There are a few incorrect statements here.....
> > Consider that additional cooling air is going to be needed to realize
> >100w extra dissipation--except perhaps in the SB-220, which has an
> >over-supply of cooling air.
Being involved with Heath, and reviewing the design of the 220, I can
assure you the air supply of the SB-220 is not "over-supply".
The SB-220 was designed to be used a 1 kW DC input power on CW,
dissipating about 500 watts for TWO tubes total. The cooling system
is inadequate for 1000 watts dissipation, since the anode and base
seals will go over temperature. The problem is so bad the base pins
often unsolder themselves.
Operating a 3-500Z at excessive seal temperature is a bad idea. The
L4B moves less air than the 220, but it moves the air more
effectively over the envelope. I have never done seal temperature vs
dissipation tests on the L4B, so I have no idea what its capabilities
> Rich...I'm running an L-4B bought in 1978 with the original tubes. If I
> chaange to the 3-600's I assume I'll get a bit more output. My question
> is, will I have to do anything to the Drake. There are fans over each tube
> pulling air upp through the chimneys and of course the original rear
> mounted fan works fine. The power supply has a fan over each diode bank
> and bleeder.
The gain of a grounded grid PA relates mostly to the driving
impedance of the tube vs output impedance of the tube. Unless
Svetlana changed the tube's internal geometry to decrease the input
impedance it won't have any more gain than a 3-500Z at the same anode
voltage. You might need to increase anode voltage or drive
power for more output.
The 3-500Z DOES, contrary to folklore, saturate at 400mA of indicated
anode current. IMD is NOT, contrary to folklore, highly dependent on
bias voltage in a grounded grid PA. So unless they made a major tube
internal geometry change, bias would make little difference. Once
the tube is conducting more than half the RF cycle, little
improvement is made by playing with bias.
Svetlana did increase filament current slightly in the 3-600, from
reports I've received from Svetlana.
It might be better to call the factory about the tube, rather
than depending on hear-say here on this reflector.
73, Tom W8JI
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