Some 12 years ago, planning to build a new HF amplifier, I was in doubt if
using a known, but a bit more "delicate" 8877, or the recent (at that time)
3CX1200A7.
Having already experienced with the 8877, I decided to play with the new
triode.
Generally speaking the amplifier came out well, unless a long fight against
a self oscillation in the near of 50 MHz.
According to a number of suggestions, reporting inherent tube problems
due to the construction (not existing in the Z7), I made some of the mods
proposed (tube base shielding).
Refining also the plate suppressors, the oscillation was reduced to a quite
low level, becoming undetectable (for my spectrum analyzer) if only a little
driving power is applied. (static biasing changes doesn't produce the same
effect)
I know this is not a new topic, but as many suggestions I got in the past
to fix this problem, I wasn't ever able to get a full success like claimed
(promised) by proponents.
May be it's impossible to stop (my ? ) 3CX1200A7, or may be didn't exactly
understand how to perform the mods.
Is there anyone who could e-mail to me pictures about those fixing methods ?
73,
Mauri
E-mail: i4jmy@iol.it
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