Another Supro... model 88T

[EFK]

I just got this amp and it is completely untouched - nothing has ever been changed. Here is a schematic from a Gretsch 6159, which is the same amp (all Valco built). I tried to go through the circuit preliminarily to verify this, and it does seem to be the same amp EXCEPT mine has a 5U4 rectifier, not diodes. It does work, but there is a pulsating bleed through which I think is coming from the trem circuit. There's an underlying power hum of course, as even with new filter caps, the filtering doesn't seem all that robust (the entire preamp through PI is all relying on one common cap only and there is no choke), but there is a pulsating loud hum which varies when I turn the trem speed control even with nothing plugged in. The intensity control is switched, but even turned off I am getting this pulsing hum. Unfortunately, I'm not a trem guy and I frankly don't understand trem circuits very well. Looking at the schematic, it appears that V3 is only used for the trem and if I yank that tube, the hum goes away but so does all the sound on both channels! Anyone heard of anything like this previously? There's no scratchies on the pots at all, so I doesn't seem like there's DC bleeding through a bad cap into the pots, anyway. There are a few small electrolytics in the preamp that I haven't changed yet, but will do that. Is it possible that a bad cap in the trem circuit somewhere is letting signal bleed into the ground, maybe one of those caps around the speed control, and thus contaminating something that is affecting the signal in the channels even with the trem off? Any help much appreciated!!!!

[Gtr_tech]

Replace all the electrolytics and see where that gets you....

[EFK]

I just replaced the old MAllory 35/50 electrolytics in the preamp but that didn't make any difference either. Been looking though old threads regarding trem problems and it seems a common culprit are the 01/02/02 series of caps around the speed control, so I think I might change those out next. Most of the caps in here are all ceramic disk Z5U caps, although the big .05 are brown paper marked "Standard", I think mylar? I definitely think the caps around the speed control may be the culprit, especially given that the pulsing that is bleeding through will change speed if I rotate that knob. Looking through the old threads, I see that Enzo fairly repetitively states to replace these (as well as all the caps - not a proponent of retaining any vintage caps, I see!!! )

BTW, what the heck kind of tone control am I looking at off V1 (which is the bass channel)? This has full plate voltage on it. There's a slight difference however, in that the schematic shows the bottom of the pot and wiper linked together through a .05 to ground, while mine has only the wiper going to ground through the cap, while the bottom lug is not connected to anything at all. IS this affecting anything?

[Gtr_tech]

The ceramics are prob'ly ok. If any are the cardboard tubular types with wax on the ends....those gotta go as they get real leaky as they age.
They look like this:
http://www.antiqueradio.org/art/recap05.jpg
Replace *all* of them no matter where they are in the circuit and don't look back. I used to check them, but I just saw the trend that 80% of them were off the scale leaky. So I yank them without thinking twice. Any mylar/polyprop type is fine for replacing them with.

[EFK]

Yep, there's about 6 or 8 of those in there, although I think the ends are filled with epoxy, not wax. I had a bunch of the huge wax coated jobs in a GA20 from the early 50s, those were ALL over 100% out of spec (upwards). Working in these Supros is a pain in the a** as it's all ptp on tag strips and everything is draped over everything else every which way. Will get these cardboard tubes replaced next....

[EFK]

This has really got me tearing my hair out. I have replaced all of the caps in the amp, every one. Probably for the best, although at least I'd like to feel as though I'm getting somewhere. The schematic is a *little* bit off, although the trem circuit seems to be the same as best as I can follow everything around. The trem tube is actually V4, not V3. It also appears as though the switch on the switched intensity pot was never hooked up, so the only think turning the trem on and off is the footswitch jack. However, it doesn;t seem to make any difference whether I stick an open plug or a shorted plug in there. If I run jumpers off the footswitch lugs over to the pot switch, it doen't make any difference either. The jack does work, however (it's a shorting jack). Also, most of the bleed through is occurring on the bass channel, which is V1. V2 (standard channel, guitar/accordian) does not seem to have this problem hardly at all. Both speed and intensity pots measure out @ 500K, which is what they are supposed to be, so I'm assuming they are good. For the present, I can make the pulsing or throbbing hum go away if I yank the V4/trem tube as it isn't serving any other purpose, although this is changing the pin 3 cathode resistor value on V2 as they are tied together, so instead of around 3K it's going to be at about 1.5K (which is what the shared resistor is). I can live with it like this as I pretty much never use the trem, but I hate to quit and hate to leave a problem.

