Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blueboozer on November 24, 2020, 12:17:03 am
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I’m building a JTM45 based amp and have an OT that has a primary if 8K. It’s supposedly tuned for KT66 power tubes. Wondering if I can sub EL34’s instead of the KT66’s. It’s a Classic Tone OT and of course it’s modeled after a KT66 version of the amp.
Thanks
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Screen grid current (and hence dissipation) at high power outputs could be excessive with such a high load impedance. There are maybe half a dozen circuit variants of used for JTM45 screen grid supplies, it would be best to use one that has plenty of resistance, in order to limit max current.
See https://mhuss.com/amps/JTM45/JTM45Classic2a.pdf
I suggest to use 1k5 or 2k2 for R32.
But really, 8k is higher then 'ideal' for any regular big bottle, my guess is that Marshall used it to limit power output, so as to avoid popping the greenbacks in the combo models.
An option would be to use a lower than intended load, in order to bring it down to 4k, eg 8ohm cab on 16ohm OT tap.
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EL34s need around 1/2 the impedance of KT66s so I would think you could just plug a 4ohm load into the 8ohm tap or 8 into the 16?
I know people do the opposite. They put KT66s into their EL34-based plexis using 8ohms into the 4ohm or 16 into the 8.
High chance I have that around the wrong way so dont do it without doing the math yourself first :)