'm hoping
that my actual footswitch is closer to 300 pF - it probably is,
assuming that most of this is due to the capacitance of the footswitch
cable itself (about 6 feet) - which SPICE expects to produce a 4 dB
boost peaking at about 165 pF. I suspect that the actual boost will
be less than this, due to stray capacitances (wiring to ground plane?)
lowering the overall high frequency response in ways that SPICE cannot
readily take into account. But yes, you're right, I should expect
peaky behavior here. It's outside the audio band hopefully, which is
good because you won't hear it, bad because it could cause inaudible
parasitics, mitigated by the fact that this peak is still less than
the response at this frequency in high-gain mode and if it doesn't
oscillate there why would it oscillate in low gain mode?

Thanks for your thoughts and advice.

[Morris Slutsky]

On Jan 24, 6:39*pm, Morris Slutsky > wrote:
> > A two device amplifier with the feedback like shown is not a
> > novel idea. *It has been used quite a bit.
>
> > The wiring of the switched to ground will add some capacitance etc
> > to that node when the switch is open. *This will cause peaking of
> > the high frequencies. *It is better to run wiring at the low
> > impedance points of the circuit.
>
> > With the switch closed, the first section is running the tube at
> > its full gain. *This means that unit to unit variations and the
> > nonlinear effects of this part will show up in the results.
>
> Moose,
>
> I just checked SPICE about capacitance to ground at the footswitch
> point. *There is a small peak at about 100 KHz from a 100 pF
> capacitance there, and a really huge one from 1000 pF. *I'm hoping
> that my actual footswitch is closer to 300 pF *- it probably is,
> assuming that most of this is due to the capacitance of the footswitch
> cable itself (about 6 feet) - which SPICE expects to produce a 4 dB
> boost peaking at about 165 pF. *I suspect that the actual boost will
> be less than this, due to stray capacitances (wiring to ground plane?)
> lowering the overall high frequency response in ways that SPICE cannot
> readily take into account. * But yes, you're right, I should expect
> peaky behavior here. *It's outside the audio band hopefully, which is
> good because you won't hear it, bad because it could cause inaudible
> parasitics, mitigated by the fact that this peak is still less than
> the response at this frequency in high-gain mode and if it doesn't
> oscillate there why would it oscillate in low gain mode?
>
> Thanks for your thoughts and advice.

peaking at about 165 KHz I mean.

[John Larkin]

On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
> wrote:

>There's been a lot of discussion on AGA lately about the Marshall 2204
>"Cascaded" preamp circuit and whether or not it sucks and how much. A
>lot of people really do like the high gain preamp. But what sucks
>about the cascaded Marshall circuit is that there's no way to switch
>the high and low gain besides physically moving a cable from one input
>jack to the other. That circuit was a long time ago, though, these
>days everyone likes channel switching amps which use relays or other
>switching logic to push the signal through different tube stages and
>recombine them at the power amp. Which is complicated and no fun to
>homebrew.
>
>Anyway I came up with this circuit, which allows easy footswitching
>with no relays or anything between a 'clean' input stage and a 'high
>gain' input stage, with appropriate frequency response voicing. It's
>built and working, as part of a homemade amplifier, and I really do
>like the sound so far.
>
>I don't know if it's original or not, in the context of a guitar amp,
>but I think it's pretty neat.
>
>http://img718.imageshack.us/img718/8861/foldedfeedback.png


Reminds me of the "GE Circuit" which was used a lot as a
mic/phono/tape head front-end in the early transistor days:

ftp://jjlarkin.lmi.net/GEcircuit.jpg


John

[Stephen Cowell[_2_]]

"Lord Valve" > wrote
> Don Lancaster wrote:
>
>> Stephen Cowell wrote:
>> >
>> > 2.) Switching the cathode cap out also rolls off lows.
>> >
>>
>> But also provides improved sound staging, reduced
>> midrange granularity, and better bass speed.
>>
>> Full details at http://www.tinaja.com/glib/marcia.pdf

> THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame?

Hey, **** off... he replied to *me*... and I
*damn* sure know who 'the' Don L. is.
Gotta be cool, dude!
__
Steve
..

