loud hum in 135 watt Twin (UL)

[phydauex]

Thought I'd throw this out to any and all ,
Working on a UL twin 135 watt model that has a constant annoying hum, that I can't sem to get rid of. It had all new supply caps put in 2005, along with new screen risistors, grid resistors and some coupling caps. I at first measured some AC after the bridge diodes and just assumed the hum was that. So I replace the four diodes and snubber caps along with the bias diode, which was passing ac. Bias is clean now and there still is the same amount of AC after the bridge rectifiers (@1 mv). I've checked all solder joints, components for noise (chopstick), moved wires around, different power tubes.
Pulled the PI tube and still have same hum. I found some questionable solder joints and redid them to no avail. Replaced some B+ wires that also looked rotted and loose. It probably doesn't help that the factory wiring is a mess. All voltages seem about right, it's been switched over to adj bias and bias's up fine. Bypassd the heater hum adj pot with 2 100 ohm resistors to ground, no change. I was going to float the heater supply next to see if that makes a difference.

Any thoughts? Also pulled the boost circuit and the 3' of associated wire, no change in hum. On the scope the ripple is gone after the first dropping resistor(2.7k / 10w).

Matt

http://www.schematicheaven.com/fende..._135_schem.pdf

[Chuck H]

Those amps are getting older now. IIRC they used those crappy IC caps. Probably time to replace ALL THE ELECTROLYTIC CAPS IN THE CIRCUIT. There are other threads about this if you search the forum.

[phydauex]

All the caps were replaced with Sprague Atoms, a couple years ago. There are no IC caps in the amp. This certainly wasn't Fenders finest moment.

[Enzo]

The first step is to isolate the problem. You have done this, pulling the PI tube and finding the hum remains. So we know the hum is coming through the final stage.

Pull ALL the power tubes. Is the hum still there? If the hum goes away with the tubes then we know it is electrical. If the hum remains, then either the OT is magnetically coupling to the PT, or there is leakage between primary and secondary in the OT, or something even less likely.

I will assume the hum needed the tubes.

Verify that ALL FOUR power tubes have B+ on both pins 3 and 4 - plates and screens.

Your bias unless the amp has been modified goes through the power tube balance control. Does rotating that control smoothly run the bias voltage up and down on both sides? Even it up. At the center tap of that balance control, measure the bias supply for AC ripple. Should be none. Just in case, the power tube balance control is NOT the heater balance control. Both are set the same way though, turn for least hum. That may or may not be with the bias equal on the two sides.

Check idle current through all four tubes individually. Make no adjustments to the bias balance control during this step. Are all very simular, or is one or two out of whack?

Go over to EG Keen's Geofex site and look up his very simple transformer tester. Make one and test the OT for shorted turns. Power off of course for that.

[phydauex]

EnZo to the rescue!

1) Hum is gone with power tubes pulled - Electrical

2) Bias supply has been switched to regular adj bias like BF amps had. No ripple on bias supply.

3) All four have B+ on plates (486v) and screens (490 v).

4) I only had one 1 ohm resistor to connect to ground on one tube, got more on order. I need to make a bias meter adapter. Bias seems to be ok, stable, but I can only directly measure one of the tubes. One of the tubes seems to be hotter than the other three? I didn't have 4 new 6L6's to pop in, but have swapped them out in pairs and hum doesn't change. All the screen resistors have been replace and measure good (468 ohms), maybe one of them is iffy or possibly just one of the tubes has started to go south. This amp is very hard on tubes. It came with Sovtek 6l6 WXT's which should be ok with the high voltage, but maybe I'll get a set of JJ's or Tungsol STR's for kicks. Both of these will handle the B+.

Thanks

[phydauex]

Just an update. Got my scope going and it seems to have about 9 volts(peak) ac after the rectifiers, (which were replaced at the start). Just out of curriosity, I added another filter stage after the stand-by switch, made up of a 75 ohm resistor and a 40uf/500v cap. Then I moved the OT center tap to that point instead of the double caps. Amp is silent! The AC ripple at that stage is down to 2 volts and after that stage non-existant. The schematic shows (2) 220uf @350v caps and who ever replaced the caps used (2) 100uf@350v Atoms. This gives only 50 uf for that monstrous power section, I think it needs more and is supported by the drastic drop in ripple with the extra stage. Think I'll get some of the F&T 220uf caps as all I have are some used IC caps at that rating. I also lowered the NFB resistor from 20k down to 10k, which still lets it breath abit more, but is quieter. There were a bunch of mods to it when I got it and some are ok, some were maybe a bit much. I would rather overbuild the power supply and make it reliable and solid, then do minor tonal tweaks. This amp runs too hot and hard to scrimp on the power supply.
Matt

[Billy R aka DynaFreak]

Sounds like you are on the right track - 9 volts AC on the first filter is too much, it is usually 2-3 VAC. If you run the amp on a dummy load hooked to a scope you can watch the top of the waveform as it approaches clipping. If it bounces (breaks up) instead of just flattening, you may need more capacitance. I usually find that a little over 100uf is needed for most 100watt amps.