Red Knob Twin 60Hz hum

[Randall]

Finished a red knob The Twin months ago, but guy doesn't come get it. Finally, he makes a day to come get it, and as per my previous thread regarding playing an amp before sending it out, I fire it up. well, it was quiet when I buttoned it up 3.5 months ago, but now it has a loud hum. Sounds like a bad ground, so I go looking and find nothing. It is in the preamp for starters, so I plug a dummy plug into amp in jack, and it is there also, maybe even louder. I pull V3 and V4, and while not as loud, it is still there. I had already set bias, so I figure I must have lost a cap with it just sitting there. The hum measures 60Hz. Which caps would be the first to swap? I hate these things.

TheTwin.pdf

[Enzo]

B+ power supply ripple is 120Hz. The bias supply is half wave and so 60Hz. Scope the bias supply.

V4 is the phase inverter, so if the hum remains pulling that, it comes from the power tube stage. SO bias is still balanced? All four power tube screens are hot?

Does grounding the power amp in jack kill the hum? Is the hum on the preamp out jack?

Power dupply D, a low volt positive is also half wave for 60Hz ripple, it is used in the switching circuits, but those LDRs would transmit that into the audio path, so is it clean?

Find the hum before we start tossing caps about.

[Leo_Gnardo]

I hate these things.

Me too. I have one that's been a headache for years, luckily its owner is very very patient.

I'd try clipping in a filter cap in parallel with the first aka "reservoir" cap section. Any working good 500V cap oughta do, say from 20 uF up to 50. It would be a shame if Fender's OEM caps are failing over the course of a couple months storage but they are 25 years old more or less, and weren't any great shakes when they were new.

[nevetslab]

I too am not a big fan of these. The one I just finally received fresh tubes for, had Loud Hum, and it was the two 47 ohm resistors on the heater coil that had burnt open. I've also seen them go open without being burnt.

Even when you get everything sorted out on this model, there is a high residual hum that is 60Hz & 120Hz in the noise floor, and it's NOT from the preamp section. I did spend some time going thru the grounding of the power supply board and the middle board in the chassis to see if I could find where the error is, but wasn't able to improve on it. And, that was with dead-patching the power amp so the preamp wasn't contributing, as far as it's signal was concerned. I was still pulling power in the preamp section, not having removed all those tubes.

Has anyone found why the residual hum is so bad on these (as well as the Twin Amp, it having the same nastiness.



The spectrum analyzer images above show the difference between the noise spectrum at the Preamp Output jack vs the Speaker Output jack. The power amp in these amps is only 28dB gain, but there is WAY MORE hum and ripple in the power amp than just adding 28dB to the preamp nose spectrum. The Twin amp behaves the same way, using the same build methods. A Fender Twin Reverb doesn't have any of these faults (usually), not that this is that much like a Twin Reverb.

[jalexquijano]

which are these 47 ohm resistor? this amp is not as clean as the twin reverb and has a lot of hum residue.

[nevetslab]

R382 & R383....on the pilot lamp side of the main series-connected buss caps. The Trn Heater Wires attach near them on the wire terminals along the edge. I just looked at my Twin Amp Service manual, and that Power supply board is NOT an exact replica as in The Twin amp, but is similar.

[Randall]

They are R302 and R303 on my amp. Maybe time for new lens?

[nevetslab]

The schematic I have is low resolution, so it's like looking at fax print...could be a 0 in the middle, could be an 8 in the middle of those. Disregarding my cataract issues awaiting surgery.

[Randall]

I can relate, I find it so so frustrating to try to work from a poor quality print. I had that one and ditched it. The one I posted above is much cleaner, grab it.

[nevetslab]

Ah......thanks, Randall....far better than what I had...totally readable!

[Jquijano]

Finished a red knob The Twin months ago, but guy doesn't come get it. Finally, he makes a day to come get it, and as per my previous thread regarding playing an amp before sending it out, I fire it up. well, it was quiet when I buttoned it up 3.5 months ago, but now it has a loud hum. Sounds like a bad ground, so I go looking and find nothing. It is in the preamp for starters, so I plug a dummy plug into amp in jack, and it is there also, maybe even louder. I pull V3 and V4, and while not as loud, it is still there. I had already set bias, so I figure I must have lost a cap with it just sitting there. The hum measures 60Hz. Which caps would be the first to swap? I hate these things.

[ATTACH]46625[/ATTACH]

I also have one under repair at the moment with several problems.

1. There seems to be some ground issue that diminishes when you touch your guitar bridge or any other metal part of your guitar with your hands.

2. If you dial the reverb pot over 4 it starts making some loud hum and intermittent crackle

3. There is some light to medium frying bacon like sound that comes and goes randomly.

Any solution to these issues?