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Old 7th June 2009, 12:01 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Default KT88 triode strapped question

I'm currently building a PP amp using original GEC TT21 (KT88 equiv) and was wanting to build it as a triode amplifier, connecting anode to G2 with a 120ohm resistor. However I've found my B+ will be around 525V, so was wondering if this is a good idea?

My output transformers have no ultralinear taps so that's out of the question, and I was only looking to build a 25W amplifier anyway; so triode rather than pentode mode suits me best. The spec sheet says 600V G2 limit, but would this ever be exceeded in heavy usage? I don't want to risk a meltdown.

The amp will be fixed bias, running about 60mA per tube.

Thanks!
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Old 7th June 2009, 02:45 AM   #2
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I had a look at several spec sheets, where all give the triode maximum voltage of a KT88 as 600V. These should apply to continous useage, so I cannot see a problem in your case.

Not too reassuring is the fact that maximum anode dissipation is given as 35W, 40W, 42W and even 50W depending on the manufacturer! Unfortubately GEC does not give triode ratings, and perhaps (hopefully! ) the more eastern manufacturers are making their tubes a little more rugged to withstand the higher dissipations. But that is not your worry here; I would say you can be confident about the 600V rating for triode (it is a voltage thing in your case, not a dissipation thing).

You also mention 'heavy usage'. If that is so time-wise it should be as reliable as any tube rating is. 'Heavy usage' as in constant high output conditions would rather cause a slight decrease in h.t. than the other way round; ventilation you will have to see to. If for guitar use, as that you might use it in various places, I would only worry about a too high mains voltage somewhere - but that will have to be fairly outrageous to endanger your equipment. There you will have to check for yourself.
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Old 7th June 2009, 03:11 AM   #3
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SHiFTY,

Wire your sockets for KT88s and use adapters for the TT21s. Sooner or later (hopefully much later) you will have to use current production "bottles" and the Saratov, Russia, made "reissue" GEC KT88 is the best current production thing available, in this category.

Use 100 Ω resistors to tie the screen grids to the plates. I don't think you will have any problems with your "tall" B+ rail. As long as the g2 dissipation limit is not exceeded, a few extra Volts should not make waves.

1974 MOV KT88 Data Sheet

1961 MOV TT21/TT22 Data Sheet
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