[WRL] GK-500B Final Plate Current, Screen Current and Power OutPut Sag

[Damon S Raphael MD (W7MD)]

This is from a current thread on the AM Bulletin Board. I am posting it here
on an open reflector so it will be available to Internet search-engines:

I have enjoyed using my GK-500B since I aqired it about 3 1/2 years ago. It
came with a sagging final plate current, screen current and power output
problem which I have been able to solve.A few days ago, I replaced the Plate
Blocking Capacitor in the final circuit. It was a HT-58 500 pf 5000 V
doorknob. I replaced it with a HT-58 1000 pf 5000 V cap which looks exactly
alike except fot the labeled capacitance value.
I have now used the GK-500B on 160, 80, 40, and 20 meters with no sign of
Final Current and Output sag. From these results, I believe that the Plate
Blocking Capacitor was the cause.
If this particular doorknob cap is an original WRL installed item, then
other GK500s may have the same problem.
I am not a expert on either tank circuit design or capacitor ceramic
properties so if anyone disagrees, please post a response..
I decided to check the plate blocking cap when I noticed its low value
compared to similar caps in other schematics. I did an internet search and
found on the AMP reflector that the TenTec Titan amp has a similar problem
related to a blocking cap. A short discussion there referred to the type 858
ceramic.
Following up on searches for 858 ceramic, I located this link
http://www.qsl.net/i0jx/capacit.html
Apparently the type 858 ceramic is not a true RF rated ceramic but is a High
Voltage rated ceramic type. You can get away with a High Voltage type if it
is a bigger chunk of ceramic to dissipate the increased heat which RF
produces or you can use a higher capacitance value which exhibits a lower
capacitive reactance. The 1000 pf HT-58 is the same physical size as the 500
pf but the lower capacitative reactance is just enough impedance drop to
keep heating within the doorknob below a critical level for this circuit.
High Energy Corp which has the trademark for this line now - limits the
designation HT-58 to caps in the range of 45 to 65 pf.
The range for 500 to 2500 pf is now called the HH58 series.
What was previously called a HT-58 500pf is no longer made. The X5V ceramic
type which is what the HT-58 500 pf was originally made of - is now only
used in the 1000 pf to 2000 pf values.
They now use X5T ceramic for the the 500 to 900 pf range. X5T ceramic does
not have the sharp dropoff in capacitance with temperature increase that the
X5V has.
If you can find a cheap old 1000 pf HT-58, or a new High Energy Corp HH58
1000 pf, they should be of the same ceramic material and should work. A
larger value and physically sized HV type cap from a hamswap would probably
work too.

73,

Damon Raphael, W7MD

Tucson, AZ