[Fourlanders] 6m Commander Amp problem update

[Tom McElroy]

That's a good feeling, solving a tough problem like that, huh?
Bob - I lost track of our previous conversation - do you want my 432 BP
filter for the contest?  It's in this picture album:
*https://goo.gl/photos/Z1v6gaj2zeXMnpwQA
<https://goo.gl/photos/Z1v6gaj2zeXMnpwQA>*

73
Tom
W4SDR

On Tue, Jan 17, 2017 at 4:29 PM, Bob Lear <w4zst at windstream.net> wrote:

> 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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