Mic Kaczmarczik's Fender Bias Pot Failures Information

Fender Bias Pot Failures

Fender

>mgarvi--(at)--anix.com (Mark Garvin) wrote:
>>Good answers from Larry. Just thought I'd mention that I usually
>>patch a relatively high resistor (usually around 100k or so) from
>>the NEGATIVE end of the bias pot to the center tap. If the pot
>>fails, the resistor will pull the grid down to a safe level rather
>>than letting it float.

Jack Price writes:
>I've seen you post this suggestion more then once. This is really good

Uh-oh...I'm repeating myself...[Uh-oh...I'm repeating myself...]

>advice. Cheap insurance for your output tubes. What I have seen several
>times now from just 1 year of a home repair/restoration business, is that
>the bias wire from the bias board to the bias pot, specially if it's solid
>wire, will break off right at the pot...instant glow.

Yes, limited mojo in those solid wires, I think. Best stay with
stranded. Another questionable design practice in Fender amps is
putting the 1.5k grid-stopper right on the tube socket. They get
heated, stressed by wiggling tubes, flamed by burning screen resistors,
etc. and they crack. Not always visible and frequently intermittent.

That's the failure that I've seen most often.

Mark Garvin

From cign--(at)--elios.phy.OhioU.Edu Wed Mar 20 20:33:51 CST 1996
Article: 7684 of rec.audio.tubes
Newsgroups: rec.audio.tubes
From: cign--(at)--elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Cherry red hot tube...
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Sender: postmaste--(at)--elios.phy.ohiou.edu
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References: <3145ED69.D8--(at)--it.edu> <4i67k4$ps--(at)--anix2.panix.com> <4imsbo$gq--(at)--tdcs5.ra.intel.com> <4injq0$ld--(at)--anix2.panix.com>
Date: Wed, 20 Mar 1996 16:13:40 GMT
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In article <4injq0$ld--(at)--anix2.panix.com>,
Mark Garvin wrote:
> [snip]
>stranded. Another questionable design practice in Fender amps is
>putting the 1.5k grid-stopper right on the tube socket. They get
>heated, stressed by wiggling tubes, flamed by burning screen resistors,
>etc. and they crack. Not always visible and frequently intermittent.

The heat is a problem, especially for carbon comp resistors. But my
understanding is that the grid stoppers need to be as close to the grid
pin on the tube as possible -- something about being at the current
anti-node of the oscillations.

-- Dave Cigna

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