[AMPS] 30L1 Grid circuit

Thu, 15 May 1997 09:15:18 +0000

Hi Pete,

> I have a Round Emblem model. I was told that the RF cable 
used to connect 
> the Xmiter (KWM2 etc) to the input of the 30L1 was of critical length. It 
> was explained that the Amp could go into runaway on 21 Mhz and above if 
> this cable was not used.

Your problem illustrates my point about the grid system in the 30L1, 
and the silly super cathode driven system applied to triodes.

The 30L1 lacks two things important for stability, it has the grids 
improperly grounded and it has NO neutralization circuit. Both are 
serious mistakes with  four 811A tubes in parallel. 

You can see, if you read the post I made on a single 811A, that the 
transmission loss through a single tube is only 25 or 26 dB on 30 
MHz, with four tubes in parallel the problem is at least six dB 
worse. It is very easy and almost guaranteed feedback can exceed gain 
under some load and source conditions.

Anyone owning a Dentron Clipperton L can see the same thing, remove 
the exciter (sometimes not necessary), key the PA, rotate the 
controls, and the amplifier oscillates on ten and 15 meters.

Good design dictates ANY PA should be totally stable under any tuning 
condition with any type of termination on the input and output at 
extremes of supply voltages. Four un-neutralized 811A's (or even two 
572B's, like the Yaesu FL-2100)  will fail this test quite often.  

The long cable is a band-aid for this stability problem, the cable 
loss hopefully gets the amps input terminate well enough to improve 
stability. 

The real fix would be to properly bypass the grids and neutralize the 
tubes, or if that is too tough, add some form of attenuator 
between the tuned input and the tubes. It would be necessary to 
add enough additional loss (close to the tubes) in addition to the 
system's feedback loss so overall gain does not exceed the feedback 
loss under ANY load or source condition. Only if gain exceeds 
feedback loss (and feedback is the correct phase) will the PA 
oscillate.

I prefer neutralization, since it also improves efficiency, but 
either method will improve IMD performance and stability.

811's and 572's  are a pretty unstable tube.  

> I was told that the designers of this Amp were under the gun to make ship 
> dates and had a problem with ( ???) above 15 meters. The Air Force wanted 
> the Amp ASAP ..the designers found that the 20.5 FT cable helped reduce 
> "the problem" 

Even Gonset neutralized their 811A's, so did Heath. Collins must have 
been in a hurry, because they had some VERY good engineers.

73, Tom W8JI 

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