From tremolu–(at)–ol.com Mon Nov 18 19:18:26 CST 1996
Article: 27822 of alt.guitar.amps
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From: tremolu–(at)–ol.com
Newsgroups: alt.guitar.amps
Subject: Re: old vibrolux reverb
Date: 18 Nov 1996 20:07:19 GMT
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> By the way, while the pull boost and master volume were
>unnecessary, silly additions to the design, it has not been my
>experience that they make the amp less reliable.

Not the master volume, but the pull boost can cause prematurre failure of
the reverb drive 12AT7, it’s cathode resistor any bypass cap. In most of
the amps so equipped, they obtained the boost signal from the reverb drive
circuit, and changed the biasing on the 12AT7. Specifically, they went to
a much smaller cathode resistor, causing the tube to run MUCH hotter.
I’ve seen these tubes fail short from overheating and take out the cathode
circuit. If you have a pull-boost amp and you want reliability, at least
rip out that pull boost bullshit and put the proper BF era biasing back on
the 12AT7 cathode.