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@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|