[Amps] GS35B Gassy, or Arcing?

Jim Thomson jim.thom at telus.net
Fri Feb 27 01:48:02 EST 2015


Date: Thu, 26 Feb 2015 14:04:45 -0600
From: Mike Waters <mikewate at gmail.com>
To: nemo zilch <nemozilch at gmail.com>
Cc: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] GS35B Gassy, or Arcing?

Sounds like you received some other info off the list.

I have a wild hunch. What happens if you don't leave the grid floating? Try
the hi-pot test with the grid connected to the cathode.

And did you try this again with the filament lit?

In your OP you said you wanted to run it at 3800 volts. It's been awhile
since I researched that tube, but isn't that kind of high for the GS-35B?
http://www.gs35b.com/gs35b/

73, Mike
www.w0btu.com

##  In normal operation, the grid is bonded to the chassis.  I believe 
a hi pot test between anode to cathode will read a lot higher vs 
anode to grid.  Heck, even a 3-500Z  will easily hi pot test to well
over 8000 volts, it has to.  In normal operation, with drive applied the
peak V on the anode will be slightly less than double the key down,
fully load B+ voltage.   IE:  with 4 kv, loaded on a 3-500Z, the peak
V on the anode will be in the range of 7.5 kv. 

##  reason is... the RF voltage developed in the tank circuit is RF....
which is just high freq AC.    RF voltage will easily pass through, and back feed
through the plate block caps..and superimpose itself upon the anode. 
The peak V on the anode is the sum of  the B+  voltage PLUS the peak
of the RF voltage. 

## A buddy runs his pair of gs35bs  on 6m..with 4400 vdc no load on em.  
Runs just fine,  but the tubes hi pot test very high.  It appears that  GS35Bs
and some other russian tubes will have varying hi pot tests results.   Some are
a lot lower..and B+  has to be lowered quite a bit.  If ur GS35B only hi pot
tests to say 4 kv...then u really cant run more than 2 kv, loaded... on the anode.
Which basicly  screws up the design of the amp.   IF u have already run the fils
and blower only,  no B+ and no drive.... for days on end..and it still doesn’t hi pot
test high, the tube is toast.  In some cases, the hi pot tester  will blow off a 
sharp point or   barnacle inside the tube..and tube is then fine..and will hi pot tests
a lot higher.    The usual deal though is... it will blow open cleanly....or weld itself
shut. 

##  U have to be real careful   with the high pot tester.  Folks have done all sorts 
of damage with them, esp if current is not limited to a  real low values.... like 20-50
microamps.     I have seen where folks have simply cranked it up on a device under 
test....and fried it asap, then wonder what happened.    The usual deal, with stuff like
vac relays  etc is... the leakage current will start to rise slowly, then more quickly. 

##  vac relay makers  etc, will spec how much leakage current it should be....with XXX 
voltage applied.     But u need fine adjustment of the V applied.  You also need to be
able to read the leakage current..in the 0-100 microamp range. 

##  ur best bet is to buys some more tubes  from the Dr..and cross your fingers. 
Does the Dr  hi pot test the damned things..b4 he ships them ??   he should,so 
he doesn’t waste his and your time.    Max gain systems and also SSON  will
hi pot test  stuff like vac variable caps TWICE.  Once when they 1st get em....
to weed out the junk, and a 2nd time, just prior to shipping them. 

##  30%  or more of surplus vac parts, like fixed and variable caps are junk, and 
ditto with vac relays, tubes etc.   A buddy has 13 of those jennings, 50-4000 pf
@ 5 kv ceramic vac loading caps.  All NIB.  Only 1 is good..and a 2nd one is sorta good, the
rest are junk.   My buddy with the YC-156 HF amp  bought a total of 12  x YC-156s.
Only 2 are good, a 3rd is semi good, reat are junk.    90% of the bad ones developed a
grid to cathode short after the fil V had been on for 5 mins.    It was always at the 5-7 min mark
the short appeared.   The other 10%  had lost their vac.... gone to air. 

Jim   VE7RF   


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