[MooseFET]

On Jan 24, 3:39*pm, Morris Slutsky > wrote:
> > A two device amplifier with the feedback like shown is not a
> > novel idea. *It has been used quite a bit.
>
> > The wiring of the switched to ground will add some capacitance etc
> > to that node when the switch is open. *This will cause peaking of
> > the high frequencies. *It is better to run wiring at the low
> > impedance points of the circuit.
>
> > With the switch closed, the first section is running the tube at
> > its full gain. *This means that unit to unit variations and the
> > nonlinear effects of this part will show up in the results.
>
> Moose,
>
> I just checked SPICE about capacitance to ground at the footswitch
> point. *There is a small peak at about 100 KHz from a 100 pF
> capacitance there, and a really huge one from 1000 pF. *I'm hoping
> that my actual footswitch is closer to 300 pF *- it probably is,

Did you measure it with a low frequency capacitance meter or use a
high frequency one. I would expect that the capacitance may be higher
at higher frequencies and that there could be a large increase in the
damping factor.

> assuming that most of this is due to the capacitance of the footswitch
> cable itself (about 6 feet) - which SPICE expects to produce a 4 dB
> boost peaking at about 165 pF. *I suspect that the actual boost will
> be less than this, due to stray capacitances (wiring to ground plane?)

Any extra capacitance to ground at that point will increase the peak.
Capacitances at other points may make it get very ugly. There is a
phase shift as you go around the loop.

> lowering the overall high frequency response in ways that SPICE cannot
> readily take into account. * But yes, you're right, I should expect
> peaky behavior here. *It's outside the audio band hopefully, which is
> good because you won't hear it, bad because it could cause inaudible
> parasitics, mitigated by the fact that this peak is still less than
> the response at this frequency in high-gain mode and if it doesn't
> oscillate there why would it oscillate in low gain mode?

In the low gain mode, you have a feed back path that doesn't exist in
the high gain mode. This means that as far as the tubes are
concerned,
the system gain (closed loop gain) is higher in the low gain case.

>
> Thanks for your thoughts and advice.

[Lord Valve]

Stephen Cowell wrote:

> "Lord Valve" > wrote
> > Don Lancaster wrote:
> >
> >> Stephen Cowell wrote:
> >> >
> >> > 2.) Switching the cathode cap out also rolls off lows.
> >> >
> >>
> >> But also provides improved sound staging, reduced
> >> midrange granularity, and better bass speed.
> >>
> >> Full details at http://www.tinaja.com/glib/marcia.pdf
>
> > THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame?
>
> Hey, **** off... he replied to *me*... and I
> *damn* sure know who 'the' Don L. is.
> Gotta be cool, dude!
> __
> Steve
> .

Here ya go: http://tinyurl.com/ygwosee

LV

[Michael A. Terrell]

Lord Valve wrote:
>
> Stephen Cowell wrote:
>
> > "WB" > wrote
> > > Morris Slutsky wrote:
> > >>
> > >> Anyway I came up with this circuit, which allows easy footswitching
> > >> with no relays or anything between a 'clean' input stage and a 'high
> > >> gain' input stage, with appropriate frequency response voicing. It's
> > >> built and working, as part of a homemade amplifier, and I really do
> > >> like the sound so far.
> > >>
> > >
> > > Did you try to simply by-pass the second tube ?
> >
> > 1.) You need a voicing change for guitar 'crunch'... lifting
> > the cathode cap gives this (necessary) change.
> >
> > 2.) 'Bypassing' a stage is not easy... pops, thumps,
> > DC states, inversion of phase... all apply.
> >
> > Your suggestion would result in 180degree phase change
> > when you stomp... not a good idea, in and of itself.
> > __
> > Steve
> > .
>
> Since this is crossposted to rec.audio.tubes, there'll be a disconnect
> between the dudes who handle their glass with white gloves and keep their
> gear on the shelf and the dudes who pound the crap out of it and pour it
> fulla beer...


Real tubes handle 50+ KW.


--
Greed is the root of all eBay.

[Morris Slutsky]

