Biasing Question

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 5:11 am

Rereading this thread, I see that the hum started after you installed 1-ohm cathode resistors attached to the back panel with RCA jacks. We might want to revisit precisely how and where these are installed. The cathode currents from output tubes can be very hummy and they may be finding their way onto the reverb recovery grid.

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 11:34 am

I drilled 2 holes over by the power cord, installed two non-isolated RCA jacks, each with a 1 ohm, 1% resistor from its tip lug to its ground lug. Soldered a lead from pin 8 of each tube to a jack tip. I use an RCA cable broken out into two "banana" plugs to plug into the test meter for biasing.

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 4:11 pm

Seems reasonable provided the ground connections are good and solid. I'm wondering because it's the one mechanical thing you did that made the hum start. You also reset the bias. I wonder what would happen if you dialed the bias back to no more than 70% of plate dissipation (you're at nearly 80% now IIRC). Most times when I crank tubes up too high, they hum, but if the static balance is "perfect" you might not hear it. Maybe it could show up in a different circuit. Just grasping at straws here ...

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 4:41 pm

I just spent about an hour dressing up and shortening leads, undid the bias test points and reconnected pin 8 of the tubes directly to ground, set the bias to exactly 70% (.7 * 14/347) made no difference at all... Could I have a bad reverb tranny?

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 4:49 pm

Celery_Strat wrote:set the bias to exactly 70% (.7 * 14/347)

I thought your plates were at 377.

My question is what made it suddenly start humming? It was quiet before, right? Or did you just not notice the hum before?

And the reverb works, but hums, right?

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 4:57 pm

Sorry, the 377 was a mistake on my part. My PV is 347. Before all this biasing crap (I could shoot myself), I had an absolutely quiet amp. I could DIME bothe the volume and the reverb and there was no hum. There was a vibrato tick, which I handled with a lead dress issue. I did the biasing rigamarole, and when I turned it on, I noticed the buzz. I tried all the pots, and it's the reverb that makes it louder. Reverb "works" just fine. I'm going to video it to convey the sound I'm getting.
http://www.youtube.com/watch?v=ifEMmPe087s
the two short buzzes toward the end are the phone vibrating because i got a message.

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 5:41 pm

Could you post your V4 voltages? And maybe try to get a gut shot of that area of the board?

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 6:47 pm

Pin 1 = 144
Pin 6 = 143
Heaters = 6.0

[img

768]http://i272.photobucket.com/albums/jj16 ... 134524.jpg[/img]

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 6:59 pm

What have you got on pin 3? Could we see a bit more surface area, including this section of the board itself.

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 7:34 pm

On Pin 3 I have 1.2V and a classic rock station...

[img:512:360]http://i272.photobucket.com/albums/jj16 ... 2707-1.jpg[/img]

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 8:49 pm

If the tank is still disconnected, you're getting radio reception because the grid is wide open. If it's connected and you've got AM, there's may be a tank problem.

I can't say for sure if it's related (especially since you didn't have hum before), but the heater wiring looks like it could induce hum in anything. Generally you want the heater leads twisted together and routed away from all signal wires. Fender did theirs "up in the air" above the sockets. Others laid them along the chassis and toward the back (still twisted though). Usually the sockets are positioned differently (rotated) to accomodate each style. Yours look like the traditional Fender orientation, so they should be up in the air.

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 8:55 pm

I'll redo my heaters. The twisting was a pain, so I admit, I gave up after V2. The tank is still connected. I only heard the station when I touched the probe to the pin. It's a local FM station. 100,000 watts and 35 miles away...

Firestorm · Post by **Firestorm** » Mon Jan 31, 2011 10:06 pm

Here's what actual AB763 wiring looks like. This one's a Twin, but has all the same preamp circuitry.

[/img]

Celery_Strat · Post by **Celery_Strat** » Mon Jan 31, 2011 11:27 pm

Well, It's fixed!!! Between all the lead dressing, and tidying up, I've got it dead quiet with reverb! The thing I think did it was, I moved the filter cap grounds off of the damned reverb transformer lug! (DUH!!!) over to the PT lug. Quiet as a mouse!! I really want to thank you guys for all that help! Awesome advice you guys gave. I'm still going to redo my heater wires, though!

Thanks again!

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