DUKANE tube PA help

[getonemaker]

Hello Everyone!

I got a nice Dukane tube PA amp. I brought it up slowly with a variac and it worked for about 40 hrs. Now there is a loud hum and some scratachy noise (as if you had a loose speaker connection, which I do NOT have). Since I tried spares of all the tubes, I know it is not that. I did replace the C6 which is two paralell 20 mfd caps and did not see a change.

Please look at the attached schematics and give some advise what other part might need to be replaced.

Thanks!

 

[Wavebourn]

Do you see any smoke going from somewhere?

Check RC1 diode, C5 caps, and R1 resistor. You should have negative voltage on the cap. Remove output tubes before powering on to measure!!!

 

[getonemaker]

more info

I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!

 

[Wavebourn]

If pilot lamp is changing it's intensity and you hear scratching sound that means you need to check power plug, power cord, power switch, everything that goes from power plug to the primary of the power transformer, it seems to me you have some loose contact there.

 

[getonemaker]

more info

I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!

 

[Wavebourn]

I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!

Forget about it, I thought that the problem may be in bias. But looks like you have the problem with powering your power transformer. The pilot lamp is powered from the same transformer with the rest of the amp.

 

[DeathRex]

If you have a dim bulb tester you can tell if your power transformer is going bad.

 

[getonemaker]

more tests

Thanks for the replies. I have checked all the connections running to the power transformer and resoldered them, with no effect.

I also removed all the tubes and turned the unit on. No scratching sound, no hum and the pilot lamp stayed the same.

Looking at the amp, I suspect that C4 which connects to R23 and R26 might be the bad apple. Would that make sense?

It is 25 mfd 25 V. I have a 20 mfd 50 V and two 2.2 mfd 63 V caps. Would it be OK putting these in parallel to replace that?

Thanks again!

 

[Wavebourn]

Interesting... So, without tubes it does not cause that popping corn sound, and pilot lamp does not dim?

Try inserting tubes back, one by one. First put power tubes and check. And so on, until it starts popping.

 

[dgta]

Yes you can use 20 uF for C4, no need to be exact. Measure the voltage at that point and use a cap with equal or higher voltage rating. R23/26 provides a reference voltage to bias the filaments above ground. I doubt that's the problem, but simple enough to replace.

 

[getonemaker]

even more tests

OK, so there are two issues. Hum and scratching sound.

If I only have the rectifier and the power tubes, there is hum, but no scratching sound. I have tried with multiple tubes. Inserting just ONE 12au7 tube at either location gives no scratching sound, but if both are in, there is scratching noise. Again, this is independent of tubes, as I tried several of them. Did this shed any light?

Note: no 12ax7 tube was inserted.

Thanks again!

 

[dgta]

Yes.
The scratching sound is likely bad components or bad pots in the tone circuit. Check everything or bypass the tone section altogether.

 

[Wavebourn]

Do timbre controls change the timbre of the scratching sound?

 

[getonemaker]

identified the bad part, how to replace it?

As I was gently poking around I found that one of C14 was falling apart. I will replace it and hope that fixes the problem. Here is a question, however:

C14 is .003 mfd, 1600 V.

I guess it is non-polarized. I can't find a capacitor of that exact value.

Would

1. a 3.3 nF 1600 V be OK
2. a .003 mfd 630 V be OK

I will need to buy those.

I have however three .01 mfd 600 V capacitors at home. If I put them in series, would that be OK? I guess I would do this and if it fixes the problem I will eventually buy option 1 or 2 if one of those is OK.

Please let me know what you suggest I do.

Again, I am really thankful for all of yours time.