would a Master Volume solve my issue?

[fastfolkert]

I use the clean channel of my Fender Champ 25 (SS pre, 12AX7 PI and 6L6 power tubes). It sounds good, but the problem is it starts to break up (or compress) only at very high volume levels, too loud for our practice room.
My buddy guitar player uses the clean channel on his Marshall JTM30 and he can get a nicely "compressed" clean sound, by setting his clean channel volume high, and master low. Without ear-bleeding volume.

I like to have a clean sound, with some compression or break up if you like. Does that mean I need an amp with master volume?
Or is the solid-state nature of the preamp preventing this "compression" or "break-up" ? In that case I consider a full tube amp.

Would it be possible to build-in a MV in my Champ 25? (I have the skills, tools and measuring equipment to do the work)
I have not tried a power soak (dummy load) or a potmeter in the effects loop yet.

Schematics of the almost identical Champ 25SE are here: http://ampwares.com/schematics/champ_25_se.pdf

Thanks!

[mozz]

Doesn't that loudness control work as a master volume?

[Justin Thomas]

Yes, the Loudness IS a Master Volume. Have you tried cranking the Volume control while keeping the Loudness low as loud as you can stand?

I have a funny feeling that the Clean channel on this amp is very much like the Silverface amp Master Volumes. That is, they're not really designed to make ANY amount of what we would call "breakup." The Gain channel is probably another matter entirely. You may get the results you want by using the Gain channel, and keeping the Gain under 12o'clock or so, and then adjusting the Loudness as high as you can get away with. It will most certainly require fine tuning, but the idea is to keep the dirt and compression at absolute minimum that you need, then keep the Loudness as high as you can, and hopefully use guitar controls to take it from there.

I'm not exactly sure when the JTM30 came out, but I'm venturing a guess almost 20 years after yours. Different music, different expectations, so different circuit. Different sound. Your friend's Marshall is probably much closer to a tweed Bassman than your Champ, which is probably closer to a late-70s Deluxe in overall design. Just some guesses...

Hope that helps,

Justin

[fastfolkert]

Doesn't that loudness control work as a master volume?

My Champ 25 has only one volume control per channel; there's no gain on the clean channel, and no overall MV.
Turning up the volume control of the clean channel will eventually give me a nicely "compressed" tone, but only at unacceptable "loudness" (for our practice room)

Pls. note it's a Champ 25, not an ordinary Champ. It's completely different

[pdf64]

Schematics of the almost identical Champ 25SE are here: http://ampwares.com/schematics/champ_25_se.pdf

That amp has a control R82 noted as 'loudness'; that is a master volume, ie if it's turned down then no signal can get through to the power amp, no matter what the other control settings are.
Does your amp have that control; it is the furthest control along from the input sockets, between the reverb control and the FX loop (pre-amp out / power amp in) sockets.
If not, then the correct schematic is needed for review, in order to ascertain how feasible it would be to add a master volume to it.

[fastfolkert]

(Shame mode on). I linked to the schematics of the Champ 25SE which indeed has a MV (shame mode off)

I have a Champ 25 (no SE), whick lacks a MV.....I have not yet found the schematics of the 25.
Hopefully it uses the same PCBoard and I can add a potmeter in that position. Thanks for pointing out!

[J M Fahey]

If you can´t fid your exact model schematic, print the SE version, if necessary enlarge and print only the power amp section, meaning the area around the power tubes and phase inverter, and start comparing yours to the schematic.

Tick every part you can identify and if found but different value, write the new one (yours).

I bet they will be *basically* the same, PCBs won´t necessarily match, nor part numbers, so you should also correct that .

In any case, it WILL have some kind of phase inverter or another, the schem shows a gain stage driving a concertina (unity gain) inverter, with a volume control between them (master) which you can add easily; the other possible PI is the long tailed pair, what Twins and such use, you can also add a Master volume there, although it´s somewhat more complex.

So now it´s your turn
Check and post the corrected schematic.

[fastfolkert]

Thanks again for your insight ;-)

I managed to find a (bad) copy of the Champ 25 schematic.
For the poweramp it is identical to the Champ 25SE, except that the 25SE has a 1M pot (MV), and 0.047uF coupling cap.

Champ 25: (my amp, no MV)


Champ 25SE:


I have the parts and skills to add that 1 Meg pot+coupling cap in my Champ 25.
However, will that give the desired "compression", since the preamp is entirely solid state ? (opamps)

[Enzo]

It is two parts. Tack them in there temporarily and find out if it does anything useful to you. If it does, then you can worry about how to do it permanently.

[fastfolkert]

It is two parts. Tack them in there temporarily and find out if it does anything useful to you. If it does, then you can worry about how to do it permanently.

I know......but I modded this amp before (tamed the drive channel) and putting it back together is a real PITA.
Especially getting the LEDs in their front panel-holes, and simultaneously lining up all those pots....
Anyhow, I try to get an impression from the "pros" whether this MV mod would bring me anything, given the solid state preamp.

Thanks again for reading!

[robrob]

I would install the "Loudness" control just like the Champ 25SE. If you can't get the breakup you want with the Loudness control turned down then just add a boost pedal in front. The phase inverter is a unity gain stage so a post phase inverter master volume (PPIMV, putting the MV pot between the PI and power tubes) won't make the master volume any more effective.

An added benefit of having the MV in front of the cathodyne PI is you can dial it down and prevent nasty PI double frequency distortion. This is the same reason the 5E3 Deluxe benefits from a pre-phase inverter master volume.

You simply replace the phase inverter's 1M grid leak with a 1MA pot:

[g1]

Is the preamp the same as the 25SE? If so, I think the preamp should sound ok overdriven.

[Justin Thomas]

I checked the Fender website, and according to them, they have schema available for the Champ 25, Champ 25G, Champ 25GR, and Champ 25SE. You have to email them, and they send you a pdf. Might be worth a shot, just so we can all be looking on the same page. Would eliminate a lot of "if, then..." stuff.

There <IS> a possibility that that Champ 25 (plain) schem is for "something else," but I think it's worth TRYING, right? What's an email cost?

Justin

[olddawg]

Does this amp have a serial FX loop or a preamp in and out? If so, just put a 25k audio log pot in a project box and plug it in. Or... Use a compatible volume pedal.

[fastfolkert]

Is the preamp the same as the 25SE? If so, I think the preamp should sound ok overdriven.

Yes it is; here's the 25SE:


The 25 is identical.
As you can see it's all opamps. That's my uncertainty: will it sound OK when overdriven; or wil it be a nasty solid state scratch sound?

@olddawg: Yes, the Champ25 has a pre-out/power-in. Not tried a potmeter there yet.

@robrob: thanks for the insight. The 25SE has a fixed grid leak, but adds the 1Meg pot in front of that, plus a coupling cap. See my earlier post with the schematics of the two poweramps.