How to Bias a Fender Princetone Reverb
The 6V6 amplifier chosen for this experiment is the ubiquitous Fender Princeton Reverb. This is going to be ‘fun’ on a few different levels. First of all, there are not a lot of choices when shopping for 6V6 tubes today. Secondly, this is a ‘fixed bias’ amplifier in the truest sense of the phrase. This amplifier does not use a bias pot, instead relying on two resistors acting as a voltage divider in order to establish the bias voltage. So, let’s pop this ‘AA165’ circuit up on the bench, and see what we can learn.
There are many bias ‘calculators’ available ‘on line’ for you to investigate. For the curious, or for those that wish to compare the parameters selected for the different calculators, you may visit the following alternatives.
- Tube Amp Bias Calculator. I haven’t checked out the other pages to this web site, but so far it looks interesting.
- Dreamtone has a lot of interesting pages to their web site, but this link is strictly for the bias calculator.
- Duncan’s Amp Pages has a plethora of information available; a tube data base, tone stack ‘calculators’, and this ‘anode load calculator’.
If anyone knows of any other similar web sources, a ‘heads up’ my way would be most appreciated. Now, for those that insist on measuring plate current (or what you believe to be plate current), please consider the following warning.
WARNING! For those of you who insist on measuring plate current, be advised of the following. Measuring by way of the ‘transformer shunt method’, the indicated plate current (or what is purported to be the plate current) will be lower than if you insert an ammeter in series with the plate connection, or use a 1-ohm resistor in the cathode of the output tubes. In one experimental setup, the transformer-shunt method indicated 33mA of idle plate current. Inserting an ammeter in series with the plate lead yielded a reading of 36mA. Measuring the voltage drop across a 1-ohm resistor in the cathode gave a reading of 31.5mVDC, translating to a current of 31.5mA through that particular tube. Carefully measuring the resistance value of the purported 1-ohm resistor, I ‘discovered’ the value was slightly higher than 1-ohm. Hopefully, you see the pitfalls with this method which is highly touted as being the only ‘accurate’ method.
For our first round of tests, I chose a set of Electro-Harmonix (Russian) 6V6 tubes, purchased from a reputable dealer. Next came the B&K E310B signal generator and a Hitatchi V212 oscilloscope. This is a basic 20Meg oscilloscope, and I can compare the quality to other 20Meg oscilloscopes. There is no ‘on the fly’ adjustment possible for the bias voltage; instead, this amplifier relies on a simple resistive voltage divider in order to establish the bias voltage. A partial schematic is seen below.
I opened up the connection for the 22K resistor (seen above), and utilized a decade box in series with a 10K potentiometer for this part of the experimentation. My choice to start off was a 47K resistor and the potentiometer turned to the mid-way point of rotation. The bias voltage was -46VDC, and the measured plate current was 12mA, with a plate voltage of 448VDC when measured with a Fluke 112, and 455VDC when measured with an ‘average’ DMM purchased at Radio Shack. The waveform is seen below.
I slowly went through the decade box, until the waveform showed the crossover notch ‘almost’ disappearing. This occurred with a resistive value of 15K, and the potentiometer was slowly adjusted until the notch did in fact ‘disappear’. The waveform is seen below.
Now, I shut the amplifier off, and measured the effective resistance to the combination of the 15K resistor and the potentiometer setting. I measured 22.5K using a Fluke 112 digital meter. Using the original standard value 22K resistor, I solder it back in place, and measured the bias voltage, plate voltage, and idle plate current. The bias was now -31.6VDC, the plate voltage was 410VDC, and the plate current was 16mA. A trip to the Weber Bias Calculator suggests 20.4mA, so I cannot see where I went drastically wrong using an oscilloscope!
I repeat the above experiment, except this time I use the Heath 4554 oscilloscope. This is a 40MHz oscilloscope, and we should note that even the crossover notch appears sharper and clearer compared to ‘lesser’ oscilloscopes.
