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Morris Slutsky Morris Slutsky is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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/img718/8...edfeedback.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 he

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.


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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.


"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/img718/8...edfeedback.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
..


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Engineer[_2_] Engineer[_2_] is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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/img718/8...edfeedback.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
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pimpom pimpom is offline
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Posts: 1
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.

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/img718/8...edfeedback.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 he

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.


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MooseFET MooseFET is offline
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Posts: 53
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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/img718/8...edfeedback.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




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Don Lancaster Don Lancaster is offline
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Posts: 7
Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

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
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WB WB is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

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 ?
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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.


"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
..




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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.


"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
..


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Lord Valve Lord Valve is offline
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Posts: 296
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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




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Morris Slutsky Morris Slutsky is offline
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Posts: 13
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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.
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Tim Williams Tim Williams is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

"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


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Too Long in The Wasteland Too Long in The Wasteland is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

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.
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Morris Slutsky Morris Slutsky is offline
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Posts: 13
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.


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.


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Morris Slutsky Morris Slutsky is offline
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Posts: 13
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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.


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John Larkin John Larkin is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.

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/img718/8...edfeedback.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


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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Posts: 18
Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.


"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
..


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MooseFET MooseFET is offline
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Posts: 53
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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.


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Lord Valve Lord Valve is offline
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Posts: 296
Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing"folded" feedback loop.

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


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Michael A. Terrell Michael A. Terrell is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.


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.


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

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.


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Bitrex Bitrex is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

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/img718/8...edfeedback.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
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John Larkin John Larkin is offline
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Posts: 151
Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.

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



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Morris Slutsky Morris Slutsky is offline
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Posts: 13
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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/img718/8...edfeedback.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.
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John Larkin John Larkin is offline
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Posts: 151
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.

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/img718/8...edfeedback.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



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Morris Slutsky Morris Slutsky is offline
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Posts: 13
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.

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/img718/8...edfeedback.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
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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.


"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
..


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Stephen Cowell[_2_] Stephen Cowell[_2_] is offline
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Posts: 18
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.


"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
..


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John Larkin John Larkin is offline
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Posts: 151
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.

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


  #30   Report Post  
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Lord Valve Lord Valve is offline
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Posts: 296
Default Novel? Guitar amp input stage, single 12AX7, footswitchableusing"folded" feedback loop.

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




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Michael A. Terrell Michael A. Terrell is offline
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Posts: 318
Default Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.


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.
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