From detritu–(at)–x.netcom.com Sun Feb 7 16:15:28 CST 1999
Article: 157492 of alt.guitar.amps
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From: detritu–(at)–x.netcom.com(Lord Valve)
Newsgroups: alt.guitar.amps
Subject: Re: Please Help with bias on Princeton Reverb
Date: 7 Feb 1999 06:50:38 GMT
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In <36BCA1B2.4F4--(at)--dt.net> Paul Cassone writes:
>
>I have a ’79 Princeton Reverb which needs a bias adjustment. The
plate
>current is 425, and the NOS 6V6’s are drawing 32 milliamps. To avoid
>having the tubes blow up, I know that I need to get the current draw
>down to around 24 – 26 milliamps. This amp has no bias adjust pot,
and
>a resistor or two will need to be changed. I thought I knew which
>resistor needed to be varied, and I used the method Weber suggests, of
>putting a 250K pot across it but I must have put the pot across the
>wrong resistor. The current draw didn’t change and I blew up the pot.

>
>What I was supposed to do, according to Weber, was to monitor current
>draw while I turned the pot until I got the current draw where I
wanted
>it. Then I was supposed to measure the value of the pot, find a
>resistor of the same value, and then solder it across the resistor on
>the board.
>
>Sounds logical to me, but I must first find the right resistor, right?
>
>So, any clues? Which resistor or resistors should be varied with this
>amp to set bias? Is the Weber method a sound approach? Any other
>methods you like better?
>
>Your advice and suggestions are much appreciated.
>
>Regards,
>
>Paul

Lord Valve Speaketh:
OK, Paul, forget all that bullshit you just wrote about and do
it right this time. You have a resistor that is in parallel
with the bias cap; one end is grounded, the other is connected
to the negative lead of the cap. The lower the value of this
resistor, the lower the bias voltage will be, and the more
current the output tubes will draw. A higher value will give
a higher bias voltage, and cause the tubes to draw less current.
Here’s what you need to do: unsolder the existing resistor and
chuck it. Whatever value the resistor is now (let’s say it’s 33K,
’cause I can’t remember and all my schematics are down at the shop)
cut it in half and put it in series with a pot that’s twice the
new value. For a 33K resistor, rounding the values off to standard
components, that gives us a 15K resistor and a 25K pot. In order to
use the pot as a variable resistor (rather than a voltage divider,
which is how pots are most often used) you’ll need to tie the center
lug to either of the outer ones. Now, you have a 2-terminal pot.
Solder one lead of the new resistor (15K) to the ground point, where
you removed one lead of the old resistor. Solder the remaining lead
to one of the terminals on the pot. Solder the other terminal on the
pot to the negative lead of the bias cap, from which you removed the
other lead of the original resistor. You now have a variable bias
supply, which you can tweak to your heart’s content. BTW, use a
trimpot, of the stand-up variety. The whole shebang will fit into
the space where the old resistor was. This rig will give you a supply
that has a voltage range of from somewhat lower than the stock resistor
to somewhat higher. The reason that you don’t use *just* a pot is
this: if one end of the pot is grounded (which it would be if
you didn’t use the 15K series resistor) then the bias supply will
go to zero if you turn the pot all the way down; that’ll smoke
the pot, and smoke the tubes, too, if you leave it that way for
longer than 10 seconds or so. If you find that you still can’t get
enough negative voltage to set your idle current correctly, raise
the value of the series resistor by 5 or 10 K.

Lord Valve
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