Hum in MV Twin Reverb

[earache]

Hello all

I have been fooling with a Silverface MV Twin Reverb. I replaced the caps in the doghouse, the three carbon Comp resisters there and also the 100k resisters at the preamp tubes. The amp hums still. Suggestions for what to look at next?

Can anyone share how they approach biasing these amps that have both "output tubes matching" and "hum balance" pots?

Thanks in advance

[Chuck H]

First things first though...

Did the amp hum before you replaced the caps under the doghouse or any work you've done?

Does the amp have proper bias voltage?

Do any panel controls affect the level or sound of the hum

Does it hum with no guitar plugged in?

[g1]

Try setting the matching pot with all volumes down, set it for minimum hum. Then turn up the volumes and adjust the hum balance.
If you still have a hum problem, you have replaced the filter caps, but not the bias filter cap so you should probably change it too.

[earache]

I am on vacation so don't have access to the amp, but can reply to the comments.

Chuck H - the amp hummed before I replaced the caps and the resisters in the doghouse. It hums whether or not anything is plugged in, and the panel controls don't affect the noise, except the reverb does affect it slightly. Can't presently measure the bias voltage but will at first opportunity.

g1 - This amp doesn't have a board with a bias cap on it like other old Fender amps. As a matter of fact, there aren't any electrolytic caps inside the chassis at all, save for the bypass caps. I replaced all of the caps in the doghouse so one of those must have been the bias cap...right?

[Enzo]

Right? I would not assume that.

This one?
http://bmamps.com/Schematics/fender/..._100_schem.pdf

Go find the bias tap on the power transformer. Follow the wire to the rectifier diode, the other end of that diode connects to your bias filter. There is a second one connected to the tap on the bias balance pot. Offhand I see five caps in the doghouse, how many do you have?

[earache]

Reviving this month-old thread because I have come back around to this amp again


There are 7 caps inside the doghouse, and I replaced them all.

[earache]

Thank you for posting that schematic, Enzo, but I don't believe it is the same amp. This one has ceramic caps paralleled with 4 of the 5 total diodes on the bias board.

[MarkusBass]

There were several different versions of the amp. Here is one with caps parallel to diodes.

Mark
twin_reverb_sf_135_schem.pdf

[earache]

Yes Markus I believe that is the one - 135 W ultra linear master volume.

[earache]

Gonna check and see if I have anything near the negative voltages that are posted for the bias and will report back.

[g1]

Did you try adjusting the balance/matching pot yet?
It sounds like you have not located the bias caps yet. If you can post some gut shots, we could point them out for you.

[g1]

Never mind about the bias caps, I see that they are in fact in the doghouse, just that someone has changed them to 450V type .

[earache]

Hi G1 - here's some pix related to the caps. I can verify for sure that there are NO electrolytics inside the chassis except for the coupling caps.
In the following shots I am taking the bias measurement at both sides of the 2k2 resister which is in the doghouse





The schematic was looking for -62 and -58 volts, I came kinda close.

[g1]

Sounds close though I'm surprised to see it low rather than high. Is the 1.2K that feeds the bias diode ok or maybe been changed to a different value?
Anyway, did you try the adjustment as outlined in post #3 ?

[earache]

G1 - I think we are in the ballpark. Adjusting the "Output Tubes Matching" with the volumes off I can get the hum to zero. Once I turn the volumes up I can get the hum to to zero by fine tuning with the hum balance...is that the procedure? If so it seems to work.

I also found that the outside power tube has a really noisy ground/ pin 8.
With that settled down there is NO noise. Wiggling the tube around when powered up showed the fault, and a chopstick to that pin produced a repeatable hum.