Since 1993 Copyright Notice

BIAS Schemes

1. Auto Bias

The classic scheme ever since, with good reason. Little can go wrong with this, and output power is the highest.

LED functions are not shown, but they all four LEDs are used.

AUTO BIAS SETTINGS OF EE61 Board

• Close Solder Jumpers: "LEDs=On", "Rk="on".
• Open Solder Jumpers: Rk=0, F-
• Use a Cathode Resistor and a Cathode Capacitor
• Connect Floating power supply to F+ and F-, which can be AC or DC.
• Calculate Rf+ and Rf- resistors by yourself, but 22 Ohms is a good value for many cases. For de-humming of an AC heater, or unstabilized DC heater, Rf+ and Rf- can be replaced by a 50 Ohms wire wound potentiometer. For stabilized DC, de-humming is not needed.
• Set points of Red and Green LEDs. For this, calculate R4 by formula, as written on the PCB. With the right value of R4, both LEDs work correct.
• For first tests, use the Rm Output
• See this note, for first switch on, of a new circuit.

2. Variable Bias with DC heater modules and grounded filament.

This is a bit "trendy" but I really have to say, this is not a very good circuit. This is only used because people who sell regulated voltage or current regulated PCB's present this too simple. You are heating the tube unbalanced, with all disadvantages resulting from an unbalanced circuit. One disadvantage is, the much higher hum susceptibility of the tube. So this can ONLY work with an extremely well stabilized heater. But what a coincidence.... that's what these so-well-designed circuits are capable of. Just what you need, right? . And it is hum free, it really is. Well yes, but it stays an unbalanced circuit.

This is not to write against such modules, because you can as well use them with mixed bias auto bias, or auto bias.

Another issue with a grounded filament is the following. For safe design, you need to bias already with an Auto Bias circuit, approximately 10% below the absolute maximum. Such as 300B in Auto Bias, though a 40 Watt tube, safe design ends at 36 Watt. WIth adjustable bias you need to go another 10% below that. Many historic tube tube sheets say so, and it is with good reason. So with adjustable bias, tubes should not be biased higher than 80%. This has nothing to do with the EE61 board, but safe design in general. What stays however, the tube is used unbalanced, and hum susceptibility will be higher by definition.

If the above is no problem for you, here is how to use EE61 with a grounded heater:

Filament and Vb+ LED functions are not shown, but they are used.

• Close Solder Jumpers: F-
• Open Solder Jumpers: "LEDs=On", Rk=0
• Connect the heater supply to F+ and F-
• The Red and Green LEDs do not work in this scheme
• Begin with maximum negative voltage on the NGV input, and reduce the voltage slowly.
• Change of bias equals the tube transconductance.
• See this note, for first switch on, of a new circuit.

3. Mixed Bias with DC heater modules, or AC heater

This is the best circuit. You can adjust the amplifier, but it still runs in Auto Bias mode. Not fully, but enough to stabilize it. The filament is heated "balanced" and such circuits are much less hum sensitive. This means, the tube can AC heated, if used as speaker output tube. Or, if heated with unstabilized DC it becomes already hum free.

To adjust the DC current, all it needs, is a small voltage to the 'VAR' input. If you unplug the voltage, the circuit falls back to normal auto bias. This allows to use any normal auto bias circuit, and decide later to change the current, without having to change the cathode bias resistor. The change is equal to the tube transconductance. For instance, the 300B tube has 5mA/V. This means for every volt on the 'VAR' input, the Auto Bias set point changes 5mA. Interesting, the current can be changed up and down, by reversing the voltage.

So to repeat it, the mixed bias circuit requires no changes to the Auto Bias circuit. All it needs, is a DC control voltage to the 'VAR' input, and Mixed Bias becomes active.

An interesting way to use Mixed bias is to build a soft start circuit around it. This is not part of the EE61 any more, but it can be added easily. For this, the auto bias circuit initially is designed for 70% of the normal current. The other 30% comes from a positive voltage on the 'VAR' input. Initially the circuit starts slower, and also does not go further than 70%. Now, positive voltage is present on Rk. From this voltage, with a voltage divider the required 'VAR' voltage can be derived. So additional components is just two resistors. The capacitors of the 'VAR' input will delay this action, and circuit start is softer than normal.

A real slow start from zero can also be made with a negative voltage on the 'VAR' input. This requires more circuitry, which is only for advanced designers. The method is, to use normal 100% bias, but cut off the circuit down to 10% at switch on, with a negative voltage on the 'VAR' input. Once the tube begins to work at 10% on this condition, first takes a while. Then, this is detected by a voltage across the cathode resistor. Now, a circuit triggers on this. The negative voltage is removed from the 'VAR' input, and it will go to zero. By adding larger capacitors across the 'VAR' input, like 200uF this action will be slowed down. By itself, this method is safe, since at any failure or design mistake, the tube will not be able to bias higher than 100% anyway.

Yet any slow start experiments should be done all in the end, when you find the circuit works well without it.

LED functions are not shown, but they all four LEDs are used.

• First set up the circuit for Auto Bias, and make it work. For this proceed as follows
• Close Solder Jumpers: "LEDs=On", "Rk="on".
• Open Solder Jumpers: Rk=0, F-
• Use a Cathode Resistor and a Cathode Capacitor
• Connect Floating power supply to F+ and F-, which can be AC or DC.
• Calculate Rf+ and Rf- resistors by yourself, but 22 Ohms is a good value for many cases. For de-humming of a AC heater, or unstabilized DC heater, Rf+ and Rf- can be replaced by a 50 Ohms wire wound potentiometer. This will help, if a tube produces slight hum, whatever the reason.
• Set points of Red and Green LEDs. For this, calculate R4 by formula, as written on the PCB. With the right value of R4, both LEDs work correct.
• Now connect an adjustable, negative voltage to NGV
• You can reduce the plate current with NGV.
• If you want also to ne able to INCREASE the plate current, attach a resistor in parallel to Rk, of 10x it's value. This will increase the plate current with 10%. We can fine adjust it with NGV, up and down.
• See this note, for first switch on, of a new circuit.

4. Tube Burn in.

Is has to be said, burn in is capable if repairing several problems or even defects. For this it has to be done the right way. This includes using the appropriate Bias Scheme: Mixed Bias, and then basically perform the procedure. This can be general, or specific with respect to a certain problem, knowing what this is, and what to do. On the other hand, not everything can can be cured by burn in, and the use hours a tube has already seen, can't be retrieved. Still there are things we can do.

For details, read the burn in instructions.