Bias on Echolette Shostar S40

[YellowScout]

Hi all, I would like to run 6L6's in this amp. (Designed for EL34.) There's about 450 volts on the plate, but the trouble I'm having is getting the bias voltage lower. The attached schematic shows a 100 ohm pot coming off the bias supply, but mine has a 15 ohm, and a 100 ohm in the balancing position as shown. As the resistance is turned down so is the bias voltage (which seems counter intuitive to me). The lowest bias voltage I seem to be able to achieve is 38 - 40 mv measured across the 1 ohm resistor coming off the cathode with the pot turned all the way down. I had been running it that way, but I burned up my tubes last night. From the bias calculator I was looking at it seemed that should be hot, but within tolerance. So I'm not sure if it's too hot or just bad tubes. But either way I'd like to be able to bias cooler if I wanted to anyways. I haven't seen another bias supply set up like this, and so I'm at a loss. Let me know what you think.

Thanks, David

[Jazz P Bass]

Yeah, 40mv's is a bit toasty for a 6L6.
There is another trim pot over at the power supply.
Will adjusting that give you more range?
Try it.

[YellowScout]

The other pot controls the balance between the two power tubes.

[Enzo]

450v 40ma, that is 18 watts. Ought to be OK.


Yes, tubes can fail at any time.

Please be more careful with your terms. "Bias voltage" is the voltage applied to your control grids. That will be something in the area of -45v or so. The voltage across the 1 ohm resistor represents tube current (in a 1mv = 1ma ratio), in this case idle current. SO you are trying to reduce the current through the tube, not the bias voltage. If you reduce the bias voltage, you will increase the tube current.

The schematic shows 100k controls, not 100 ohms, very different. Are you sure the contro you have says 15 ohms on it?


The control in the bias supply sets the voltage. Turning its resistance lower will increase the bias voltage reaching the tubes. That in turn will lower the tube current. The balance control just shades the bias more toeards one side or the other to make them even.

The schematic shows the power transformer making 50VAC for the bias supply. That is enough to make about 70VDC, which is more than enough to chill the 6L6s right down. What is the range of voltage at pin 5 of each power tube as the R53 control is turned back and forth? You can remove the 6L6s to do this test if you like.

[Jazz P Bass]

Oh.
What voltage do you have at Point 3.
With the 100K pot at zero ohms, if you still are not negative enough, then I would say you have to change out one of the sets of resistors at the "balance" adjust.
Try lowering R40 & 41.

[Enzo]

Or raise R51.

[YellowScout]

Thank you for helping me use the proper terms. I love amps and I'm desperately trying to learn all I can. So, after rechecking I have 480 v on the plates, and the lowest I can get the current down to is 36 mv. It defiantly has a 15k ohm pot for the bias supply pot. At 0 r on the supply pot there's -42 v on the screen and gets down to under -20 v close to the 15k mark. I think maybe it was just bad tubes, but I just wanted to make sure I wasn't over working them, although I didn't think I was judging from the Jim Jones bias table.