[Amps] 3CX1500A7/8877

[John Lyles]

Around 1980-84, I was an RF engineer on a team that built a series of 
new FM broadcast transmitters in Quincy, Illinois. All were tetrode 
designs using the newest tubes with handles from Eimac at the time, 
4CX3500A, 4CX7500A, 4CX12,000A/8989, and 4CX20,000A/8990. At the time, 
solid state devices were mostly bipolars and only NHK in Japan had just 
started experimenting to build a kW with MOSFETs. We built one triode 
amplifier for the lower power broadcasters, the Broadcast Electronics 
FM1.5A. It had a transmission line PA made from copper water pipe, 
folded around a U shape with a 3CX1500A7/8877 at one end. It was a 
grounded-grid circuit usable to 108 MHz, with 1.7 kW CW output. 
Efficiency wasn't bad, as it was a cavity circuit, and the input tuning 
was very close. The grid to ground inductance was very low with the tube 
socket that we made.

A few years ago I traded out a scrapper Medium Wave transmitter to a 
fellow, who sent me the PA drawer out of a FM1.5A. That brought up lots 
of memories and deserved photos.

It was always pointed out by Sutherland and Orr at Eimac to get the 
cathode inductance contribution in the circuit as small as possible, or 
to tune it out. Otherwise a portion of the drive voltage between cathode 
and grid is across that inductance. But we were talking about VHF.

73
John K5PRO



> Date: Sat, 11 Feb 2017 11:06:28 -0600
> From: Kevin <kstover at ac0h.net>
> To: amps at contesting.com

> Jim brings up a good point that didn't cross my work addled mind last night.
>
> On 10m FM you aren't going to get 60+% efficiency. Add to that 1500W
> continuous carrier on FM for lets say an hour? The Pi-L tank will be a
> puddle in the bottom of the cabinet. What's an 8877 rated for in CCS
> service (since ICAS is now verboten)?
>
>
> On 2/11/2017 10:51 AM, Kevin wrote:
>> All day, key down, maximum smoke? I doubt it.
....
> R. Kevin Stover

> Message: 3
> Date: Sat, 11 Feb 2017 10:44:40 -0800
> From: "Jim Thomson" <jim.thom at telus.net>
>
> ##  8877 is rated for 4 kv at 1 amp CCS..provided you blow enough air through it.
> Even Dick Ehorn said you wont get more than 50% eff on any of these bandswitched amps,
> on 10 and 12 m bands.   68%  on 160-80 40m. 65%  on 20m.   60% on 15M.  Those are his
> words not mine.  But I would agree with that general statement.
>
> ##  at one time there were several 1.5 kw  CCS  FM  broadcast TX in the 88-108 mhz band,
> that used a  single 8877.      Single  3CX-800A7 were used in FM broadcast at 750 w.   Pairs
> of 3CX-800A7  used at 1.5 kw on FM broadcast.   24 and 365.


On 2/11/17 4:26 PM, amps-request at contesting.com wrote:
>> ##  8877 is rated for 4 kv at 1 amp CCS..provided you blow enough air through it.
>> Even Dick Ehorn said you wont get more than 50% eff on any of these bandswitched amps,
>> on 10 and 12 m bands.   68%  on 160-80 40m. 65%  on 20m.   60% on 15M.  Those are his
>> words not mine.  But I would agree with that general statement.
> I built my first bandswitched 8877 amp back in the seventies for
> publication in the Bill Orr Handbook (which paid for the parts). When
> bench-testing the amp, I was surprised to see the efficiency on 10m
> was only 50%, even though the tube was rated to 250 MHz.
>
> Bill put me in touch with his colleague Bob Sutherland (w6UOU and
> later W6PO), who suggested rewiring the input pi-net so that its
> output capacitor was soldered permanently to the cathode pin of the
> tube socket, with no more than 1/4 inch lead length to ground. I was
> dubious that this change could possibly have any effect at all, but
> when I followed Bob's suggestion, the efficiency jumped up about 10%.
>
> I never did understand exactly why this cap placement had such a huge
> effect on the amplifier's efficiency, except that it had something to
> do with the parasitic inductance of the capacitor leads and the
> importance of having the stored charge in the capacitor readily
> accessible during a large fraction of the 8877's conduction cycle.
> Maybe somebody can explain it to me. Bob really knew the ins and outs
> of vacuum tube amplifiers.
>
> I miss both those guys.
> 73,
> Jim W8ZR

> Message: 4
> Date: Sat, 11 Feb 2017 21:32:42 -0600
> From: "Gary Schafer" <garyschafer at largeriver.net>
> Hi Jim,
> My understanding is that the cathode needs a low impedance to ground in
> order for the high frequency plate current harmonics to develop properly.
> 73
> Gary  K4FMX