Vibro Champ capacitor questions

[Vito]

I hadn't used my 1966 Viibro Champ for several years, and when I fired it up recently, it showed these symptoms:

1. Low volume (very weak signal), high distortion – It's not pleasant distortion...more like what you get when there's a dirty volume pot or an intermittent connection and the sound is barely getting through. I have to crank the amp up to 8 to get any decent volume, but it's still badly distorted. The amp should be taking my head off at 8. Yes, I've cleaned all the pots, so that's not the problem. I've also replaced all the tubes, so it's not that either. The amp is 46 years old, so it wouldn't surprise me if the caps were a little weary. Also, when I crank the bass control up to 10, the low end actually decreases, suggesting that there's a problem in the tone circuit caps.
   
2. Tremolo doesn't work - Even with the intensity on 10, there's barely any tremolo. (Yeah...I know Fender calls it "vibrato", but that doesn't change the fact that it's tremolo — a periodic fluctuation in volume. Vibrato is a fluctuation in pitch.) Here again, it can't be the tube; the vibrato circuit has a new 12AX7. So, it seems likely that capacitors might be at fault here.
   
3. Constant low-level 120Hz hum – Actually, this has always been a problem in this amp. It's really not loud enough to be annoying when playing, but it's annoying for close-mic'd recording. At first, I thought that the filter ("can") capacitor might be the problem, but after reading more about it here it appears that it might just be a result of the amp's (single-ended rather than push-pull) power supply design. Anyhow, if there's any way to reduce the hum or eliminate it altogether with new or different capacitors, I'd llke to do it, since I'm going to replace all the other caps anyway. I'm handy with a soldering iron and wiring in general, and I have no objection to bypassing the multi-section filter ("can") capacitor with suitably high-voltage discrete capacitors, if that would help.

I've been searching for an online source for capacitors, and I've been surprised to find that there are only a couple of places that have everything I need in stock. Prices vary widely; in some cases it might be due to quality differences, but in most cases there's no apparent reason for the price differences. It's a little disconcerting. Are the less expensive ones junk? Are the more expensive ones overpriced? Do some affect the sound quality more critically than others? Beats me...

One thing I've noticed about the can capacitor: The AA764 schematic (the CBS version) shows three 20 µF sections of the filter cap, but my amp has a GE can with four 20 µF sections, two of which are wired in parallel (to make 40 µF) and connected to the +355V supply. So the schematic is "wrong" compared to the factory wiring on my (pre-CBS) '66 Vibro champ, which evidently requires a 40 µF (450V) cap, plus two additional 20 µF (450V) caps for the high voltage supply.

Anyhow, all of the Vibro Champ capacitor kits I've seen online are wrong...at least, they don't have capacitors that exactly match the values in my amp. That's no sweat; I can piece together my own kit for all the small caps. (MojoTone has them; Mouser has everything but the can caps...incredibly, they don't carry multi-section capacitors!) But my main uncertainty is about the can capacitor. I can get a JJ can cap (40/20/20/20, 500V), but apparently it's fat, and wont fit the existing hole. (Yeah...I know I can use a clamp.) The corresponding CE can is smaller and should fit without modifying the chassis, but it costs more than twice as much. Apart from the fact that I don't know whether more than twice the cost means more than twice the quality (it probably doesn't), I'm not even sure whether replacing the can cap is going to provide any benefit, especially as regards reducing the hum. I'd appreciate any guidance anyone can provide.

Thanks!

[leydenjar]

If your can cap is original it needs to go. It's more than likely why your amp is humming. That hum is ac you're probably hearing.

I use ce can caps and sometimes weber. Or just put individual caps in the chassis carefully.

Champs are difficult to screw up. I'd use a 40uF on that first node and 20uf on the subsequent.

You should replace your bypass caps as well if they're 46 years old.

I don't think I can address your questions concerning cap prices and quality other than to say passive electronic component manufacturing is a world I know a little about but not enough to comment on.

You'll need a large iron to get the can cap off of the chassis and to properly install a new one so you may just want to go the individual cap route.