On Jan 24, 8:24*pm, MooseFET > wrote:
> On Jan 24, 3:39*pm, Morris Slutsky > wrote:
>
>
>
> > > A two device amplifier with the feedback like shown is not a
> > > novel idea. *It has been used quite a bit.
>
> > > The wiring of the switched to ground will add some capacitance etc
> > > to that node when the switch is open. *This will cause peaking of
> > > the high frequencies. *It is better to run wiring at the low
> > > impedance points of the circuit.
>
> > > With the switch closed, the first section is running the tube at
> > > its full gain. *This means that unit to unit variations and the
> > > nonlinear effects of this part will show up in the results.
>
> > Moose,
>
> > I just checked SPICE about capacitance to ground at the footswitch
> > point. *There is a small peak at about 100 KHz from a 100 pF
> > capacitance there, and a really huge one from 1000 pF. *I'm hoping
> > that my actual footswitch is closer to 300 pF *- it probably is,
>
> Did you measure it with a low frequency capacitance meter or use a
> high frequency one. *I would expect that the capacitance may be higher
> at higher frequencies and that there could be a large increase in the
> damping factor.
>
> > assuming that most of this is due to the capacitance of the footswitch
> > cable itself (about 6 feet) - which SPICE expects to produce a 4 dB
> > boost peaking at about 165 pF. *I suspect that the actual boost will
> > be less than this, due to stray capacitances (wiring to ground plane?)
>
> Any extra capacitance to ground at that point will increase the peak.
> Capacitances at other points may make it get very ugly. *There is a
> phase shift as you go around the loop.
>
> > lowering the overall high frequency response in ways that SPICE cannot
> > readily take into account. * But yes, you're right, I should expect
> > peaky behavior here. *It's outside the audio band hopefully, which is
> > good because you won't hear it, bad because it could cause inaudible
> > parasitics, mitigated by the fact that this peak is still less than
> > the response at this frequency in high-gain mode and if it doesn't
> > oscillate there why would it oscillate in low gain mode?
>
> In the low gain mode, you have a feed back path that doesn't exist in
> the high gain mode. *This means that as far as the tubes are
> concerned,
> the system gain (closed loop gain) is higher in the low gain case.
>
>
>
> > Thanks for your thoughts and advice.

Hear you, Moose, so I SPICEd it again. I've been using LT-SPICE with
Duncan Munro's tube models, it's easy to use and the price is right!
Anyhow, modeling an open switch as a 300 pF capacitor, which is
reasonable from what's known about guitar cables, I can snub out the
ultrasonic peak pretty much completely with a 30 pF cap across the 680
K grid resistor on the second tube. The cost is the loss of 1 dB or
so, at 20 KHz in high-gain mode. Probably I could live with that.
Although some guitar amp purists would complain about loss of
'sparkle', guitar amp lovers have some hatred towards snubber caps due
to their abuse by vendors in the past to compensate for really bad
wiring layouts. This ain't a huge snubber though. I could put one
in. The thing is, though, it's such a small capacitance value that I
have no idea how much stray capacitance is currently there and what
it's doing, I don't have a spectrum analyzer setup here, I make amps
in my kitchen mostly. But it's a reasonable thing to do. If I get it
open again, I might put a small snubber there.

[Bitrex]

John Larkin wrote:
> On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
> > wrote:
>
>> There's been a lot of discussion on AGA lately about the Marshall 2204
>> "Cascaded" preamp circuit and whether or not it sucks and how much. A
>> lot of people really do like the high gain preamp. But what sucks
>> about the cascaded Marshall circuit is that there's no way to switch
>> the high and low gain besides physically moving a cable from one input
>> jack to the other. That circuit was a long time ago, though, these
>> days everyone likes channel switching amps which use relays or other
>> switching logic to push the signal through different tube stages and
>> recombine them at the power amp. Which is complicated and no fun to
>> homebrew.
>>
>> Anyway I came up with this circuit, which allows easy footswitching
>> with no relays or anything between a 'clean' input stage and a 'high
>> gain' input stage, with appropriate frequency response voicing. It's
>> built and working, as part of a homemade amplifier, and I really do
>> like the sound so far.
>>
>> I don't know if it's original or not, in the context of a guitar amp,
>> but I think it's pretty neat.
>>
>> http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
>
> Reminds me of the "GE Circuit" which was used a lot as a
> mic/phono/tape head front-end in the early transistor days:
>
> ftp://jjlarkin.lmi.net/GEcircuit.jpg
>
>
> John
>
>

I have seen similar circuits somewhere on the 'net where I think there
is positive feedback as well as negative feedback used. I guess the
positive feedback was used to kick up the gain of the crappy early
transistors. Here's a two transistor circuit from 1964 that uses "DC
positive feedback and AC-DC negative feedback":

http://www.freepatentsonline.com/3267386.html

[John Larkin]

On Mon, 25 Jan 2010 04:20:42 -0800 (PST), Morris Slutsky
> wrote:

