How to kill hum in a Vibro Champ?

I have a blackface Vibro Champ (date code PI, 1966, September) that has always had a low-level 120 Hz hum for as long as I can remember. The hum is constant, and is independent of the volume setting, or even whether there's any input. It hums even at idle, with nothing plugged in. It's not a loud hum, but it's audible, and it makes the amp unsuitable for recording at low (clean) volume settings. The amp is stock (no mods), with factory wiring.

I have just replaced all the tubes, so the 5Y3GT rectifier isn't a suspect. I'm more inclined to suspect the original GE "can" filter capacitor (it's a 20µF/20µF/20µF/20µF, 450VDC, with two sections wired in parallel to make 40µF). After 46 years, some leakage is likely. In fact, there are other problems with the amp that new tubes didn't solve, so I'm going to replace all the capacitors anyway.

I'm seriously considering disconnecting the can cap and substituting individual individual 500V electrolytic caps for filtering. I've been advised that it might reduce the 120 Hz hum somewhat, but I've also seen posts (elsewhere) that say the hum is an intrinsic result of the power supply design, and even new filter caps won't eliminate it entirely.

I'm committed to the recap job anyway, but I'm curious as to whether anyone knows if it's possible to eliminate the 120 Hz ripple entirely — I mean short of completely redesigning the power supply.

 

Thanks for your reply, DavidP.

Yes, I'm sure it's 120 Hz .

Just to be sure I'm not hearing the first harmonic of a 60 Hz fundamental (which the little 8" speaker might not be able to reproduce with great volume in an open backed cabinet), I plugged the output into my Tremolux cab, which has two 10" speakers. There's no 60 Hz there; it's definitely 120 Hz hum. Actually, that makes sense considering the way the 5Y3GT full-wave rectifier works. It seems most likely that what I'm hearing is 120 Hz ripple coming off the rectifier tube.

Perhaps I should clarify that you're not talking to an amp genius here. I have a decent knowledge of physics but only a rudimentary understanding of amplifiers in general, and relatively little knowledge of Fender tube amps in particular. So your statement...

...doesn't mean anything to me. I'm guessing that what you're talking about has something to do with the transformer leads that power the 5Y3 rectifier, but if I'm mistaken, that just goes to show how little I know.

All I can tell you is that the amp has factory wiring. It has never been modified. Except for a couple of differences between my blackface amp and the Fender/CBS AA764 schematic that is ubiquitous on the Internet (on my amp, the first filter cap node is 40µF, not 20µF; and there's no 330pF cap between pins 5 and 8 of the power tube), my amp is otherwise wired exactly the same as shown on the schematic...if that's any help.

As for the dropping resistors and plate resistors, I guess I can figure out what those are, but there again you're over my head as regards their effect on the hum. The new can capacitor you mentioned — now THAT makes some sense to me. If my existing 46-year-old can isn't doing its job, then it makes perfect sense that a new can would improve the filtering. I guess I won't know for sure until I complete the recap job.

Thanks again.

 

Yeah, that's pretty much what I figured. I probably should get that done first and then see what happens.

I still haven't decided whether to use individual caps or replace the can. If I go with the can, I had intended to use a CE Manufacturing 40/20/20µF 500VDC can. But I'm intrigued by your idea of slightly attenuating the first stage voltage and "beefing up" the second stage. Maybe that resistor in series with the first stage filter cap would make the power tube run a bit cooler; it has always seemed to me that it runs very hot—much hotter than the 6L6 power tubes in my other Fender amps.

...er, when you say "beef up", I assume you mean increasing the capacitance of the second stage filter cap, right? I guess I can do that pretty easily just by adding another capacitor in parallel with the second stage tap on the can.

Thanks!

 

OK, thanks. Now I'm with you.

You've described it correctly. One of the green wires from the power transformer secondary is grounded to a lug on a transformer mounting bolt. The other goes to the pilot lamp, and then to pin 2 of the power tube, and to pins 4 & 5 on each of the 12AX7s. And yes, power tube pin 7 and pin 9 on each of the preamp tubes are wired to chassis ground.

