Mojotone Princeton Reverb Clone Project

On this page I explain the issues I had with my Princeton's 6V6 fixed bias circuit and what I did to address them. The fixed bias circuit is different from the cathode bias circuit used in the other tubes in that the fixed bias circuit has its own power supply whose control voltage is independent of the flow of current through the tube.

Initial bias measurements

When I initially checked the bias on the 6V6 tubes I found them to be running "hotter" than I expected. The target power dissipation for the JJ 6V6 is 70% of the maximum value of 14W, or 9.8W. Instead I found that my 6V6 tubes were running over 12W. In fact, my measurements showed:
Item6V6 (brown wire)6V6 (blue wire)
center tap to plate voltage drop (Vdc) 4.615.33
plate (pin 3, Vdc) 366365
grid (pin 5, Vdc) -26.6-26.7
center tap to plate resistance (ohms) 138.9155.5
current (amps) 0.0331890.034277
power dissipation (watts) 12.1512.51
The full spreadsheet is here: xlsx or pdf

After some debugging and checking of the circuit, I decided that the bias voltage on the grid of the 6V6 tubes was not negative enough resulting in too much DC bias current flowing through the tubes. This resulted in the tube power dissipation shown in the table above.

As describe here, the bias voltage applied to the 6V6 grids is generated by the diode acting as a half-wave rectifier in the DC bias supply circuit. The exact value of the bias voltage is determined by the resistors in this circuit. In particular, the 22K ohm resistor controls this voltage. When I went back to check the DC bias power supply, I discovered that the 22K ohm resister I had was out of spec and actually measured 20.4K ohms:


The smaller the value of this resistor is, the closer the bias voltage will be to zero volts (i.e. ground). The larger the the value of this resistor is, the more negative the bias voltage will be. The -26 Vdc bias voltage being applied to the grids of the 6V6 tubes resulted in the 12W power dissipation shown above. Clearly the resistor needs to have a larger value in order to get the power dissipation closer to the target value of 9.8W.

Resistor substitution

To verify that the out-of-spec 22K resistor was an issue, I removed it and used alligator clips to try different resistor values. The results:
Resistor 6V6 (brown wire, watts)6V6 (blue wire, watts)
20.4K original resistor12.1512.51
21.6K resistor10.7610.90
26.6K resistor7.007.19

At this point I had two options: I could solder in a new resistor into the bias power supply, or I could install a potentiometer in the bias power supply. Adding a potentiometer is more work, but it makes it easier to adjust the bias voltage in the future (e.g. when changing output tubes). I decided to go with the potentiometer using one of Uncle Doug's excellent videos as a guide.

Bias potentiometer modification

The bias potentiometer modification is shown below:

bias fix figure

The AC voltage (shown in red) comes from the power transformer into the 100K resistor and half-wave rectifier diode to convert it into negative DC voltage (shown in blue). We remove the 22K resistor and replace it with a 10K linear potentiometer in series with an 18K resistor. This allows us to have a base-level resistance of 18K and by adjusting the potentiometer we can add in as much or as little of the 10K resistance in the potentiometer as we want (e.g. from 18K to 28K which covers a good range of values from the 22K in the original design). In addition, we add a larger 220K resistor between the diode output and the wiper of the potentiometer. Under normal use, the 220K resistor is so much larger than the 10K and 18K resistors that it has minimal impact on the circuit. But if the potentiometer has mechanical issues and the potentiometer's wiper loses contact with the rest of the potentiometer the 220K resistor provides an extra voltage reference for the bias voltage to protect the output tubes from losing their bias voltage and conducting too much current. Note that no DC current flows out of the DC bias circuit since it is just connected to the grids of the 6V6 tubes. The goal here is to provide a negative bias voltage to the 6V6 grids.

The bias circuit is on the small board in the corner of the amp near the pilot light. In the following figure, the 100K resistor and diode are at the top of the figure, and the 22K resistor is under the blue 100uF capacitor.


The 10K linear pot used in Fender-style amps is show below. To adjust it, you put a screwdriver down in the hole and turn the wiper. This design prevents you from accidently changing the bias adjustment.


To secure the pot to the chassis I used a piece of sheet metal I got from the Home Depot. I made a mounting bracket by cutting the sheet metal with tin snips, using a vice to bend it to the desired shape, and drilling out holes in which to attached the bracket to one of the power transformer screws and the pot.

IMG_5926-df24a5 IMG_5927-013236 IMG_5929-e53ea6 IMG_5932-89b419

Once the mounting bracket was complete I was able to install the pot in the amp by screwing down the bracket and soldering it in I used a marker on the bracket to indicate what direction to turn the pot's wiper to make the 6V6 tubes hotter.

IMG_5931-a0490f IMG_5934-611dfc IMG_5935-d27035 IMG_5936-26b37f IMG_5938-b54fe3

Having successfully installed the bias pot, I adjusted its value to get the power dissipation of the 6V6 tubes to its current value of around 10W. See the testing page to see the current bias measurements.

Amp warmup impact on measuring bias

One thing to be aware of when using the resistance of the output transformer winding to compute the tube current is that this resistance is not constant. As the amp warms up, the resistance of the output transformer increases. So it is best to try to take your voltage and resistance measurements at about the same time so that they match the current temperature of the amp.

To test this, I started with a cold amp and measured the output transformer resistance at 1 minute, 5 minutes, 10 minutes, 15 minutes, and 20 minutes of operation. The results are in the following table.

Transformer side1 minute5 minutes 10 minutes15 minutes20 minutes
brown wire (ohms) 133.8136.7138.4140.1141.6
blue wire (ohms) 149.9153.9156.4157.3158.5

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This page maintained by Chuck Cranor