>
>Hi everybody,
>
>I picked up a Mcmartin broadcast transmitter, I'am going to put it on
>10-160m.
>Anybody ever converted one of these before?
>
>Pair of 4cx5000a7
? 4CX5000A/8170?
> in the final, all overload circuitry is done very nicely so
>I will leave it as is. The input circuit is going to be a 9:1 xfmr and 450
>ohm 100watt resistor parelled and grid driven. Plate circuit will be a roller
>inductor and vacuum caps for a pi network.
>
>Anybody have any thought on this amp?
>
Assuning that you are going to use in Class AB1: The paralleled grids
total c. 240pF, so needs to take care to reduce conductor losses on 10m
and 15m. // 9:1 trifilar transformers are a sticky wicket for wide
freq. coverage. I was able to construct a successful one (using few
turns) for 20m to 10m. To cover 160m to 40m, more L was needed.
However, more turns also added more C, which caused unacceptable swr esp.
above 20m. (see the AB1 grid circuit on my Web site for details) Thus,
you might wind up with two, switched trifilar input transformers, one for
the lower bands and one for the higher bands. // My guess is that you
will need 200w to drive it on 80m and probably more higher up due to grid
circuit losses.
- 8170s must be neutralized or the input swr will be bad news, except on
160m. .
- For optimal VHF stability, put the 450 ohm grid termination close to
the grid and use minimal lead lengths to the grid connections on the
sockets. Also, triple bypass the cold end of the resistor to gnd.
(note: the cold end does not connect to gnd because this is where the DC
grid bias supply connects.)
- With a pair of 8170s, one can wind up with 80a of RF circulating
current, so, for ssb, the tank inductor needs to be at least 1/2 inch
copper tubing, with forced air cooling. If a pressurized chassis is
used, an easy way to cool a tank inductor is to drill a grid of c.
20mm-spaced small holes in the chassis underneath the coil.
- If the amplifier is going to be driven by a modern radio, high speed
switching would be appropriate. Three RJ-1A vacuum relays in parallel
will be able to handle the output current. In order to speed 'em up, the
power source needs to be approx. 100VDC with a suitable resistor to limit
coil current to 0.08A.
- good luck/have fun, Mike.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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