At 01:16 PM 2/16/98, you wrote:
>
>I'm not familiar with the MLA 2500, but if the HV metering arrangement is
>anything like the SB-220 then the "1500v" may be the result of bad
>multiplier resistors rather than genuinely low plate voltage. But can
>anyone tell him what the grid current's supposed to be at rated output?
>That's what I'd be really worried about, given the price of 8875s.
>
It's also possible that the MLA-2500 in question is one of the original
designs with the dual voltage (SSB/CW) supply. In that case '1500V' is
about right for the CW setting. However, since the multiplier resistors
in the MLA2500 were the typical 2W carbon comp resistors (3 x 1 M plus
100K) and were mounted on the rectifier PC board where they received no
cooling, I would not trust them.
As to the grid current ... the 8875 has a 5 WATT grid just like the 8874
and the 3CX800. Eimac rates the grid of the 3CX800 at 36 mA max ... I
would hesitate to show more than 60 mA grid current for the two tubes.
Since the MLA2500 uses a 1A full scale meter for grid current, any more
than a "wiggle" on the stock meter is too much ... I would recommend
increasing the existing grid current shunt (R19) by a factor of 10 so
the grid current meter would be 100 mA full scale.
>From the Varian Power Grid Tube Catalog:
8875 as cathode driven RF Amplifier at 30 MHz
class of operation - AB2
plate voltage - 2000
plate current (A) - .50 (single tone intermittent voice)
- .31 (two tone plate current)
drive power (W) - 26
useful power out - 587
Note: "usefull power out" is the power measured at the load and does not
include power lost in the output circuit (pg 4, Interpretation of Catalog
Data).
Based on Eimac/Varian published data, anyone seeing 1500 Watts out of an
MLA2500 is seriously abusing the tubes and probably generating a lot of
garbage on the bands. Even at the Eimac ratings, the tube's rated plate
dissipation is being exceeded (ca. 400 watts) in high duty cycle operation
(heavy voice processing, RTTY, etc.) operation.
73,
... Joe Subich, W8IK ex-AD8I
<w8ik@ibm.net>
"Hindsight is an exact science."
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