Fender Silver Face 100 Watt Twin Hum

[dpm309]

Working on a 100 Watt Silver Face (MV model) that has an annoying hum. Replaced all of the tubes (they were all original) and am still getting the hum but when I turn the balance pot all the way to the right, it decreases somewhat. I don't think this amp has a bias pot. The volume pots do not change the level of hum even when turned down to 0. Will pull the chassis and to see if I see anything. A blackface conversion was done by the previous owner and the hum developed over time. I have attached what I think is the correct schematic.Any ideas/suggestions?Thanks,Danhttps://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf

[DeepBias]

Leaky or failing filter capacitors ?

[tubeswell]

Leaky or failing filter capacitors ?

^This is what I’d look at first^ … Including the bias supply cap (if it’s the original white Mallory cap, it will be way past needing replacing by now)

[dpm309]

I don't see any leaking or bulging filter caps. Just to be sure, I jumpered each with a 100uf/500V know good filter cap with no change in the hum level. Some one changed these out previously with 2 80uf/450V and 3 22uf/ 500V. Stupid question but which cap is the bias supply cap? This doesn't have a bias pot, only a balance pot.

[tubeswell]

Stupid question but which cap is the bias supply cap? This doesn't have a bias pot, only a balance pot.

There’s two of them (80uF 75V) on that schematic

[mresistor]

Can you post a photo of the innards, especially a higher resolution photo of the power supply side? Thanks.

[dpm309]

Here are some pictures of the innards. One of the photos shows a trim pot installed on the back of the chassis and is wired into the heaters of V2. Turning this pot does nothing to the hum level. This amp supposedly had the blackface conversion done to it and I don't know if this trim pot was part of it. Also showed a picture of under the cap can where there are 2 80uf/450V and 3 20uf/500 filter caps. These values do not match the schematic. The filter cap on the bias board and is 33uf/160V and again does not match the schematic. What would be the purpose of the trim pot?

[shooter]

disconnect the NFB, any change?


have you identified it as 60hz, 120hz, some random hz???

[dpm309]

I am not seeing the NFB on this schematic. Also, sounds like 60Hz hum but with the MV turned up, it sounds like 120Hz. Also, this amp does not have the push pull MV, as shown on this schematic.

[dpm309]

Another observation, the hum does not disappear when pulling the preamp tubes one by one (except for V6 - phase inverter). Also, am getting -50VDC on pin 5 of the power tubes with the tubes removed.

[SEL49]

I am not seeing the NFB on this schematic.

Look at the horizontal line just above V7. That's the NFB loop.

[dpm309]

Thanks, I see it now. I'll disconnect it at the output jack and see what happens. Also noticed someone clipped the ground (see picture) below the PT. Can't see what was wired to this ground.

[dpm309]

OK, that did not change the hum level at all. Will reconnect.

[mresistor]

Not a fan of those puny radial filter caps on the filter cap board.  I would replace them with some good F&T axial capacitors.   

[dpm309]

I did jumper them individually with a good cap and it did not change the hum level. What value should I replace them with since they don't match the schematic at all. I also disconnected the trim pot for the heaters for V2 with no change in level.

[dpm309]

 Just ordered 3 22uf/500V F&T axial caps. Will report back when I install them.

[stratomaster]

I did jumper them individually with a good cap and it did not change the hum level. What value should I replace them with since they don't match the schematic at all. I also disconnected the trim pot for the heaters for V2 with no change in level.

Do you have any kind of center tap on the filaments?  Check the pot to see if it hasn't shorted or gone open.

Removing the pot should only be done if you have another CT in place.

[shooter]

the hum does not disappear when pulling the preamp tubes

Guessing that included V5 the trem tube??

[Dave]

There is a 100 ohm resistor in the PI circuit connected to ground. Remove the grounded end of that resistor and connect an alligator clip lead to the dangling end. Connect the other end of the alligator lead somewhere else to ground. If it has an effect, disconnect the ground end and probe around until you find the quietest spot to ground it.


Dave

[dpm309]

Yes, still getting hum when I removed V5. I have no idea what the extra trim pot for V2 was for. Disconnecting it from the circuit did not change the hum level. There is no center tap on the heaters and I do not see a pair of 100ohm resistors as shown in the schematic. I looks like they might of been connected to the ground lug located below the PT. Will wire a pair of 100ohm resistors to create an artificial center tap,

[mountainhick]

Yes, still getting hum when I removed V5. I have no idea what the extra trim pot for V2 was for. Disconnecting it from the circuit did not change the hum level. There is no center tap on the heaters and I do not see a pair of 100ohm resistors as shown in the schematic. I looks like they might of been connected to the ground lug located below the PT. Will wire a pair of 100ohm resistors to create an artificial center tap,

Pretty sure that pot is a humdinger and the center tap for your filaments. It may be the problem if there's crud built up in the contacts or the pot is failing. I had an identical setup on a stock 70's silverface bassman ten, took it out and replaced with two 100R resitors at the pilot light like 60's versions.

[dpm309]

Wired a pair of 100ohm resistors to the pilot light and disconnected the humdinger pot. The has hum been significantly reduced. Will remove the 100ohm resistor from the PI circuit as Dave suggested and try to find a quieter grounding spot. Also hoping the the new filter caps will reduce the  hum even further. Looks like I am getting closer to fixing the hum issue.