|[AMRadio] Re: [Boatanchors] 1625 tube, 807, 837|
k4kyv at hotmail.com
Sun Oct 10 15:24:14 EDT 2004
The 837 WAS an option with that amplifier. I recall a friend of mine bought
one of those amps to run AM. He planned to drive it with an Eico 720, plate
modulated. Not understanding the principles of low-level modulation, he
thought he could amplify the 40 watts or so output from the Eico to 800
watts modulated carrier. His amp came factory built, with 837's. It was
rated at 800 watts p.e.p. INPUT not OUTPUT, as was the standard back then.
On his first try, he burnt holes in the plate of one of the 837's. Those
tubes were very cheap on the surplus market (20c or so each), but cost about
$8 each (1960 dollars) at the local electronics store. He bought an
expensive new replacement, and although he avoided toasting any more 837's,
he finally came to the conclusion the hard way that that amp would not
produce an AM signal with any improvement over what the 720 would do without
If I recall correctly, the 837 is rated at 15 or 20 watts plate dissipation.
Four of them would have 80 watts plate dissipation at best. Running 100%
modulation, the most output you could get with four, without exceeding plate
dissipation rating, would be 30-40 watts carrier.
The plate dissipation rating of the 837 is about the same as that of the
802, but the plate structure of the 837 (although similar) is about twice as
long as the one in the 802, so in fact it probably would be safe to run it
at 30 watts dissipation, about the same as with the 1625. The 837 didn't
require any modification, but the 1625 requires rewiring the beam-forming
plates so that they are directly grounded instead of returned to the
cathode. Some brands of 1625's brought the BF plates out of the envelope
with a separate lead, and they were grounded to the cathode at the tube'
base pin. The modification required removing the base, or grinding a hole
in the side of the base, to gain access to the BF plate lead, and bringing
it out to a separate pin, or wiring it to one of the other grid pins. In
grounded grid service, leaving the BF plate connected to the cathode
bypassed the shielding effects of the control and screen grids, causing
internal rf feedback, identical to running a triode without neutralisation.
The 837 (same pin connections as the 802) is a power pentode, not a beam
power tetrode, and all 3 grids come out on separate pins, so it is easy to
wire up as a grounded grid amplifier. The 837's probably worked about as
well as 1625's, without all the hassle, and were cheap and widely available
as surplus at the time.
The amplifier was well constructed with good components (mostly WW2
surplus). My friend eventually sold his to another ham who used it on SSB,
and the friend used it for several years with good results.
My friend's disasterous experience with the amp was the result of the
deceptive advertising, common at the time, using p.e.p. ratings to inflate
the power capability of ham equipment running SSB. As commonly in the case
of sweep tubes, 800 watts p.e.p. input with that amplifier was achievable
only by running the tubes at excessive plate voltage (1000 volts as I
recall), relying on the short duty cycle of SSB to avoid melting down the
tubes. I doubt you could even run it at 800 watts DC input continuous on CW
in linear service without overheating the tubes. That would be 100 watts
per tube. I think you can run 100 watts DC input on cw with a 1625 or 807
in class-C service if you are careful.
This is an example of why p.e.p. is totally inappropriate to use for rating
the power output of a transmitter. It gained popularity in the amateur
community when SSB first came into widespread use because the manufacturers
could inflate the power ratings of equipment in the magazine ads, and the
pro-SSB lobby could add a bogus extra 3 dB to the alleged power advantage of
P.e.p. is useful for rating the undistorted peak output rating of an
amplifier, to indicate the power level where flat-topping occurs.
Understanding the principle of p.e.p. is essential in understanding how AM
linears and grid modulated amplifiers work, but it is meaningless when used
to rate the actual strength of a transmitted signal. Signal strength is a
function of the AVERAGE or MEAN power output of a transmitter.
That is why the present-day FCC definition af amateur radio power levels is
> From the P&H Electronics advertisement on page 144 of
>the November 1960 issue of QST:
>NEW modern styling! NEW high efficiency 3 element band-switching pi net.
>Puts more power into any antenna or load from 50-70 ohms. For SSB, DSB,
>Linear AM, PM, CW and FSK. All bands 80-10 meters. May be driven to 800
>WATTS PEP with popular 100 watt SSB exciters. Uses four modified 1625's in
>grounded grid. On customers order, will be furnished with 837's. (note:
>1625's and 837's are not directly interchangeable, since sockets are
>different.) Typical P&H Low Z untuned input. TVI suppressed. Parasitic
>Free. Meter reads grid drive, plate current, RF amps output. Heavy duty
>power supply using 816's. NEW modernistic grey cabinet measures approx. 9"
>x 15" x 10 1/2". Panel is recessed. WANT TO SAVE OONEY? BUY IT IN KIT
>FORM. It's a breeze to assemble and wire. BEFORE YOR BUY - SEE THE NEW
>LA-400 AT YOUR DEALERS.
> ...The 837 was definitely used in linear amplifiers in
>place of the modified 1625s.
...> > I don't think it was the 837 however.
>I recently had to look up specs on the 837 for a TBW transmitter I put on
>I was suprised to see how weak the 837 is compared to the 807 - much less
>power from a 837.
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