Help! My amp sounds like a ring-modulator.

[Humbucker]

I got a '63 Fender Bassman piggyback amp in a trade a few weeks ago and I just noticed tonight that when I play notes past the twelfth fret on the B string I get this weird out of tune, fuzzy harmony note that appears in the background right after the note's transient tapers off.

It's hardly noticeable if you're using the bridge pickup but it's really noticeable when on the neck pickup, it seems that the bassier it is, the worse the problem becomes.

At first I thought that it might be the speakers causing the problem but I tried this head with another cab and it's the same deal. Then I thought that the problem might disappear if I'd turn down the amp's volume but it's still there.

I know that filter caps can cause this type of problem but half of the caps in my amp are pretty close to being new already. All of those big electrolytic caps in the "doghouse" have been replaced with Sprague Atoms, so that should be alright. All of the remaining five or six electrolytic caps inside the chassis are original Astrons and I just haven't gotten around to replacing them yet, do you think that that might be my problem? Or are my Spragues less then healthy? Any chance that a bad tube might be the culprit?

What about adding grid-stopper resistors? I seen a chassis picture of an amp like mine and the guy had some grid-stopper resistors installed on the two power tubes. Do you suppose that that might cure my problem?

I'd appreciate any opinions with this. Thanks!

[Steve Conner]

My magic 8-ball says it's likely to be a bad filter capacitor in the bias supply.

[Humbucker]

Hey Steve,

Thanks for chiming in!

That could definately be the problem since this cap is the original Astron 25/50 cap. I've got to order myself some more caps and replace all of these electrolytics that are in the chassis.

I've heard that some people upgrade the bias cap to a 100/100, would that be okay to use in this amp?

[Steve Conner]

I can't think of any drawbacks to using a bigger cap in there.

(FWIW, if that cap dries out, the resulting ripple modulation of the bias works out no different to a ring modulator with your guitar going in one port and 60Hz going in the other.)

[Humbucker]

Out of curiousity, I'm going to replace the stock 25/50 cap in the bias supply with a 47uF/65V cap that I have in my junk bin and see if anything changes. Soon, I'll get myself all the new caps that I need to get this amp healthy again.

I'll post a follow-up soon. Thanks!

[J Martin]

Did you measure the raw bias voltage? 50 volt caps are often too low a working voltage since the mains voltages have come up a bit since 1963. I use 47/100v as that covers all Fenders, even the ones that have two 75 uf caps.

[Humbucker]

For the bias supply, I'm getting appx. -55 volts DC, I'm measuring this with the negative lead of my meter to ground BTW. I'll probably get a 100/100 cap for the bias supply circuit but that 47uF/100V sounds like it would be even better.

[J Martin]

You don't need 100, it just takes longer to charge up. You probably don't even need 50, but thats a safe practical value for the small currents in that circuit. When I am working on those old Ampeg B-15s with the 100/100 bias filters I see the bias current soar up to twice its idle current while the cap charges up when the standby is turned on. The Ampeg V4 amps only have two 10/100 caps and a resistor in their bias supply. The later Fenders went over to the two cap/resitor method also.