Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on October 30, 2018, 03:24:03 pm

Title: Extreme Hiss at turn on/turn off Fixed.
Post by: Willabe on October 30, 2018, 03:24:03 pm

I have another M520 stereo power amp, 4 x EL34, 1 x 12UA7, 1 x EF86. It's never been used, sitting in box as back stock.

Sounds real good, all dcv's are good. This amp has a 50dcv stand off on the heaters.

BUT, the left channel at turn on/turn off has extreme hiss/wind storm sound.

Pull the 12AU7/PI tube, on that channel no hiss/wind storm.

I flip/flipped the EL34's, left channel with right channel, same hiss, tried several known good 12AU7's, same hiss, I cleaned the tube ceramic socket, same hiss.

Schemo attached below. 

Title: Re: Extreme Hiss at turn on/turn off
Post by: Willabe on October 30, 2018, 05:09:35 pm

I think I found the problem, C21 - .01uF/600v cap on the PI 2nd triode grid to ground. It reads open on my meter in and out of circuit.

It's labeled .022uF/600v.   

I'll see what I have here and clip it in and see it that get's it. 

Title: Re: Extreme Hiss at turn on/turn off
Post by: Willabe on October 30, 2018, 05:45:19 pm

Yes, that fixed it.     :icon_biggrin:

I'm taking some pics to post.