Super Champ XD Donor

[JustMike]

 I got one of these that had the bad DSP board. I bypassed the DSP board and made it work, but it had other issues. After it warmed up the level dropped and it got distorted. So I decided to put another tube in it and make it a full tube amp. I kind of hacked my way through it and left the power stage intact. I installed a socket and I'm using 1/2 of a 12ax7 for the preamp. I followed Rob Rob's Tweedle Dee Mod schematic Bright channel (33k grid stopper vs 68k on the guitar input..does this really matter as far as guitar sound?).
 Obviously, there's no tube rectifier, but I decided to leave this as is. It sounds OK, maybe a little bland and I suspect it could get louder with another preamp stage. I'm not looking to HOT ROD this into a high gainer. I'm going for Fender or maybe "not so TWEEDle-D" sweetness. So. for a single channel amp, what can I do with V1 stage 2?


Thanks,
 

[vampwizzard]

can you piece together an existing schematic for what youve got? Thats easiest to work around. One input, 33K. two inputs, 68K, thats the difference.

[JustMike]

V1 is a 12ax7 I wired just like Tubenit's Tweed OD special. Pin 6 feeds the P.I. in the Super Champ (pin 7 of V1B). I went ahead and used both halves of V1 since the original post. It sounds better and louder. A little farty when I turn it up and there's a ground buzz I need to get rid of. I grounded the cath filters right to the chassis as well as the volume and tone pots.

[vampwizzard]

Sounds like the right thing to do with that second available gain stage. Getting rid of the fartiness can be done via coupling cap values or increasing speaker freq response. Ground wise, Ive tried to follow Merlin's grounding schemes. Ive always kept the preamp and power amp sections separate and grounded to the same point via a dedicated post on the chassis (separate from safety ground from mains).

sounds like a great project. surely youll get the kinks worked out.

[JustMike]

 I've got some issues I can't figure out. It's got a pretty substantial hum with no input and the volume on 0. And when I turn the volume or tone up past 50% it squeals (with the volume up a little). Any ideas before I just gut the PC board and scratch build it? I do have some wires from my new tube socket running over the PC board, but when I chopstick them around, there's no change in the hum.
 One thing to note; I'm tapping my B+ for the new tube right off of the PI at pin 6. No new filter node and I'm getting about 190v at my 100k plate R's.

[PRR]

> I'm tapping my B+ for the new tube right off of the PI at pin 6.

What a strange thing to do. How is that working for you?

[JustMike]

 OK. So I decided to scrap most of the PCB and put in some real sockets and start from scratch. What's left is the on/off switching circuit and the B+ bridge rectifier. I started building on a turret board with a 40u and 2 22u caps. And while I was at it I ended up building Rob Rob's Simplified 5E3 Tweedle Dee Mods amp with a few deviations, namely my filter caps are 40, 22 and 22 rather than his 33,33,33.
 The transformer puts out 307 VAC and after rectification I'm getting 378V B+, node 2 is 353, node 3 is 283.
 The amp works, it's very quiet but it has almost no headroom, meaning no clean sound when the volume is past 1-1/2. Is this just the nature of a low power tweed?

[RadioComm]

My 5e3 has no mods, but starts breaking up around 6+. Real loud at 6+. After that, doesn't get any louder just more compression and saturation. Plenty of headroom before 6.

On another note, I also have a champ XD that I tried to fix, then bypassed the DSP, but then like you, I just degutted it saving some parts. Ended up as Doug's 6V6 Plexi. Here is the link to the post plus some pictures:

https://el34world.com/Forum/index.php?topic=26053.msg283789#msg283789

ttfn

[JustMike]

I've tweaked it some more and I like it. But now I don't know what I have. It's S.S. rectified and cathode biased. Filter caps are 40/22/22/22. My DC voltages are 378, 340 and 285. V2-6 is 246, V2-1 is 201, V1-6 is 204 and V1-1 is 170.
 I split the cathodes, normal is 520/25u and I Marshallized the bright input with a 2.2k/.68. For my coupling cap values, I followed Rob Rob's Tweedle Dee mods. I guess it's somewhere between a Super Champ and a Tweed Deluxe. Oh. I put in RR's NFB switch and while I was at it, I changed the speaker to a Celestion G10-35. It sounds good but it's still too loud for the house.
 The volume controls are very interactive, especially when I use the bright input. I've heard that's normal, but with these mods (Mostly the split cathode I'm thinking) maybe something's not right?

[JustMike]

Is it possible to separate the channels and give each of them their own tone control? I see both channels share the wiper of the tone pot which ties to the input of V2a. I'm guessing not, but would the solution be to just put coupling caps on the outputs of the volume pots?