Posted by PakProtector [ 152.163.96.144 ] on September 16, 2005 at 19:32:24:
hey-Hey!!!.
I am preparing a modification to the cascode front end of the Merlin amp. As drawn it shows a twin triode cascode. I am preparing to substitute a MOSFET for the upper triode section. An IRF820 to be exact.
It shall be connected in exactly the same fashion. the two gates tied together with a gate-stoppe of 150R and referenced to the lower source of the voltage reference CCS. No change to the voltage, as this parameter is critical to the performance of the lower input section.
It will offer a moderate increase in available voltage swing, since it has a very low voltage drain-source minimum. Say 10V instead of the 6H30's 50 Volts.
Anyway, it is an interesting looking mod, and offers a means to execute the amp a little bit differently. It also removes the slightly hard to source 6H30. With HY1269's as finals, a 6J6A in the bottomof the cascode, and a pair of 12AX4 damper diodes for the rectifier, the 6H30 is the most expensive tube in the line up. OTOH, IRF820's are more expensive than 6J6A's....
I'll let you know how it turns out soon.
cheers,
Douglas
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