[600MRG] WD2XSH/6 transmitter failure advice please

[Pat Hamel]

Dave and all,
I laid out the amp for a "pretty" front panel so the four push-pull-parallel
tubes glow through a hardware-cloth screen. That, the three-inch-diameter
plate coil, and the oil-filled tuning caps make space tight.
There were two caps that failed. The variable and one of the two blocking
caps (one blocking cap feeds each of the 811 grids so I can individually
meter them). 
One Cardwell variable has arced before a few years ago, which is why I put
the disk blocking caps in series with them.
The arcing is not directly between the plates, but apparently on or in the
insulating bars (not ceramic) that hold it together. 
They are about 3/8 inch by 3/16 inch by 2 inches long.

So - now comes the question that I should have asked last time it happened: 

Given that the bars are small, straight, and rigid, I should be able to
duplicate and replace the insulating bars easily.

Given: Here on the humid gulf coast, I don't think that varnished Popsicle
stick is a good idea for 1250 volts DC.

Given: When I was a kid, in the 1950's K8DHN rebuilt a capacitor for me. The
first time he used the wrong plastic which broke down, then he found the
right plastic which lasted until the amp was disassembled.
I never did know the correct plastic type.

TO find: Recommended plastic or ????
This group always has a good answer.

Till then, I will put the parallel power supply back to the exciter and 4
watts ERP tonight.

Thanks and 73,
Pat /6


  

-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of dave.riley3
Sent: Saturday, September 07, 2013 9:52 AM
To: 600mrg at w7ekb.com
Subject: Re: [600MRG] WD2XSH/6 transmitter failure

add:

RG8 is about 17 pf per foot ( styrene type ), so pre figure length and 
then some, connect, then clip to tune ( with power OFF fer sure )...
Use both connections at one end, other ends open, not connected for EZ 
cut...

With power off, tune final tuning cap with good load on final to see 
minimum final grid current change...
Yes, driver connected and hot, final HV OFF, this will let you tune 
neutralization real fine in safety...
SNIP