[Amps] SB-220 relays fixed; questions

[Pete Smith]

Thanks for comments on my previous post.  I replaced both vacuum relays in 
my modified SB-220 this weekend, and have just been testing.  As a result I 
have a few questions. I'm really just curious, rather than looking for 
solutions to a problem per se.  The amplifier seems to be stable and 
comfortable at full output, as before.

First, a bit of context; I installed the vacuum relays - an HC-1 and a Hi-G 
d'Italia - roughly where the old T/R relay was, so that I could leave the 
coax cables from the input jack, from the input tank, and to the output 
jack in their original positions.  By revisions, I mean really small 
things, like lengthening the connection from the common pole on the HC-1 to 
the output coax by 1/2 inch or less, using one less short ground between 
the chassis of the Hi-G and chassis ground, etc.  These don't seem like 
they should be significant at HF.

This is the third time that I have revised the connections between the 
relays and the existing wiring, and each time the amplifier has behaved a 
little differently.  Before the relay failure, the input SWR was a bit high 
on 40, and I found that using the tuner on my Mark 5 resulted in usefully 
higher output.  After the repair, the input SWR is nearly perfect, and I 
have bags of output.  40-15 are unchanged, while 10M now shows about 1.5:1 
SWR and slightly reduced output power, but the exciter is not folding back 
its output at all, so that's acceptable.

Also slightly puzzling, the grid current meter is now deflecting slightly 
in the negative direction when I turn on the amplifier without applying 
drive, regardless of band.  Resting plate current seems nominal at about 
130 ma.  I can't be sure that it hasn't been doing this for some time, 
because I have treated this amplifier pretty much as "given" until the 
recent failure and hadn't been paying much attention to the grid meter.

The numbers look like this -- into a dummy load, constant 100W drive, input 
SWR <1.2:1 except where noted.  The dummy load measures to be slightly, 
progressively more reactive above 14 MHz, and I have not corrected these 
figures for reflected power:

80 -- 1250W out; 750 ma plate current, 150 ma grid
40 -- 1240W out; 690 ma                145 ma grid
20 -- 1200W out; 720 ma plate current, 140 ma grid
15 -- 1050W out; 650 ma plate current, 140 ma grid
10 -- 890W out; 680 ma plate current, 140 ma grid (input SWR 1.5:1)

Should I just shrug and leave well enough alone?  Are variations of this 
sort to be expected with minor changes in the length and configuration of 
RF wiring?


73, Pete N4ZR
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