Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Duncan on November 15, 2025, 04:01:31 pm

Title: Fender Bantam Bass oscillation coming from a wire
Post by: Duncan on November 15, 2025, 04:01:31 pm

Howdy.

An acquaintance brought me a 1969 Bantam Bass combo for some work (retube, caps, etc). Pretty standard stuff.

So I went and did the servicing I’ve done a thousand times, fired it up, and was presented with some pretty bad oscillation (helicopter noises).

I narrowed it down to this wire:

https://imgur.com/a/1MGKYY1

I’ve tried running it under the board, over the board, around the board, and all I’ve been met with has been different versions of terrible oscillations. I’ve checked the resistors in the path of this wire, and they’re all fine. I’ve also swapped the wire with a new piece

Any suggestions for this? I have some 600v shielded wire that I might have to try next but I wanted to engage the deep knowledge base here before I potentially waste more time.

Here’s the schem: https://el34world.com/charts/Schematics/files/Fender/Fender_bantambass_cfa7003.pdf

Thanks!!

Title: Re: Fender Bantam Bass oscillation coming from a wire
Post by: tubeswell on November 17, 2025, 11:45:32 am

Maybe a non-functioning cathode bypass cap on the shared cathode resistor going to pin 8 of both 12AX7s. (Combining the cathodes there through a shared resistor is a recipe for oscillation if the resistor ‘becomes’ unbypassed). Failing that, add more filter capacitance to that preamp supply node.


Also when you did the recap, did you also change out the filter cap(s) in the bias supply?