[stokes]

Its advantageous to use individual caps in place of that can.The preamp cap shouldnt be grounded by the higher current power and screen taps.Another trick to quieting these amps is to lift the ground reference on the 6.3v heaters and "ground" it on the power tubes cathode.That will put your ground potential on the heaters at approx. 25vdc,or whatever the volts on the cathode are.This keeps the hum from the heaters away from your signal ground and out of the signal chain.I like Sprague and F&T brand caps,I have used them for many years and have never been disappointed.I would suggest just leaving the can that is in the amp where it is for "vintage appearance" and just disconnect it.Find a place to fit your new caps inside the chassis and ground the preamp 20uf down by the preamp grounds and the main and screen tap by the PT and power tube cathode.FWIW 20uf is plenty of filtering for these amps,all the schems I've seen show 20uf,never seen a schem with 40uf.You can use either,if you like,but there wont be much difference between the 20 and 40.Although the 5Y3 can take the 40uf okay,its design max is rated at 20uf.

[Vito]

Thanks for your replies, Leydenjar and stokes.

...FWIW 20uf is plenty of filtering for these amps,all the schems I've seen show 20uf,never seen a schem with 40uf.You can use either,if you like,but there wont be much difference between the 20 and 40.Although the 5Y3 can take the 40uf okay,its design max is rated at 20uf.

I've never seen a schematic with 40µF for the first node either. Mine says 20µF on both the schematic and the layout...but then, it's a CBS schematic, and I'm not sure whether the original schematic for my blackface amp might have shown a different value.

The filter can cap is definitely original, and the amp has never been modified. The original yellow cloth-covered wire has been stripped back enough to pass through two terminals of the filter cap, and is soldered to both. The GE can is a 20µF/20µF/20µF/20µF (450VDC) multi-section cap, so two 20µF sections wired in parallel definitely makes it a 40µF cap. That's factory wiring. I'm not opposed to modding the amp for better or quieter performance, but there's no question that this particular amp (date code PI, which makes it 1966, September) is effectively using 40µF/20µF/20µF for filtering, irrespective of what the schematic says.

For my part, I don't know enough to say one way or the other whether 20µF or 40µF would be better. However, it stands to reason that 40µF can hold twice the charge of a 20µF cap, so it can deliver twice the current. Whether that's desirable is another matter, on which I'm not qualified to speak.

...You'll need a large iron to get the can cap off of the chassis and to properly install a new one so you may just want to go the individual cap route.

Its advantageous to use individual caps in place of that can...I would suggest just leaving the can that is in the amp where it is for "vintage appearance" and just disconnect it.Find a place to fit your new caps inside the chassis and ground the preamp 20uf down by the preamp grounds and the main and screen tap by the PT and power tube cathode.

Unsoldering the original can wouldn’t be a problem, but I’m inclined to use individual caps and disconnect the original can, leaving it in place. I’ll save all the original parts. In that way, if I ever decide to sell the amp, the new owner can make his own choice. I was thinking of installing a small turret board or breadboard and mounting the individual caps on that, but I’m not sure there’s enough room. I’ll figure out something.

F&T makes a 22µF, 500V axial electrolytic that seems to be highly rated, so I’ll probably go with four of those (with two of them in parallel for the first power supply node).

Some things I’ve read on this forum and elsewhere lead me to suspect that even changing the filter caps might not significantly reduce the doggone 120Hz hum. I guess it’s the nature of the power supply itself, but it seems to me that there ought to be a way to filter out that 120 Hz ripple. Maybe it’s time for more research.

Thanks again for your replies!

[Jazz P Bass]

Have you measured the Vac ripple on the power supply nodes?
Putting in new caps to fix a none existent problem is not the way to repair something.
If the caps are indeed 'bad' it will show up as a gross ripple on the nodes or worse, a drastic Vdc drop of the node.
And yes, I agree that a bad cap can make the amp hum.
Troubleshoot the problem.

[Vito]

Have you measured the Vac ripple on the power supply nodes?
Putting in new caps to fix a none existent problem is not the way to repair something.
If the caps are indeed 'bad' it will show up as a gross ripple on the nodes or worse, a drastic Vdc drop of the node.
And yes, I agree that a bad cap can make the amp hum.
Troubleshoot the problem.

I agree that throwing parts at the problem is not an ideal strategy. However, I do not have a "non-existent problem". I have three very specific problems, which I described in detail in my original post in this thread. I'm in the process of determining the causes of those problems.

The tubes in the amp were over 20 years old. It was time to change them anyway. So I did that first, in case any of the problems were caused by the weary tubes. As it turned out, that wasn't the case.

There is no doubt in my mind that there are capacitor problems. The problem with the tremolo, the distortion and weak signal, all point to it. I have had 46 years of service from the existing caps. It is time for new ones.

