[Drake] Neutralizing Drake Transmitter Final Amplifier Tubes

[Barry L. Ornitz]

Bill Campbell, WD4HEN,  asked about the final output tube's 
neutralization procedure, noting that he was unable to get the 
factory procedure to work.

Bill Abate, K3PGB, gave an excellent suggestion and noted that 
with some tubes, it was impossible to obtain proper 
neutralization.

> I found it was easier to adjust the neutralization cap for 
> minimum RF feed through (no plate or screen voltage) as a 
> first step to get it in the ballpark.  I then followed the 
> book procedure for max output and dip in plate current 
> occurring at the same point.  Also, some finals cannot
> be neutralized properly and have to be replaced.

Something that some people tend to forget is that you have to 
start with matched tubes.

With the sources of these tubes drying up, matched tubes are 
getting even more difficult to find.  Matched pairs are 
difficult enough for the T-4X transmitters, but getting three 
matched tubes for the TR-3 and TR-4 is even more difficult.

Also a limited number of sweep tubes were made with an 
exceptionally long internal cathode lead.  They worked fine 
for television sweep service, but they could not be used much 
above 40 meters in transmitters.  You can often spot these 
tubes with a quick visual inspection, _IF_ the metallic getter 
does not block your view which it often does.

Having had my T-4X since 1965, I have been through a number of 
tubes.  I never had problems with RCA, Sylvania, and GE 
tubes.  I think I once even used some International tubes from 
Japan.  But I always used pairs of the same brand, and these 
were matched for equal transconductance in a Hickok tube 
tester.  Neutralization settings might be quite different from 
one manufacturer to the next, but I never failed to neutralize 
them.  Mixed tubes, i.e. those from two different 
manufacturers, may or may not neutralize properly - even if 
their transconductance is matched.

Another trick to try is to neutralize the tubes on a lower 
band such as 20 meters.  This will often get you in the 
"ballpark" quickly.  Then move up to 10 meters where only a 
slight additional adjustment may be needed.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net