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[Amps] 8877 on 6

To: amps@contesting.com, n3jpu@speakeasy.net
Subject: [Amps] 8877 on 6
From: "John T. M. Lyles" <jtml@lanl.gov>
Date: Mon, 24 May 2004 11:11:45 -0600
List-post: <mailto:amps@contesting.com>
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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