All,
I have a 8877 amplifier for 6m [the one which Steve G8GSK mentions
in his postings] which has an Eimac 8877 tube manufactured in 1993
in it. The amp is to the GJ4ICD/G3WOS design.
When initially built I couldn't get the loading control into range at
higher powers. Inm the end I took it down to steve GSQ's place and
got it on a network analyser and found that the pi-tank was a little
short of L. we added about 1/2 a turn and this fixed it... well at least
it appeared to...
I have run the amp from several different HT packs - the really big one
you can only just pick up and is aboput 3900V off load and 3600V on
load (has a 2750VAC at 2A transformer in it). This HT pack tends to
live in the contest van due to the fact that its difficult to pick up, carry
upstairs and install in the shack [next time I buy a house the shack will
be on the ground floor!]
When running from the big HT pack it tunes up fine and delivers about
1800W carrier with 900mA before the onset of too much grid current,
however I have noted that the lid (anode compartment) gets quite warm
above the blocking cap - which is a 1000pF 20KV HT57/58 (can't
remember) about 1" dia and 1.5" long.
Never had any flashes or bangs (I have a 1/4 stub across the RF output
to deck to keep HT off the antenna if the blocking cap were to go short)
but I do wonder if there is appreciable loss in the blocking cap and hence
the heat rise.
Last week I put the amp back into service in the shack with a much
smaller supply (3200V at 600mA) originally used on a single 3-500 for
6m. When tuning up the amp I have noticed that:
1. I can only get 400W out before high grid current
2. the standing current is very low (about 30mA)
3. gain appears to have gone up (10W in for 400W out => 16dB?)
4. it will take off and oscillate quite easily if mis tuned
Still helped me to work some K3s VE3s, VU2ZAP and UN6P recently ;-)
Any thoughts on:
a) blocking cap and heating/loss
b) current state of the amp
appreciated.
Mike G8TIC/M0VHF
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