Help with old Mystery Supro

[Deric]

Working on a cool little Supro.  No idea of the model #.  Serial number tag says "1247".  PT fires up.  Rectifier and first filter cap are working.  Amp uses a "fused" mains plug with a fuse on each leg of the AC.  Looking for suggestions on fuse size for testing.  If everything checks I out plan on re-capping and adding a 3-prong cord and proper fuse.


Closest schematic I can find is a National-Dobro 36-6.  It uses a fused plug but does not list the fuse used.  Multi-section cap in this amp is the HUGE cardboard box.  Box has no markings on it although I have not pulled it out yet.  36-6 uses 8u, 8u and 2u.  Any suggestions?


Pics and schematic (my trace of this amp) attached.


Any suggestions/info appreciated.


Thanks!

[Deric]

More...

[sluckey]

I'd try 1 amp fuses. Do you have a larger size schematic?

[Deric]

Do you have a larger size schematic?

Doh!


Updated.

[eleventeen]

Wow, that may be the oldest amp I've seen posted here.


1 amp or 1.5 amp fuse is probably right but with semi-sorta leaky caps, etc; you're probably gonna blow some number of those while you shake the thing down.

[PRR]

It is a DeLuxe with pre-6V6 tubes and circuit.

'42 is a 6-pin 6F6 which is what the 6V6 displaced. '80 _is_ a 4-pin 5Y3. 6N7 has no exact descendant.

It matters if you are in 120V land or 240V land. You don't say. Supros are surely uncommon outside North America, but we get all types here.

Fuse like a DeLuxe.

In any case, Use a 100 Watt Incandescent Lamp Limiter!! As said, this will be an ongoing flame-test.

The "??" cathode cap must be very large, over 50uFd. And the 2nd grid resistor can not be 10K, is likely nearer 1Meg. Then the "??" coupling cap should be a bit small, 0.005uFd. If these two R-C networks are not staggered, V1 is an oscillator.

[Deric]

Thanks for the replies!!!  I am indeed in N. America so 120v and I am using a bulb limiter for all testing.


I'll double check the 2nd grid resistor when I get home - thanks for the heads up.  You can see the large cathode cap and smaller coupling cap in this picture.  I can't move them around enough to read any values.  If they are leaky and I have to change them they should be easier to read when removed from the amp.  If I manage to verify them I'll update the schematic.

[PRR]

> If they are leaky

My 2 cents says all the electrolytics are useless, 99.9% of the wax caps are bad now or shortly after you put power on it, and the resistors may be drifted 678% off spec. ('42 cathode R may be wirewound and still OK.)

Sure is tight in there!!

I think the basic circuit is still valid. The lack of a Volume knob is odd but probably not a big deal. Your axes have knobs and this is not a high-gain amp. You may need a booster in front to OVER-drive it. If you do, the old speaker will soon shatter. So good parts of reasonable values. Start with the power supply because it has the most smoke (if it isn't already toast).

As a fine-trim, a 2Meg first grid to ground may be nice if a leaky pedal is used in front. Supro relied on "all sources" being passive conductors to hold 1st grid bias, and this is no longer for-sure.

[Deric]

Double checked the 2nd grid resistor and sure looks like 10k to me (brown, black, orange) and measures 11.8k with my meter....

[sluckey]

Wow! Been a long time since I've seen that "body-end-stripe" coding system. Thanks for the memory.

[PRR]

> sure looks like 10k to me

Yeah, I see.

But it makes V1a have low-low gain. V1b hasn't enough gain to get to a happy point, even for days when guitarists were background players.

Yet it is no recent hack.

Maybe that's all the gain they thought you should have? Maybe a factory screw-up, the original owner didn't know the amp was flawed, and it never got fixed?

Leave it for now. If it winds up otherwise OK but needs 10X the strum force to get a gutless sound, try 100K or 1meg there. (If you can, tuck it in series with the 10K, it is too pretty to remove.)