From: "John Smith" on Sun,Jun 5 2005 12:39
pm


Yes, I should suspect that a lower freq for a high freq tube is no
problem... I think why you don't see them ever used at 144 Mhz is that
there used to be VERY expensive--you would only pay the money if
necessary and there were much cheaper alternatives for 144...

The 2C39 was never intended for lower-frequency "CW" or other
narrow-band modes. General Electric used them in 1800 MHz
pulsed microwave radio relay terminals having peak pulse
envelope output of 12 Watts...with forced-air cooling on
the integral anode fins. There were six 2C39s in every
GE 1.8 GHz terminal designed about 1950 (!).

The transconductance is very high (14,000 umho?) and the
grid-cathode, grid-plate spacing very small. The only sure
way to hold such tolerances in varying heat environments is
to use non-standard glass or ceramic tubing sections in the
inverted lighthouse structure. Those who want to use it have
to have an equally-expensive socket arrangement or be a
fairly competent machinist have good tools at their disposal.

The GE microwave terminal used integral socket and tuned
cavity assemblies at 1.8 GHz. The GE transmitter was a
chain of 7-pin and 9-pin tube oscillator-multipliers to
about 255 MHz, then into a 2C39 septupler, that into the
2C39 final amplifier (pulsed anode). Receiver LO had a
similar arrangement but only one 2C39 in the chain. "Hot
spare" duplicate assemblies for the RF doubled the 2C39
totals. They worked and worked and worked in 24/7 operation,
nine terminals where I was the operation and maintenance
supervising NCO in the Army. There were hundreds of them
used in many kinds of multi-channel radio relay, including
long pipeline telemetry and control in the USA.

The only thing "bad" about them was due to another tube
manuafacturer's dimensional conflict with GE cavity design.

Why that particular tube? Because it "looks sexy" with
the anode fins so nicely machined? There's lots more
half-century-old vacuum tube designs available somewhere.
Western Electric 416s and 417s are out there in larger
quantities. Geez, 829 duals have been used at 144 MHz
with more output than one can safely run a 2C39 at that
frequency. Socket assemblies for an 829 are easier to
get than for an inverted lighthouse structure.

A year or so ago I happened across a couple of non-USA
sites giving nice details of 2C39s used in ham VHF
transmitters. Nice photos and dimensions available.
Both of them used the triode "pushed" beyond its safe
limits of power. Think about replacement costs.

At 2 meters, a better way using vacuum tubes (besides
going to Eimac goodies) is in a "distributed amplifier,"
the grids of many 9-pin tubes connected to an L-C
delay-line-like structure, the plates connected to another
L-C network. The Tektronix 540 series scopes had those
for the vertical amplifiers, good beyond 50 MHz, back in
the 1960s. No tuning required...except in how to couple
the medium-Z output to a low-Z antenna.

Or...one can get transistors that have worked in their
ratings at 150 MHz VHF "business band" with all the nitty
and gritty details in old Motorola Application Notes.
CW operation at 2 meters no sweat at up to 150 Watts,
more than a 2C39 pair can do safely. Lots of alternate
choices...such as solid-state Cellular Telephone cell
site ("base station") transmitters and equivalents that
don't have internal networks to optimize operation in the
1 GHz band. [but, you knew that already, right? :-) ]