Fender Twin Reverb

[Dano]

I'm working on a Fender Twin Reverb, it has the numbers A775176 on the inside. The schematic I'm using is on page 456 of the Groove tubes 4th edition tube amp book.

This unit exhibits a slight fuzz sound when notes are played at low volumes.

I have taken voltage measurements on all the tubes and find nothing abnormal.

I have substituted new tubes (both preampt and output) and the problem still exists.

I have substituted the 3 20uf/500v caps and the problem still exists.

I have not been able to substitute the two 220uf/285v caps, I do not have anything of that size in my stock. Could they be going soft?

Measuring the resistance of the output transformer with RED being the center tap, the results are (I am using a Fluke 87 multimeter) :

Between RED and BLUE 39.6 ohms
RED and BLUE/YELLOW 4.7 ohms

RED and BROWN/YELLOW 5.3 ohms
RED and BROWN 37.4 ohms

Is the output transformer bad?


Any thoughts or suggestions?

Thanks,

Dan

[Tom Phillips]

Dan,

I have experienced the symptoms you describe when a voice coil has a certain slight rub. The bad sound can be masked when you play loud. In my case it only showed up when I played quietly or when the note decayed down to a low level. The first thing I would do is test the amp with a different set of speakers.

If you don't have another set of speakers then disconnect them one at a time to isolate the bad one. Short across the terminals of the disconnected speaker to damp the cone movement. Otherwise the disconnected speaker could resonanate and still make the sound. This, of course, assumes that they arn't both bad. (Unlikely, but possible)

Good luck,
Tom

[Dano]

I did eliminate the speakers, I use another speaker when working on the bench. The sound was still there in the bench speaker also.

Thanks for the reply.

Dan

[Andy]

it's possible that it's biased too cold and you're hearing crossover distortion. when you turn it up, it may be smoothing out and making it less audible.

[Enzo]

Scope the output and see if you can spot the distortion. That it seems to be a steady level regardless of signal level is a clue... of something. VC rubbing was my instant thought too.

I doubt the cap is bad, but scope it for ripple. Since the power stage push pull cancels power ripple hum, as much as 20-30 volts doesn't cause trouble. But there should be almost no ripple on the following B+ nodes.

Apply a steady sine wave and see if you can hear it on that at some level or other. If you can, then that should be a lot easier to SEE on a scope. For example little ripple bumps along a sine waveform.

Welcome to the group, I'm in Lansing, myself.