6au6 triode connection preamp

[apehead]

I pulled an AO-29 amp from a Hammond M3 that I wanted to convert to a guitar amp (schematic). There is a "phono" RCA input that can be used for a guitar, but it is only a single 12AX7A gain stage (V4B) before the 12AU7 cathodyne phase splitter (V3B). No tone controls, and the input is downstream from the volume pedal pot, so no applicable volume control. Quiet and no potential for overdrive. I've been mulling over some modification options, and I wanted to get some opinions on the forum.

Tubes/sockets V3, V4, V5, V6, and V11 can be used to get a circuit similar to the layout of the "normal" channel of a Fender AB763 Deluxe (schematic). Power transformers for both amps are close in output voltages (Hammond 350V, Fender 325V), yet somehow the Fender is sending 420VDC to the OT, whereas the Hammond only sends 320VDC to the OT. I don't see any voltage doublers in the Fender power supply or anything like that, what would account for the higher output voltage of the Fender power supply?

There are also many now unused tube sockets that I'd like to make useful. V1, V2, V7, V8, V10 are 7 pin sockets, and V9 is a 9 pin socket. For the sake of cost, I wanted to use the original tubes for the 7 pin sockets. V8 and V9 will likely be a reverb circuit. How can I determine if T4, T5, or T6 would be suitable for the reverb transformer?

That leaves four pentodes, maybe to be used as onboard gain channels. Should I strap them for triode operation, or keep them pentode? The odd order harmonics of pentode operation would better simulate transistor distortion. Should I put pots in between the gain stages for wave shaping and feedback prevention? Thoughts?

Thanks!

[Helmholtz]

Power transformers for both amps are close in output voltages (Hammond 350V, Fender 325V), yet somehow the Fender is sending 420VDC to the OT, whereas the Hammond only sends 320VDC to the OT.

Hammond schematic says 2x320VAC. A 5U4 drops around 40VDC more than a GZ34. Add another 4VDC across the 30R resistor.
Generally DCV depends on total current load at idle. The 140mA of the Hammond is more than twice the current of a Deluxe.
Expect B+ to rise if you use fewer tubes.

[apehead]

Hammond schematic says 2x320VAC. A 5U4 drops around 40VDC more than a GZ34. Add another 4VDC across the 30R resistor.
Generally DCV depends on total current load at idle. The 140mA of the Hammond is about 3 times the current of a Deluxe.
Expect B+ to rise if you use fewer tubes.

Thanks for the correction. The voltage drop would probably explain why the power tubes started cherry plating immediately when I tried using a plug in solid state rectifier instead of the rectifier tube! I think I still have it around somewhere. I also have a bunch of tubes the previous owner of my house left behind, are voltage drops typically listed on rectifier tube datasheets? Almost all the amps I have are solid state power supplies. I still don't understand how a B+ voltage would exceed the peak AC voltage of the power transformer, though. Shouldn't the maximum possible rectified voltage of the Fender be lower than 325V? How did you determine the current of the amp circuit? I don't see it on the schematic.

[g1]

The 325V is not peak, it is RMS. So around 460V peak.
See attached for tube rectifier voltage drops.

[Helmholtz]

I still don't understand how a B+ voltage would exceed the peak AC voltage of the power transformer, though. Shouldn't the maximum possible rectified voltage of the Fender be lower than 325V? How did you determine the current of the amp circuit? I don't see it on the schematic.

B+ can't exceed the peak ACV. The peak value of 320Vrms is 453Vp. Loaded DCV will always be lower than that.

Regarding current: The Hammond schematic states 140mA total. Idle current of a 6V6 should be around 25mA and for preamp/PI tubes I assume an average value of 1mA per section/triode.

Regarding the reverb transformer you will want a small power (as opposed to signal) transformer, like a 1W or 2W OT. From looking at the schematic it seems that only T6 could transfer a little power.
But as it's a PP transformer you would need to use the PP driver and phase inverting interstage transformer as well.

[apehead]

Thank you all for the clarification. I didn't even think of RMS! If I were to use the plug in ss bridge I have, the only drop would be the 2.4v from the four diodes? If I added a bias adjustment for the 6V6 power section, would it be possible to run them cold to avoid excess plate wattage? I could spring for a pair of the higher plate wattage JJ 6V6GT tubes, but I have many old 6V6 tubes laying around and would like to put them to use. I also have some Chinese 5881 tubes, would it be safe to substitute those (given an appropriate circuit design) without blowing the OT?

There were also a whole pile of transformers in the house, many look like PTs and OTs for old television sets radios, etc. I have a variac so I can use to determine their step-up, but no hipot. Is there a way to test them?

[Helmholtz]

If I were to use the plug in ss bridge I have, the only drop would be the 2.4v from the four diodes?

No, the SS rectifier will drop around 1.4V (only 2 diodes conducting at a time).
But there's more to it.
DCV also drops by PT sag (with the lower current of a Deluxe the PT secondary voltage will increase) and increased filter cap discharging at higher currents.

My advice: Use a GZ34 if you want the sound of a Deluxe. I'm pretty sure your B+ will be around 400V once you built the Deluxe circuit and removed unused tubes.

BTW, the tricky bias supply in the Hammond also steals 22V from the B+. As there's no bias tap on the HV winding, you will need to use a Marshall type bias supply.