[Amps] Safety choke question

[Jim Thomson]

Date: Mon, 9 Jan 2012 02:20:34 -0800
From: Jim Barber <audioguy at q.com>
Subject: [Amps] Safety choke question

So what do you all think about using an RF Parts RFC-3 (225 uh 1.5A 4KV) plate choke as the safety choke for a large HF amp? 

The supply is 4KV at up to 2A, bridge rectifier with 32uf of filter. Although the supply *can* source 2A CCS, the duty processor will drop the mains relays if more than 1A flows for more than roughly 100us. Whether that happens as designed or the mains fuses blow first I don't know yet.

Anyway - the circulating current in the choke should be low enough to ignore on 160m, and the effect on the tank should be manageable, especially since this amp won't run above 20m. (in this incarnation at least) I *think* the wire should be large enough to hold until the mains fall out and the caps discharge, but I'll have to think up a way to test that without stressing the supply or myself too much.

I got the idea from an old posting of Ian's, in case anyone experiences deja vu... ;-)

Thanks and 73, 
Jim, N7CXI

## I assume by safety choke, you mean the choke that goes directly across the load cap ?  This is used in case the plate block capS short out, and you don't want 4 kv going up past ur 
t/r relays and up the coax to ant, etc.   If that's the case, use a 45 uh choke, wound with 18 ga magnet wire, on a 1" diam solid teflon/delrin rod.  Then tap the base of the rod with a 1/4-20  or  1/4-28 thread. 

##  Now bear in mind, that safety choke will DC grnd ALL  your tank components like tune+ load + band switch + tank coil +  some of your t/r relay contacts.  It will NOT dc grnd your coax on RX !
I always install a 2nd, identical safety choke, between hot side of output RF connector..and chassis.  OK, now  you are dc grnded on RX as well as TX.  On TX, both 45 uh chokes will effectively be in parallel.

##  IF it's a PI-L  tank set up, I'd install the safety choke at the far end of the L coil....and not across the load cap.  Reason is, the load cap in any PI-L will be at a 200-350 ohm point....and not a 50 ohm point. 
At a 50 ohm point,  45 uh is plenty  for 160-10m, and no series resonances <30 mhz.  It won't affect the amp's tuning either.  You won't destroy 18 ga magnet.  I still don't know what ga wire is used in that 225 uh 
Ameritron/rf parts choke. Does anybody know?  [I think it's small like 24-28 ga /]      It will work too, but eats  up more space.  

later... Jim   VE7RF