Biasing the output tubes ‘properly’, I get the following waveform. The resistance value measured was 18.13K when measured with a Fluke 112 DMM, and since 18K is a stock value, that is what I will put in the circuit.
The plate voltage is measured at 408VDC, and the plate current is measured at 18.2mA. I could add ‘a pinch’ to my tweaking of the bias voltage by paralleling resistors until I achieved the bogey value of plate current, but that wouldn’t be an exercise in practicality. Using a stock resistor value that leaves a little elbow room, I shouldn’t have too much trouble using different tube brands in this amplifier.
What, if anything, can be concluded from all of this? I have a few points to make, and you may come away from this ‘Lesson’ with a few of your own observations. This is all perfectly acceptable, and hopefully this will lead you toward doing your own experimenting.
- The control settings on the amplifier made a tremendous difference in how accurate the bias adjustment can be. It cannot be over-emphasized; we must set the controls for a symmetrical waveform, and maximum unclipped output.
- The accuracy is also affected by the meter you use, and the method you use to measure idle plate current. If you are using 1-ohm resistors in the cathode, the 1-ohm resistors have to be matched, and be exactly 1-ohm. I opened up the plate connection, to avoid the screen current and insure accuracy.
- Although the bias voltage is reduced (less negative) until the notch ‘just’ goes away, it returns ‘softly’ when the waveform is seen to start clipping from increasing the Volume control. Also, you can ‘tweak’ the bias voltage, by decreasing it slightly after the notch has just disappeared; beyond a point the waveform will not increase in size, and the top half will begin to flatten out. This is a good time to stop, and increase the bias voltage slightly. Remember to make sure your output tubes plates are not glowing a nice cherry-red, or that you have exceeded maximum plate dissipation. This is how you will learn exactly what you are doing, and the possible consequences.
- One critic argues that the negative feedback in the amplifier affects the bias adjustment when biasing with an oscilloscope, and removing the feedback loop negates the bias adjustment. I suppose if you cannot dazzle them with brilliance, the next best thing is to baffle them with bullshit. I haven’t met a Princeton Reverb yet having a feedback loop that liked to go AWOL when I least expected it, negating my bias adjustment.
- Oscilloscopes with a lower bandwidth and/or accelerating voltages yielded a trace which appeared ‘fuzzy’, making it difficult to distinguish exactly when the crossover just disappeared. As a theory, in many cases the crossover may not have completely disappeared, resulting in an amplifier biased on the cold side. Truth or fiction? The oscilloscopes with a good, clear trace showed the crossover notch disappearing more clearly, and resulted in an amplifier biased much ‘better’. If you learn to understand the waveform presented to you from your old Heathkit oscilloscope, you can ‘compensate’ for the fuzzy waveform, and make sure the crossover notch has disappeared. This is a criticism to the oscilloscope ‘method’, and I cannot argue that criticism. However, until you own a ‘good’ oscilloscope, you will simply have to learn to make compromises and compensations. Using your ears to fine tune the bias adjustment is another way to make sure you have it ‘right’. In the end, you will almost always have an idle plate current that falls somewhere between 10mA and 40mA, so why criticize how I got my highly accurate number?
- The ‘dummy load’ you use will only affect your work if you are ‘sweeping’ the audio range, and plotting frequency response. A ‘better’ mousetrap involves a ‘load’ designed to emulate the speakers frequency response and varying impedances. Do you really need this kind of accuracy? Not really, but if your are the curious sort, investigate this circuit by CLICKING HERE.
The whole point of this exercise was to show you that using an oscilloscope to bias your amplifier is not as bad as many gurus will have you believe. However, there are pitfalls that have to be addressed. Back at Articles That Didn’t Quite Make The Cut, you were given a hint that any oscilloscope can lead you astray. Make sure the oscilloscope you use is in good operating condition, and use the appropriate probes. The same carries over to your signal generator. Misunderstanding or misusing these techniques can also result in a very disappointing experience.