Is there an issue running long OT wires?

[glebert]

Could it be that the output was always unstable, but sometimes it wasn't so bad that some signal could get through (with thin sound), and at other times was just full on oscillation? I have an amp that had an oscillation problem in the reverb circuit but some signal would still get through.

[nickb]

Hmmm... it might show up as a higher than usual leakage inductance as a consequence of a poorly assembled core. you could measure the inductance of the primary with the secondary open and then shorted. I'd expect the ratio to be well over 100 but you'd really need to measure a good transformer to make a meaningful comparison and that ship has sailed.

[Randall]

Something else to consider here is as I mentioned, the transformer leads were left untrimmed and coiled up. No apparent oscillation. I plan to have him bring it back in after a spell so I can trim them back.

[Helmholtz]

Hmmm... it might show up as a higher than usual leakage inductance as a consequence of a poorly assembled core. you could measure the inductance of the primary with the secondary open and then shorted. I'd expect the ratio to be well over 100 but you'd really need to measure a good transformer to make a meaningful comparison and that ship has sailed.

Yeah, that's what I thought, too.
The increased airgap would cause a drop in primary inductance, so poor bass response. And the reduced coupling is likely to increase leakage inductance.

For reference: My Fender Princeton Rev. OT( 022913) has a (serial) leakage inductance of 9mH between 1kHz and 10kHz. Actually a very good value.
Total primary inductance measured with my LCR meter is 17H (@ 1kHz), but that value isn't very reliable/comparable because (other than with leakage inductance) it strongly depends on measuring current. It actually increases to around 100H at higher current (more exactly voltage-time area).

[Chuck H]

Randall notes in post #1 that this is a long standing, repeat return issue, so perhaps this is one of those Friday afternoon amps that never worked quite right
Maybe it happened to have been fitted with tubes that had a lowish gm, so there wasn’t quite enough gain to oscillate?

THIS^^^^^^^^^^^^^^^^

The user also noted that the amp always sounded a little thin too. Maybe the amp was fine (ish) when it left the factory because of the particular tubes and the particular resistor values, but value drift, tube changes any maybe even some board conductivity, etc. had tipped the scale. If the new OT exhibits a difference in phase at frequency of a few degrees that may be all it took to correct the problem.