Adding a master volume to a Carlsbro

[Ell]

So, it's not technically a pedal, but I've got a big old Carlsbro 50 Top head. I'm not an expert on amps, but it's a pretty simple circuit... It's an ultra clean amp. If I want a bit more distortion out of this thing, where would you add a master volume? I was thinking, after the second half of V1, before going into the phase inverter. Would it just be as simple as wiring up a pot there? Or, should I try to add another gain stage with another 12AX7?

[antonis]

It's an ultra clean amp. If I want a bit more distortion out of this thing, where would you add a master volume?

Passive Volume control can only make it "cleaner"..

[Ell]

Passive Volume control can only make it "cleaner"..

Hmm, so I'm wrong thinking I can put a volume control at the red circle, and then crank the old "volume" control (which would now become the gain control) at the blue square? Wouldn't that drive the first preamp valve?

[Ell]

Another potential source of gain that I've seen talked about is "negative feedback". As I understand, Fenders (and this) tend to use negative feedback to keep the amp clean, while Vox and Marshall amps remove this so that there's more distortion.
So, either removing this, or altering this is an apparent source of gain, but I don't really know what to look for on a valve. Can anyone elaborate on how I would go about modding this aspect, and if it's worth doing?

[GibsonGM]

It kind of 'is what it is'...for what they were built to do, they're good amps.  I wouldn't mess around w/the feedback without some good advice (you can get bad runaway issues if you don't know what's going on) - but yes, you COULD remove it.   The 68k running back to the PI from the OT secondary.

Have you tried slamming the living crap out of the front end with a boost?   I mean - the way to 'make this thing a beast' would be to add more triode gain stages to the preamp; it's just not how it's designed.   So I think  a pre-input boost might get you something more to your liking.   Even something tube-based (in fact, I love using a tube driver-type boost in front of a Fender to juice it up).   Or a mosfet boost, LPB-1, TS, what have you...the amp isn't doing anything wrong, stop picking on it!   

[Ell]

It kind of 'is what it is'...for what they were built to do, they're good amps.  I wouldn't mess around w/the feedback without some good advice - but yes, you COULD remove it.   

Have you tried slamming the living crap out of the front end with a boost? 

Hmm, okay. Yeah, I tried hitting the front end with a Matsumin Valve Caster clone, and a Boss OD-3, and it was pretty cool. But, I get the feeling that no matter how powerful the boost, it isn't actually clipping the input, but there's still headroom.

Do you know where the feedback loop is in the circuit?

Thanks for the responses! Apologies that this isn't a pedal... y'all are just the cleverest people on the internet.

[printer2]

I think a dual pot after the PI would give a more pleasing distortion. On the NFB, I do not know of any Marshalls that do not have NFB and some Vox amps have it, some not. The NFB is coming from the output transformer to the PI.

[GGBB]

I think you could lower the headroom of the two input stages by increasing the 47k dropping resistor. Not sure that's a candidate for a pot though, but maybe a toggle, or simply a one time mod to give you a range of tones you can live with.

Otherwise, as mentioned already, a post phase inverter trainwreck style MV should work well, assuming that distortion is happening either in the preamp or the PI. If this amp distorts primarily in the power tubes, a master volume alone won't help.

[GibsonGM]

It only has 2 stages in the preamp...it's not clipping a lot, in the modern understanding.  And it takes several stages and tone shaping to get there too. It SHOULD clip much More by hitting it with a booster...what kind of tones you looking to get out of this - I hope not death metal     

It's a Valve Jr. with a poweramp, lol. 

I'm with Gord - PPIMV.  Great amp to learn on!    Me, I'd be audio probing that preamp - but do not get into this end of things if you don't know what you're doing!!  You could easily kill yourself.  You can remove the FB, but it's not going to sound how you want - the 68k I referred to earlier.  If you play with that and get the values wrong, you might end up with a squealing, horrible mess...PPIMV.