Bogen PA head for guitar? Help with pentode preamp stages.

[MikeH]

Have you simply tried putting an OD pedal into it? Fresh tubes?

Power Tubes read fine on the tester, and the preamp tubes are brand new, so that bit is ok. Obviously putting an OD pedal in front of it would work, that's not the solution I'm going for though.

[MikeH]

Does anyone think replacing the 5uF filter caps with 8uf caps would have negative consequences? I'm having trouble finding 4.7 or 5uf caps from a brand that I like or trust.

[Enzo]

Ann Arbor? OK, I will type slower then...

8uf???? That is an ancient non-standard value. Probably pay "vintage" prices for them. Caps in that era had tolerances of like -20/+80%. Seriously. Just use common, standard value 10uf caps and don't look back. A lot of 8uf caps would measure 10 anyway. No, I don't think it would change things.

The ones on either side of R21 are there for decoupling the preamp stages, increasing 5 to 10 won't hurt anything. Then note the two 5s in the power supply are paralleled into 10uf anyway, so replace them with one 10uf cap and you haven't changed anything anyway. All those 5ufs appear to be in one can cap.

[MikeH]

Ann Arbor? OK, I will type slower then...

8uf???? That is an ancient non-standard value. Probably pay "vintage" prices for them. Caps in that era had tolerances of like -20/+80%. Seriously. Just use common, standard value 10uf caps and don't look back. A lot of 8uf caps would measure 10 anyway. No, I don't think it would change things.

The ones on either side of R21 are there for decoupling the preamp stages, increasing 5 to 10 won't hurt anything. Then note the two 5s in the power supply are paralleled into 10uf anyway, so replace them with one 10uf cap and you haven't changed anything anyway. All those 5ufs appear to be in one can cap.

Bustin my chops, eh? I actually work in Lansing so, you can't hold it against me that much. In fact, I don't know if you know Steve O from the repair shop at Elderly, but this is his amp that I'm tweaking for him.

Anyhoo- yeah the caps are in cans, but I was just going to disconnect the cans and fly wire the new caps inside the chassis, as there is plenty of room, and caps that low in value don't take up much space.

Ok- thanks for all the help, everyone. First things first, I'm going to do the recap, then play around with raising the plate voltage in the preamp, and seeing which is more suitable, pentode vs triode, as that is easy to flip back and forth for AB comparisons. If that doesn't get it there I'll look at lifting the tone control for something more guitar amp suitable.

Lowell mentioned lifting the C18/R19 bit on the OT secondary - what purpose does that serve in this circuit?
I generally don't like to mess around with things in the power section if I don't know what they do, as that is where all of the expensive parts usually congregate.

[Enzo]

C18 is probably more for stability that anything. It is just a little RC, so disabling it certainly wouldn't hurt anything, expensive or not. At the absolute worst, it might allow the amp to oscillate. if that happens, you turn it off and put the cap back. it might have some small tonal contribution, so you find out if you like it or not.

[Pdavis68]

You might try jumpering a smaller resistor across R5 (say 220K) and seeing if that improves things a bit. The 6SJ7 is set up almost exactly like a champ (5C1) except for the plate voltage, which the champ has at about 130V. But otherwise it's pretty close (for Champ: screen at 21V, 5M grid leak). You might also put a pot on the NFB so you can limit it.

[lowell]

The feedback db down frequency for C18/R19 is the following, taking 6V6 plate resistance and capacitance into account.

6v6
Ra=10k
Ca=8.5pf

1/(2pie*R*C*)
1/(6.28*14700*.01uf)
= 1.2k

Must be linearizing above that frequency point right? Not really sure what to make of that as everything above that is linearized 100% so I'd think lifting it would have a very audible effect. If there's squeal then C18 can be adjusted to only affect those higher frequencies. .....unless i missed a zero in my calculations.

[sgelectric]

Correct me if I'm mistaken but wouldn't simply raising the screen voltage in relation to the plate voltage result in more gain which is part of what the OP is looking for?

SG

[sgelectric]

Correct me if I'm mistaken but wouldn't simply raising the screen voltage in relation to the plate voltage result in more gain which is part of what the OP is looking for?

SG

[MikeH]

Finally got around to doing the recap, and it definitely helped out a lot. Getting a little breakup now, plate voltage is closer to normal on v1 now (it was at like 20v before). Only thing is now on the brightest tone setting I'm getting a high pitched squeal. It's the rotary setting connected to c13 and r28. Could just be a tube issue and now that I recapped it it has enough gain to get microphonic and squealy. Any other thoughts on what it could be?

[trobbins]

I'd suggest temporarily disconnecting the feedback from speaker to tone circuit, and C17, and C18/R19, and checking on the basic performance of the amp with guitar. That config will give the most gain, and hence indicate if you can overdrive the output sufficiently for starters.

C18/R19 is likely needed because the main feedback path is being used.

[teemuk]

There's only a -single- gain stage (and even that for "mic" channel only) in this thing before the signal gets significantly padded down by volume control after which it's already feeding the power amp stage(s).

The unfortunate fact is that you are not going to get much "preamp" -based distortion/gain from this kind of circuit with typical guitar input signal levels. If you pump up the headroom of the 1st stage by increasing plate voltage and lower the gain by using triode mode you get even -less-.

My advise would be to either rebuild this thing competely to "guitar amp specs" or use outboard effects for distortion. One gain stage isn't going to cut it. You probably won't even nearly overdrive it and the volume control heavily attenuating the signal after that gain stage also ensures you get very little ovedrive in the preceding stages.

[olddawg]

Wouldn't cost much to populate a turret board and would save you a lot of time and headache. If you have to rewire the tube sockets, replace them with new ceramics. They are $1 each or so.

[lowell]

I'm a fan of the jfet input for said instance. Just a few components. Kevin O'Connor has a circuit in his first Ultimate Tone book that's easily incorporated into a tube amp.

[Pdavis68]

You might consider pulling the 6SJ7, putting in an octal to noval plate and changing the preamp to a 6U8 (or 7199 or something of that nature). Then you'd get a pentode preamp as well as a triode second stage. This is a variation of the AX84 blues preamp. Just do the first two stages. Since you don't have the lossy tone stack, you probably wouldn't need that third stage.