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TedNugent's picture

Mod necessary for KT88's or KT90's?

Hi All,

Can someone please tell me if KT90's will work in my HD - 130 (chassis 2275-130) without a mod? Some posts have people installing with nothing more than a bias adjustment while others refer to a 'KT88 Mod'. A few posts seem to refer to tube size/length as a potential problem?

Confusingly there are posts which show people just popping them in with no problem.

I'm working under the assumption that KT90's will be not much different than KT88 and if a KT88 works fine a KT90 will as well.

Electro Harmonix KT90's rated for plate voltage of 850V so I'm good on that front. I've read posts cautioning about 'current draw' stating it might be a bit much for the MM transformer. Any thoughts on this? I don't know what that means.


The Nuge




you hve to alter the grid-resistors and, important, die BIAS-Voltage. For EL34 the range is about 28-44V, for the KT's you need up to 100V. From where? Difficult. And remember OT Raa, might be a different OT necessary.

TedNugent's picture


I've had it running at 25mV with no problems for a couple months now. Do you think I could safely boost it to 45 Mv? Where do you get the 100mV figure from? (I'm sure you don't mean 100V). Also - I didn't make any modifications whatsoever. Just biased properly (25Mv) and that was it. So far so good.


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