Supro Help

[Appaloosa]

Hi I recently acquired an Old Supro amp. I'm trying to identify it and find a schematic if thats possible. the amp has a little gold badge on the back the says "V14791" is this the model? the amp has Two 6V6 output tubes, one 5Y3G rectifier, One 6J7, and one 6SC7 preamp tubes. the output tranny is located on the speaker basket.

The amp works with little hum buts seems to be overly distorted to me, and output volume seems to be weak. when you turn the volume up past 12 o'clock the amp makes a siren type tone. I assume I should replace filter/coupling caps and check resistor values and tubes but I need help identifies cap values.

thanks for any help! check the pics for identification thanks!

[trobbins]

Check out the photo albums in Magnatone-Valco : A forum for the discussion and appreciation of the unique amplifiers made by Magnatone and Valco., and post to that forum would be my suggestion for starters.

Ciao, Tim

[Appaloosa]

Thanks I will try that

[riz]

The filter cap values are printed on the cap can. You don't need a schematic to fix this. Although helpful, and sometimes reassuring, not necessary, especially for a relatively simple circuit like this. Takes just a few minutes to draw your own, and expect voltages to be high: 110v yesteryear vs. 120v modern mains. Then you can sort of guesstimate what the original voltage values were, if it matters. You'll want to check all those wax caps in there for leakage, too, they're known for it as they age. Cool amp!

[trobbins]

There are many things to check when wanting to use an old amp like the one shown. Not the least is to have the power cable replaced is a proper earthed 3-pin cord, and ensure the chassis earth, and all the mains side wiring is either confirmed as ok or re-insulated or replaced. Sometimes the mains switch can be 'marginal' with respecty to earth leakage, and trip earth leakage breakers (RCDs), especially if a MOV is not placed across the mains primary.

Ciao, Tim

[Appaloosa]

I think I have found the values of all the wax caps, but their is also a tubular looking cap that looks almost to be hollow, I can't read what the values are on it. The only thing I can make out is 200? is this Voltage? is this even a cap? I can take a better picture of it and locate what tube pins its connected to, if anyone can help identify it. thanks

[riz]

Don't take my word for it but that is probably a 200-ohm power resistor, likely used as a cathode resistor. Yes, all information would help. What tube, what pin, where it goes. trobbins added some good info too. 3-prong cord is a must, and often the switch and fuse should be moved over to the hot leg of the mains, if they are not there already.

[trobbins]

I agree with riz on the cathode resistor, as it appears to be from a 6V6 to chassis. Best to use an ohm-meter to check all resistor values - and replace any that are more than say 10-15% from value, as that may indicate a poor tolerance part, or a drifitng part - you may want to replace with a similar carbon comp part if it is one of the other resistors. Unfortunately the wax capacitors are almost invariably leaky.

[52 Bill]

I love those old Supros. This one is just like my very first amp I got from my dad. Same color chassis, but no silver paint on the case. I learned a lot tearing that old thing apart, making it "better".

A great sounding amp when it's set up well. Congrats!

[Appaloosa]

I'm trying to order some new caps for this Supro. What brand/type is recommended? I heard sprauge or Mallory is best for filter caps but I think I need a 40, 450v canister type mallory filter cap but I cant find this value in a single value cap. Can I buy a multi value canister type cap and just use the one 40 mfd lug? There is also one paper covered electrolytic? cap on the far left of my attached picture that has no legible writing on it, does anyone now what this might be? where is the best place to order caps? CE Dist? Antique Amp supply? thanks again.

[trobbins]

Aesthetically, I wouldn't touch the old can chassis mount caps - they look great. Pragmatically, under the chassis is a different matter. I personally would solder a small tag strip to the can cap terminal, and connect a replacement electrolytic plus the connection wiring to that tag strip. And given that you have 'lifted' the chassis ground, then I would attempt to implement a better star ground scheme - which should be relatively easy given the tag strips already in place. Try and get cap values of similar capacitance, but don't fret if say you use a 47uF cap or a 33uF cap for a nominal 40uF replacement - you shouldn't go outside that sort of range or you will get issues of rectifier rating and ripple current limits. Your modern replacement will be a large improvement on what the amp started with any way. My choice is to use any 'branded' cap which you can find a spec sheet for on the web, and with a 105C rating.

If you can identify what the unknown cap connects to, and the local circuitry, then should be able to advise - it may be a cathode bypass cap on an input stage.

Ciao, Tim

[Appaloosa]

Thanks for the help! thats a good idea about just leaving the old canister caps and using new ones underside. The cap in question is connected to pin #5 and #8 of the 6J7 preamp tube, then hooks to a .02 mfd cap, then back to pin #1 of the same 6J7 tube, then pin #1 is hooked back to pin #5 via a 3,300 ohm resistor. Can you tell which way the polarity goes on this cap? what value should I replace it with?

My other question is what is the third 1/4" female jack used for that is also in the same location? you can see it in the picture I have attached. you can also see the two input jacks in the background on the first pic.

I'm just a novice but what is involved with updating the power cord and ground circuit? add a 3 pole A.C. cord, connect green to chassis ground? must I re-rout other factory connections? i attached a picture of my current power input section with the stock 2 prone cord. thanks

[52 Bill]

The cap in question is connected to pin #5 and #8 of the 6J7 preamp tube, then hooks to a .02 mfd cap, then back to pin #1 of the same 6J7 tube, then pin #1 is hooked back to pin #5 via a 3,300 ohm resistor. Can you tell which way the polarity goes on this cap? what value should I replace it with?

I can't tell from the photo, but pin 8 is the cathode of the 6J7 tube. The other end of the cap seems to go to a terminal strip which in turn appears to be grounded through a wire to the longer terminal strip. If this is the case, I would guess that the value would be 25uf@25v and the positive end should go to the tube cathode and the negative to ground.

My other question is what is the third 1/4" female jack used for that is also in the same location? you can see it in the picture I have attached. you can also see the two input jacks in the background on the first pic.

This jack has been added to the amp and is not original to the circuit. This includes the brown rectangular cap that connects to the volume pot. As to what it does, it appears to be an output from the preamp stage, before the volume control.

I'm just a novice but what is involved with updating the power cord and ground circuit? add a 3 pole A.C. cord, connect green to chassis ground? must I re-rout other factory connections? i attached a picture of my current power input section with the stock 2 prone cord. thanks

To bring the amp up to modern standards, you will need to rewire the existing circuitry. The ground connection (green wire) must be connected to the metal chassis. The neutral connection (white wire) will go directly to one end of the transformer primary winding. And the hot connection (black wire) will go to the fuse holder then to the power switch and then to the other end of the primary winding.

I will dig through my files and see if I have a copy of the schematic for this amp.

[riz]

Make sure the "hot" side of the mains goes to the tip of the fuse holder, not the side. That way there is reduced risk of shock when replacing fuses. It's backwards now. I have been gluing in caps with hot glue when there is not a more practical solution. Sometimes there is a good place to mount one or two, and the rest can be stacked on with glue. Other times glue to the chassis. Works pretty well, and I've been using cheap Nichicon caps-- they work fine and they're SMALL, making them well-suited to this kind of work. Boy, there's a bunch of caps in there that might be living on the edge! When you update the power cord, make sure to clip out the "death cap" to the chassis from one side of the mains, if it's in there still.

[Appaloosa]

thanks again for the tips, I'll try and get some time tonight to work on the power cord/ground. I still have to order the caps and check all resistors. What the scoop with "the Death Cap"? if its their do I simply remove it? what is its purpose? thanks!