• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fixed bias vs. auto bias

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CCS bias fixes the wrong thing. Fixed bias sets the quiescent current, and lets the average current vary as it will (generally upwards). CCS bias sets the average current, and lets the quiescent current vary as it will (generally downward if bypassed). Cathode resistor bias is somewhere between these two extremes; the average current rises with signal, but the quiescent current falls so you have to start with the valve biased too hot.

The reason for the change in current with signal is the second-order nonlinearity present in all devices. It is quiescent current you set from the datasheet load lines, but you measure average current with a meter. The two are only the same with no signal present.
 
CCS bias fixes the wrong thing. Fixed bias sets the quiescent current, and lets the average current vary as it will (generally upwards). CCS bias sets the average current, and lets the quiescent current vary as it will (generally downward if bypassed). Cathode resistor bias is somewhere between these two extremes; the average current rises with signal, but the quiescent current falls so you have to start with the valve biased too hot.

The reason for the change in current with signal is the second-order nonlinearity present in all devices. It is quiescent current you set from the datasheet load lines, but you measure average current with a meter. The two are only the same with no signal present.

Also interesting observations.
Perhaps this parasitic and unwanted effects was stronly reduced through Tim de Paravicini's topology used by the EAR integrated Amplifier "V20". 10 pairs of small signal tubes works parallel for driving the loudspeaker output transformer. Read more about follow URL's:
EAR V20 integrated amplifier | Stereophile.com
http://www.diyaudio.com/forums/tubes-valves/94403-ear-yoshino-v20-amplifier.html
..:: EAR Yoshino, professional audio equipment,Cambridgeshire,UK ::..
V20 Vollverstärker (German language)
 
Hello,

I have just bought a pair of custom made mono 300b SE amps(no schematic). Also, bought a new pair of 300b tubes for these amps, but did not know how to bias/adjust plate current and/or voltage! Your assistance and instruction on how to bias the 300b tube amps are greatly appreciated.

Best Regards,
Sb
 
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Some information on who made these and what sort of provisions there are for adjusting bias would be helpful. For example if autobias you just plug them in, no adjustment is required or possible as there will be no provision for adjustment - if fixed then adjustment is required, and pots and test points for doing so should be provided. Sometimes contacting the original designer would be most appropriate in such instances... Should sufficient information be known about the particular design any of us ought to be able to advise.
 
Other solution is micro controller variable autobias. It don't have side effect of Servo type autobias. It can keep 100% sound charateristic of fixed bias but easy to use. More safety to tubes & amplifiers. This topology can combine with power tube protection circuit to protection your amplifiers. You can use A/D converter or comparator to detect plate current and adjust it automatic by micro controller. It can adjust to very precise & easily. I design & sell to some tube amplifiers mfy already. It only need microcontroller + A/D converter or comparators + Digital potentialmeters.
 
I wish I could remember where I saw it but I remember reading an old article from an electronics magazine where they experimented with fixed bias and resistive cathode bias and found that in a dynamic sense the cathode bias provided greater output. As I recall the experiment was done to try to find out why cathode bias amps subjectively seemed to be able to play louder. IIRC the test amp was a PP EL84.

Anyone seen this and remember what the source was.
 
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I wish I could remember where I saw it but I remember reading an old article from an electronics magazine where they experimented with fixed bias and resistive cathode bias and found that in a dynamic sense the cathode bias provided greater output. As I recall the experiment was done to try to find out why cathode bias amps subjectively seemed to be able to play louder. IIRC the test amp was a PP EL84.

Anyone seen this and remember what the source was.

Interesting, and completely counter to my own experience.. :D
I wonder what the story is...
 
iam sure this question has come up before or is even unanswerable but i will ask regardless.

Iam planning to build a kt88 amp, i found the circuit for the modified williamson circuit on the Plitron website. A friend mentioned to me that fixed bias was for guitar amps and i should convert to auto bias. Should i keep the fixed bias or convert it back to the williamson? Your thoughts, opinions and past experiance would be greatly appreicated.

Fixed bias , is useful on high-power tubes (amplifiers) to avoid the wasting of power (and heat) on big cathode resistors (KT88 , EL34 , 6L6 ,etc) BUT for low power tubes like 6BQ5/EL84 , 6V6 , ECL82 , I always prefer cathode bias . Simple as that !!!
 
I have always wondered about the phenomenon of reaching a clipping point then having to back way down below the clipping point to get things back into line.

While having a big cathode resistor in a high power amp is a bit annoying, it does provide some helpful resistance to runaway.
 
I found this thread interesting, because it was unknown to me that fixed bias could på problematic. In the start of the thread, it seems that some preferred cathode bias because of subjective matters and the measuring types stated that it was oppesite to measurements...then the article came up with an explanation, but then the thread died...

I am hoping for some further discussion :)
 
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