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Yet Another Output Tube BIAS Servo Schematic from DIYAudio.ru
Yet Another Output Tube BIAS Servo Schematic from DIYAudio.ru
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Old 18th September 2018, 07:48 PM   #11
DF96 is offline DF96  England
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I can't see the point of a bias servo, but they are popular so a lot of people think they need one. About the only people who might actually need one are those with toroidal OPTs, as they can saturate easily if unbalanced. However, few people use toroidal OPTs.

Some people want to exactly set output stage bias current. Why? Valves are not exact devices, and their datasheet graphs are not very exact either, so who cares if the bias is 40mA or 50mA? 40mA might be correct for one set of valves; 50mA for the next set. In either case you won't get much difference in power or distortion if you get it wrong, just a slightly different compromise.

Others want 'fixed' voltage bias, but without the effort of adjusting it for new sets of valves or as valves age. They need a servo which only looks at or around signal zero-crossing; such servos do exist, but they are more complex.

Some go for CCS bias, which achieves much the same thing as a simple servo: setting the wrong current (average instead of quiescent). Worse than a bypassed resistor!

In my view the best option is what people used to do: bypassed resistor for Class A; fixed bias for Class AB and B. In some cases a mixture of the two can be used.

For those who are puzzled about the difference between quiescent current and average current, be aware that valves generate significant second-order distortion. This has two components: second harmonic (and IM for multitone inputs) and DC. Yes, an equal amount of DC shift is produced. The second harmonic may cancel out in a push-pull OPT so you don't hear it, but the extra current still remains. For fixed bias it does nothing, except heat the valve a little. For resistor bias it causes extra voltage drop, which has the result of biasing the valve a little 'cooler' during loud music; the usual solution is to set the bias a little 'hotter' for silence and quiet music. CCS bias (and simple servos) are worse, as they are in effect infinite bypassed resistors. Roughly speaking, 10mA of second-order distortion DC causes:
fixed bias: no change
resistor bias: 5mA bias shift during loud music
CCS bias: 10mA bias shift during loud music

I find it slightly amusing that the people who want to exactly set their output stage bias with CCS or simple servos are guaranteeing that their bias will shift by the maximum amount whenever music is present; it is only what they set it to during silence!
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Old 18th September 2018, 10:54 PM   #12
TonyTecson is offline TonyTecson  Philippines
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Yet Another Output Tube BIAS Servo Schematic from DIYAudio.ru
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Old 19th September 2018, 07:40 PM   #13
petertub is offline petertub  Sweden
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Quote:
Originally Posted by DF96 View Post
I can't see the point of a bias servo, but they are popular so a lot of people think they need one. About the only people who might actually need one are those with toroidal OPTs, as they can saturate easily if unbalanced. However, few people use toroidal OPTs. <snip>
Spot on!

In addition: if using new good tubes bias won't change except for mains voltages fluctuation. If one uses tubes from various sources ( to save money?) you might want to invest in a bias servo. But return of investment ??
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Old 19th September 2018, 09:48 PM   #14
Sodacose is offline Sodacose  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
Sodacose,
Like a resistor, it will only work for Class A output stages".
I'm not gingertube to defend this schematic drawing, but I believe it is similar to the Tent Labs bias servo. The diode clamps sample a window about the idle point, allowing it to work just fine in Class AB according to several users who have used it. I have not personally experimented with this circuit but I have taken a passing interest if I ever build something that I want to add a servo to.

Not quibbling with the average vs quiescent distinction though. It's a fair point with the time constants involved.
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