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Immediate Feedback at Startup

:

8/20/1999 1:33 PM

Immediate Feedback at Startup

This applies to my homebrew (2xEL84) fixed bias.

When I turn on the amp, after rectifier and heater

warmup, It starts to feedback wildly....

I checked R.G.'s Debugging page and tried the

2 most suspect items:

1) input jack - I pulled the 1st preamp tube and the

feedback remained, so I can rule out the jack.

2) bias supply diode or cap - well, I'm getting a

consistant and steady negitive bias, so I'm assuming

the bias supply is working properly......

Any ideas what would cause a very low pitched feedback?

I'm going to pull the rest of the pre-amp tubes tonight in

hopes to try and isolate the feeback to a particular section.

Mook

8/20/1999 3:07 PM
R.G.	Very low pitched oscillation, right? In a fairly new amp that has worked correctly before, that's a tough one. Is it an oscillation or motorboating?

8/20/1999 3:08 PM
R.G.	Wait a minute - is this the same amp that was frying bias resistors? You could have fried other power supply parts as well.

8/20/1999 7:24 PM

Actually, after disconncting the CT for the 5V filament (yellow wires), things seem OK.

I cliped the CT from ground, applied a few layers of heat shrink, and hot glued the CT lead to the (Hammond) aluminum chassis.

I "fired" the amp up again with a new 5Y3 and 1A fuse. It "held" nicely for 10 minutes before I turned it off. I checked the voltages all over the power supply and they seem fine. The bias supply is nice and steady at -13V. It is adjustable to about -38V.

Mook

8/20/1999 3:37 PM
Doc	Is this a push-pull amp with a negative feedback loop off the speaker winding? If so, interchange the plate leads on the output transformer primary.

8/20/1999 7:25 PM
Mook	I'll try this tonight.... Thanks, Mook

8/21/1999 1:34 AM
Mook	Yes. I changed the OT Primaries and the feedback stopped. Thanks. Now I have another problem. The Plates of the EL84s are up to 392V! YUK! It's Zener time! Or, I might just spring for a lesser PT..... Thanks, Mook

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