Fender Studio Bass Huummm...

[Tim Skaggs]

I have a late 70s early 80s fender studio bass amp that I converted to a "head". I've had the amp several years and its performed well into a variety of different speaker cabs.

A couple years ago, it developed an illness of crackling / popping when turned from standy to "on". This went on for a while, then developed into a very annoying hum. The amp still "works" well, despite the hum, but the hum seems to be getting a little louder. The hum is present when the amp is on and not in standby. The hum is the same with the volume & output wide open, or at minimum, or anywhere between. It's also present whether or not there is a cord in either input jack. The volume of this hum isn't too loud, but is easily noticeable.
I have a tuner app on my phone that displays frequency, and the hum is exactly 120.0 Hz and does not vary in pitch.

I've done some very minor troubleshooting, by changing out preamp and rectifier tubes, which had zero effect on the hum.

I think I know where this is heading; mass capacitor replacement, but I thought I'd ask the collective first to see if anyone had any suggestions that could be done by a beginner / intermediate level amp tech for which I might qualify....

 

[Stumbo]

Good amp techs!

 

[pfschim]

best of luck working out the hum issue with your Studio Bass.
I have one too, but it is dead silent (for an older tube amp) - and I am not enough of a techie to offer any suggestions.

 

[Tim Skaggs]

Well, as it turns out, I've fixed my amp. The down side is, I don't know what was wrong.

When I wrote the original post, I had removed the amp from the cabinet and had changed the aforementioned tubes. Two of the preamp tubes were microfonic, one much worse than the other.

Those of you who are familiar with these amps know that they have a Hum Balance pot and an Output Tube Matching pot (poor man's bias) on the rear panel. Before removing the amp from the cabinet, (and many other times before) I adjusted each of the pots for minimum hum. Both pots work as designed. The hum balance control would introduce hum at each extreme. The output tube matching pot did the same, but neither one would eliminate the hum at any setting. I could make the hum louder at each extreme, but couldn't eliminate the hum at any point in the middle range.

After changing the preamp tubes, and re-seating all the power tubes, I still had hum. I also cleaned the contacts on the foot switch jack for the EQ (shorting contacts allow the EQ to work when nothing is plugged in the jack). Before I put the amp back in its cabinet, I thought I'd twist on the hum balance and output tube matching pots one more time. Hum balance did nothing, but this time, the output tube matching pot did some good. I was able to eliminate pretty much all the hum. I don't know what changed. My suspicion is that one of the output tubes wasn't seated well.

Whatever the cause, it's much better now....

 

[Tim Skaggs]

The hum problem returned on my Fender Studio Bass amp some time ago (probably close to a year ago), but I just got back to work on it after finding & viewing some troubleshooting techniques on YouTube to solve hum problems. I found a couple capacitors that appear to be in major decline & wanted someone else’s opinion on the appearance of these caps. I think it’s obvious they need to be replaced, but wanted input from those who have seen the same. Are these leaking caps the possible cause of a moderate hum I’m hearing that increases significantly when the output is increased (hum does NOT change when the “volume” is increased).

Thanks in advance; Tim

 

[Tim Skaggs]

Any suggestions where I can get a couple 220uF, 275VDC capacitors?

 

[Wasnex]

Yes...those could possibly be the source of the hum.

I believe those are the filter caps for the HT supply, so be careful. The schematic shows 220uF 285V and the expected voltage across both is 500V.

These capacitors are stacked in series, so it should read 250V (or half the HT voltage) across each capacitor. I suggest checking the voltage to make sure it's not over 275V.

The 39k 2W resistors may need replaced as well. These resistor pass a little current which does two things: 1. It insures each capacitor sees approximately 1/2 the total voltage. 2 It bleeds down the charge when the amp is turned off.

Various options on this page:
Invalid Link Removed

A higher voltage rating is fine.

 

[Tim Skaggs]

Yes...those could possibly be the source of the hum.

I believe those are the filter caps for the HT supply, so be careful. The schematic shows 220uF 285V and the expected voltage across both is 500V.

These capacitors are stacked in series, so it should read 250V (or half the HT voltage) across each capacitor. I suggest checking the voltage to make sure it's not over 275V.

The 39k 2W resistors may need replaced as well. These resistor pass a little current which does two things: 1. It insures each capacitor sees approximately 1/2 the total voltage. 2 It bleeds down the charge when the amp is turned off.

View attachment 4953574


Various options on this page:
Invalid Link Removed

A higher voltage rating is fine.

Great info & thanks for the response.

 

[Tim Skaggs]

Replaced two of the larger and most swollen caps & didn’t gain anything on the hum. I finally got back on the project and changed out the next two larger caps (last week, 12/4/23). The hum is gone, the amp is dead quiet excep for a slight hiss that changes with the treble level, which I believe is probably normal. Played a gig with the amp on 12/8/23 driving a single 8-ohm 15” Eminence in a ported cab. It sounded absolutely wonderful. Next gig, NYE….