>On Jan 25, 4:02*am, Fred Abse > wrote:
>> On Sun, 24 Jan 2010 18:46:38 -0700, Lord Valve wrote:
>> > we're just ignorant guitar pickers
>>
>> So why crosspost to sci.electronics.design?
>>
>> sci - the hint's in the name.
>>
>> --
>> "Electricity is of two kinds, positive and negative. The difference
>> is, I presume, that one comes a little more expensive, but is more
>> durable; the other is a cheaper thing, but the moths get into it."
>> * * * * * * * * * * * * * * * * * * * * * * *(Stephen Leacock)
>
>I'm the one who crossposted. Guilty! I just wanted to hear from
>these 3 different groups. It's a guitar amp circuit so it belongs on
>AGA. It's a tube circuit so it belongs on RAT. And since it's
>design, feedback, all that complicated stuff, I wanted to hear from
>SED. Fair enough I guess! I understand that the perspectives here
>are all going to be different. As to the Lord, he is well known to
>dislike "3 legged fuses" and anything digital - but oddly enough he
>does know quite a bit about the old-fashioned FETs of the 70s so go
>figure, it's his personal opinion and not his lack of knowledge, it's
>an informed opinion even if we may disagree. Me, I don't always
>dislike solid state guitar amps, and nobody has ever had a problem
>with transistorized guitar effects such as the classic Fuzz Face,
>although I do dislike digital guitar gear probably because I first saw
>the stuff in the early 90s when it had latency so bad that you could
>hit a note and go make a sandwich before coming back and hearing it
>come out of the speaker. So that's where I'm coming from and why I
>crossposted.
>
>As to the Lord's client list - never heard of ANY of them? Really?
>Not even Meatloaf or Fleetwood Mac or Huey Louis? I can understand
>that most people wouldn't like Yo La Tengo but they are cool I think.
>And definitely the Dirty Dozen Brass Band rules - check out "Brooklyn"
>I think that's their main popular tune. As to the Flobots - I love
>the Flobots. My band actually covers "Handlebars" and they let me
>play the horn solo (I only play guitar but I use an octave fuzz
>there. It's kind of close. Kinda.) I mean, it's not like he has
>done work for the Beatles but hey you know the Beatles are half-dead
>already, life is like that, there's plenty of good music out there
>today.

Electrical engineers tend to be suspicious of the subjectivity of
audio, not to mention the multitude of bad, bad circuits. Our world
involves measuring things, not asking drugged-out, hearing-impaired
rockers how they like the sound of our gear.

I designed one guitar amp. and that was plenty. Not only did some
goofy music store owners decide how it should work, but they wanted to
make all the money, too.

John

[Morris Slutsky]

On Jan 25, 7:31*am, Bitrex > wrote:
> John Larkin wrote:
> > On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
> > > wrote:
>
> >> There's been a lot of discussion on AGA lately about the Marshall 2204
> >> "Cascaded" preamp circuit and whether or not it sucks and how much. *A
> >> lot of people really do like the high gain preamp. *But what sucks
> >> about the cascaded Marshall circuit is that there's no way to switch
> >> the high and low gain besides physically moving a cable from one input
> >> jack to the other. *That circuit was a long time ago, though, these
> >> days everyone likes channel switching amps which use relays or other
> >> switching logic to push the signal through different tube stages and
> >> recombine them at the power amp. *Which is complicated and no fun to
> >> homebrew.
>
> >> Anyway I came up with this circuit, which allows easy footswitching
> >> with no relays or anything between a 'clean' input stage and a 'high
> >> gain' input stage, with appropriate frequency response voicing. *It's
> >> built and working, as part of a homemade amplifier, and I really do
> >> like the sound so far.
>
> >> I don't know if it's original or not, in the context of a guitar amp,
> >> but I think it's pretty neat.
>
> >>http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
> > Reminds me of the "GE Circuit" which was used a lot as a
> > mic/phono/tape head front-end in the early transistor days:
>
> >ftp://jjlarkin.lmi.net/GEcircuit.jpg
>
> > John
>
> I have seen similar circuits somewhere on the 'net where I think there
> is positive feedback as well as negative feedback used. *I guess the
> positive feedback was used to kick up the gain of the crappy early
> transistors. *Here's a two transistor circuit from 1964 that uses "DC
> positive feedback and AC-DC negative feedback":
>
> http://www.freepatentsonline.com/3267386.html

Very interesting! I looked at that circuit and definitely that's DC
coupled positive feedback and AC coupled negative feedback. The
positive feedback increases input impedance (so I guess that's what
some people call 'bootstrapping') while the negative feedback
counteracts it to provide linearity. Neat idea. However I'd probably
not want to use that circuit myself - having positive feedback at DC
is just begging for thermal drift once the transistors heat up, isn't
it? It would worry me. And in these times of high-impedance input
devices like FETs, there's no need to use this circuit I don't think.
Nice idea, though, I did check it out.