I don't believe that the filter caps are "bad", actually...or at least, I don't think that simply replacing all the caps is going to solve all the problems. I think it's likely that there are intrinsic problems with the amp's design. For one thing, I have been researching the specifications of the 5Y3GT rectifier, and the factory-wired 40µF capacitor in the first stage of the power supply is just wrong. The 5Y3 specs call for a capacitance of 20µF. I suspect that higher capacitance might do more good in the second stage (for the preamp tube). I also think the first stage voltage is higher than it needs to be.

Nevertheless, I do think it's likely that the filter caps are weary. The ripple voltage is about 9 VAC. I believe that new caps will help. But I'm looking into other potential causes of noise...for example, the heater circuit, which uses the chassis to complete the signal path. Why? To save a little bit of wire? There is an unused center tap on the stock power transformer, cut back and capped with electrical tape. With a bit of extra wire, I can run a dedicated circuit for the heaters.

My initial questions reflected the honest ignorance of one who has little experience and has much to learn. I knew that going in. I'm not afraid of ignorance. I acknowledge it, but I don't stay in it.

Thanks for your comments.

[stokes]

Nothing about these amps power supply that makes them more prone to 120 cycle hum at the output.Good,properly working caps will sufficiently reduce 120 cycle hum.The fact that the outputs are single ended causes them to be noisier than a PP output.Elevating the heater supply ground reference as I described earlier,whether it be a CT or a pair of 100 ohm resistors, goes a long way to quiet things in this regard.Grounding the preamp cap away from those higher current parts of the circuit is also helpful.Many older BF amps had the screen cap grounded with the pre's and causes problems.Its more of a problem in higher power amps,but if you are taking steps to eliminate noise,it makes sense to take this step as well.

[stokes]

Apparently,I was typing while you posted your last post.If your heaters dont have the "twisted pair" arrangement,it will help a lot to redo that.You will need to either connect the CT,or you can add a 100 ohm resistor from each side to ground,better to connect this or the CT to the cathode of the power tube,as I've already pointed out.As for the tremolo,I would suggest just replacing all thre caps instead of removing/replacing one at a time till you find the bad one.It'll save you time and the cost of these caps is insignificant.These amps can be made dead quiet,taking the steps I've described,I've done many of them over the years.

[Jazz P Bass]

I am not against a total recap.
I try to isolate issues one at a time.
Clipping a known good cap across an unknown one is a good strategy.
If the ripple drops, that cap is bad.
As to the coupling caps, they must block Vdc.
If they 'leak' they are bad.
9Vac ripple on the power supply is bad.

[Chuck H]

If the ripple drops, that cap is bad.

MMMkay

If they 'leak' they are bad.

MMMkay

9Vac ripple on the power supply is bad.

MMMkay

[g1]

9Vac ripple on the power supply is bad.

Not sure where in the power supply you are referring to. At the first node I would think the ripple could be worse than that with good caps. For the preamp stages it would be unacceptable.

[stokes]

Wouldnt want to see much more than 5vac at the first tap.

[Vito]

Update after Vibro Champ recap job

Thanks to all who have replied thus far.

I have changed all the caps in the amp. First I replaced the filter "can" cap with a CE 20µF/20µF/20µF/20µF, 475VDC multi-section cap. The new cap did not solve the 120 Hz hum problem (more on that below).

I did replace the other caps one by one as planned. After replacing each one, I plugged in a guitar, powered up the amp, and listened to the effect of each change before replacing the next cap. Yes, it took more time to do it that way, but I wanted to hear for myself what were the effects of each new cap in solving the problems listed in my original post.

I started at the preamp and worked my way downstream, replacing the caps in the trem section last. There was an immediate improvement in the volume after I replaced the tone caps (.1µF and .047µF), although there was still significant distortion ("dirt"). There was a big improvement when I replaced the 10µF cap on pin 8 of the preamp tube, and an even bigger improvement when I replaced the .02µF cap that couples the preamp to the power tube. The original was a ceramic disk, and I replaced it with a 600V orange drop. Between those two caps, that brought back most of the volume, but there was still some dirt in the signal.

Replacing the 25µF cap on pin 3 of the 12AX7 in the trem circuit restored the tremolo to its former glory. Replacing the other caps in the trem circuit cleaned up some of the dirt.

