Fender Silverface Problem and Solution

[bradicusmaximus]

Hey there everyone -

I wanted to post this little story up in the event it might help someone else along the way. Although it's kind of like the Deluxe Reverb Hum topic, I thought it was different enough to not muddy up that one with my rambling.

About a month ago I found a fairly decent deal on a 1976 Pro Reverb so I picked it up for myself. I really like the Pro Reverb so it was a good buy for me. Someone long before had jammed it into a Super Reverb cabinet and at some point the past, a tech had wired in an effects loop.

When I tested out the amp at the shop it was in, the amp was fairly quiet though the reverb wanted to run wild when it got over around 3 and half on the dial. No big deal, it looked and sounded workable. And it's a Silverface, so there's not much in my mind that can't be fixed on these things if there is a problem.

I get the amp home and the first order of business is to check it all over and probably do a cap job. After pulling the chassis and looking under the dog house, I see one of the filter caps had a blow out sometime in the distant past. Interesting since the amp was as quiet as it was. But stranger things have happened.

Long story short, I replaced all the filter caps and then went about fixing the mess of a job that was the effects loop. While in there, I did some black facing to change out the bias balance to a real adjustable configuration. Since this was far from pristine, I also did a few other minor mods (term circuit off switch, mid control on the normal channel, add reverb and them to the normal channel, negative feedback control, as well as putting some values in line with blackface specs) and retubed the entire amp.

When all was said and done, I had a very noticeable hum on both channels. It was very much a 60 cycle drone and was loud enough that it needed to be taken care of. I chopsticked the various wires and nothing. This is very odd. Of course my thoughts started turning to some problem with a component added - probably a filter cap or something like that. Bad soldier joint? Broken wire? Bad transformer? What could it be?

Then I realized there was a small balance control on the back of this model amp connected into the filament wiring. After sweeping it through its range of motion, I did detect the hum changed. It did not go away, but it made an audible difference. So, before I go crazy redoing work already done, I'll explore the heaters and that wiring network.

I've worked on a bunch of Silverface amps and they definitely aren't of the quality of a 60's Fender. One of the most obvious places is the heater wiring, which was installed with the bare minimum of care and attention. This was no different.

My first order of business was to eliminate the pot from the network. They installed it right between V1 and V2 and had a whole slew of loosely twisted filament wires hanging out there. With that done, I went about tracing the filament connecting pin 9, which should connect to other pin 9s across the network. And everything was good until I got to the phase inverter. The wiring was reversed both from the previous preamp tube and to the adjacent power tube.

After cleaning this up by realigning the wires to consistently hit the same pins as well as giving a few additional twists to dress up the wires, I added an artificial center tap off the junction at the lamp holder to ground.

With all of this complete, I fired up the amp and low and behold it was significantly quieter. There is still some high end fizzy-ness, but I attribute that to having not gotten around to swapping out some of the ceramics for silver micas.

This was long, but I'm hoping that someone, sometime can gain some insight into how the wiring of heaters affects the noise. I attribute the solution less to tightening the heater wires and more to making sure they were consistent and well dressed in combination. Something to look into for some of these 70's Fenders when Quality Control took the decade off.

[pdf64]

Thanks for documenting it so thoroughly!
I know it's with the benefit of hindsight, but it might have been useful to have tried the artificial CT ground reference on the heaters as they were, instead of the humdinger, as those controls often get damaged / become fragile when there's a tube short.
Have you still got the old control available to test?

It's just that my suspicion is that heater wiring polarity doesn't really matter, but wiring then re-wiring heaters is such a drag that I've never got around to verify it.

[bradicusmaximus]

Thanks for documenting it so thoroughly!
I know it's with the benefit of hindsight, but it might have been useful to have tried the artificial CT ground reference on the heaters as they were, instead of the humdinger, as those controls often get damaged / become fragile when there's a tube short.
Have you still got the old control available to test?

It's just that my suspicion is that heater wiring polarity doesn't really matter, but wiring then re-wiring heaters is such a drag that I've never got around to verify it.

I considered that, but I figure if I'm in for a little I might as well go in for it all. I hate wiring heaters myself, but once I started really looking at the way it was wired, I realized they needed dressing. I left the pot in place. Maybe if I get really ambitious I'll set things back and test the hypothesis - or I'll just wait until the next silver face pro reverb finds its way to my workshop! lol

[ampgeek]

Very nice Brad.
Thank you for sharing that with us!
Dave O.

[bradicusmaximus]

Very nice Brad.
Thank you for sharing that with us!
Dave O.

No problem. I've asked enough questions and taken enough from this forum that even if it's a minor contribution, I'd like to at least give something back. Hopefully it saves some hair on someone's head from being torn out!

[Stevem]

If the amp still has the factory ciraimic on the input to the PI then that's the fuzzy fizz!

[bradicusmaximus]

If the amp still has the factory ciraimic on the input to the PI then that's the fuzzy fizz!

When I was doing some blackfacing, I swapped out the PI Input Cap for an Orange Drop.

I pretty much went through the normal channel end to end and swapped various parts. Some of the leads were looking kind of sketchy for whatever reason. Maintaining vintage value wasn't even remotely on my mind as the entire amp was kind of beat up and a nice "player". The vibrato channel is box stock, which means it's got the ceramics in place. Neither channel had the bright cap on the switch swapped so those are still ceramics.

At this point, the fizz is very, very quiet and hardly noticeable unless I crank the treble way up. I only notice it because I'm kind of obsessively listening for noises. I'm sure to the casual listener it's a very quiet amp. Since I don't have much intention of recording with it, I think it's about as good as I'm going to get it unless I get a bug to do something else to it!

[pdf64]

Regarding the fizz, scope the output with a real speaker as the load, to check for any hint of oscillation; it may only manifest as a parasitic at high power.
If clear, look to get any ceramic caps replaced, eg bright, treble control, with a film type.