Ideas? aka JudyBox power amplifier fun

[pbrommer]

Hey everyone-
I don't post a lot, but when I do, it's to this board.

Anyways, I took advantage of the cheap Judybox amps from a while back, and surprisingly, mine was in 'okay' condition (same crappy soldering, wire all over, etc.). It did work out of the box and sounded nice, but I thought it was a bit bright in one of the channels. Long story short, I pulled all of the board components and rebuilt/rewired everything (so stock layout/values, just rewired and cleaned up) and found a missed connection. So, all is well.

But, I can't leave it well enough alone: I have to tinker. What I'd like to do is make it so it can be 6L6/KT66/EL34 drop in compatible (has 6l6GC now) and maybe move it to Class A (or AB, or whatever).. So, here's what I have voltage-wise for now:

B+1: 346 (feeds OT B+ and plates)
B+2: 329 (feeds screens -- pin 4)
B+3: 241 (feeds preamp and PI)

Currently, it is cathode bias (which I'd like to leave) with 250 ohm resistors on each tube. That makes the bias a bit hot according to Weber's bias calculator (about 80%). Yep, I need to change the cathode resistors. What I'm thinking is put a 250 ohm resistor attached to the cathode then a 500 ohm rheostat attached to the 250 ohm resistor going to ground. Adjustable cathode bias for each tube. Feasible? Or am I just crazy? Oh yeah, OT can be 8k or 4k, depending on secondary (16/8/4 for 8k and 8/4/2 for 4k)

Is there anything interesting that could be done? I like the sound, I just want to open it for some flexibility and have fun and learn about the changes. Shoot me some ideas. I can draw up a sketch of the schematic, get other voltages (what they are now), or get pictures. Throw me some ideas, help me have some fun and make this amp really interesting. Thanks in advance.

Patrick

PS: I probably won't make clips because (a) I don't have mics and (b) I am a terrible guitar player.

[xtian]

Let me point you away from the power section. You're not going to make much difference in the amp's sound and performance there, especially when there are so many, much more dramatic things you can to with the preamp.

Likely areas for investigation are the NFB circuit, tone stack values (or tone stack bypass), cathode bypass switch, Rp/Rk values, etc.

[M Fowler]

rjguitars had an amp where he did what your talking about ?200R bias resistor plus rheostat to bias the power amp tubes.

Maybe rj will see this and reply.

[matt h]

(deleted)

[pbrommer]

Fun ideas guys. I like where this is going. The preamps aren't too bad -- more tweed so I hear (there's a clip on the internet: https://www.youtube.com/watch?v=vIEIyrsl5Fs). Not bad, and I'd like to keep it (unless I get bored). The two preamps don't utilize both sides of the PI: they link together right before the PI input.

Now the ability to switch shared cathodes, that'll be interesting and flexible. I was trying to figure out how to do switching between cathode bias for differing tube types. Thank for the help. I know that there isn't much to do with the power amp and changing the sound, but thanks for all of the ideas/notions for this amp. We'll see how it goes.

I'm thinking I could wedge in a cut control and a PPIMV, maybe.

Thanks for the ideas guys. I'll give it a shot and report back.

Patrick

PS. I need to finish my other judybox 6v6 ax84 october.. hehe. PPIMV, cascade/parallel switch....