Mesa Triple Rec Mains Current at Idle

[tdlunsfo]

Are you using the correct 8 Ohm output?

I think so. Outputs are labelled 16,8,8,4,4. I am using one of the 8 ohm outputs.

[tdlunsfo]

If your signal generator is able to provide a few volts, feed your signal to the PI input before the coupling cap (make sure to measure 0VDC there). Turn all MVs fully up and repeat output power measurement.

I ran this test and got 30Vrms at clipping. Bottom half is clipping before the top but not by much. This is with 1.54Vrms audio generator voltage.

[Helmholtz]

See if the PI can supply around 106Vpp or 38Vrms clean sine to the power tube grids on both sides.

Also make sure to measure amp output voltage at the output jack terminals (not at the load resistor).

[tdlunsfo]

It can't get there. 30Vrms is best it can do clean and yes measurements are at the output jack. The bias in this amp is really cold. Some of these tubes are only at 30% dissipation..

[Helmholtz]

It can't get there. 30Vrms is best it can do clean and yes measurements are at the output jack.

To achieve full output the power tube grid signals must be able to drive the grids to 0V.
This means a peak-to-peak grid signal of twice the grid bias.

What are your no-signal PI DC voltages (supply, plates, cathodes)?

[tdlunsfo]

V5 DC Voltages:
1. 285
2. 33.5
3. 51.8
6. 286
7. 31.9
8. 51.8
Heaters - 6.4Vac

Swapping the PI for a new 12AX7 gives me about 31Vrms output. After some research, the triple rec has a EL34/6L6GC bias switch and they are biased really cold for 6L6's and warm for EL34's. I'm starting to think that this is how this amp is made and the customer is hearing a slightly worn PI tube. I unfortunately don't have six EL34's lying around to test the theory.

[Helmholtz]

You forgot to post the PI supply voltage. Or even better post the voltage drop across the PI plate resistors.

I think the 6L6s are at least partly worn and certainly not matched.
You could try to add a say 180k resistor between bias supply and ground for a little warmer bias.
But I think the amp needs fresh and matched power tubes.

[tdlunsfo]

82k drop = 143V
91k drop = 145v

[Helmholtz]

PI looks fine. I guess it could deliver more than clean 30Vrms at the plates with power tubes pulled.