Interesting autobias..

I was thinking of trying my (inexperienced) hand at designing an auto bias ckt and then I came across this

http://www.curcioaudio.com/BCS_3.htm

Anybody have experience with these?

mcgyver74 wrote:I was thinking of trying my (inexperienced) hand at designing an auto bias ckt and then I came across this

http://www.curcioaudio.com/BCS_3.htm

Anybody have experience with these? :)

Looking at the schematics found at curcio , the grid resistors for the el34 is
220k , which is adequate for EL34 tubes but not 6550 tubes.

No, i have no experience with this, but sure i would like to test it.

Analyzing the circuit it seems that there is 4 comparators, one for each tube.
The comman catode resistors (11.2 ohm) is replaced by individual 5.6 ohm, they are
used as voltage measuring points, affecting the amount of bias ( negative voltage) that
is supplied to each tube.
Time constant seems fairly short ( 0.1uF / 33k ) which indicates a rapid response,
needed as there is no memory, at power on it starts to regulate bias and keep it
on the prescribed value, 45mA as adjusted with P1.

As long as the board is below 70 degree Celsius it should regulate fine, and keep the
tubes fine. Maybe the short time constant can affect lout passages in music, only
test will tell. If so, one could increase the C1-C4 somewhat.

Tho note about the grid resistors ( R1 R3 R5 R7) are 220k, and prohibits use of
6550 or KT88, i see no reason they cannot be reduced to 50k as needed for
fixed bias operation. But test is needed to confirm that that transistors will
drive this when grid current starts to flow.( in normal circumstances nothing bad
will happen, but when the tubes get real hot, then the risk of grid current and redplating,
and here is where lower grid resistors will save the tubes)

Go ahead, install and report back!

I recently ripped the Curcio autobias circuit out of an amp at a friend's request.........

audiobill wrote:I recently ripped the Curcio autobias circuit out of an amp at a friend's request.........

Ok..

Why ? What was the problem ?

peterh wrote:

audiobill wrote:I recently ripped the Curcio autobias circuit out of an amp at a friend's request.........

Ok..

Why ? What was the problem ?

That would be my question as well.

I have these thoughts on a feedback-activated bias system as the Curcio device seems to be:

a) We can probably agree that a few percentage variation in feedback voltage is inaudible. Using the 1.56 (indicated), between say 1.50 and 1.58V would not be detected.
ai
b) Meaning that if the chip on the board can manage to keep the swings based on line-voltage within the inaudible spread, there would be no material effect on the sound.

c) Leading to the conclusion that moment-to-moment, tube-to-tube stability would be achieved leading to greater amp stability and longer tube life. All good so far.

Now, what happens if the bias oscillates sufficiently to be physically safe for the tubes, but audible to the listener?

So, what was the reason this board was ripped out? Inquiring minds want to know!