NADFTSABD

SG John said:
They're a sterile industrial tube with not very good musical properties. I was told by a couple of techs that Marshall installs them to make sure the amp gets out of the warranty period for new amps. They are identical to an EL34, so all you have to do is bias the amp. There is no changing of resistors or other stuff involved, as if you were going from 6550 to EL34, or to a 6L6. Maybe Plexi, Ivan, or someone else with better qualifications can chime in. When I went from 5881 Sovtek tubes to EL34s (first JJ, then old production RSD) in my '89 JTM45 reissue, it was a night and day change in how it behaved and sounded. It was good before, and great after.
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OK, I’ll give em a shot. Just ordered a set of JJs. There’s a set of RSDs in Germany, but the guy is on vacation.
 
steveb63 said:
Read the same thing myself, of course at your own risk........

My knowledge of amps is very rudimentary, don't know about having to check bias etc.... way out of my league.
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To run EL34
you have to change the screen resistors and change the bias voltage.
The screen resistor for KT66 / 5881 / 6L6 is 470 ohm.
(KT66 5881 6L6 is basically all the same tube) 5881 is the military 6L6. KT66 is the British 6L6.
The screen resistor for EL34 is 1000 ohm.
The bias for 5881 / KT66 / 6L6 is about - 52 to -57 VDC.
The bias for EL34 is about -42 VDC. So you have to change a resistor in the bias circuit and reset the bias to run EL34.

But what you have now:
you can change KT66 to 6L6 or 5881 without doing any mod. Should be able to set it no problem the way it is.
 
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