# Questions about tube conversion in a Marshall 6100



## incidentslip (Dec 29, 2009)

Hi, I have a 30th Anniversary 100w Marshall head, I was wondering what's all involved in converting the head from 5881's to EL-34's


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## JMC Amps (Dec 19, 2009)

Easy swap:

Your screen resistors should be replaced from 470R to more EL34 friendly 1K 5W (R 251, 252, 253, 254). These values can be left there for 5881s also.

Adjust bias pot RV201 to attain proper bias. You may get lucky and find the proper bias range with the stock bias circuit, but may have to adjust R212 from 33K to 22K to shift the V-. This is assuming a –57v to –60v supply. I would be cautious about using Marshall’s –42V and –50V recommendations. I found these to be too hot. The best way to bias an amp (IMO) is to check for actual current through the tubes via a 1ohm cathode resistor by using ohm’s law. Assuming a 475 plate voltage, aim for about 32-36mA.

Many EL34 amps see 220K grid leaks in place of 150K (R234, 235). I’ve used either.

BTW, The heater current of EL34s is WAY more than 5881s, so this assumes Marshall uses the same PT (Dagnall T5826) for either tube.

See:

http://www.drtube.com/schematics/marshall/6100-63-04.pdf

http://www.drtube.com/marshall.htm#AN30

Regards
-JMc


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