My re-leasson in thermal expansion!

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Stevem
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Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

My re-leasson in thermal expansion!

Post by Stevem »

Last week I gave back a customers 135 watt silverface twin that I repaired and recapped.

It played like a clean machine monster outputting 110 wats of RMS power , and I played it for atleast 1/2 hour just before the customer picked it up.

Two days ago I get a call from the fellow that it blew the fuse at a gig so I had him drop it back off to me.

Upon testing it out after replacing the fuse it played fine for some 45 minutes and then blew the 4.5 amp fuse again!

I took out the chassis and found that it's normal level of bias voltage was down to only -27 volts and after snooping around enough I found the cause.

It seems that when I changed out the 25 uf caps off of the tube socket near the bias balance pot I unknowingly bent in one Lead of that 33K resistor that goes to the wiper of the pot so close to the pots case that when the amp heated up enough and things like that pot case expanded that resistor shorted out to the pot and there by put that dip in the bias voltage level!

So for me it was another reminder to always good back and carefully look at areas of any amp that you work on even if the amp seems to be 100% for 30 minutes of play time!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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