Ad Widget

**MWJB** · 06-11-2009, 03:10 PM

What do the other 2 power tubes read?
What happens when you swap the tubes in V4 & V7?
What's the screen voltage at pin 4 of each of the power tubes?

It is possible that you do not have a matched set (the 2 tubes nearest the PT should draw the same +/- 15% and the 2 nearest the PI tube should read the same +/- 15%), or that your bias balance pot is not set for an even current draw on both ends of the OT primary (2 power tubes at each end). If you can adjust the bias balance pot so that you get a matched reading accross all 4 power tubes (whatever that reading might be) that will be a start. Then you might have to adjust the value of the grounded resistor on the bias balance pot to raise the current overall (smaller resistor = more current), see what voltage you end up with at 35mA.

**Enzo** · 06-12-2009, 01:46 AM

My very first reaction was open or off value screen resistor.

**smokey** · 07-05-2009, 09:49 PM

Thanks to both of you for you suggestions.
I finally got along with it. I checked the secreen grid resistors and didn't find any opened or off I've changed them anyway. I also did a cap job with no success except a slightly higher B+. So I finally changed the bias' pot resistor for a lower value (10k) and ended with a good reading of 36mA/tube onced balanced. Was the stock bassman 100 underbiased or did I miss something?

thanks again

**vintagekiki** · 07-06-2009, 01:28 AM

10k is the balance pot (aka OTM or output tube matching) is not a bias pot.
10k balance pot set on the middle value.
Adjust bias (cathode current value at 25 mA) is to be made by change the bias resistor 15K.
Resistor higher than 15K = high bias = less cathode current = cold tube
Resistor lower than 15K = low bias = high cathode current = warm tube

Attached Files

Ad Widget

Announcement

Bassman 100 tubes drawing low current

Bassman 100 tubes drawing low current

Comment

Comment

Comment

Comment