Some tube amps, like the Fender Silverface Princeton Reverb, do not have a pot to adjust the bias. Several times, I read in forums, that you can simply change the tubes without the need for a further fine-tuning of the bias.
Actually, this is wrong.
The bias is the quiescent current of the tubes. Every tube has a certain operation range where it works without any problems.
If it is set too high, the tubes will not last very long and in the worth case, the amp can be destroyed.
A bias that is adjusted too low, will make the amp sound thin and harsh.
The adjustment of the bias is absolutely essential for this amp.
The Princeton Reverb has a firmly soldered resistor which controls the bias. It is allocated near the light on the small circuit board. By changing the tubes, you can calculate the needed value of the resistor. Personally, I recommend to install a trim pot, thus the bias can be adjusted quite easily.
So, changing the tubes becomes less cumbersome and it is (to a certain extend) adjust the bias by ear.
Something, that also doesn’t make sense is to assert, that a Fender Princeton Reverb always sounds best with a bias of 21mA, for the voltage of these old amps can vary a lot.
Therefore, it is necessary to check the plate voltage. Ideally, it ought to be around 420 Volts in a Fender Princeton Reverb.
If you use a Fender Princeton Reverb with two 6V6 tubes, the perfect bias would be 20mA. This corresponds a load of 60%. In the extreme case, you should never exceed a load of 70% of the maximum performance level of a tube. Regarding an amplifier with a plate voltage of 420 Volts, this would be 23.3.mA.
Altering the bias causes in turn a slight alteration of the plate voltage. Consequently, you should, after the amp has heated up, check the plate voltage again and, where appropriate, re-adjust the values. I have already experienced, that the plate voltage initially was about 450 Volts and then went down to ideal 420 Volts. Thus, you can make sure that the amp works safely and the sound is at its optimum.
But now, let’s have a look at the fine-tuning.
The perfect operation range of the tube is not necessarily the best area for a perfect sound.
You ought to use the amp with a connected measuring device and set the bias higher and lower.
A higher bias makes the amp sound a bit richer, more trebly, compresses sooner and will overdrive earlier.
The sound of a lower bias will be clearer and sonically appears a bit cleaner. And the headroom as well is somehow higher.
So, you can easily adapt the amp to your individual sonic ideas. But you should definitively check the adjusted value to make sure that it stays within the frame.
Regarding the adjustment of the bias, and the modding of amps in general is, ensure that you know what you are doing and that you use the right tools and measuring devices.
The voltage inside of the amp is lethal!
Last but not least: there is no need for adjusting the pre-amp tubes. They can be substituted by any 12AX7 tubes problem-free.