From: Tim C.   (tcowles@cableone.net)
Date: 5/17/2000 3:58 AM
Subject: Fender Vibroking Mod

 
There has been much discussion in the past about the lack of headroom in the
Fender Vibroking. They also have a tendency to sound constipated.

Recently one of our Fender dealers gave me a Vibroking and the license to
do whatever necessary to make it sound better. It is series one with the
6BQ5 reverb driver tube. I came up with the following modifications to
increase headroom and make the overall tone better.

1) At the plate of V4A are two coupling caps in series, a .22mfd. and a
.0068mfd. Remove the .22 cap and replace it with a straight wire. It is
identified as C5 on the circuit board. After each of these caps is a 470k
resistor to ground. Unsolder and remove both resistors from the circuit. They
are R21 and R15 on the board.

2) At the cathode of V4B is a 1.5k resistor and a .47mfd. cap. Remove the
.47 cap identified as C10 from the circuit.

3) Change the treble tone cap in the tone stack from 150pf to 390pf. It is C7
on the circuit board.

4) From the wiper of the treble cap to the phase inverter is a 100k resistor.
Remove it and replace it with an 820 ohm 1/2 watt resistor. 

5) Change the output coupling caps, C13 and C14 from .047mfd to
.033mfd.

6) Change the cathode bypass cap at V4A from 22mfd to .68 or 1mfd. It is
C4 on the board.

That's it.

Set the treble on about 6 to start with. You should note that the amp will
bloom more and the bass is much fuller and richer sounding after the mods.

Of course tone is subjective but I really think it is a huge improvement over
the stock wiring.

Respectfully submitted,
Tim Cowles