Biasing is a topic that always comes up in conversations with players when talking about amp maintenance. I will attempt to give some tips on biasing Dr. Z amps.

First, all of my EL-84 amps are “self biasing,” so no adjustment is needed when installing new EL-84 power tubes. All you need is a very closely matched set of new 84s, plug them in and start to play. As for current production EL-84s I prefer JJ/Tesla brand. They are, in my opinion, the stoutest, longest lasting, well built 84 currently available. For the last five years I have been shipping new amps with Groove Tubes EL-84-S tubes, and the reliability has been great. The EL-84-S is a Tesla EL-84, and the rating I select is a #6.

Before I continue with simplified bias setting instructions on other Z-Amp models, maybe an explanation of the need for bias adjustment is in order. The simplest way to think about biasing output tubes is to use an analogy of your car’s idle adjustment. To optimize the performance of your car, the idle speed in RPM must be adjusted correctly. If it is too high you’ll need to apply more pressure to the brake pedal when stopped at a light, RPM’s will be too high and your car will want to race. If the idle is set too low, the car will want to stall because the RPMs are too low. This is the same with your output tubes. Bias too high and the amp will sound a bit harsh and distort too quickly, and will lose all focus at high volumes. If the bias is too low the amp will sound cold, brittle, and at higher volumes will have excessive crossover distortion, will sound dissident, and your guitar will sound out of tune.

The next three amps I will discuss are “Fixed Bias” amps, meaning the bias is FIXED by adjusting a pot on the board.

DISCLAIMER: Tube amplifiers contain potentially lethal, high voltages even after they are unplugged, that may cause personal injury or death. Do not attempt to repair, modify, or work on any amplifier unless you are absolutely certain you know what you are doing. If you are not absolutely certain about what you are doing, please consider the following a reference sheet for your amp tech.

The “Route 66” amp normally ships with Groove Tubes KT-66-HPs with a rating of #5 or #6. To adjust the bias we will try and set each tube’s idle current (or plate current with no input signal, Power and Standby on) into a fixed load measuring from each plate PIN3 to Center Tap of Output Transformer. This is called the Shunt Current Measurement, because you are shunting your Current Meter across high voltage. This is a dangerous setup and SHOULD ONLY BE DONE BY EXPERIENCED TECHNICIANS. I have found an easier way to dial in the Bias, and it will bring the correct operating bias point to within 97% of optimal.

The safer method is to monitor the Center Tap Voltage, located on the RED and WHITE STRIPED wire that attaches to the Multi Can Capacitor. Your volt meter would be set to DC range. Ground your Common lead to the chassis and carefully measure the RED/WHITE striped wire with your highest DC Voltage range. Adjust the bias pot till the Voltage at the RED/WHITE striped wire is at 475 Volts DC. If you are using Groove Tube KT-66-HPs rated at #5 or #6 with a 5AR4/GZ-34 rectifier in the power supply then the voltage reading of 475 volts should be the optimum setting. Remember to have a fixed resistive load, or if not, a speaker will do to properly adjust the bias.

The KT-45 is the same basic setup and adjustment as the Route 66, with one small deviation. KT-45s can have a Solid State Rectifier or a Tube Rectifier; it is a matter of taste. The Tube will soften the amps response and lower its overall wattage. With the Solid State Rectifier installed, adjust the bias pot for 480 Volts DC at the Center Tap. With a 5AR4/GZ-34 Tube Rectifier, set the center tap for 440 Volts DC.

The final Fixed Bias amp currently in production is the Z-28. It uses a different color coded Output Transformer so instead of the RED/WHITE striped wire from the Center Tap, it is a solid RED wire, and it is placed in the same location. Perform the set-up as noted above, with optimum voltage set at 375 Volts DC. This will be correct if Groove Tube 6V6-R tubes (with a rating of #5 or #6) are installed with a 5AR4/GZ-34 Rectifier in the power supply.

These methods are used on my bench. I also spot check for correct current settings, then “ear test” the amps by playing them to assure correct operation.

For those interested in bias current, the settings are in the ranges below:

Route 66 set between 34 and 38 mA per tube.
KT-45 set between 28 and 32 mA per tube.
Z-28 set between 24 and 28 mA per tube.

DR. Z