Silver Face Twin Reverb Bias Control Mod

Last Updated 02/08/05
By Paul J. Marossy


 

One of the main differences between a black face and silver face Fender Twin Reverb is the bias adjustment arrangement. In the black face and early silver face amps, there is a power tube bias adjustment pot. In the silverface amps, this bias adjustment pot was omitted and became a hum balance pot instead. The hum balance pot is used to minimize the amount of hum between the pairs of power tubes. In my own Twin Reverb, I have a hum balance pot and a bias adjustment pot. If you are happy with the silverface Twin Reverb sound but want to have a bias adjustment pot as well, it's very easy to add one. This way, you get the best of both worlds.


Here's how to do it. In the picture below, the hum balance and associated components are shown in white. The hum balance pot will remain as is with the exception of removing the 15K resistor (shown in greenish brown) from its connection to ground. In place of this resistor will go a 100K 1/2 watt pot, wired as a variable resistor (shown in red). This will give you a bias adjustment pot. Make sure you use a resistor between the pot and the connection to ground to prevent the power transformer from being damaged by preventing the negative bias voltage from being directly shorted to ground. Use at least a 3.3K resistor here, maybe even a 4.7 to 6.8K.

The next part of this mod has to do with measuring the bias current conveniently. This is another very simple thing to do. Find Pin 1 (cathode) on the power tubes (V7, V8, V9 & V10). These pins should be directly connected to the chassis (unless your amp has aleady been modified). Between Pin 8 and ground, we will insert a 1 ohm / 1 watt resistor (also shown in red). The purpose of this resistor is to make measuring bias current easy. Setting your DMM to measure millivolts, measure the voltage across the 1 ohm resistor. If you measure 34mV, then thanks to Ohm's Law, you will have effectively have 34 milliamps of bias current flowing through that power tube. Adjust the bias control as required for your power tubes. They most likely won't all be exactly the same, but they should be within a few milliamps of eachother if you really do have a matched set of power tubes and all of your resistors are good. The hum balance pot will also have some small affect on how the power tubes bias as well as minimizing hum. Generally, the lower the hum, the lower the bias current will be. A small amount of hum is pretty normal, so don't get worried if you hear a little bit of it. Also, one thing to note is that a few of the resistor values shown on the layout below differ from the schematic. A previous owner had them changed for some reason. I didn't feel the need to change them to original specs.






Below are some pictures of how this mod has been implemented in my own amp.


This is the back of the master volume control. The hum balance pot will be nearly directly behind this pot.
Here is the bias adjustment pot. A knob really isn't necessary, but I think it's convenient to have one on there. You can see the 15K resistor on the far side of the hum balance pot. The connection to ground was simply clipped and the resistor was left in place.

Here is a view of the 1 ohm resistors installed. These make measuring bias current a snap!

Of course, it almost goes without saying, be very careful inside your amp while measuring the bias as there is line voltage and high voltage DC in close proximity to the resistors you will need to measure!

 

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