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

[MikeH]

A friend of mine came into a bogen HE10 PA head. It has a push-pull 6V6 output stage which would be nice for guitar; I'm going to clean it up and recap it for him as he wants to play guitar through it. With a guitar plugged in to the mic channel it's 100% clean all of the way up the dial, which makes sense based on its original intended use. I want to tweak this thing to get some grind out of it. The input stage is a 6SJ7, which I have no experience with.

The first thing I noticed is that the plate voltage is super low (37V), and I figured raising that would be a good place to start. I looked up the datasheet for a 6SJ7 and it said these things can handle up to 200V. So I was thinking I could accomplish this in a few ways; I could lower the value of R5 or lower the value of R21, or perhaps both.

Can anyone tell me if I'm on the right track? Any other ideas how to coax a lot more gain out of this thing?

Thanks in advance, here's the schem:

[olddawg]

You could make an Ampeg Jet, which uses that tube complement: Image from http://toneclassics.com/Ampeg%201959...0schematic.JPG.
Or a number of other small amps of that vintage.

[lowell]

Could have some fun with that pentode. Raise plate voltage, rewire to triode mode with a largish Rscreen for some creamy squish!

If it were me I'd start with removing the feedback loop in the PA. That'll open'er up. Then toy with what I mentioned above with the 6v6s. Best to have the PA distort first b4 the preamp. The magic happens in the Pentodes.

[Leo_Gnardo]

That low plate voltage on the pre tube, with grounded cathode, gets you some serious compression. Before you change it to "better" try as is, might like it a lot. Bound to be different that all the standard pre's we're used to.

IIRC Terry Kath (Chicago) used either a Bogen or Knight PA, got some ferocious sounds with that. Listen to him melt it down, Feedback on the first album. Plus a string of hit songs. Let's put it this way, Hendrix was impressed. 'Nuff said.

[MikeH]

Could have some fun with that pentode. Raise plate voltage, rewire to triode mode with a largish Rscreen for some creamy squish!

If it were me I'd start with removing the feedback loop in the PA. That'll open'er up. Then toy with what I mentioned above with the 6v6s. Best to have the PA distort first b4 the preamp. The magic happens in the Pentodes.

Thanks for the replies everyone.

The negative feedback is going the PI via R22 right? The rotary switch for the EQ seems to be tangled up in there as well, so it's not entirely clear to me.

Also could you point me to where I could learn how to rewire the pre to triode mode? What you're describing sounds like what he wants it to do.

Thanks!

[lowell]

Yes the rotary would be disabled. It'd be easy nuff to replace that rotary with a 1meg pot. Then use that as a 5E3 Tweed Deluxe style tone control between the pentode and 2nd gain stage. Look up a schematic for it. Not sure how well that pentode will drive the tone control, but it's not the hungriest of tone stacks either.

Might wanna leave that pentode as is for now per Leo's suggestion. Could be "different" and unique.

If you wanna go for the triode mode though, you'll connect the plate to the screen grid with a resistor. Which means, move that 2.2meg so it's between pins 8 and 6.

[MikeH]

I wouldn't be opposed to keeping anything about this amp stock, but right now it's just too clean and not all that loud. Which is to say it sounds nothing like a guitar amp, lol.

So I can't eliminate the NFB without disconnecting the tone control (which is actually kind a cool one), is that true?

[Enzo]

The NFB IS the tone control here. You have a switch marked tone, but it is nothing more than a presence and resonance control in switch form. Before getting rid of it, how well does the stock circuit act as a tone control? regardless of the gain structure.

A common tone stack is subtractive, meaning adding one to the circuit will result in substantial signal level loss, and if you already consider it weak, well...

You could probably stand to raise the B+ in the preamp stage some.

[MikeH]

The NFB IS the tone control here. You have a switch marked tone, but it is nothing more than a presence and resonance control in switch form. Before getting rid of it, how well does the stock circuit act as a tone control? regardless of the gain structure.

A common tone stack is subtractive, meaning adding one to the circuit will result in substantial signal level loss, and if you already consider it weak, well...

You could probably stand to raise the B+ in the preamp stage some.

Ah... Looking at the schematic that makes way more sense now.

The tone control works pretty well now, some of the settings are a little bit too dark, I was thinking about playing with cap values on some of them to make them more useful. But my friend really digs the rotary switch tone control so I'd really like to keep it if possible

[olddawg]

You could still use that rotary switch as standard tone control circuit pot. Just use a handful of resistors to match the pot's resistance at various position of rotation.

[lowell]

Keeping that tone control = keeping linearity/clean sound. Wonder if it'd work in a local feedback config. Hmmm...? Be a fun experiement as I personally have never seen a guitar amp with a local feedback tone circuit. If it worked you'd then still be able to have the PA open loop!

[lowell]

C18/R19 on OT secondary seem like a negative feedback source too. Try lifting that string.

[madkatb]

I see lots of suggestions to rewire the pentode as a triode. It's unlikely that you will use the second input so you could always use R2 to make a variable pentode/triode control. Here's Merlin's circuit:
Triode/Pentode Photo by merlinblencowe | Photobucket

[Enzo]

If his pentode leaves him with not enough gain, then a triode will provide even less, wouldn't it?

[olddawg]

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