[Amps] 4CX300a Seal Leaks

[texasrf at aol.com]

John, this where you pull out your trusty hv leakage tester and check for  
leakage between the anode and screen grid. If there is onset of leakage at a 
 voltage of 1000vdc or so, there is a very high likelihood that the tube  
is unusable. On the other hand, if the leakage shows up at 3 or 4 kv, there  
is a good chance the tube is ok.
 
You don't have a hv leakage tester? They are easy to cobble together  from 
an old microwave oven transformer, diodes and filter caps. The  transformers 
are usually in the 2100vac range so a half wave doubler using  two 
microwave oven diodes and two filter caps will get you up to about 6  kV. A 4 X 
multiplier using four diodes and four caps will get you to about 12  kV. A small 
variac on the primary will allow voltage adjustment. The only other  parts 
needed will be a micro ampere meter and current limiting resistor to  
protect the uA meter. An old Tripplet 630 VOM with a 6kV scale will serve very  
well for both the uA meter and the limiting resistor. Just use the 6 kV range  
and connect in series with one of the hv tester output leads.
 
Old microwave oven parts are always listed on Ebay for reasonable prices. I 
 have purchased several complete but defective ovens for as little as zero  
dollars only paid for shipping and all had good transformers, diodes and  
capacitors.
 
A local appliance repair store might well have some they would like to get  
rid of.
 
If you have a hv power supply with enough voltage, that can also serve as a 
 leakage tester. Just use the same VOM/6 kV scale in series with the B+ for 
 testing. Beware high voltage! Be careful!
 
73, Gerald K5GW
 
 
 
 
 
 
 a message dated 1/15/2010 8:24:39 P.M. Central Standard Time,  
finnerjf at worldnet.att.net writes:

I read a  post by Carl, KM1H on August 10, 2009 that read:

There are several  ceramic tubes that are notorious for seal leakage. The  
4CX300A is a  classic.

This post is directed to Carl. The list might also be  interested. I would 
be interested in a brief explanation of the  problem.

I have 7 NOS JAN 4CX300A's an am building a 3 tube passive  grid driven 
amp. The date codes are 7204, 7432 and 7634 if this makes a  difference. Are 
the JAN Eimac tubes less problematic? 

The amp is in a  simibreadboard state with 4CX250B's. Since I now have 3 
sockets for the  4CX300A's, I will switch tubes and raise the Ep to 2500 V. I 
do not want to  waste my time if I am going to have a problem with this 
change.

John  WA4EPI 

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