Hum issue in PAS Aikido

Wondering if anyone could give me a hand in tracking down a low level hum in a recent PAS Aikido build? It does increase with volume. How do I go about tracking down the culprit?

Thanks,
Mark

I have not tried other tubes yet, I will try that on Monday. I will try to post pictures tomorrow.

Thanks,

Mark

I think this will give you the pictures.


Not sure why this isn't working.

email me a photo at info@tubes4hifi.com

I have emailed the pictures.

Side note the hum seems to vary in how loud it is. Sometimes relatively quiet then suddenly it increases for 10-20 minutes then gets relatively quiet again. Does that likely indicate a tube issue?

Thanks,
Mark

Are all covers installed and screwed ?

Yes, all covers installed and screwed in place.

from your note I would suspect bad tubes

Swapped all 12au7's and it is the same. Hum variable to what volume is and at times louder than other times.

I have 7 days and still cannot post pictures.

did you try the filament mod that I sent you via email?

Yes I did. And it helped but did not get rid of it.

Someone suggested that I short the inputs to the preamp. In order to show it was the preamp or something else. I had disconnected them while working on this but that is not good enough. Shorting them stopped the hum. So evidently the hum is before the preamp.

Mark

I to do an update for the sake of others that may have a hum problem as well as for Roy's name sake.

After doing the shorting of the inputs to eliminate the preamp from the equation, I came to find out at least part of the hum problem was caused by a CAPS 2 music server I had put together. When I unplugged the USB from music server to DAC  the hum stopped. After a period of time I narrowed it down a bit more. I had hooked up a digital antenna to the CAPS server for recording some TV. (That is a no no but I did it anyway) When I disconnected the antenna the hum decreased but was still there on a low level. So I came to find out there is an isolation transformer that is used for cable TV. I bought one and hooked it up and all hum is gone!!

Hope this might help someone digging to find out how to solve a hum issue.

had that same problem with a Heathkit AA-151, hum would increase when the volume is increased. You will find the problem using a Heathkit signal tracer, hovering the prob over the negative side of the electrolytic capacitors.
If you don't have a signal tracer, I would start grounding the negative side of the electrolytic capacitors to chassis ground, using heavy stranded wire like number 14 or 16 gauge wire. Solder one end of the 14 gauge to the negative side of the cap or it's grounding lug and and the other to ground on the chassis.
If this unit was recapped by you or someone else that has replaced the cap cans with individual electrolytic capacitors, that is when hum will usually start to be a problem. I guarantee that this will solve your problem.
Remember that the original cap cans have a very short distance to ground and they are usually grounded well and hum is not at all a problem.
 
In some cases, adding other components increases the hum,is because the original path to ground in your amplifier is not good and the problem becomes much worse.
Inline Issolation transformers are perfect when connecting a laptop audiocard to your amplifier which can cause severe hum, this will totally issolate both ccomponents and the hum will totally dissapear and is dead quite.

May I ask what model signal tracer you used? Just curious.

Heathkit IT-12.