Fender Blackface Deluxe Reverb Hum/Buzz problem

[Westrek]

I finally found the problem and I as I thought, it would be something that was overlooked by me and two other techs before.

This amp has had the power transformer replaced and although it is a genuine Fender, it did not have a center tap for the heater.
I just noticed this and after fitting a virtual center tap with 100 ohm resistors, the hum/noise is gone and the amp is very quiet now.
I also installed a buss wire ground for the preamp side similar to what amp builders suggest for the AB763 circuit.

I would appreciate any parting thoughts and very much appreciate the input of those here. Thanks! Mike

[Chuck H]

I wonder if anyone has insight as to why the missing virtual CT would manifest itself as 120Hz hum and buzz.?. I'm asking honestly, not sarcastic. There was another recent thread where someone installed a "humdinger" circuit. And though they haven't reported back to my post yet it was implied that adjusting the humdinger got rid of buzz as well as hum in their case. I've never had trouble with noise from a filament supply and I always just assumed it would invariably be 60Hz hum only.

[Westrek]

Yes, I hope others chime in with some thoughts on the matter.

[Enzo]

I would expect 60Hz. The only thing comes to mind is maybe we got fooled by the counter. Your counter said 120Hz, but any distortion COULD possibly fool the counter. I would scope the hum and verify 60Hz or 120Hz.

Only way I get 120Hz is using a convoluted rationalization. Like this. MAYBE.... heatr hum was getting into more than one tube, and by the nature of the amp circuit, that hum can be at different phase one from another. That might cancel if everything was all even and at the same level, but things are not, so teh 60Hz lumps at alternate phase combine to make a 120Hz-like noise signal. I told you it was convoluted. The only support for this theory is his report in post #1.

Raising the volume pot on the vibrato channel increase the volume of the noise, but approaching the midpoint of the pot, the noise is almost nulled out. Continuing cw the noise is further increased.

That is the classic symptom of two sources of hum in the circuit, out of phase with each other. One source is always in the circuit after the volume control in question. I'll call it noise B. The other out of phase source comes from earlier in the circuit, noise A. SO from zero volume control, noise B is heard alone. As you advance the control, noise A rises to the point it is equal to noise B in level, so they cancel at mid point. COntinue to turn up and the amplified noise A gets louder than the portion cancelled by B so the overall hum increases again.

I have seen that many times. I don't expect it to be 60Hz out the speaker, but then when I see that situation, I am not thinking Hertz anyway.

I told you it was convoluted.

Your money back if not completely satisfied.

[Justin Thomas]

My Bassman 100 lost one of the 100Ω resistors once & buzzed like mad. I didn't really look into why, but I replaced the resistors & it went away. I didn't look into the frequency of the noise, but it was definitely buzz & not hum.

I thought the "tone" of the noise was more dependent on waveshape more than frequency? My Tele picking up noise from the air always cones out as buzz instead of hum, & I didn't think that was 120Hz...

Jusrin

[Westrek]

I would expect 60Hz. The only thing comes to mind is maybe we got fooled by the counter. Your counter said 120Hz, but any distortion COULD possibly fool the counter. I would scope the hum and verify 60Hz or 120Hz.

Only way I get 120Hz is using a convoluted rationalization. Like this. MAYBE.... heatr hum was getting into more than one tube, and by the nature of the amp circuit, that hum can be at different phase one from another. That might cancel if everything was all even and at the same level, but things are not, so teh 60Hz lumps at alternate phase combine to make a 120Hz-like noise signal. I told you it was convoluted. The only support for this theory is his report in post #1.



That is the classic symptom of two sources of hum in the circuit, out of phase with each other. One source is always in the circuit after the volume control in question. I'll call it noise B. The other out of phase source comes from earlier in the circuit, noise A. SO from zero volume control, noise B is heard alone. As you advance the control, noise A rises to the point it is equal to noise B in level, so they cancel at mid point. COntinue to turn up and the amplified noise A gets louder than the portion cancelled by B so the overall hum increases again.

I have seen that many times. I don't expect it to be 60Hz out the speaker, but then when I see that situation, I am not thinking Hertz anyway.

I told you it was convoluted.

Your money back if not completely satisfied.

I'll bet you're correct. I'm going to un-do the fix and run some additional tests. I'm curious to know if the Vibrato channel "nulling" that occurs at mid point is related to the fact that the 2 channels are out of phase with each other at the mix resistor point. Will report back. Great opportunity for continued learning!

Thanks so much for the input!

[Helmholtz]

In his book Merlin explains how heater buzz/hash (as opposed to hum) is produced. Reason is that heater voltage is typically clipped and thus contains higher odd harmonics. The capacitive coupling between the heater and grid of preamp tubes prefers the higher harmonics.
Electrical heater balancing is most effective to compensate this effect.

BUT, as the symmetrical heater voltage can only contain odd harmonics, this does not explain 120Hz (being an even harmonic of 60Hz) heater hum.

So my question is: How does the heater voltage look on the scope? If it actually has 120Hz content, it must look asymmetrical.

[Enzo]

2 channels are out of phase with each other at the mix resistor point

perhaps, but that wasn't where I was coming from. In one stage, the signal at the grid is out of phase with the signal at the plate. SO the next triode stage will be working at opposite phase from the first. So if one stage is getting heater hum, and the next stage is picking up hum from the grid wires, those two hums can be at opposite phase. All within one channel.