would a Master Volume solve my issue?

[J M Fahey]

Sorry, but "the other" solutions don´t cut it, you should do exactly what Fender did in the SE version.

besides:
* that the preamp is SS is irrelevant, you won´t be overdriving a preamp but a triode.
* it will give you the exact same compression as any other triode, because that´s what it is.
* a volume pot in the loop will control the volume of either the clean SS preamp or that of the built in distortion pedal, again SS, while the SE way will add true tube distortion and compression.

It IS a PPIMV ; don´t get tangled in the definition, both triodes are part of the PI and the first is the one which gets the NFB, proof that it´s part of the power amp and not just the last preamp triode.

[fastfolkert]

Sorry, but "the other" solutions don´t cut it, you should do exactly what Fender did in the SE version.

besides:
* that the preamp is SS is irrelevant, you won´t be overdriving a preamp but a triode.
* it will give you the exact same compression as any other triode, because that´s what it is.
* a volume pot in the loop will control the volume of either the clean SS preamp or that of the built in distortion pedal, again SS, while the SE way will add true tube distortion and compression.

It IS a PPIMV ; don´t get tangled in the definition, both triodes are part of the PI and the first is the one which gets the NFB, proof that it´s part of the power amp and not just the last preamp triode.

OK, OK, I'm convinced ;-)
I fetch my amp from the practice room and will mod it this weekend. I try your suggestion first (clone according 25SE), and will also try robrob's suggestion.
Will be back after the weekend Thanks again!

[robrob]

It IS a PPIMV ; don´t get tangled in the definition, both triodes are part of the PI and the first is the one which gets the NFB, proof that it´s part of the power amp and not just the last preamp triode.

Are we looking at the same schematic? I see a driver feeding a cathodyne phase inverter with a master volume between them. Why would you call the driver part of the phase inverter? I don't see any NFB either. The negative feedback loop can reach deep into a preamp so I don't feel comfortable calling everything after the NFB injection point "the power amp."

fastfolkert, I missed the fact that the phase inverter's grid leak was separate from the master volume. I'd keep the mod simple and just replace the PI grid leak with a 1MA pot for the master volume.

[fastfolkert]

fastfolkert, I missed the fact that the phase inverter's grid leak was separate from the master volume. I'd keep the mod simple and just replace the PI grid leak with a 1MA pot for the master volume.

I'll open up the amp next weekend and will try two things:
1 - add the 1Meg pot + coupling cap on the position Juan suggested (= cloned from 25SE)
2 - add the 1Meg pot on the position you (robrob) suggested (= replace existing grid leak resistor).

To me, the differences are subtle in the design, but whatever sounds best, wil be made permanent.

Thanks again.

[Alex R]

Are we looking at the same schematic? I see a driver feeding a cathodyne phase inverter with a master volume between them. Why would you call the driver part of the phase inverter? I don't see any NFB either. The negative feedback loop can reach deep into a preamp so I don't feel comfortable calling everything after the NFB injection point "the power amp."

fastfolkert, I missed the fact that the phase inverter's grid leak was separate from the master volume. I'd keep the mod simple and just replace the PI grid leak with a 1MA pot for the master volume.

Doesn't the NFB come in via R86?

R81 on the v3B grid looks like it's providing some feedback around v3B, this might create some odd results if you replaced it with a pot?

[fastfolkert]

Doesn't the NFB come in via R86?

R81 on the v3B grid looks like it's providing some feedback around v3B, this might create some odd results if you replaced it with a pot?

You could be right; that's perhaps why Juan suggested to leave R81 in and just add a pot, like in the 25SE power amp:

[J M Fahey]

Are we looking at the same schematic? I see a driver feeding a cathodyne phase inverter with a master volume between them. Why would you call the driver part of the phase inverter? I don't see any NFB either. The negative feedback loop can reach deep into a preamp so I don't feel comfortable calling everything after the NFB injection point "the power amp."

fastfolkert, I missed the fact that the phase inverter's grid leak was separate from the master volume. I'd keep the mod simple and just replace the PI grid leak with a 1MA pot for the master volume.

Lots of work to deliver today, don´t have time now for one of my long winded explanations, will answer just 2 of the simplest points:

1) don´t you see the NFB?
check what R86 and R85 do: take a sample of voltage present at the speaker out, attenuate it whatever´s necessary and apply it to the cathode of V3A, which is the inverting input at the beginning of the Power Amp.
The non inverting input is V3A grid.

2) NFB does not "run deep into a preamp" at all.
Tube amps are hard to apply much NFB, if any at all, because transformers with their huge phase shifts complicate things, turning them into oscillators.

