Scratch Build Reverb and PI Questions

[womantone]

After building a 5e3, 5f2 and rebuilding aa764, ab763 I have decided to move on to trying to build my own. I know the basic concepts but I just wanted to verify a few things before going forward. My concept is to use a fender deluxe chassis as a launching point for a two channel amp. First channel will be based off the classic ab763. Second channel based on the jtm45. Channels switchable using A/B box. And I want to have global reverb on the amp. For global reverb would I just route both coupling caps together? I am using Fender's tube reverb off the ab763 so I refer to the schematic as a reference. So would I just route both coupling caps to the 1m to ground as well as pin 7 on the driver tube (v3)? And do I have to use both triodes of the driver tube (v3) for the reverb drive?

I am also trying to understand the phase inverter. I realize it splits and flips the signal to feed each individual power tube. But does it use both triodes in the process? I am basically just trying to maximize use of every triode and leave no triode unturned. And does it matter if I use a 12ax7 or 12at7? Obviously the circuit is designed for use with a 12at7. But I am asking this more from a sound perspective. Or does this make little to no difference?

I will let you guys know when I have the schematic and layout complete. I am still tinkering with some stuff to find the optimal tone stack and bias combination for each channel.

[MWJB]

In short, it's not going to be easy to accomodate in a DR chassis, you really need 3x 12AX7 before the reverb circuit (JTM has a cathode follower tone stack). You will really need another gain stage for the JTM circuit, after the tone stack, then mix the 2 channels after the coupling caps, tap the reverb signal, then reverb recovery/mix stage, then PI. Where will you put the reverb transformer & cables? Your preamp tubes are all going to be bumped up one socket to the left?

You will most likely lose the tremolo.

If you went with a scratch chassis you could do a better job of incorporating both designs...others have already done it but typically leave off reverb on the JTM channel.

If you want to use the DR chassis & just want to give the Fender normal channel a bit more of a JTM/tweed amp type vibe (25K middle, 33K treble slope & JTM tone stack values, perhaps "texas tea" control) that will be easier (leaving out the cathode follower), then you can mix channels after the 2nd stage coupling caps with 220K resistors and tap the reverb signal off there.

This route would also keep the 2 channels in phase, allowing you to select one or the other, or both.

The AB763 PI circuit is equally at home with 12AX7 or 12AT7, for the JTM channel you really want to keep a 12ax7 in the PI. The difference between the 2 tubes makes a significant difference to the tone. To "correct" the tone on the AB763 channel replace the 1.5K/25uf cathode assembly at V2, pin 3 with 2.7K/15uf, or else it may be too stodgy.