Awful noise from Fender Bantam Bass

[Prairie Dawg]

hey guys: should I recap the amp before or after retubing it? and is the bantam a self biasing amp or should it be biased?

I would replace the power supply and other electrolytics before I replaced the power tubes. According to the schematic the Bantam is a fixed bias amplifier.

What that means is that the bias voltage is supplied externally from a separate source (a cathode biased amp is a self biasing amplifier)-in this case a separate winding on the power transformer, conditioned with a diode and a 50 uf capacitor. The setup is a hum balance, but you will notice that there is a 3.3k resistor upstream of the hum balance.

It takes a little bit of doing, but the first thing to do is get a pair of tightly matched power tubes. Then, while you're in there redoing the ele3ctrolytics, take the time to blueprint the bias network and all the stuff downstream/associated with it. Pay special attention to the 470 ohm 1w screen and 1.5k grid resistors, make sure they're all spot on.

Once this is all taken care of you can be assured that the matched tubes you paid good money for are doing what they're supposed to do when they're in your amp.

Then, you can set the balance so that they're exactly the same, and then you can adjust the bias level as desired by adjusting the 3.3k resistance.

So.....to summarize. You can get an exact balance with the potentiometer and set the level exactly where you want it. The best of both worlds? I think so.

http://schematicheaven.com/fenderamp...ss_cfa7003.pdf

[the.reveller]

Ok...the ampīs problem is definitely capacitor related. I havenīt been using it lately but this week I replaced the driver tube, and while some microphonic issues were resolved, the amp still produced some wierd sounds...it sounds like a faint vibrato going on in the background...what to do what to do...

[Dave Curtis, dB AudioTech]

Although I'm not the one you asked I thought I'd chime in.
The schematic suggests two 70uF filter caps @ 350V in series and three 20uF filter caps @525V. The transformer is a 125P5D which has a secondary voltage of 325V (got this value from the Hammond replacement PT which is a 290DX). Rectified this would be around 460V at no load. So 500V caps would be sufficient IMHO. You might check your actual voltages to see if my calculation is OK.
Higher rated caps (600V) are more expensive.
Depending from where you order, it shouldn't be more than 30-35$. You need one 40uF@500V and three 20uF@500V. Besides you need 3 cathode bypass caps (for the preamp tubes) which should be no more than one or two bucks each.

Hey! Don't forget the bias circuit! I lost count of the number of amps I've opened up where all the 'lytics have been replaced. Except in the bias circuit.