Marshall JMT45 Reissue PTP Turret Board - Need More Bias

[MeanMisterMustardCap]

Hello All!

I'm new to the forum, so please forgive me if this is a redundant post--I've been combing the forum for over an hour now and haven't found any other posts covering my topic/question.

I have one of these installed in my Marshall JTM45 reissue: http://metroamp.com/wiki/index.php/JTM_ ... kit_layout" onclick="window.open(this.href);return false;

I just put in some new tubes and the bias trim pot maxes out at roughly 29 mA. By my calculations (for KT66 tubes) I need to be up around 42 mA - 45 mA. I was going to solder ("jumper") an additional 47K 1/2 watt carbon comp resistor where the existing bias resistor is. Is a 1/2 watt resistor sufficient for this mod?

Many, many thanks for your input.

Chuck

[toner]

Depending on your plate voltage the bias current should be up around 40mA so yes, you need to decrease the 47k resistor. Parallel-ing another 47k may take you too far. I would try 100k or 150k or just replace the resistor with a 33k. I'd use a CF or MF rather than CC. 1/2w is fine.

[MeanMisterMustardCap]

Thank you so much for your reply!

My plate voltage is currently 422 volts. I have some 1/2 watt resistors lying around but they look "skinny" compared with the stock 47k resistor already in the bias circuit and was afraid this might give me some issues once wired in. Is is plausible that this is a 1 watt resistor?

Thanks again

Chuck

[toner]

Sure, the existing R could be 1w. 1/2w is more than enough because there is so little voltage there. One reason to avoid CC's is because they can drift high in value over time which will obviously affect the bias voltage to the tubes.

[MeanMisterMustardCap]

VERY interesting re: MF v. CF v. CC resistors. Thank you very much for the supplemental info. I'll be sure to ask specifically for MF or CF resistors when I hit up Radio Shack later today.

You've been incredibly helpful. Thank for sharing--I really appreciate your assistance.



Chuck