Fizz rding on note

[JammyDodger]

Hey All,

I have a '63 Bassman. I have recently noticed that if I strike a note it rings clear, then as the note beings to fade a noticeable fizz appears. It is not a sub-harmonic or Intermod. In fact it seems to be either a fundamental or a higher-order harmonic of the note. It persists even when the original, clear note has almost vanished.

The amp has new Power Supply Caps and Bias Cap. The Cathode cap (25/25) has been replaced as well. I have tapped on the tubes and none of them are microphonic.

Any suggestions as to the cause / fix???

Cheers, Mike

[Firestorm]

Sounds like a minor parasitic oscillation. What model? '63 should still be a 6G6B, yes?

[JammyDodger]

Yes - that would be a 6G6B circuit.

[Structo]

Hmmm, I'm following this thread.

This seems to be what I am experiencing with my amp.

[Merlinb]

Does it sound a bit like rushing sand, blooming as the note decays? That's a common complaint, and is caused by cold solder joints. (Often around the cathode components I find). Try reflowing those joints.

[JammyDodger]

To All,

In another forum someone suggest grid stoppers (1.5k). Tubes are good, no microphonics. I might just try the resistors.

Cheers, Mike

[JammyDodger]

To All,

In another forum someone suggested grid stoppers (1.5k). Tubes are good, no microphonics. I might just try the resistors.

Cheers, Mike

[Firestorm]

Check the rest of the grounds too; a bad ground and a cold solder joint do the same thing and typically the preamp and presence grounds were made to the back of the pots. If there's corrosion between the lock washer and the brass grounding plate, you'll have resistance. Also, go through the amp and see if any of the grid wires can be shortened. Even a little bit helps. If you add grid stopper resistors and they help, you may be able to ditch the 100pF snubber cap between the legs of the PI. Fender stopped using it when they added the 1k5 grid stops.

[JammyDodger]

Hey All,

I am leaning towards trying a 1.5k grid stopper on the 6L6's. Another thought, the amp originally used 6V6's. I have NOS 6L6's in it now could this casue the fizz????

Cheers, Mike

[Firestorm]

I'm pretty sure that amp originally had 5881s which are a lower-power tube than 6L6G- tubes, but that isn't likely to be the source of the problem. By all means try the grid stops -- once Fender put them in. they never took them out. BTW, you didn't say which channel you're using -- Normal I assume, but the Bass channel is a unique circuit, so if that's where the problem lies, there could be other issues.