| Back
to Library page |
|
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. |
 |
|
The areas in red are new or have changed from the stock plexi layout.
Compare the two layouts side by side. |
| - |
 |
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. |
| 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. |
| 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. |
