Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Hellawatt on September 11, 2017, 10:45:49 pm

Title: 100 Watt Plexi Bias Circuit
Post by: Hellawatt on September 11, 2017, 10:45:49 pm

I recently acquired a 100 watt plexi that somebody built from a mojotone kit. Everything seems to match the Marshall 1959 schematic. It came with 6L6s, which I swapped out for a new set of EL34s. Now I can't get them to bias hot enough. The most I can get is 465V on the plate and 0.033A cathode current for about 14 watts dissipation. I want to get closer to 37mA for 17W . The bias circuit has a 22k, a 15k, a 10k pot, and a 47k resistor.

I tried increasing the value of the 22k resistor, which shifted the bias range up, but it also narrowed the range considerably. Is there a better way to increase the range, or shift it up without narrowing it? Do I need a different pot? I can only find these big bias pots in 10kl. Why aren't the stock values for this circuit giving me the right range if this amp was meant to run EL34s?

Thanks

Title: Re: 100 Watt Plexi Bias Circuit
Post by: sluckey on September 11, 2017, 11:38:23 pm

Change the 47K to 33K or 22K. That will give you a wider range. Then set the bias pot to mid range and fiddle with the 22K resistor value to give your desired bias point. Now you should be able to adjust above and below your desired bias point.

Title: Re: 100 Watt Plexi Bias Circuit
Post by: 2deaf on September 11, 2017, 11:42:39 pm

Put the 22K back in and reduce the value of the 47K.  If you have a 220K sitting around, try tacking it in parallel with the 47K.  If it still isn't enough, try tacking in a 100K.  If you find a parallel value that is acceptable, you could put it on a switch so that you can bias 6L6 or EL34.

Edit:  Sluckey responded while I was still typing/watching football.  Sluckey knows way more than me, so do what he said.