The final improvement in cleaning up the distortion came with replacing the 25µF cap that connects in parallel with the 470-ohm resistor to chassis ground on pin 8 of the power tube. That completely eliminated all the dirt. When I got old the cap off, I could see that it had suffered some heat damage from the 470-ohm resistor, which it was touching. I felt that resistor while the amp was on and it gets very hot. In fact, the yellow,violet,brown color coding is so badly discolored (presumably from the heat) that it's not possible to identify it by color reliably. I intend to replace that with a precision 470-ohm, 2 watt resistor. Anyhow, when I installed the new cap, I stood it above the circuit board and well away from the resistor. The amp sounds just like its old self now—plenty of volume, and all of the crackly, dirty distortion is gone.

Unfortunately, "its old self" includes the 120 Hz hum. I tried using the filter can cap with one section (20µF) and two sections in parallel (40µF...which was the way the original cap was wired). There's no question that the 40µF filter reduced the 120 Hz hum considerably relative to the 20µF filter. The new cap has quieted the hum relative to what it was with the old filter cap, but it's still audible.

I'm convinced that the hum is an effect of the power supply design. It's not 60 Hz hum, and not related to anything in the signal path. The hum is there regardless of the setting on the volume control, even when nothing is plugged into the input. I don't have an oscilloscope, but I can find the pitch of the hum on my guitar, and it's approximately B2 [i.e., 123.47 Hz — 1/2 step below C3, which is an octave below middle C (C4)]. That makes it 120 Hz hum, so it has to be ripple voltage coming off the rectifier tube.

Here are the ripple voltages I measured, at 20µF and 40µF for the three taps off the filter can capacitor, measuring from each tap to chassis ground:

Tap	20µF filter	40µF filter
1st stage	4 VAC	2 VAC
2nd stage	0.5 VAC	0.3 VAC
3rd stage	0.1 VAC	0.09 VAC

By way of definition, the 1st stage tap is connected to one side of the output transformer primary, the 2nd stage tap is connected to pin 4 of the power tube, and the 3rd stage tap is connected to the preamp tube's plates (pins 4 and 6) through 100K-ohm resistors.

I checked voltages in various places around the amp relative to the voltages shown on the schematic. Everything is higher than the schematic shows. Here are some comparisons (line voltage = 120.6 VAC, 60 Hz):

Location	Schematic	Measured
Rectifier cathode	315 VAC	355 VAC
Rectifier anode (1st stage filter cap tap)	355 VAC	430 VDC
Power tube anode	342 VDC	415 VDC
Power tube cathode	21 VDC	30 VDC
2nd stage filter can tap	340 VDC	421 VDC
3rd stage filter can tap	320 VDC	403 VDC
Heater voltage	6.3 VAC	6.93 VAC

It has always seemed to me that the amp runs very hot. If the AA764 schematic is supposed to be correct, all the voltages on my amp are way out of spec (too high). I can't do much about the power transformer, but it seems to me that it wouldn't hurt anything (and it might help a great deal) to feed one section of the filter can cap with the rectifier output through a resistor to drop the voltage (effectively creating an RC filter), and then feed the 1st stage filter can tap with that RC filter's output through another resistor to add a second RC filter.

The difference between spec and measured rectifier anode voltage is 75 VDC. If each RC filter drops the voltage by about 37 VDC, that should bring the 1st stage tap voltage into spec. I'll have to measure the rectifier output current, but the maximum plate current is 300 mA for the 5Y3GT Sovtek I'm using (...although I'm not sure I trust Sovtek's data sheet, and in any case the current could be higher in this amp). It's higher for other tubes (e.g., 440 mA for a GE 5Y3GT). Using those figures as a design spec (for now), I would need a 123-ohm, 3.3 watt resistor to get a 37 volt drop across the first RC filter at 300 mA, and an 84-ohm, 7.2 watt resistor to get a 37 volt drop at 440 mA.

So, I figure a 100-ohm, 10 watt resistor for each RC filter should do the trick. (V = IR; P = I²R)

The cutoff frequency for a 100-ohm, 20µF RC filter is about 80 Hz. I ran a Bode diagram for the circuit and it shows that the amplitude should be about 10 dB down at 120 Hz, so two such RC filters in a row should quiet the hum considerably. I could reduce the cutoff frequency by lowering the R value, but then I wouldn't get the voltage drop I want.

Anyhow, that's what I figure I should try, but I'm new at this, so if anyone spots any errors, I wouldn't be surprised. Please straighten me out.

Thanks!

[echuta13]

When CBS took over they made a couple of modifications to the VibroChamp. One was to use a 40/20/20 cap-can. This is the schematic for the later variants.

http://www.thevintagesound.com/ffg/s...b764_schem.gif

[Chuck H]

Wa there ever a pre CBS Vibro Champ???