I don't know about the L-75, but it's a single 3-500Z. The L-4B uses a two 3-500Z tubes, and the
recommended max Grid current is 220 mA for the pair. The Grids are pretty tough, dissipation is
about 20W, so there is a little leeway, as opposed to external anode tubes which tend to 'poof'
Grids if you overdrive them just slightly.
The L75 has about the same plate voltage as the L-4B, so a Grid current in CW/TUNE would be 120 mA.
If the amp is loaded correctly for 1000W _INPUT_, ~2600 VDC @0.375 mA, and 120 mA of Grid current,
it's right. You switch to SSB by increasing the Plate voltage, and in the L-4B at least, that
implies that the proper operating conditions are met. You can't use the meters to measure anything
under voice conditions anyway!!
IMPORTANT! Don't forget that the L-75, like all other ham transmitters of the era, are rated in
INPUT POWER to the PA. When the L-75 is loaded properly into a good 50 ohm load, the CW OUTPUT
power should be ~550 - 600W MAX!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
BARRY SPEISER wrote:
I have finally found a L75 after a long search. I am driving it with my TR7. The manual gives
maximum output on CW at .375 ma plate but no grid current value? Same for SSB, no grid value. RTTY
and SSTV have values listed for both plate and grid. Any one have a L75 and can supply what plate
and grid you are running? Thanks.
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