Loud Hum - 75 Silverface 135W Twin

[vshe...@aol.com]

My 75 Twin ( 135 W version ) recently developed hum that can't be controlled - suspect one of the transformers - any ideas how to determine if either one is the culprit?  Also, distortion at medium volume - spiking with bassier notes. Sounds lousy - I think one of the transformers affects harmonics as well.

 

Thanks

Vince
vshe...@aol.com

[Mike Marsh]

Did you check the Filter caps?

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[vshe...@aol.com]

Hi Mike,

 

Yes, just replaced 4 out 6 of them a weeks back ( two smaller ones are hard to find ). I found a bad 750Ohm resistor ( it had read  0 ) on the V7 tube and voila - hum is gone. Still a little hissy, but I can live with it.

 

Thanks for getting back. Next mission is to see about reducing the hiss and the Vibrato circuit of course.......

Vince Shemansky
vshe...@aol.com

[Paul Angle]

If that tube was a phase splitter tube, I recommend you do not turn on the stand by switch at the same time as the power switch,  Turn on Power first wait at least a minute then turn on the standby.

Otherwise the plate resistor will open occasionally due to voltage stress.

Good Luck

Paul Angle

pa...@acetroubleshooting.com

[vshe...@aol.com]

It is the phase splitter tube and thanks for the advice.  Have always turned power on first then standy - but probably not as long as you recommend so thanks.