[Amps] 8877 on 6

["John T. M. Lyles" <jtml@lanl.gov>]

Gary

The Harris socket arrangement is definitely for lower frequency, and 
would need some improvements to build a stable 8877 amp for 50 MHz.

There are papers on buiding a 6 meter amp using this tube:
W6SAI Eimac Amateur Service Newsletter AS-42
(reprint of Ham Radio Feb 1971 article)
probably same in Orr Radio Handbooks of the era

I helped design a broadcast transmitter for 88-108 Mhz using the 
8877/3CX1500A7 in 1983, for 1.5 kW CW output 24/7. We started with 
the Johnson socket as it is inexpensive. The trick is to lower the 
socket under the deck, for the cathode input circuit, placing the 
grid ring at about chassis level. provide a solid ring of contacts 
around the grid connector, (could use that part in the Harris socket 
for that). The grid needs to be grounded for VHF here, not with the 
thin spiders of metal I see in the photos of the Harris socket you 
posted. Use either a solid ring of copper or aluminum, or four wide 
strips as a minimum, from the ring/plate you have to chassis. Since 
you have to provide cooling through that, there obviously needs to be 
some holes or slots, but be aware of the RF current paths.

The Harris socket you showed is similar to many 1 MHz designs, that 
won't perform well at VHF due to the excessive hole sizes (leakage 
through) and high inductance at VHF. Just use the porcelan part for 
the cathode circuit and perhaps the grid ring, with the modifications 
suggested.

The big chimney you showed should be fine. However the anode 
connection looks marginal. You need to provide a more solid 
connection, either a wide connection going around the tube, 
preferable with a clamp to the anode or perhaps a strong fuse clamp 
on the top cap of the tube going to a wide strap to a tank circuit.

73
John
K5PRO




> Date: Sat, 22 May 2004 21:04:13 -0400
> From: "Gary Mitchelson - N3JPU" <n3jpu@speakeasy.net>
> Subject: [Amps] 8877 Question
> To: <Amps@contesting.com>
> I have been looking for a good quality 1KW class amp for 6M but so far no
> real luck (not interested in using 3-500's on 6 again).
>
> I do have a couple of nice 8877's and a bunch of parts from some Harris
> RF-354 HF amps I scrapped but had some questions on suitability of some of
> the parts for 50Mhz.
>
> 1) All the 6M 8877 amp designs I have seen use the SK2200 tube socket. What
> I have from the Harris amps are Johnson 122-247-202 sockets, they have a
> full 360 deg silver finger stock Grid Ring. Are there any issues with using
> this for 50Mhz?
>
> 2) The chimneys from this are made by Harris. They are not like the straight
> wall chimneys I have seen on 8877's, these have a 5" opening in the base and
> flair down to around the tube (the RF-354 uses a single muffin fan for the
> cooling, although they spin it at 11,000RPM !) I plan on using a quiet
> squirrel cage type of blower. Any problems with using a chimney of this
> design?
>
> 3) Lastly the Chimney has built in Plate connection made of silver finger
> stock. All the other designs I see use some sort of clamp around the plate.
> Any issues using this type of connection over the clamp?
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