Date: Mon, 17 Dec 2012 17:36:26 +0000
From: Chris Wilson <chris@chriswilson.tv>
To: Amps <amps@contesting.com>
Subject: [Amps] Choke input filters, design criteria?
Having built a big 5kV 1.8 amp supply that seems to work safely and
well, I want to build a lesser one for a single GS35B 2 meter amp. I
have a suitable transformer with a good range of taps to be able to
vary the output voltage through a fair range, and have acquired a 4
Henry 2 Ampere choke (not a swinging choke, a "normal" one). I am
wondering if I should attempt to use it rather than a capacitive input
filter? What design criteria can I follow to check if it's suitable,
and if so, to choose a suitable cap and bleeders to go with this? It's
from a dismantled and parted out Hirschmann 3500V DC supply. My supply
will be run off single phase, 50Hz UK 240 volt mains. Thanks.
--
Best Regards,
Chris Wilson.
## After building 21 HV supplies over the years.... I would suggest not
using the choke.
And that’s in either the normal choke input scheme..... or using the resonant
choke scheme like
henry radio did.
## You say you have a xfmr with a good range of taps.....so why not use a
lower tap....and a C input filter.
Most of the GS35B HF and 6m amps that I have been involved with work good with
aprx 4000-4400 vdc
no load.
## heck, you don’t even need to build a new supply. All you really need is to
use a HV tap switch, ( or a spst vac relay)
to change the HV taps.... and route the lower B+ to the GS35B amp. You
could also use a 2nd spst vac relay in series with the
B+ feeding the GS35B amp. Then with both vac relays operated, the 5 kv is fed
to the big amp. With relay’s de-energized, the
4 kv is fed to the GS35B amp. You see what I’m getting at. Do it in such a
way that it’s fail safe...and the GS35B never gets
5 kv.
## 5 KV no load will not blow up a GS35B.... not on RX anyway. On TX, it
would be over the top.
## Bottom line is... chokes are a pita. The dynamic V regulation even on a
resonant choke set up like henry used is nothing to
write home about. A big plate xfmr + high C filter, with big ga wire back to
main panel is the ticket these days.
## Lose a bleeder on any choke config..and the no load V will skyrocket to
dangerous levels, creating more problems.
Instead of the usual .9 x sec V..... it becomes 1.414 x sec V if a bleeder open
up.
## IMO, your real NO load B+ is what you have..when drawing idle current, not
what you have on RX. IE: hit the ptt,
and let the amp draw normal ZSAC. The tube has to draw normal ZSAC b4 you can
drive it. With any big C filter config, it
will not drop down very much on ssb/cw.
later......... Jim VE7RF
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