Driver Tube Bias?

[Mikey]

Hi All,

If I were to use a TL431 chip to bias the driver tube (instead of the supplied LED), what would be good
values to choose for the two resistors (R3 and R4 on the C4S board) to set the bias voltage properly?

Assume I will eventually end up with a 6SN7 in the driver tube position...  ;-)

Thanks,

Mike

[Paul Joppa]

Get the newest board (used in the Paramount "soft-start") which has 2.49K for R3 and a 10K trimpot for R4. Bias voltage is 2.5*(R3+R4)/R3, for a range of 2.5v to 12.5v, and the tube current must be at least 2mA (1mA for R4/R3, plus 1mA minimum for 431 operation).

[Mikey]

Hi PJ,

I'm assembling the Speedball upgrade right now, and I'm going to add the 431 bias scheme at the same time.
Since the driver tube bias in the stock configuration is 1.5 VDC, I thought I'd get as close to that as possible, 2.5 VDC.
In that case, it seems that R4 wants to be a very low value, even a jumper wire...correct?

As you're probaly aware, I've converted my Crack driver tube to a 6SN7.  Is 2.5V a decent bias voltage in this application?

Thank,

Mike

[Paul Joppa]

... Is 2.5V a decent bias voltage in this application? ...

No, it's not. It's a direct coupled circuit, so the bias controls the operating point of the output tube. 2.5v will move the driver plate from 70v to around 100v. It may "work" - tubes are amazingly tolerant of abuse - but it's a major change.

[Mikey]

Thanks Paul,

I guess I'll scrap the 431 bias idea and leave the LED's in place...

Mike