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Gain stage mod added to plexi board
kit. |
Note: This is not a channel
switching mod. The volume difference between the
switch off and switch on is very large. You will
probably end up just leaving the mod switched on
all the time once you hear how cool it sounds.
This mod is here for information purposes only. I
do not offer technical support on how to install
it in your amp. You have to be able to figure it
out from the drawings below. Basically you will be
eliminating 2 of your input jacks because you will
be using the 2nd half of valve one as a gain
stage. I have also converted the 2nd volume pot
into a master volume because it is very handy. The
mix or combination of the volume and master makes
the amp have tons of different tones. I have
re-voiced the first half of the V1 to not be a
super bass or treble channel. You will need a good
quality DPDT toggle switch, 2-2.2k 1/2 watt
resistors, 2-470k 1/2 watt resistors, 2-.022/630v
caps and some shielded cable. Mount the DPDT
toggle on the front panel in one of the old input
jack holes or use a relay switch. Keep all the
wire leads as short as possible. The 470k resistor
connected to the end of the .022 sets the gain
level that you send to the valve two tube. When
the toggle switch is off, the gain stage is
switched completely out of the signal path and you
are back to a stock pre-amp. |
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The areas in red are new or
have changed from the stock plexi layout. Compare
the two layouts side by side. |
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How it works.
Follow the signal paths on the drawings. The red
lines are internal switch connections when the
switch is thrown one way or the other. |
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Normal gain: The signal leaves the
.022 cap at point one. It then goes to the volume
pot. It leaves the center lug on the volume pot
and goes to pin 5 on the switch. It goes through
the red line to the pin 4 switch contact. It exits
the switch and goes to a 470K resistor. It then
goes to pin 2 of valve two. Note that the switch
also grounds the input grid on valve one/pin 7.
You cannot have an input grid just floating, it
must be tied to ground or the tube will go into a
run away current situation. |
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Added gain stage: The signal leaves
the .022 cap at point one. It then goes to the
volume pot. It leaves the center lug on the volume
pot and goes to pin 5 on the switch. It goes
diagonally to the upper left pin 3 switch contact.
It goes through the red line to the pin 1 switch
contact. It exits the switch and goes to pin 7 on
valve one. It exits valve one and goes through the
.022 cap. It then goes to the upper right pin 6
switch contact. It goes through the red line to
the middle pin 4 switch contact. It exits the
switch and goes to a 470K resistor. It then goes
to pin 2 of valve two. |
