Kind Of a Champ Project

[zendragon63]

Oh happy day--the home computer has finally been rehabilitated and I am able to lurk at will. I am kindly asking for some critique on a Kind-O-A-Champ project that I have been messing with on and off for a few months.

The preamp is a 12AX7 feeds a Bax TS and then split into a 12AX7/5879 paralleled (kinda along the lines of the fatness control geezer put into the Super Tweed Overdrive) and while it is very tonally tasty to use one or the other or any amount of each, I am suspection a ridiculous output impedance imbalance between the triode and pentode into the power tube. Am I overlooking or avoiding using mixing resistors?

Also I would take any and all suggestions on the spartan circuit; every day I find out how little I know and how much there is to learn. Attached schem and voltages as currently built. Thanks in advance guys. Regards

dennis

[tubenit]

Hey that is a cool looking amp!  I like that idea.

Perhaps the only thing I would consider differently is a .01 going into the James tone stack & on the second 12AX7 triode cathode maybe a 2.2uf instead of a 10uf with a coupling cap off that V2-b plate being .01 instead of .022.

I've used the James tone stack quite a bit on my builds and have often changed the .02 going into it to the lesser value of .01, but I also use a James tone stack that has more mids emphasized than you're doing with yours. IF you like strong mids, try plugging in the values I have listed into the Duncan Tone Stack Calculator and take a look at it.

My thinking for that is approaching it that way might give you even more contrast between the pseudo channels where the 12AX7 second triode will sound significantly cleaner and the 5879 will give a much fatter tone.  Maybe use the .001 cap across the 5879 plate IF you need the tone smoothed?

However, the way you have it already, I think would sound absolutely great.

With respect, Tubenit

[stingray_65]

Now that is a new take on a Champ!

I can't wait  to here a sound clip or a description of how it sounds.

This could be a side project for me while I build the TOS.

Ray

[zendragon63]

Thanks guys. I really do appreciate the input. I am trying to be minimalist on parts so hence the impedance question. I will admit I am an impedance moron.

And my apology carelessly interchanging the Bax and James terminology.

It actually sounds pretty good right now into a single Celestion Blue; maybe a little woofy with the Les Paul. The coupling cap changes may just take care of that. Has a good, natural sustain to it and hardly needs a VVR but balancing it against preamp and MV gives a nice grind and will make my bride a little happier at 10:30 PM. It just needs a cabinet built for it this weekend.

Now that the computer is working again, I'll see if I can get the sound recorder software to work and get a clip. Thanks again. Regards

dennis

[stingray_65]

maybe a little woofy with the Les Paul. The coupling cap changes may just take care of that.

That should work quite well.

If I may direct you to one of Tubenits schematics, you'll see a full/bright/fat switch on the cathode bypass cap.

http://www.el34world.com/Forum/index.php?topic=5621.0

I incorporate it into just about every amp I build, it's very usefull when you have many different guitars to play.

Humbuckers really get boomy on preamps like yours (IMHO) and that bright setting works perfect (.68uf like a Marshall) and the fat setting is very single coil friendly like a Fender.

Ray

[Fresh_Start]

Just curious - is the 240K grid return resistor between the master volume and the power tube grid stopper redundant?  IOW can't the master volume serve as as the grid return anyway?

However, if it works fine right now, a 250K master volume pot would be electronically equivalent to what's in the amp (I think).

Cheers,

Chip

[zendragon63]

The 240K grid leak resistor in and of itself with the MV does seem redundant except I understand that if the pot were to open up for any reason, it would potentially present infinite grid resistance to ground and allow the current to creep up and damage the power tube and the OT. Haven't had one go south yet but it would do more damage there than losing a preamp pot. Cheap insurance or maybe just a personal quirk--though I could be all wet. Regards

dennis

[Fresh_Start]

Dennis - that makes plenty of sense from a safety net perspective.  Hadn't thought of that angle. 

I was just thinking about the 240K grid return resistor in parallel with the 1 meg pot and how that might affect the pot's taper and effective value.  It's not worth the effort since you're happy with your amp, but I'd try a 250K audio taper pot and a 1 meg resistor just for fun if I were building the amp.

Chip