Distortion as note fades out.

[Axtman]

I'm repairing a Fender Bassman Ten amplifier (later model 70 watt model with ultralinear transformer). The amp sounds good on the initial attack of the note but as the note fades out distortion creeps in. It's like the sound of a solid state amp when suddenly turned off while someone is still playing. The distortion is on both channels.

The amp is going to get all the electrolytic capacitors replaced since they are all original. Hopefully that will cure the problem.

BTW, all the tubes test good except though the preamp tubes are a little weak.

[Jazz P Bass]

Speaker check?

[Axtman]

Yes I plugged into another cabinet and got the same results.

[Jon Snell]

Both 6L6s conducting? Try a new pair.

[J M Fahey]

Also play a 400 or 1000Hz tone, amp loaded with speaker, not resistor load, and rise/lower volume while you scope output.

You might find cossove, assymmetry (as Jon suggests) or instability/oscillation.

The scope is your Friend.

[Axtman]

I violated Enzo's Law which states, "Always check the easiest first." The problem turned out to be voice coil rub in one of the speakers. Doh!

[g1]

But you said trying it into another cab gave the same results?

[Axtman]

Well......that's an interesting question. I think because the other cabinet was near the first there was some sympathetic vibrations going on. Plus I did recap the amp so that did help. Oddly enough the amp sounded worse after I recapped it (with, unbeknownst to me, blown speakers.) I only figured something was wrong when I plugged a solid state amp into different cabinets and found the cabinet I had been using had blown speakers. I removed all four speakers and found that two were dead (no reading on ohm meter or 9v battery) and the other two had coil rub.