Supro 1970 RK - Hum - burnt resistor - no schematic

[spunko]

The complain was bad hum and low or no output. The story tells the amp started to fail shortly after purchase, the seller told to the customer it was just bad tubes, and send a pair of 6L6 instead of the 6v6 the amp uses. The client changed the tubes and the amp worked fine for very little, then it failed again.

When I got it, I noticed R18 was burned so badly it couldn't be read. I replaced it mirroring R22, with a 1k/1 watt resistor (no schematic), but I don't have 6v6s at hand, so I used a new pair of 6L6s I have and the amp worked fine, no hum, super clean, tube probes read good values.
I tested the amp for 2 or 3 times, no more than 10 minutes everytime, and everything worked perfect, so I orderded a pair of 6v6s.

This is the fun part. The new 6v6 arrived overseas, so I installed them and as soon as I hit the standby switch ON, R18 burnt again immediately. I was even using a bulb limiter.

I replaced R18 again, and tested with my new 6L6 and everything is fine, no smoke, no hum, clean sound.

Do I have bad luck and got a bad 6v6? Why R18 again?    coincidences

What would you suggest next?
- R18 has "bias" printed above, R22 does not.
- All the wires in the grey sleeve go to the master volume potentiometer

[acheld]

Maybe I'm being naive, but I'd call Absara Audio to request a schematic.  I think they are still in business.

[Latole]

I will never replace burned tubes with new tubes without checking the bias

[mresistor]

Maybe I'm being naive, but I'd call Absara Audio to request a schematic.  I think they are still in business.

Absara Audio and Pigtronix were acquired by D'Angelico in June 2020.  I read that the amp is 2x 6V6 running class A. 

[Latole]

R18 and R22 seem to be screen resitors by their value and if they are connected to leg 4 of the 6V6,

A defective tube may have damaged it.

Class A or AB, I always check the bias, it's so easy and it avoids problems

[spunko]

Could it be a bad tube or a bad connection on the socket?

When I replaced R18 and used it with my new 6L6 and the tube probes, everything seemed fine:
HT: 339VDC
V1: 39.5 mA
V2: 41.1 mA

The moment I installed the new 6v6, R18 fried again. I also noticed that one of the "new" tubes had another silver spot on its side, just like the one on the top of the tube (Flashed getters), but the other tube doesn't have it. And that was the tube installed on R18 socket, maybe an over heating mark? 

[Latole]

Silver spot is normal.
Some bad contact / solder on circuits ?

[spunko]

Silver spot is normal.
Some bad contact / solder on circuits ?

Thanks!! I was just thinking about that. Why R18 again?. Maybe bad contact between tube/socket, bad solder joints.

[shooter]

can't see past the wire;
does R18 say "bias" above it?
what value is it?

[spunko]

I got a reply from Supro, R18 is 1k 1/4W resistor. Yes, it says bias.

[Latole]

Bias ??

It is not a Class A amp ?
If it is Class AB amp , you bias is a way too hot ; It should be less than 28 ma

HT: 339VDC
V1: 39.5 mA
V2: 41.1 mA

[shooter]

without a schematic, it's pretty hard, like shooting at noises in the dark.


did the amp come with a 6L6 or a 6V6?

[spunko]

The amp comes with 6V6. The supro web page says it is class A. They didn't provide the schematic, just told me the value of the resistor.
I used 6L6 because I didn't have 6v6 at hand at that time. The amp is runing fine, but the fact that R18 burned out again makes me think of bad connection / solders as Latole said.

[Latole]

You have to do a sketch / schematic for the output tubes and bias circuits

I do that many time. If you don't, you'll (and us ) work like a blind man

[shooter]

where does the blue wire that connects to R18 come from?  A pic of the "other end" would maybe help.
I'm not seeing how a 1k R fits into "monitoring?" bias since a typical 6V6 cathode R is ~~~ 500 ohms

[mresistor]

a few pics of the whole amp and boards would be helpful..   to put the closeups in relation to the rest  and see how things are oriented..

[Latole]

I ask for the schematic.

It look it is a 2 circuits board amp , not only one ?

[Latole]

Received 1-2;

[Latole]

2 of 2

[ac427v]

Those schematics are golden. Thanks Latole!The power amp diagram seem to match the pictures except for the question of 6V6 or 6L6. There is agreement showing cathode bias with a 270 ohm 15 watt resistor. The ads for the amp claim 25 watts of Class A power from 6V6 tubes. I doubt that unless they snuck 4 power tubes in that amp. Did the ad folks just get the wrong tube?

Anyway, the OP can trace the wires back from the burnt resistor and verify that it isn't miswired or connected to other damaged components. Have at it!