LTP PI - Is this normal?

[MWJB]

Oops! Pardon me, just noticed that you mix the channels before the reverb and both channels are actually in phase. Apologies.

[Daedalus]

Your 2 channels are out of phase with each other, running them in parallel will degrade the sound.

Put 100mV 1KHz (confirmed with a meter) on one input at a time (tone controls fully clockwise), scope the output at the dummy load, what kind of W do you get with a clean sinewave?

I have the "equal weighting" mod in there; the two channels are connected together at the plates of v1b and v2b and both go through the reverb return stage.

The input for the measurements I have been doing is a clean 60mV sinewave at various frequencies (I think it was at 500Hz for the traces I posted here). My pre-amps seem to "dirty" the signal up pretty early.

Thanks

[MWJB]

"The input for the measurements I have been doing is a clean 60mV sinewave at various frequencies (I think it was at 500Hz for the traces I posted here). My pre-amps seem to "dirty" the signal up pretty early." - What value cathode resistors do you have at V1a & V2a at the moment (560ohms/triode is very low, but I see 1.5K in red?) and what plate voltage?

A typical Deluxe amp may have 220vdc+ on the input tube plates, even with 1.5K per triode your unlikely to get more than 190vdc? As your amp has lower 6V6 plate voltages than a BF/SF Fender (so they will distort earlier) you may want to experiment with reducing the value of the 10K preamp dropping resistor in your power supply...if you want to clean up the front end of the amp?

[Daedalus]

In the AB763 at Deluxe style voltages the 12AT7 PI tube will dissipate 1.5 to 2mA per triode, yours will be a little less as you have an unusually low B+.

Actually my voltages are right on with this model:
http://www.fenderholic.com/schem/del...b763_schem.gif
Thanks

[MWJB]

"Actually my voltages are right on with this model:
http://www.fenderholic.com/schem/del...b763_schem.gif Thanks"

Fair enough, it's just the schem you posted listed the 6V6s as having 333v on the plates.

[Daedalus]

"Actually my voltages are right on with this model:
http://www.fenderholic.com/schem/del...b763_schem.gif Thanks"

Fair enough, it's just the schem you posted listed the 6V6s as having 333v on the plates.

That's correct and that's what I measure there even though my power supply voltages are the same as on the Fender. Something to do with the cathode biasing perhaps?

[MWJB]

No, it's because you have a lower voltage PT. Converting a BF deluxe to cathode bias won't knock 100v off the plate (to ground voltage). As your power tubes are running much less voltage than AB763 they will distort earlier...hence the suggestion to clean up the preamp by raising preamp plate voltages to compensate.

Personally. I would be looking for a little more current at the power tube plates using a 200-250ohm cathode resistor.

It's worth noting that real BF Deluxes (as with all 50's & 60's Fenders) run voltages a little higher than those on the schematic.

[Daedalus]

Thanks to everyone who contributed to this thread. I did learn some things even though I still don't understand if the early clipping on the cathode waveform is normal or indicating a problem. I need to open up another LTP-PI amp and do some comparison probing. (Lack of workbench space!)
I'm still pretty confused with the very high measured ac and dc levels at pin 2, yet I don't see any clipping on the grid signal (no blocking distortion).
Well, this is only a hobby, I'll get it straightened out eventually; if not, I'll just close it up and play it!
Thanks again and best regards...