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[Amps] 3-500's and stability

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Subject: [Amps] 3-500's and stability
From: (Roger D. Johnson)
Date: Thu, 22 Aug 2002 21:21:08 -0400
 I'm sure I've posted this before but for the benefit of the new
guys I'll do it again. 

 I have on old Drake L-7 that has been my workhorse for many, many
years. The amp is extremely stable. There has not been one pop, snap
crackle or bang as far back as I can remember. I've used it from 160m
thru 10m including WARC bands (30m into dummy for test) without a
problem. What is the secret? I installed a QSK system based on ideas
from Rich's website. Why is this good? I have a decent dual trace
scope which I hooked up to monitor RF input and output for testing
before and after the mod. The original old 3 pole TR relay is WAY too
slow! The TS-940 exciter starts producing RF about 6ms after keydown.
The old relay hasn't even started to budge at this time! The scope
shows the 100 watt output of the 940. At about 12ms the relay has
closed but the contacts are bouncing. The scope shows that the RF
is cutoff during the contact closing interval and then there is a
God-awful mess as the contacts bounce, connecting and disconnecting
the input and output. How any amp could survive this without
triggering some sort of nasty oscillation is beyond me. Pure luck I
think. With the QSK system, all is sequenced nicely. No hot switching
or nasties anywhere. 

 Two other aspects. I directly grounded the grids with copper strap. No
more resistors or RF chokes. Is this good for stability? I don't know
for certain. I know it never solved the problem in any of the other
amps I've worked on. The QSK system allows you to separate the input
and output relays. I'm not certain if this helps either but I've always
been told to keep inputs and outputs separated to avoid oscillations.

 Mine is the only Drake I've done but I nearly went crazy trying to
tame friends TL-922's. The only thing that worked was the QSK system.
I'm sure that there are other relay sequencing schemes out there but
QSK sure is nice for CW ops!

73, Roger

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