[Amps] 4CX300a Seal Leaks

[John Finner]

Thanks Gerald.  I did not think of hipotting them.  I have both a 5200 V supply that I can variac control and a Tripplet 630 for indicating leakage.

I will be safe with the high voltage as I learned my lesson at 11 back in 1953 with a 300 V supply.

John WA4EPI
  ----- Original Message ----- 
  From: texasrf at aol.com 
  To: finnerjf at worldnet.att.net ; amps at contesting.com 
  Sent: Friday, January 15, 2010 10:00 PM
  Subject: Re: [Amps] 4CX300a Seal Leaks


  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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