1965 Supro

[Doc Stitches]

What tube goes in socket between the 3 12AX7 sockets ? Directly under the foot switch and intensity.

Bottom tubes are 6L6GC, 6L6GC, and 5UA

It is a 1965 Supro with a 15 in. speaker.

I have yet to find a useful schematic.

NEVER MIND ... Figured it out ... duhhh: rolleyes:

Thanks
Doc

[nevetslab]

The only Supro Amp I've worked on was the Thunderbolt, which we still have in our rental inventory. After replacing a bad pair of 5881 power tubes, I found the amp was really noisy, and had plenty of gain. I ended up swapping out the input preamp tubes (12AX7) with a 12AY7, which calmed it down, still with plenty of gain. Input jacks on it were not grounding-types, so the amp would hum/buzz if nothing was plugged in and the volume turned up.

[mozz]

The model number may be painted on the wood by the lower chassis.

[Doc Stitches]

There is a small silver label with the numbers 1-22738 What is that ?

There is NO volume, hum, or hiss and the speaker and speaker wires are OK

Could bad main filters cause this ? There is also a large single filter under the bottom chassis which is bulged at both ends.

Just wonder what caps to check and if caps can cause absolutely NO volume ?

Doc

[mozz]

The silver tag is the serial number. If you keep plugging it in and there are problems, you may burn out the transformer. You won't find another. Filter caps, especially if they are bulging, need to be changed. Pull all the tubes and measure the HV.

[nevetslab]

In looking at the right photo, I'd guess the 12AX7 at the far left is your input tube, then next to it is the rest of the preamp stages, and teh tube on the other side if the I/O power supply connector is the driver tube for the power amp/power supply sitting down in the floor of the cabinet. What skill level are you? I see you've been posting on the forum quite a bit, so I assume you are technically skilled. Are you set up with a test bench and basic instruments.....multimeter, signal generator, scope, variac, etc? Do you have heater glow on the tubes when it's powered up? (both up in the preamp/driver stage above and below in the power amp/power supply? Do you have the means of measuring AC mains current/wattage, like thru a metered variac?

[Doc Stitches]

In looking at the right photo, I'd guess the 12AX7 at the far left is your input tube, then next to it is the rest of the preamp stages, and teh tube on the other side if the I/O power supply connector is the driver tube for the power amp/power supply sitting down in the floor of the cabinet. What skill level are you? I see you've been posting on the forum quite a bit, so I assume you are technically skilled. Are you set up with a test bench and basic instruments.....multimeter, signal generator, scope, variac, etc? Do you have heater glow on the tubes when it's powered up? (both up in the preamp/driver stage above and below in the power amp/power supply? Do you have the means of measuring AC mains current/wattage, like thru a metered variac?

You are correct on the far left tube being the input. My skill level ... I have bought, repaired and sold around 20+ tube amps ( mostly Fender, Marshall, Silvertone ) I use a Multimeter, and Variac. Test bench with basic items, but no signal generator or metered variac. All tubes glow, preamp and power amp.

[nevetslab]

With the speaker disconnected, (1 terminal open is fine), do you get 'nominal' DCR on the voice coil. Around 6 ohms if it's an 8 om speaker.

Assuming yes to that, we can deduce a number of things already. The I/O cable that plugs into the socket from the power amp/power supply carries heater voltage successfully, two wires (at least) carrying one side of the AC mains to the on/off switch. Beyond that we'd have to begin probing. I'm always a big fan of close inspection of the wiring of connectors like that. We don't yet know if the plate voltage to run the preamp and driver tubes is present. There may be just one DCV line, and it gets further filtered and dropped down from the one level up in the preamp chassis, as the driver tube susually runs on somewhat higher voltage than the preamp tubes do. There would be a pair of signal leads, no doubt twisted that come from the driver tube (push-pull to the power tubes), probably AC coupled, though no guarantee on that. And, there would be at last one ground wire.

Do all the connections into the mating plug appear to be solid? The other end probably goes thru a rubber grommet or strain relief, and into teh back end of the chassis down below.

We'll need to know that you have the plate voltage supply present up in the preamp chassis. I'm guessing the plates on the power tubes to be around 400VDC, and the plate voltage on the driver tube to be around 3--=350V, where the plate resistors connect. The plate voltage on the tube itelf will be much lower, lke around 180VDC. The Preamp tubes will be running at around 250V-300V where the plates connect, and supply voltages at the plates in the 150V area.

Let's see what you find up to this point.

What is the Tube Rectifier type? 5UA is not correct. 5U4 or 5U4BG would be, though physically, that looks more like a 5AR4 in size.

[Doc Stitches]

Thanks nevetslab

Rectifier is 5U4 ... I'll start getting the info tomorrow.

[mozz]

Doesn't look like a 5u4 in the picture. Have you measured the HV yet?

[Doc Stitches]

hey mozz ... you have a good eye my friend ! ... actually that was an old picture with a 5Y3

Will check the HV soon

2 heads are better that one, especially when one is mine

[Doc Stitches]

7.3 on the voice coil ... tried speaker on other amp and it sounds great ( lower volumes )

[Doc Stitches]

Wires all tight and connections into mating plug seem solid.

Inside the 8 pin plug from power supply chassis, my readings show several wires are connected ... I am assuming this is correct ?

Plates on power tubes is 490v

Preamp plate pin 1 is 423v
Preamp plate pin 6 is 202v

As I was checking pin 1 on preamp tube

1 ... bottom of multicap capacitor started smoking

2 ... then I heard a pop and saw the 450v cap had ruptured the + side

The amp was on for about one minute when this happened.

[nevetslab]

Well, we were sort of suspecting that power supply cap to be dodgy, based on the physical description early in this thread. Prior to the multi-section filter cap blowing, you were reading 423VDC on Pin 1 of one of the 12AX7's. That's much too high, but may be a result of a decoupling resistor having failed as well. Is there just one high voltage wire coming up from the power amp/power supply chassis down below? The voltage on Pin 6 seemed reasonable.

Otheres here on the forum have alternate sources for multi-section filter caps. I've usually been buying them thru Antique Electronics Supply in Arizona. when I need them. Staying witing that mechanical format makes them expensive. I've often changed the mechanical packaging to allow better quality radial or axial leaded single-section power supply caps, of where there are a number of sources. You'll have to assess that when you open up the power amp/supply chassis.

You'll need to assess the damage in the power amp/supply chassis for other parts besides the multi-cap. Once a solution is decided and parts are on order, you can still move forwards from there, to see where you've lost signal.

While I have the luxury of fine lab power supplies, allowing me to power up sections of an amp that I'm working on, after disconnecting the section that has gone down, awaiting parts, I can still make forward progress to see how the front end is working. If you're careful, you can also do this with another working tube amp that's similar enough. It's a whole lot easier with known accurate schematics of both units, so you can marry the front end of one, and substitute it for the front end of a different amp, allowing you to see what else needs attention I'd only do this if I had 100% confidence in what I"m doing.

At any rate, forward progress being made, in spite parts failure. That's to be expected.

[Doc Stitches]

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