Phase Inverter Test

[merlot]

I have a JTM60 that has lost volume and has high distortion. If I scope both plates of the phase inverter at the same time, shouldn't the two sine waves be opposite each other? (Correction--scope was reading at the grid resistors of the power tubes. Same result looking at the output tubes' plates.) Looking at both simultaneously, their waves are identical as far as timing, not inverted. Is this the cause of my loss of output? If so, where do I look to correct this? I've tried different 12ax7 tubes in that socket, replaced the socket recently, and checked all the resistor values (I think) and they seem dead on. Thanks in advance for any help with this. I'd love to get this amp healthy again.
Bob

[merlot]

OT problem instead?

Never mind about the inverter. Answered my own question after a re-check with the scope. However the problem still persists. Every thing up to, and including the power tubes checks out okay. What are the chances this could be the OT? I did all the usual resistance checks--each primary (plate) terminal has around 105 ohms. The secondary has almost no resistance on any impedance tap. With the variac test, I come up with a winding ratio of 10:1 (2vac on the secondary gave 20vac on the primary).

[pdf64]

'What are the chances this could be the OT?'
It's unusual but possible, and would correlate with the symptoms.

'each primary (plate) terminal has around 105 ohms'
What's the resistance p-p? If less than 210ohms there may be a primary short.

Are there 1ohm cathode resistors? You could monitor the Vac on the cathode and compare to the Vac on the plate, at high signal levels.

[J M Fahey]

Hi Merlot.
Since you have a scope, load appropriately the secondary, hook your scope there, expecting to read around 60Vpp and inject signal into the PI.
Raise the volume from 0 and watch he waveshape:
Is it symmetrical?
At what point does it clip?
Do both halves clip somewhat symmetrically? Or very unsymmetrical?
What power are you getting?
If unsymmetrical: exchanging output tubes changes the symmetry problem?
Do you have a pair og "good" tubes to test there?
What about the screen voltage, both with and without signal?
If you want to see the unmasked truth, lift the NFB, although even "good" amps look ugly, every blemish appears.
Good hunting.
PS: JTM30 and 60 are very good sounding, but their transformers are underrated.