• 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.

Tube auto fixed bias

Status
Not open for further replies.
Jump to Latest
#2
"Noise reduction by at least 40db (100 times) resulting with brilliant sound and clearer high-frequency tones
Clear undistorted sound for a great listening experience"

Fantastic.

"Furthermore, tube operating points need no further re-adjustments. The module constantly creates optimum conditions for the output stage, independently of tube aging and consequent changes in their parameters."

How about drive signal for class AB operation?
 
#3
Ah, in Class AB the bias goes up and down, it needs to and can with fix bias, if the board tries to 'fix' the bias it is going to break the party.

I know that you can pull down the bias with such a circuit if the tube is detected by the current sensor to be over maximum dissipation.

This could prevent an early tube failure of $$$ 300Bs
 
Last edited:
#4
It works splendidly. Apparently, the interval is long enough to ignore the signal and take the average of the cathode. Pavel also says it continually adjusts, rather than one time at power on. I'm just thinking it would be cool to remake something like this using only tubes just as a brain exercise. There are surface mount parts on the bottom of the board not visible in the pic on the site... It uses 5 opamps, and 4 transistors for the biasing from what I can tell which makes me think the tube version would be more complex than the rest of the amplifier.
 
#5
If you listen to class AB loud the bias jumps up at least 30%, for sustained periods of time, there is not very anything harmful to bringing down bias during that class AB from the auto-bias.

It is harmful if the class AB is going too high or in SET if the current is too much in the grid and the power dissipation raise too much.

I think it is legitimate to question auto-bias for tubes pushed to higher power.
 
#6
Agreed, but this module strives to keep the bias at whatever you set it to, regardless of the music program. I don't know how that works on a long loud passage though. Perhaps I should test it using sinewave drive... But my question, even if this board doesn't work ideally, is to simulate it's operation using tubes.

FYI: Operating data: 6P45S in triode, 300V B+, 120ma idle, 700ma peak. Calculated 9W Class A limit, 80W total. RLa-a=1k3:8R

What would be interesting would be if the bias circuit used the audio signal to calculate the required offset...
 
#7
The circuits i have seen use P transistor that pulls current through a resistor, bias voltage is taken from the collector. The emitter goes to gnd, and the base is a couple volts negative. This is great, as it allows the control circuit to be done (DC coupled) around amps ground +- couple volts for the opamps. As there are no P tubes, the whole thing should be shifted to a B- voltage, hm, complicated.

But IIRC, there is a schematic somewhere where a tube controls the bias.
 
#8
kodabmx said:
Here's a question for those who have more knowledge than I.

Would it be possible to create a circuit like this autobias board from Pavel at audioamp.eu without using transistors? Like an all tube version?

Automatic bias control module for four push-pull (PP) tube AB-4. Dimensions 74x77 mm. TES

Cheers
Koda
Click to expand...

Yes! Build the amp with a cathode resistor that supplies bias. This is self-adjusting. It will however cost some power. But it's reliable!

See The Valve Wizard -Push-Pull for a description
 
Last edited:
Status
Not open for further replies.
Top Bottom