[Ericus Rex]

No bottom cover?  Hummm!

I wonder if that possibly is the issue.  I can note that when we bench test an old or rebuild Dyna PAS-3 preamp without the bottom cover installed we then see excess hum from its outputs with the scope.  This hum goes away as soon as the metal bottom cover is installed.

Where is the amp located in relation to near by AC power cords or other equipment that might be radiating electromagnetic fields into the unit, made much easier to happen due to the lack of a bottom cover?

Meanwhile, the use of shorting plugs on the inputs will help tell us if the hum is being picked up at the inputs or being generated internally.

Regards,

Frank Van Alstine

But wouldn't that kind of hum be in both channels?  I thought the OP's hum was channel specific.

[fz1jmp]

No bottom cover?  Hummm!

I wonder if that possibly is the issue.  I can note that when we bench test an old or rebuild Dyna PAS-3 preamp without the bottom cover installed we then see excess hum from its outputs with the scope.  This hum goes away as soon as the metal bottom cover is installed.

Where is the amp located in relation to near by AC power cords or other equipment that might be radiating electromagnetic fields into the unit, made much easier to happen due to the lack of a bottom cover?

Meanwhile, the use of shorting plugs on the inputs will help tell us if the hum is being picked up at the inputs or being generated internally.

Regards,

Frank Van Alstine

Frank;

It did not have a bottom cover. I wrote to the prior owner and he bought it from the owner after John and there was no cover.

I had the amp on, very college days, 2 cinder blocks. I did put a nice wood board under it. I'm looking for a suitable stand, but still have not hit the one I want.

JakeJ mentioned I should just DIY the RCA shorts, I will do that this weekend, I saw some decent plans.

I've received a response email from one of the repair sites I wrote to, SG Custom Sound. They were recommended and he thought it sounded like a power supply issue, but would of course have to look at it. I can certainly double box it like its irreplaceable glass.

So here is where the dilemma comes in; Shipping is such a violent process, so many things can happen. I loathe shipping every day items, and now I have this amp, that to me is precious. The shop is in PA., but its roughly 4 hours away. Not crazy to drive, but....

So, do I just live with it? I can. I'm listening now, Leonard Cohen, and can't hear it at all, the music is not loud at all. I know no one can predict the future, but, if it does not get worse, and I can live with it, do you think I would be OK and not have a catastrophic failure? And please I understand its only opinion, but I value people with experience in this area. YOu have computer questions, I can probably help, tube amp, I'm asking you all!

[avahifi]

I doubt if it is a power supply issue since it is a single channel issue.  Try setting the amp on a metal cookie baking pan as a bottom shield and see if that helps. Make sure no exposed wiring or components can touch the metal surface.

Continuing to run it as is with the low level hum?  I can't tell if that is a good idea or not.  The hum may simply a minor layout or grounding layout issue, or it could be something less benign.  Hard to tell without having the amp on the test bench.

A inexpensive temperature probe would be useful to aim at identical left and right channel parts on the circuit side to see if anything is getting unusually hot.  You can get one for under $20 from Harbor Freight tools. We use this tool all the time.

What kind of speaker cables do you use?  If they are simple two conductor cables with side by side conductors (recommended) then twisting them about three twists per foot by chucking them in an electric drill will help eliminate RFI pickup in them that can cause hum.

Finally see if other owners of this amp have any hum issues?  It simply might be a design issue not unique to your unit.

With this I am pretty much out of do it yourself helpful ideas.  Good luck finding and eliminating the issue.

Frank Van Alstine

[fz1jmp]

I doubt if it is a power supply issue since it is a single channel issue.  Try setting the amp on a metal cookie baking pan as a bottom shield and see if that helps. Make sure no exposed wiring or components can touch the metal surface.

Continuing to run it as is with the low level hum?  I can't tell if that is a good idea or not.  The hum may simply a minor layout or grounding layout issue, or it could be something less benign.  Hard to tell without having the amp on the test bench.

A inexpensive temperature probe would be useful to aim at identical left and right channel parts on the circuit side to see if anything is getting unusually hot.  You can get one for under $20 from Harbor Freight tools. We use this tool all the time.

What kind of speaker cables do you use?  If they are simple two conductor cables with side by side conductors (recommended) then twisting them about three twists per foot by chucking them in an electric drill will help eliminate RFI pickup in them that can cause hum.

Finally see if other owners of this amp have any hum issues?  It simply might be a design issue not unique to your unit.

With this I am pretty much out of do it yourself helpful ideas.  Good luck finding and eliminating the issue.

Frank Van Alstine

Frank, thank you again for taking the time and effort to help. I will try your suggestions and of course let everyone know. I poured through other hum issues and learned quite a bit.

I have Acoustic Zen Satori speaker cable, its a nice sounding wrapped cable.

I never thought of asking the t=other users about them, I know where they "hang out".

I will vigilantly monitor and see if any change in hum. Today I thought what if the cinder blocks were crooked, then the amp may have stress from being crooked. Well I put a board over the blocks and I think the hum lowered slightly. When I used the Radio Shack meter, it was -2 as opposed to 0, so maybe it was an improvement.

Thank you all again!!!!

[fz1jmp]

Hey All;

I thought it somewhat fitting to post a bit of a follow-up. The hum is inaudible from more then a foot away and the amp sounds fantastic, so I decided to leave it be. But, I went back to another SET - centric forum and read through what the builder of my amp, John Hogan, wrote. LOL, a few comments like this "my opinion is the hum actually enhances the sweetness and richness of the music.. just my opinion...  john" made me feel completely at ease.

John passed away a few years ago, but I'm guessing if I asked him, I know what he would say. So, pardon me while I go back to listening to Warren Haynes in my living room playing Stella Blue from "Live from Bonnaroo".

Thanks all!
Jack

[Ericus Rex]

A little hum within a foot of the speaker should be considered normal for an older tube amp.  Don't sweat it.