Speaking of Hiwatt's hum

[drewl]

This is a late 70's design, Harry Joyce wired 50 watter with v2 cascaded for higher gain.
And that is where the noise is coming from.
Once the master is up past half way the noise becomes too much.
Remove v2 and its fine.moving the shielded input to v2 helped but I can't imagine this much hum is normal on these (around 400mv with pre amp pots turned down)

I'll try and find the correct schem and post it, there are so many weird versions with no documentation.

[drewl]

Okay,
so it's a 60Hz hum coming from V2, ground pin 7 and the hum is gone.
Somehow it's picking up the 60Hz, possibly from the heaters.
It's a shielded input, coming from two 470K mixing resistors after the volume pots.

The volume pots don't effect the hum, the 470K's are under the pc board, about 1/4" off the board, the shielded wire is shielded at that point.

Same schematic as the other current Hiwatt thread.
I'm going to try remounting the 470K mixers, maybe on the top side of the board, and replacing the shielded wire to pin 7, get it away from the filaments.
Any other ideas?

http://hiwatt.org/Schematics/DROL_2InputPre.pdf

[drewl]

Well that didn't work.

Elevate the heaters?

[TimmyP1955]

Are both ends of the shield connected or just one? If both, try just one. If no joy, they the other end instead. Also play with where it actually connects (the farther from the circuit the better?).

[drewl]

Crap, that didn't work.

Tried moving the shield, removing shield, just made it worse.

Somehow that stage is acting like an antenna.
Ground either end of the shielded cable and it kills the 60Hz hum.

Any other ideas?

[52 Bill]

Any other ideas?

Change V1? Replace the shielded cable with a different one. Try temporarily tacking in a 1 meg resistor from pin 7 to ground, any change?

[drewl]

sweet!
The 1Meg from pin 2 to ground severely reduced the hum.

How'd you know?

The hum started to come up on power up, then went vroop and quiet.

[g1]

Pin2 or pin7 of V1 or V2? Is it wired with the same pin #'s as the drawing, or are the triodes reversed?

[drewl]

It's pin 7, which the first stage of v2.

[g1]

Yet turning down both the pre volumes didn't help like the resistor did?

[drewl]

No, not at all.
The wiper of each volume pot has a 470k mix resistor tied together under the PC board.
The shielded cable attaches to their junction, the shield connected to ground there, and the other end to pin 7, the input to v2 A.
Somehow that junction of the mix resistors acts like an antenna picking up 60hz.

[52 Bill]

Somehow that junction of the mix resistors acts like an antenna picking up 60hz.

No, I think that there may be a problem with the grounding of the pots. Those sorts of hum symptoms are kind of typical when there is no ground reference to the input grid of the tube. Normally the 470K resistor through the 500K pot should be enough, but there must be a problem there. If you read the resistance from pin 7 to ground what do you get? Try rotating the pots while you read the resistance.

[drewl]

I checked that.
The low end of both pots are grounded, I even cleaned the inside of the front panel and the pots so they had a good ground to the chassis.
The resistance from pin 7 to ground was also correct, varying with the pot settings.

Maybe a buss bar connecting the pot housings to ground could help, but I tried grounding the volume oot chassis with no change.