Blackface Fender Bassman AA864 low, distorted output

[jvm]

I have a 64 Bassman, AA964 circuit. New electrolytics, new power tubes, grounded ac cord. Amp worked beautifully for about a week then output became "garbled" and compressed. I can audio signal trace to 6L6 grids and have a clean signal but have this nasty "garbled" signal at the plates even with the volume pot set very low. Preamp is working fine and I have clean signal up to the output tubes. All voltages are fine. I have checked every connection in the output section and even traced each wire connection. I have subbed new 6L6's with no change in the output issue. 1.5k's and 470 screens are new as well but I checked them anyway. Checked output jack wiring and connections, they are fine. I rarely get stumped by a vintage Fender but I'm baffled by this one. Output transformer? Fender-Bassman-AA864-Schematic.pdf

[The Dude]

Do you have a similar OT laying around you could temporarily sub to see if there is an improvement?

[loudthud]

Did you inadvertently plug into the Extension Speaker output instead of the main Speaker Output ? The Speaker Output has a jack that shorts the output unless that jack is used first to protect the output transformer in case the amp is operated with no speaker connected.

[jvm]

Dude, I am planning to sub another OT to see if that is the issue. Loudthud, I am in the proper output jack.

[jvm]

Problem solved. Blown speaker in my test cabinet. Never assume your test equipment is good! I appreciate the suggestions, guy.

[The Dude]

Ha! Yes, that sux. Just last week I was tracking down a reverb problem. I kept getting intermittent readings on the tank transducers. I chased the problem around for about a half hour, checking cables, tank wires, etc. The wire on one of my meter probes had broken. It was still hanging on, but intermittent. It's definitely a pain when your test equipment fails!

[Chuck H]

I can't count how many times I've scratched the hair right off my head looking for a failure in an amp when what seemed like an important symptom was actually peripheral and part of the testing. Well... I can't count because I didn't keep track. But it's happened to me at least five times. Sometimes with whole days lost to the experience

[pdf64]

Have 1ohm cathode resistors been fitted?
If the signal appearing there is good on the scope, then the valve must be working properly.
Hence a bad signal at the anode (use a 100x probe!) is most probably due to a bad OT.
Alternatively / also, if the OT is tested by feeding in a signal to the secondary, the voltage between CT and each anode should be very close.