Cascading gain stages on AB763. Any issues with that?

[ad30]

Can I take the normal channels output to the PI (where the Dan Torres mod starts) and send it to the input of the bright channel (pin 2, V2) for extra gain? So plugged into the normal channel, I'll have twice the gain (plus reverb, bright cap bypassed) and plugged into the bright channel, there will be no change.

Does that sound right? Are there any issues with this I've not spotted? Has anyone tried it?

Thanks.

[NorCalTuna]

Yah, it's fun to try... But it but it sounds like buttock- wayyyyyy too much bass. Here's what I did on my Deluxe Reverb- I used the second normal input jack as a switching output, pulled (iirc) from that spot, after the blocking cap. When nothings there the amp is stock, when it's got a plug in it mutes. Then, I have a cable and a box thats got a rotary switch that can put anything from .01 (stock mb) to .047 inline to (clutch part) the LOW GAIN input on the vib channel. I suppose I could do something trickier but that works fine. Technically. It'd be better if it only used the first triode- two fender eqs makes a lotta scoop. V1a with eq bypass switch to .01 out would be cool. The same switch could change that stage's cathode cap. Why haven't I done this?

Honestly? cascaded fenders (mark 1 mesa, groove tubes stpg-1, etc) need to be tailored to be what they are. Just hookin' one up doesn't really work so great. Too much blocking cap, cathode bypass cap, too much driver cap, low gain driver, feedback ciruit.... you end up compromised. Either you end up with a wimpy fender (love that mesa clean!?!) or a flatulent, portly high gainer- that's unstable if you're unlucky. You could use a bunch of switches to lighten/fatten up the cathode bypass (the resistor values are close enough iirc) but that still leaves the first stage blocking cap- which, depending on the AB763 (oh yah ps? thats just the date it was designed- jul, 1963. The AB763 Bandmaster I have sucked. It's now a "mesa". The aforementioned AB763 deluxe reverb reissue I rebuilt os style is remarkable and nearly stock. So, specifying the model is important- tube count, transformers, details, etc.) could be anywhere from 10x the size on down, and, like salt, you can't really cut the bass back out later. While I'm there, You're not going to have twice the gain. The gain of the eq recovery stage, itself, has a gain of 59; luckily you're going to throw a lot of this away. Check this out:

Tube Circuit Gain Calculator

As for has anyone tried it? Around the same time we were making moon landings, I can think of at least one guy here in NorCal wiring up the channels of a fender, not that I "was there" or anything. I'm sure theres another thread about this?

[Chuck H]

+++
Nor Cal has it absolutely right. Simply cascading the channels fails to compensate the EQ of the circuit for the requirements of high gain. The Torres mod is supposed to be pretty good. I haven't tried it. But why mess with what's known to work? Dan's other OD circuits sound pretty good IMHE. He's been at it a long, long time. You can always tweak coupling and bypass cap values to your tastes after it's in and working.

[ad30]

I appreciate the input here. It's clearly not as simple as diverting a wire or two.

It'll take me some time to look into the above (I'm just starting out). It's given me a lot to think about

Thanks.

[Gtr_tech]

If it were me I'd build an outboard preamp and leave the amp itself as is. Small plate/filament pwr xfmrs are cheap and the parts needed (less enclosure) won't add up to much. I've done this for customers and myself and they're handy to have around for those vintage amps you don't wanna mess with. You can even recycle enclosures if you're crafty. I have one I built inside an old C band sattelite reciever box. An electronics parts dealer I frequent recently gave me a bunch of (apparently outdated) commercial video stuff thats all in rack enclosures, so I may do another one sometime and add an active tone stack this time.

[ad30]

Gtr_tech, have you got any schematics. That sounds really interesting.