Re: [Harp-L] Low plate voltage on the preamp tubes?

[G <gigs@xxxxxxxxxxxxxxx>]

Date: Mon, 15 May 2006 22:23:28 -0500
From: Dina Janzen <dlj@xxxxxx>
Subject: Re: [Harp-L] Low plate voltage on the preamp tubes?
To: harp-l@xxxxxxxxxx

>I agree that reducing the plate voltage of Fender and similar amps 
>warms up the sound and makes it less stiff and gainy.  I have been 
>happiest with 12AX7 voltages of 100 to 110V.   It is not hard to do, 
>but involves dealing with a portion of the circuit that has REAL HIGH 
>voltage.  A simple way to dial in the voltage and sound is to 
>temporarily replace the power supply dropping resistor feeding the 
>preamp tube plates with a potentiometer, adjust the pot to taste and/or 
>voltage and then insert a fixed resistor of appropriate value.  You 
>might be able to find a tech that will do just this as you play and 
>listen.  It turns out that increasing the value of the last power 
>supply dropping resistor has little effect on the upstream (power tube) 
>voltages and power tube bias.  Allen Amplification makes an guitar amp 
>with switch selectable final resistance.  Seems like one could also 
>control this with a suitable control panel pot.
>Jim R

Hi Jim,
  Basic question: What kind of wattage do the resistors or POT have to be
for this sort of thing?
-- G.