unloaded voltage readings

[mesherm]

What happens to the hum when you short the inputs?
Worse, better, no change?
You have a lot of stuff packed in that chassis, chasing down the source might be difficult.
Have you compared UL mode with Triode mode yet?
Rolled any 6CA7s or KT77s in place of the 6550s?

[TomMcNally]

I would have the same suggestion as Mike - try it with the inputs shorted. If you have anything connected to a computer, or anything that connects to cable TV in any way shape or form, you could have a ground loop. Often a preamp with a three wire power cord can even introduce hum. Isolate it and let us know.

[erichayes]

Hi All,

Jeff, you need to identify the type of hum you have or you'll drive yourself nuts trying to track it down.

If the hum is smooth and predominantly one frequency (120 ~, very close to B flat an octave below middle C), it's power supply hum and can be caused by insufficient filtering--for whatever reason--or excessive current draw due to insufficient bias voltage or defective tubes. Even though it's in both channels, it only takes one tube to drag a common power supply down.

If the hum has a 60 ~ fundamental frequency with lots of harmonics, it's audio hum, and could be caused by a ground loop, heater-to-cathode leakage in the first audio tube, unbalanced heater to ground, blah blah.

Do what the guys suggested and short the inputs. Listen for what type of hum you have, do a little poking and prodding, and let us know what you find.

[jeffdavison]

Tried the shorting plugs... nada effect.
lifted the ground from the wall.... nada effect.

it was definatley hum from a ground loop, internal to the amp.
Played around with the grounding scheme and a suggested tweek from curcio's forum.

My amp is really a true dual mono amp. each channel has it's own star ground on each underside of the chassis.

The driver board was supplied with a ground "buss" for both channels.

I originally connected the front end ground buss and a line from each channel star ground to a point on the Corcomm line filter which is connected to the 3rd prong(ground) of the power chord.

What I did was to break the front end bus into two halves completely seperated and tied each half to the corresponding channel star ground point thus making the ground a true dual mono as well up front.

I removed the leads from each star that went to the Corcomm filter and just made a single connection from the Corcomm to a seperate nut/bolt connection on the rear panel of the chassis.

The tweek that was suggested on the Curcio board was to take a ground lead from the star ground and make a twisted pair with the global feedback line from the 16 ohm tap that goes to the driver board for each channel.

With all this I got about 95% of the hum taken care of. If I hold my ear to the speaker, it is still audible but at a really really low level. It's even below the hum of my Tice Power Block / Titan line filter. So it is very very livable.

Nothing like being able (with some outside help) to take care of problems with one's own hands.

Not to be too proud , but in triode mode this amp has Balls! It's fairly tight down low (Bela Flecks Cosmic Hippo plays very nice) and is extremely 3 dimensional. I am very happy with the project and again I have to thank everyone for the help, advice and guidence.

JD

[mesherm]

I can well imagine! You have enough power supply iron and filter caps in it to light up a small city Yellow_Light_Colorz_PDT_16
That was quite a project you completed...congrats
I am partial now also to triode mode. I don't recall but did you install adjustable negative feedback? On one of my triode mode amps I built on an old ST70 chassis I used the front switch for two different negative feedback levels.