Talk:Heathkit SB-221 HF Amplifier
Using 3cx1200a7 ceramic tubes
http://www.redwaveradio.com/8_cd6d4ebd835b1158_1.htm
-- Is this direct replacement, which would require no modifications? Just -- plug and play?
It is a good replacement but not quite plug and play....
The basing of both tubes in the same, so the same socket could be
used.
I originally thought the same filament transformer could be used, but
it's voltage would have to be increased by 2.5 volts AC.
The filament current required by the 3CX1200A7 is 21 amps; but the
original filament transformer for the pair of 3-500Z's was 30 amps., so there is enough current available.
I have seen people use the same filament transformer and it may have
decreased their output power as compared to what would be possible if the filaments were run at full voltage; but if you start out at 5.0 volts and then unload the transformer winding by reducing the current drain by 1/3, the voltage may go up to 6.0 volts or more.....
The plate voltages are compatible at 3000 to 3500 volts.
The bottom of the chassis would have to be pressurized to force air
through the cooling fins of the ceramic triode. that was not necessary with the 3-500Z tubes.
And the PI-TYPE output tuning network may have to be changed to match
the new value of plate capacitance and the new operating point (plate impedance) of the ceramic tube.
--- if the drive into amp was not change is it a significant power increase? --- 1% 5% --- ??
According to the Eimac Catalog, the 3CX1200A7 should produce about
1500 watts output with only 85 watts of drive (with 3600Volts on the plate).
Eimac would rate a PAIR of 3-500Z tubes at about the same output if
we run everything within the Eimac specifications.
We all know both tubes can be 'pushed' to much more than that.
My experience is that a single 3CX1200A7 can as well or better than
(3) of the 3-500Z's....
Henry 'conservatively' rates a pair of 3CX1200A7's at 5000 watts
output in one of their 'export' linear amplifiers.
And I have seen over 15KW PEAK from one of those 'export' henry
amps..
Dennis
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