[John Larkin]

On Mon, 25 Jan 2010 07:31:38 -0500, Bitrex
> wrote:

>John Larkin wrote:
>> On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
>> > wrote:
>>
>>> There's been a lot of discussion on AGA lately about the Marshall 2204
>>> "Cascaded" preamp circuit and whether or not it sucks and how much. A
>>> lot of people really do like the high gain preamp. But what sucks
>>> about the cascaded Marshall circuit is that there's no way to switch
>>> the high and low gain besides physically moving a cable from one input
>>> jack to the other. That circuit was a long time ago, though, these
>>> days everyone likes channel switching amps which use relays or other
>>> switching logic to push the signal through different tube stages and
>>> recombine them at the power amp. Which is complicated and no fun to
>>> homebrew.
>>>
>>> Anyway I came up with this circuit, which allows easy footswitching
>>> with no relays or anything between a 'clean' input stage and a 'high
>>> gain' input stage, with appropriate frequency response voicing. It's
>>> built and working, as part of a homemade amplifier, and I really do
>>> like the sound so far.
>>>
>>> I don't know if it's original or not, in the context of a guitar amp,
>>> but I think it's pretty neat.
>>>
>>> http://img718.imageshack.us/img718/8861/foldedfeedback.png
>>
>>
>> Reminds me of the "GE Circuit" which was used a lot as a
>> mic/phono/tape head front-end in the early transistor days:
>>
>> ftp://jjlarkin.lmi.net/GEcircuit.jpg
>>
>>
>> John
>>
>>
>
>I have seen similar circuits somewhere on the 'net where I think there
>is positive feedback as well as negative feedback used. I guess the
>positive feedback was used to kick up the gain of the crappy early
>transistors. Here's a two transistor circuit from 1964 that uses "DC
>positive feedback and AC-DC negative feedback":
>
>http://www.freepatentsonline.com/3267386.html

Both the feedbacks are negative in the GE thing. The lower one is DC
feedback to bias the first transistor. The one through Zf is negative
feedback to set the gain and do equalization.

John

[Morris Slutsky]

On Jan 24, 11:51*am, Engineer > wrote:
> On Jan 24, 10:24*am, "Stephen Cowell"
>
>
>
> > wrote:
> > "Morris Slutsky" > wrote
> > ...
>
> > > I don't know if it's original or not, in the context of a guitar amp,
> > > but I think it's pretty neat.
>
> > >http://img718.imageshack.us/img718/8861/foldedfeedback.png
>
> > I like it! *Two thoughts:
>
> > 1.) *If you replace one gain stage with this, you'll
> > * * invert the signal... this can cause problems sometimes.
> > 2.) *Switching the cathode cap out also rolls off lows.
>
> > I'd like to hear it in action.
> > __
> > Steve
> > .
>
> Looks good! *More thoughts... (for a flat frequency response
> application)
> -- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode
> decouplng (and feedback) over a much wider frequency range
> -- plate load on the 2nd triode is 100K//47K (at least when switch is
> closed), or about 32K; this seems a bit low.
> -- bypass the 2nd triode cathode resistor (and make it smaller, e,g.
> 2.7K) for more gain, then increase the above 47K for the right amount
> of feedback
> This design might serve to switch a single audio input from microphone
> gain to AUX gain (not tried)
> Cheers,
> Roger

Hi Roger,

If I was to design a circuit for hi-fi work, I'd never want the
feedback loop to be broken at all. Perhaps I'd be swapping resistors
within the feedback loop, but I wouldn't want to break the loop
altogether. Only guitarists want the gain and distortion to go up at
the same time, along with a band-limited frequency response - that
really is just for this one specific application.

Yeah, I load that triode heavy. It's a tradeoff. The 12AX7 has a
pretty high rP, I think about 68K. But it has a ton of mu! The other
12A*7 tubes have lower rP but a lot less mu. It probably all works
out the same in the end, except for bias levels. But the 12AX7 is the
tube that I can conveniently buy, so I'm using it. Output impedance
would be expected to be 68K || 100K = 40K, so my 47K load does eat
about half the signal, about 3dB of gain. There's still plenty gain
there to go around, you just may have to turn the pot for the next
stage to 10 instead of 9, no big deal. And it is a 47K load under all
circumstances - with the loop open it's 47K to ground, with the loop
closed it's 47K to what amounts to a virtual ground.

