2 channel champ with BF tonestacks

[H Chandler]

I'm just about ready to put this project together, and I'm having a bit of a dilemma.

I'm building a champ style output circuit and I'm planning on feeding it with a pair of BF bassman normal channel preamp circuits. In this circuit, the tone stack is sandwiched between the 2 triode sections and ends in a 220K mixing resistor. I originally had thought I could just connect the junction of the 220K mixing resistors directly to the coupling cap at the grid of the output tube.

Well, then I got to looking at the NFB circuit. On the champ, the feedback resistor connects to the cathode of the second triode. Where do I connect the NFB if I have two channels?

Or, should I mix the two channels directly off of the volume wipers into a common second gain stage and connect the feedback resistor to its cathode? In which case, what can I do with the extra triode .. a switchable gain stage?

Thanks,

Howard

[MWJB]

"What can I do with the extra triode?" - leave it disconnected, ground off the unused plate, grid & cathode. This will keep your 2 channels in phase if you wanted to bridge them/run them in parallel. Might not be a great deal of gain though (may or may not be an issue?) as you'll lose a little signal back through the unused channel, turning it up to prevent losses may increase background noise level (again, may or may not be an issue) - this is pretty well what Fender did with the tube PA heads?

But what I might try is have your typical BF style preamp channel with T(M?)B controls and a 2nd triode for tone stack recovery. Then just run the 2nd channel as a simple tone & vol channel with no recovery triode. Both channels then feed your last triode (where you have your NFB connected to the cathode) via 270K mixer resistors (and a coupling cap for the TMB channel) and a grid load resistor at V2, pin 7. You'll have 2 different sounding channels (BF & tweed), but you won't be able to run them in tandem as they will be out of phase (well you will, but you'll get a drop in gain), but switching between them with an A/B box will work fine.

Not quite what you asked but might do the trick unless you have a specific reason for wanting 2 identical channels.

[H Chandler]

Thanks for the reply.

I'm not married to identical tone stacks, but this is for an application where both channels will be used at the same time. It's for a unique stringed instrument that has one set of guitar strings and a set of short treble strings, each set with its own pickup and eq requirements.

So, what if I mix before tone stack recovery, a la plan B, recover half the gain with the third triode and follow that with the remaining triode, tying the feedback resistor to it's cathode. Probably have to fiddle around with different stage gain and feedback resistor values. But the channels will be approximately phased, not taking into account filter phase response of each channel.

Makes sense?

[The Radium King]

is your pa cathode biased? check at ax84.com, as they are doing stuff like bringing the nfb to the cathode of the output tube. this way you avoid the additional preamp tube. if you do go with the additional gain stage (the more gain the better ...) you could use the remaining triode to build a fold-back reverb; tap a signal off your ot secondary into a reverb tank, take the reverb tank output through a gain stage and insert it into a split grid to ground resistor on your last preamp gain stage (you can put a pot here to control reverb mix). check tut1 for more info. you could also build a simple vibrato circuit; check out the princeton 6g2 schematic at www.schematicheaven.com.

[MWJB]

Chandler, yes that should work (mix V1a & b before V2a, then additional stage with V2b w/NFB).

[Bruce / Mission Amps]

Originally Posted by H Chandler

I'm just about ready to put this project together, and I'm having a bit of a dilemma.

Well, then I got to looking at the NFB circuit. On the champ, the feedback resistor connects to the cathode of the second triode. Where do I connect the NFB if I have two channels?

Or, should I mix the two channels directly off of the volume wipers into a common second gain stage and connect the feedback resistor to its cathode? In which case, what can I do with the extra triode .. a switchable gain stage?

Thanks,

Howard

Look at the schematic of a Fender tweed Super 5D4.
Note the way the NFB connects to the grid of the actual power tube... now just imagine getting rid of the bottom power tube in the push pull amp and making it a single ended amp.

[bsmco]

I am trying to learn as much as I can about amp building before I start my first project. I would love to see the schmatic of this amp as I would like to add a 2nd channel to a vjr.

thanks