[Amps] parasitic oscillation techniques

[Dave white]

That's a great story, Paul!

Reid Brandon at Eimac (W6MTF to thee and me) has given me a lot of help too When I built my 3CX3000 amp a few years ago I had problems with very low gain having built a traditional parasitic suppressor (probably badly). I described the amp, how I'd very carefully isolated input and output circuits and used half the world supply of finger stock and feedthru capacitors.  Reid suggested that I run without a parasitic suppressor at all.

Expecting some sort of loud and bad stuff to happen as my amp turned itself into a 10kw vhf oscillator I took him at his word.  Viola. 13+dB gain, clean as a whistle, no violet glow on the little neon bulb I'd put in the plate tank as a 'canary', all working as it should.

W6MTF is indeed a helpful man of great knowledge and I wonder what on earth Eimac will do when their 'old guard' who have been there for years and years take their well earned retirement?

Cheers
Dave G0OIL

-----Original Message-----
From: Paul Christensen <w9ac at arrl.net>
Sent: 24 August 2009 23:08
To: amps at contesting.com
Subject: Re: [Amps] parasitic oscillation techniques

> If you will recall, in the mid-1980's, there was a massive recall of all
> 8877's by Eimac.

Apologies if I've repeated this story in the past...

There was a wave of contaminated Eimac 8877s that preceded the 1985 event. 
It nearly cost me my college job when I was working at a small FM station in 
Rockford, Illinois between 1978 and 1979.  Long story short: An AEL FM-2.5K 
transmitter used the 8877 to produce approximately 2250 watts into a 
three-bay circularly-polarized antenna.

One day, an air blockage caused the 8877 to overheat -- to the point where 
the anode turned pitch-black.  At the same time, grid current was running 
dangerously high in order to produce the full licensed TPO.  I drove to 
Richardson Electronics in the Chicago suburbs and purchased a replacement 
8877.  Upon installing it, no amount of drive, nor tuning would bring it 
near full power and cavity tuning adjustments were way outside of normal. 
Purchased a second tube.  It behaved exactly the same as the first. 
Purchase a third tube.  It too behaved the same...

Thinking that the problem was me and not the 8877, the station's owner flew 
in a consulting engineer and paid him several $K to tell him what I already 
knew, but being only 20 years of age, I was beginning to question my 
analytical troubleshooting skills.  There's nothing like the pressure of 
being off the air with zero ad sales to test one's ability to work under 
extreme pressure.  And extreme it was.

Somehow, I ended up speaking with Eimac's Reid Brandon who informed me of 
the bad batch of 8877s.   He sent me one directly from Eimac's Salt Lake 
City facility.  I installed it in the AEL transmitter and it performed 100% 
I was tempted to tell the owner "told 'ya so," but reserved that remark for 
another occasion.

Needless to say, grades suffered that week but it was quite an experience 
I'll never forget.  I still use an 8877 in my Alpha 77Dx.  I'm reminded of 
the Rockford event every time I put my hands on an 8877.

Paul, W9AC 

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