From Dr.Distortio–(at)–bs.mhv.net Tue Aug 22 21:45:48 CDT 1995
Article: 3041 of alt.guitar.amps
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From: Dr.Distortio–(at)–bs.mhv.net (Dr Distortion)
Newsgroups: alt.guitar.amps
Subject: Re: Bias adjust safeguard and Fender failure modes
Date: 22 Aug 1995 15:57:05 GMT
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Mark Garvin (mgarvi–(at)–anix.com) wrote:
: wiggled, of course. So these get their own terminal block *real near
: the output tube*.

I leave them hanging off the grid pin, but I shrink-wrap the resistor to
the wire coming from the bias supply/phase splitter network. I want to
keep that stopper as close as possible to the tube…

: Also, I usually replace the screen resistors with
: heavier wirewounds which I glue down with epoxy.

The heartier resistors are a good idea, but epoxy? 🙂

: Anyway, I realized that the Fender bias pot circuit could be altered
: easily with the addition of a single resistor from the wiper to the
: negative bias voltage. A high enough value will hardly affect the
: pot’s operation.

I actually saw this done in a Traynor amp; they used a 1M resistor going
>from the raw bias supply. As for me, I just set up the bias pot as a
rheostat with the wiper tied to the high side:

R1 VR1 R2
Raw bias—–////–|—////—-////—Ground
|————-)|———-|
C-
If the pot goes intermittent, the C- simply goes high. Is this the kind
of setup you’re describing?

: Another interesting thing I’ve never heard mentioned about the Fender
: circuit: there is a signal path from one signal phase to the
: other–thru the 220k (or 68k or 100k) grid bias resistor and up thru
: the other one.

I’ve been thinking about the same thing recently, myself. Note the bypass
cap in my little schematic…

From mgarvi–(at)–anix.com Wed Aug 23 19:54:24 CDT 1995
Article: 3062 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Bias adjust safeguard and Fender failure modes
Date: 22 Aug 1995 01:44:06 -0400
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This subject has been touched on here before (by Dr. D and myself),
but is probably worth repeating.

In my own designs, I normally use the bias adjust pot as one side
of a voltage divider, with a resistor to the neg.bias as the other
side. Reason? The pots are one of the components likely to fail
due to cracked elements or dirt. If that happens in a conventional
design (Fender), the wiper arm of the pot may get lifted-> No bias->
burnt output tubes. In the circuit I described, the bias will just
go full negative and will protect the output tube.

So I was going thru a Fender amp looking to improve weak spots. This
particular amp had failed due to a crack in a 1500 ohm grid block
resistor. The little bahstid was intermittent. Fender mounts these
between two pins of the output tube socket, so they get heated nicely
by the tube below. And they get stressed when output tubes are
wiggled, of course. So these get their own terminal block *real near
the output tube*. Also, I usually replace the screen resistors with
heavier wirewounds which I glue down with epoxy.

And I connect pins 1 and 8 (after other wiring is moved, of course),
just in case anyone plugs in an EL34. (EL34’s don’t have internal
connection between suppressor grid and cathode as 6L6’s do)

Anyway, I realized that the Fender bias pot circuit could be altered
easily with the addition of a single resistor from the wiper to the
negative bias voltage. A high enough value will hardly affect the
pot’s operation. If the pot fails and the wiper opens, the resistor
will supply -volts to keep the tubes from melting down. Readjust
the bias afterward, of course. The resistor will scew it slightly.

Another interesting thing I’ve never heard mentioned about the Fender
circuit: there is a signal path from one signal phase to the
other–thru the 220k (or 68k or 100k) grid bias resistor and up thru
the other one. Attenuated by the impedance of the bias-adjust pot,
but there nonetheless. In the ’63 amps, it looks like about 5% of the
signal could get thru, depending on setting of the bias pot. This can
be bypassed by use of an electrolytic in parallel with a mylar cap, for
complete isolation. Not absolutely necessary of course. I have not
checked sonic differences–probably not huge, but it looks like a
Fender cost-saving omission to me.

Comments and follow-ups are welcome, and as always–be careful.

Regards,
Mark Garvin

From mgarvi–(at)–anix.com Wed Aug 23 19:54:47 CDT 1995
Article: 3060 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Bias adjust safeguard and Fender failure modes
Date: 23 Aug 1995 03:18:32 -0400
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In <41cuoh$5q--(at)--ver.mhv.net> Dr.Distortio–(at)–bs.mhv.net (Dr Distortion) writes:

>Mark Garvin (mgarvi–(at)–anix.com) wrote:
>: [grid blocking resistors] get their own terminal block *real near
>: the output tube*.

>I leave them hanging off the grid pin, but I shrink-wrap the resistor to
>the wire coming from the bias supply/phase splitter network. I want to
>keep that stopper as close as possible to the tube…

Good idea of course. I solder one end to the terminal block and the
other end to the tube socket pin.

>: Also, I usually replace the screen resistors with
>: heavier wirewounds which I glue down with epoxy.

>The heartier resistors are a good idea, but epoxy? 🙂

Or hot glue. I don’t trust the heavier resistors when supported
by only one lead. Paranoia induced by brutal roadies and bad van
suspensions .

>: Anyway, I realized that the Fender bias pot circuit could be altered
>: easily with the addition of a single resistor from the wiper to the
>: negative bias voltage. A high enough value will hardly affect the
>: pot’s operation.

>I actually saw this done in a Traynor amp; they used a 1M resistor going
>from the raw bias supply. As for me, I just set up the bias pot as a
>rheostat with the wiper tied to the high side:

1 meg seems a bit high, but better than letting them float. Hadn’t
noticed that Traynor did this. I use something closer to 100k.

>R1 VR1 R2
>Raw bias—–////–|—////—-////—Ground
> |————-)|———-|
> C-
>If the pot goes intermittent, the C- simply goes high. Is this the kind
>of setup you’re describing?

Yes. Provides lower impedance to the V- supply.
The single added resistor does it for quick patches, tho.

Regards,
Mark Garvin

From mgarvi–(at)–anix.com Wed Aug 23 19:55:03 CDT 1995
Article: 3055 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Bias adjust safeguard and Fender failure modes
Date: 23 Aug 1995 03:22:20 -0400
Organization: PANIX Public Access Internet and Unix, NYC
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>mgarvi–(at)–anix.com (Mark Garvin) wrote:

In rfrie–(at)–etcom.com (Robert Fries) writes:
>Could you explain this bit about the electrolytic in parallel with a
>mylar? Where would you put these

Hi Robert,
See Dr. D’s ascii schematic in adjoining post.

>, and why these two types in parallel?

Electrolytics are good for lower frequencies, but are not that good at
bypassing full audio bandwidth. Mylar covers a wider range.

Regards,
Mark Garvin

 

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