[Fourlanders] 6m Commander Amp problem update

[Bob Lear]

We had a problem with my 6m Commander VHF 2000 amp on the mountain 
during the September contest.  I had been off and on working on it since 
then and consulting with Ray, Dick and Ron about the problem.  Several 
theories were put forth and various tests were made here by me.

The amp was showing increased resting plate and grid current after 
transmitting.  Sometimes, the idle plate and grid currents would be 
normal on PTT before RF and sometimes elevated.  Other stations that 
heard us on the air did not see any problem with our signal but Ron was 
getting some kind of noise level increase on 432 when we were 
transmitting.  I did duplicate the same problem after bringing the amp 
home and testing it on a dummy load.  It did not act up until it had 
been warmed up and in use for some time.

One idea was whether the cathode bias was possibly changing and I ran a 
jumper out to an external meter and verified that it was steady and not 
changing even when seeing the elevated idle currents.

Another thought was with the control board or metering functions.  I 
pulled the control board out of the amp and tested it off-line and it 
appeared to be working properly,  I even redrew the schematic form the 
circuit board to make sure I had done it correctly the first time as the 
manual schematic was not drawn in typical fashion even though it was 
basically correct.  Just was harder to follow.

A third thought was that it may have been one of the tubes.  I was going 
to try that next, but first thought also that it could have been a 
'flake' in one of the tubes also causing the problem. So I ran it with 
filaments and blower only for about three days, the usual process for 
reconditioning a tube, especially ones that hadn't been in use for a 
long time.  Even though these had been in use, the amp has been 
transported back and forth to the mountain so I felt it couldn't hurt to 
just try this.  I also re-seated the tube pins a few times just in case.

After the reconditioning, I ran it again and noticed just a slightly 
different behavior.  Now, there was no elevated resting grid current and 
the resting plate current was normal every time the amp was keyed up but 
would then remain high after applying and stopping RF before unkeying 
the PTT.  Some of you may immediately recognize this problem but I 
didn't so I called Ray again.  Well, he says, that's classic 'Parasitic 
Oscillation' signature.  Even though it was doing that all along, I 
guess my latest description sounded more clear.  Ray suggested that I 
needed parasitic suppression resistors on the plates.  Even though the 
amp has about a two turn ribbon coil from the HV and Tank circuit to the 
tube caps, there were no suppression resistors.  Ray was surprised at 
that but that is the original design of the amp.  He thinks maybe just 
lucky that it had not had a parasitic problem before.

Then we remembered that there were modifications to the plate circuit 
that I made last year to repair the amp after one of the plate blocking 
caps blew.  That happened during a contest, probably due to WSJT duty 
cycles in an amp that was designed long before JT modes came around.  
The original two plate blocking caps were connected top and bottom with 
1/4" wide silver-plated thin copper straps and also similar strap from 
the caps to the tank coil.  With Ray's advice, I replaced the _two_ 
door-know style caps with _three_ that were mounted on triangular copper 
plates that were 1/8"  thick and also silver-plated.  A new thicker and 
wider strap was run from the caps to the tank coil and also 
silver-plated.  This appeared to work fine at first but then the problem 
developed.  Even though we had helped one situation by giving the 
blocking caps some heat transfer relief, that did make a change in the 
plate circuit.

Now on Ray's suggestion, I crimped and soldered two 33 ohm 2 watt 
resistors in parallel with each of the ribbon suppressor coils to the 
tube plate caps.  After reassembly, this immediately solved the problem 
and I have run the amp a good bit to test it and make sure it is not 
still oscillating after it's warmed up more.  I ran it with the contest 
CW CQ messages on repeat for quite some time and it has not acted up again.

Ray said he'd never seen a 6m amp without parallel parasitic suppression 
resistors along with the parasitic coil and now I don't have one anymore 
either.

I've learned a lot again this time from taking the amp apart, testing 
various components and maybe now I'll be able to recognize parasitic 
oscillations from the now obvious behavior of the resting plate current.

With 20/20 hindsight!  73, Bob


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