So, you mentioned "the twisted pair configuration". Is that a modification to the stock wiring configuration — one that will reduce the hum?

 

Aaaccckkk...I got out of sequence with my replies, but no sweat.

OK...first off, I might not have made it clear, but there are two discrepancies between what's in my stock '66 blackface amp (which is an AA764, according to the sticker inside the cabinet) and what is shown on the standard AA764 schematic that shows up everywhere on the web. The discrepancies are:

1. The first stage filter capacitor is 40µF, achieved by wiring two sections of the General Electric 20µF/20µF/20µF/20µF (450VDC) can in parallel. The AA764 schematic shows that the first stage filter is 20µF. I'm certain that the 40µF is not a modification. The factory-installed cloth wire is stripped back and soldered through two lugs on the cap, and the wire is dressed in such a way that it looks like it was done that way at the factory.
2. The schematic shows a 330pF cap between pins 5 and 8 of the power tube, but there's no such cap in my amp. My understanding is that the 330pF cap was necessary in the silverface amps because of other changes CBS made in the amp, which caused high-frequency parasitic oscillations, but the pre-CBS amps supposedly don't need it. In any case, mine doesn't have it, and it sounds OK without it.

So, in this pre-CBS AA764, someone at the factory decided that the first stage capacitance should be 40µF, for whatever reason. I've read elsewhere that some blackface ampmeisters believe that the 40µF filter creates too high an inrush current on the 5Y3 rectifier, and that makes sense to me. So I like the idea of getting a 20/20/20/20 can and using only three sections to start with. As you say, I can always add a section later if needed.

As for your cautionary note...thanks; I'm well aware of the potential for zapitude from charged capacitors, but it never hurts to be reminded.

 

Wow, printer2...thanks for that! Power Supply Designer looks like a handy tool, but it appears it's Windows only, so I snooped around and found a similar tool for Mac OS X. Cool!

Anyhow, I'm not going to worry about losing a few volts on the supply if it will reduce the hum. I'm assuming that cutting the amplitude of the ripple voltage almost in half will also cut the amplitude (volume...as in loudness) of the hum by a corresponding fraction.

This idea is sounding better and better the more I hear about it. And it gives me plenty to play with. YOW! I'm going to order the caps and a few resistors and have some fun.

Thanks again!

 

The "twisted pair+center tap" modification

Hi David:

By way of follow-up, I've done some preliminary research on the "twisted pair modification". Apparently, it requires creating a "virtual center tap" using a 100Ω resistor between the pilot lamp and chassis ground if there's no center tap on the power transformer. However, there is a yellow/green wire on my power transformer (although it has been clipped and capped with some electrical tape), so maybe that's the center tap. There's enough of it sticking out that I could connect to it.

I have more to learn about it, but I think I see how it works. Essentially, it's giving the heaters their own dedicated circuit path, rather than having the circuit completed through the chassis ground, which I imagine can pick up all kinds of noise. I guess it saved Fender a little bit of wire to use the chassis as a conductor for the heater circuit, but I don't see how it saved them any time; they still had to solder the power tube (pin 7) and preamp tubes (pin 9) to ground. It seems to me that it would have been just as easy to run the separate wire from the center tap, especially if it made the amp quieter. Go figure...

Interestingly, in the process of researching the twisted pair thang, I've also found some information about changing the stock two-wire (ungrounded) power cord to a three-wire (grounded) cord—something I've been meaning to do. Apparently, it can eliminate the need for the .047µF, 600V capacitor between the fuse block and the chassis. I snickered when I saw that the unofficial name for that capacitor is "the death cap".

 

Thanks, I'm still researching it, but it does sound like it should reduce the noise, and it seems that there are many who have done the mod who agree.

I'm definitely looking into it. Thanks again!