A new hum...

[Cagey]

Reading resistance across C47 would have surprised me more than not reading any. If nothing else, you have all your tube heaters in parallel across it. Figure an avg resistance of 20 ohms per heater, you'd only see a few ohms at best. Plus you've got part of the PS rectifier in there, etc. Zero or close to it sounds about tight, especially if it's developing the 6.3vdc it's supposed to. That's a pretty heavy winding in the power transformer - it'd burn if it was feeding a short.

 

[BigSteve22]

Reading resistance across C47 would have surprised me more than not reading any. If nothing else, you have all your tube heaters in parallel across it. Figure an avg resistance of 20 ohms per heater, you'd only see a few ohms at best. Plus you've got part of the PS rectifier in there, etc. Zero or close to it sounds about tight, especially if it's developing the 6.3vdc it's supposed to. That's a pretty heavy winding in the power transformer - it'd burn if it was feeding a short.

Ok, but then what explains the continuity test showing a short? The "Power On" voltage is reading DC, so the rectifier isn't shorted, and the indicator LED works. Only other path to ground I see is through the pre-amp heater common grounds at pin 9?  :icon_scratch:

 

[Cagey]

Right, but if you've got N heaters in parallel and each one is only 20 ohms or so, you've got 20/N minimum - whatever other nominal resistance is in that branch. It's gonna read very low, perhaps even zero, depending on your meter. Digtals are notorious for indicating continuity when it's really just close. Plus, caps can hold a nominal charge, which will throw the meter off, and so on.

But, it's still a good theory, so for the cost of a cap and whatever hardship you'd have installing it, it may be worth a shot. Especially if you don't have a 'scope to tell you for sure.

 

[mayfly]

I don't think that the heater cap (C47) is shorted.  Those heaters are very low impedance, and they are all in parallel with that cap.  If there is 6.3 V on it, it can't be shorted.

To confirm that, pull the tubes, which will remove the low impedance in parallel with that cap, and test it again.  Bet it's fine.

 

[BigSteve22]

Especially if you don't have a 'scope to tell you for sure.

No scope, radio! (Sorry, showing my age, and my poor taste in humor....)

But, it's still a good theory, so for the cost of a cap and whatever hardship you'd have installing it, it may be worth a shot.......

My thoughts exactly. Not many places to order a .01F, 16v electrolytic cap with axial leads, but Allied has a few. Order placed. Thanks, keep them thoughts and ideas coming!

I don't think that the heater cap (C47) is shorted.  Those heaters are very low impedance, and they are all in parallel with that cap.  If there is 6.3 V on it, it can't be shorted.

To confirm that, pull the tubes, which will remove the low impedance in parallel with that cap, and test it again.  Bet it's fine.

I'll do that tomorrow. Thanks! If it tests good, I'll just have a couple of more caps in my parts drawer.

 

[BigSteve22]

Update....
I tried as Trevor suggested, and he's absolutely correct: Removed Per-amps, short disappears. But, I did notice something I missed before: As I said the amp functions normally, but I noticed today that the frequency response(?) is altered. There is a definite "Tremolo" to the low "E", which is gone by the time you move up the scale to the low "A" and does not reappear. Does this suggest anything specific?  ???