Probably something very similar could work for hi-fi gain control, I
think it's probably pretty common in that context actually to do gain
by switching feedback resistors.

Thanks for your advice

[Stephen Cowell[_2_]]

"Lord Valve" > wrote
> Stephen Cowell wrote:
>> "Lord Valve" > wrote
>> > Don Lancaster wrote:
>> >
>> >> Stephen Cowell wrote:
>> >> >
>> >> > 2.) Switching the cathode cap out also rolls off lows.
>> >> >
>> >>
>> >> But also provides improved sound staging, reduced
>> >> midrange granularity, and better bass speed.
>> >>
>> >> Full details at http://www.tinaja.com/glib/marcia.pdf
>>
>> > THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame?
>>
>> Hey, **** off... he replied to *me*... and I
>> *damn* sure know who 'the' Don L. is.
>> Gotta be cool, dude!
>
> Here ya go: http://tinyurl.com/ygwosee

Everyone knows that Ex-Lax destroys the
'air' in the stage space... Dulco-Lax maintains
the phase resilience in the trans-uranic highs.

However, true hardcore aficianados work it
out with a pencil... No. 2 Eberhard, pre-Sanford
NOS... no mechanical sharpening!
__
Steve
..

[Stephen Cowell[_2_]]

"Michael A. Terrell" > wrote
> Lord Valve wrote:

>> Since this is crossposted to rec.audio.tubes, there'll be a disconnect
>> between the dudes who handle their glass with white gloves and keep their
>> gear on the shelf and the dudes who pound the crap out of it and pour it
>> fulla beer...
>
>
> Real tubes handle 50+ KW.

And you'd *damn* sure better handle *them*
with white cloth gloves... or clean them in
solvent before installation!
__
Steve
..

[John Larkin]

On Mon, 25 Jan 2010 09:44:18 -0600, "Stephen Cowell"
> wrote:

>
>"Michael A. Terrell" > wrote
>> Lord Valve wrote:
>
>>> Since this is crossposted to rec.audio.tubes, there'll be a disconnect
>>> between the dudes who handle their glass with white gloves and keep their
>>> gear on the shelf and the dudes who pound the crap out of it and pour it
>>> fulla beer...
>>
>>
>> Real tubes handle 50+ KW.
>
>And you'd *damn* sure better handle *them*
>with white cloth gloves... or clean them in
>solvent before installation!
>__
>Steve
>.
>

I use only liquid-nitrogen cured National Union 6SN7s. Be careful
about the direction of the filament current.

John

[Lord Valve]

Stephen Cowell wrote:

> "Lord Valve" > wrote
> > Stephen Cowell wrote:
> >> "Lord Valve" > wrote
> >> > Don Lancaster wrote:
> >> >
> >> >> Stephen Cowell wrote:
> >> >> >
> >> >> > 2.) Switching the cathode cap out also rolls off lows.
> >> >> >
> >> >>
> >> >> But also provides improved sound staging, reduced
> >> >> midrange granularity, and better bass speed.
> >> >>
> >> >> Full details at http://www.tinaja.com/glib/marcia.pdf
> >>
> >> > THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame?
> >>
> >> Hey, **** off... he replied to *me*... and I
> >> *damn* sure know who 'the' Don L. is.
> >> Gotta be cool, dude!
> >
> > Here ya go: http://tinyurl.com/ygwosee
>
> Everyone knows that Ex-Lax destroys the
> 'air' in the stage space... Dulco-Lax maintains
> the phase resilience in the trans-uranic highs.
>
> However, true hardcore aficianados work it
> out with a pencil... No. 2 Eberhard, pre-Sanford
> NOS... no mechanical sharpening!
> __
> Steve
> .

Well, you'd know more about that **** than I would.

LV

[Michael A. Terrell]

Stephen Cowell wrote:
>
> "Michael A. Terrell" > wrote
> > Lord Valve wrote:
>
> >> Since this is crossposted to rec.audio.tubes, there'll be a disconnect
> >> between the dudes who handle their glass with white gloves and keep their
> >> gear on the shelf and the dudes who pound the crap out of it and pour it
> >> fulla beer...
> >
> >
> > Real tubes handle 50+ KW.
>
> And you'd *damn* sure better handle *them*
> with white cloth gloves... or clean them in
> solvent before installation!


No need, unless you don't like fingerprints on the stainless steel
body, but you absolutely have to make sure that there is enough water
flow to keep them from melting down. One system i worked with had
three 65 kW tubes, for a 195 kW output.


--
Greed is the root of all eBay.