5881 to kt66

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5881 to kt66

Postby nyazzip » Sun Mar 08, 2009 2:04 am

i just got a marshall jtm45 reissue for a pretty darn good price. all the "gear heads" say i Must Replace The Stock Tubes With KT66, at all cost.
to be honest, i think the amp sounds pretty darn fantastic as is. but, fine, i'm curious, and i'm a big jimi hendrix fan to boot.
heres the thing: i have been unable to find any stats on the stock reissue "drake" transformers; i gather the kt-66 tube requires maybe 60mA more filament juice than the stock (5881) tubes, ideally.
i have searched around and really found no definitive answer. so my question is, what do i need to worry about to switch a stock marshall reissue jcm45 from 5881s to kt66s, using the stock transformers? i'm not inclined to get into the transformer "upgrade" game at this juncture.
[on a side note, i ran this amp via the 8 ohm tap into a 25+ ohm(on paper, 32 ohm) cab for several hours, much of that on full volume, and its still fine...don't ask. actually i've had this "32 ohm" cab for few years and used it with everything, and thought it sounded fine, but maybe a lil quiet for what was going on. revisiting my traynor "motorboating" issues from last summer, well, i think this miswiring is why.... long story short, seems like tube amps can handle a hell of an impedance mismatch, for extended periods]
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Postby dcgillespie » Sun Mar 08, 2009 6:51 am

The KT66 tubes will require an additional 0.6 A to power the heaters (not 60 ma), which is nearly an additional 4 watts drawn from the power transformer. Lacking any transformers specs, all you can do is check the effect that the increased current draw might have on the heater voltage and B+ voltage as well to see if the extra draw is excessive. I would also monitor the transformer temp. If the heater winding can't maintain 6.3 vac with the bigger tubes, that's a sure sign of overload.

Running a VT guitar amp at high power with a significantly higher load on the transformer secondary than it's rated for is surely playing with fire. It can take out the output transformer and output tubes from internal arcing in worst case scenarios.
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