Since I've replaced all the caps, the amp has quieted down a bit and seems stronger, but this throbbing is still there. I will admit, you have to turn the volume (mostly channel 1 volume) up to around 3 oclock + to hear it, but by time you dime the amp it's fairly noticeable. I almost feel like this could be a lead dress issue, but no matter how much I move some of these wires around, nothing changes.

Any more ideas, anyone?

[BiBi]

I would try to decouple V1 from the other tubes. Like you said, having all the preamp tubes and the oscillator tube on one isn't going to do such a great job. For that matter, why not decouple all three from one another?
If you're not a stickler for keeping the vintage amps exactly as they were, then I'd also parallel V1 as the unused triode doesn't make much sense and it will sound a bit fuller

[EFK]

I generally like to leave the circuits *mostly* stock, especially with the weird stuff like these valco-made amps as they really don;t sound like anything else. Your idea of paralleling V1 is interesting, and this is accurate to the schematic as the 2nd half is not being used. Not sure what you mean by paralleling it, though - I understand cascading circuits, and I don't want to add extra stages as I'm not after gain. Do you mean running both halves of the triode in parallel somehow? (I know, duh, could it mean something else? But I'm just an amateur!) This won't affect phase will it? I'll do some searches and look into it as I'm sure you don't feel like spelling it all out for me.

I like the idea of decoupling things - I'll have to go through this rats nest and check to see what is tied together and what can be separated. Unless one of these trem pots is bad somehow and I'm overlooking it, at this point I don't see what else it can be other than something bleeding into something it shouldn't be via shared components. Thanks for taking the time to respond!

[52 Bill]

If you turn down the intensity control the throbbing still happens, right? And the throbbing is changeable with the speed control? Is the intensity pot actually grounding the input to the grid of the trem tube?

You could rewire the switch/footswitch to kill the oscillator instead of interrupting the oscillator output. This would allow the trem to work when you want it to and stop the throbbing when you didn't want it. Try lifting one end of the cathode cap on the oscillator circuit. Does this stop the oscillator from running?

[EFK]

Hi 52 Bill - this is interesting and I'm trying to digest it (as I mentioned, I don;t much understand trems, although I'm getting there...) When you mention to try lifting the cathode cap, do you mean pin 8 on V3 (on the shcematic I posted - on mine it's V4 but still all the same values)? I can lift pin 8 cathode cap but pin 3 cathode is tied to the cathode on V2, so I can;t lift that without affecting that channel operation.

You're saying the intensity pot *should* be grounding pin 2 on the oscillator tube? The pot is wired exactly as the schematic shows, so I assume it is being grounded? Or should this not have been wired this way?

[52 Bill]

The cathode cap of the oscillator section, pin 8 of V3 on the posted schematic.

I meant, did you check to see that the pot was actually grounding the grid. Is the pot working as it is supposed to be working not as it is wired?

[EFK]

Gotcha, I will try lifting the cathode cap.

As far as the intensity pot issue, I have to admit ignorance and tell you I don't understand *how* it is supposed to be working. It does measure @ 500K, the wiring follows the schematic and I checked the solder. Other than that, should it ground out the pin 2 grid when turned up or turned down? And, how do I check this?

[52 Bill]

As far as the intensity pot issue, I have to admit ignorance and tell you I don't understand *how* it is supposed to be working. It does measure @ 500K, the wiring follows the schematic and I checked the solder. Other than that, should it ground out the pin 2 grid when turned up or turned down? And, how do I check this?

You know how a pot works; a wiper runs across a carbon trace that varies in resistance, etc., etc.

All I asked was is the wiper actually reaching 0 ohms to the grounded side of the intensity pot?

[EFK]

The pot does ground out the center wiper to 0 ohms. Here's something interesting - the pot should be grounding out grid 2, the way it is wired, but even if I use a jumper and directly ground pin 2, the pulsing is still coming through the background. So, grounding grid 2 doesn't really seem to be the issue. However, if I directly ground out grid 7, beautiful silence - no pulsing at all. Shuts it right up. I don;t know what the footswitch jack is supposed to work on, but it's not a grounding function, it's being used as a connect/disconnect between components that don't go to ground. I'll bet if I rewire the footswitch jack to ground/not ground grid 7, I'll be set. Is this feasible?

[52 Bill]

So you have proven that the thumping and hum is being transmitted through circuit leakage or through the power supply. You have also found that stopping the oscillator will stop the noise.

When you ground out pin 7 you are killing the oscillator, which is what I was suggesting you do when I asked you to lift one end of the cathode cap in the oscillator section. Did you ever try that?