Typical SS amps can easily have 40dB or more of NFB ; tube amps must be happy with , say, 12dB, maybe a little more if very good iron is used.
So NFB (the little you can use) can usually be applied to the PI .... and not much beyond it.

Add just one extra stage (which adds both gain and phase shift) and the thing becomes absolutely uncontrollable.

Will deal with the rest this night.

Maybe just one little point:
the master volume as used in the SE version is fine and does its duty in a predictable way.
Not surprisingly, Fender chose it.

Turning the grid reference resistor into a MV control has at least 2 problems, may also have subtler ones:

a) a regular 1M pot, with one leg grounded, is always 1M , and you can predict highpass frequency when coupled with its coupling cap, in this case .047uF

2) if grid to cathode referenced as you suggest, remember :
* this cathode is unbypassed and carries a high AC voltage, easily 40 or 50V RMS, whatever´s needed to drive power tube grids
* said cathode is bootstrapping the 1M pot, since it floats and is connected to cathode, so effective impedance is WAY higher than 1 M
* we can easily calculate highpass frequency plugging .047 and 1M in the formula ..... but ...... what happens when the 1M pot is bootstrapped? What value do we use? 10M ? 40M? which value?

3) said pot is floating some 100V above ground, and all of it carries some 50V RMS or more.
It scratches me the wrong way to have it mounted on the front panel when carrying such DC and AV which belong elsewhere.
All this just to save 1 cap and 1 resistor?

Yes, I know it will work, specially in simulation, but on a real chassis I don´t like it that much.

Admittedly, a Master Volume will also receive 50/60V RMS ... but on 1 lug only , the other end is grounded, no DC present.

A dirty (conventional) MF may scratch or have dead spots, a dirty pot/grid resistor will be much worse because it will leave the cathodyne grid without reference.

[Dave H]

Doesn't the NFB come in via R86?

R81 on the v3B grid looks like it's providing some feedback around v3B

It will 'bootstrap' the input making the input impedance much higher than 1M.

Edit: I see Juan beat me to it

[fastfolkert]

Lots of work to deliver today, don´t have time now for one of my long winded explanations, will answer just 2 of the simplest points:
[....following a comprehensive lecture on tube amplifier theory...]

My tube-theory-knowledge is clearly not sufficient here and I'm glad I have the opportunity to learn.
Looking forward to the "proof of the pudding" next weekend!

[robrob]

Yep, I missed the R86 feedback (embarrassing).

J M When I said, "The negative feedback loop can reach deep into a preamp. . ." I meant in general guitar tube amps, not this particular amp.

I agree with the points you made in your latest post.

fastfolkert, J M is right, the "Loudness" control as wired in the 25SE with coupling caps on both sides of the pot to keep DC off the pot is the way to go.

[fastfolkert]

fastfolkert, J M is right, the "Loudness" control as wired in the 25SE with coupling caps on both sides of the pot to keep DC off the pot is the way to go.

OK, that will be the mod I'll do next weekend. Will keep you all informed with the results and many thanks to all of you for the insight in this issue!

[olddawg]

Seriously... Before you go to all of this trouble, stick a volume control in the pre out/main in. What have you got to loose? In would take 5 minutes. If you have a compatible volume pedal, even an active boost like an LPB-1 laying around and adjust it below unity it, even less.

[fastfolkert]

fastfolkert, J M is right, the "Loudness" control as wired in the 25SE with coupling caps on both sides of the pot to keep DC off the pot is the way to go.

Despite the fact that my attic was close to 35C (approx 90 F), I spent some time there today to add the Champ25SE Master Volume into my old Champ25.
I had a new Bourns series 81 pot and a 400V 0.033uF filmcap laying around. I decided to add the wires to the solder-side of the board and therefore the result may perhaps not win the "Best Amp Guts-contest".
However....., the result is exactly what I expected (and you guys predicted)! Thanks for the advice.

[robrob]

Cool, congrats on a successful and very useful mod.

[jejesixtiessix]

Despite the fact that my attic was close to 35C (approx 90 F), I spent some time there today to add the Champ25SE Master Volume into my old Champ25.
I had a new Bourns series 81 pot and a 400V 0.033uF filmcap laying around. I decided to add the wires to the solder-side of the board and therefore the result may perhaps not win the "Best Amp Guts-contest".
However....., the result is exactly what I expected (and you guys predicted)! Thanks for the advice.

[ATTACH=CONFIG]40426[/ATTACH]

Hi, i'm from france, i have the same champ 25 fender, i saw your mod, congratulations ;-) now do you have a nice crunch sound with "volume" high and "master" low ? im mean normaly its very difficult to have a nice crunch because its very loud...i try to add a volume box with preamp in - preamp out, yes its like a master volume BUT it doesn't crank and dont make nice crunch...