[Boatanchors] Tetrodes as Triodes

Wilson Lamb infomet at embarqmail.com
Thu Feb 23 09:36:06 EST 2012

Well, I couldn't leave it alone!
Thanks for all the info and encouragement!
There were at least two correct suspicions; load capacity error and metering 

I cleared a bench and rigged up the 4-125 amp, IC-730 driver, small MFJ 
tuner to input, large MFJ tuner from output to dummy load (cantenna).

The MFJs are obviously not lab instruments, so both have uncertainty.  I 
also didn't measure B+ at full load, BUT the exciting news is that I got 
almost "700" Watts out, with "about 750W" in., a remarkable testament to MFJ 
I doubt I'm seeing 93% efficiency.

At least it's behaving reasonably now.  Actually, it probably was all along, 
except for not having enough load capacity, which was mentioned by at least 
one correspondent.  So I was running the output overcoupled, on the wrong 
side of the loading curve.  I've run the handbook numbers and they indicate 
the original cap is OK, but the amp doesn't seem to agree.  The cap is an 
old four sec. BC tuning cap, which I suppose could be something less than 
365pF per sec..  Yes, I have a bridge and will eventually measure it.

When I put the fixed 300pF the builder had on the plate cap over onto the 
load cap, things brightened up a lot, and I don't mean the plates either.  I 
am running a bit over  the plate cap value from the handbook, so the tuning 
is quite sharp (high Q), but the efficiency seems good.   At 750W input, the 
plates are a pleasant red, not bright at all.  When I get a Bird hooked in, 
I'll be able to tell you the real numbers.  The small MFJ says the drive 
power at 750W input was 50W.  The match to the 730 was perfect.

Unfortunately, I was having so much fun watching the plates heat up and 
messing with the tuning that I apparently overheated the pwr xfmr, or 
something under the chassis..  After a minute or two key down, a whisp of 
smoke came up through the chassis, beside the xfmr.  I never considered xfmr 
capacity, because it is absurdly heavy, at least 40lb!  Miraculously, no 
harm is evident.  I let it cool and things seem quiet under there.  It's 
possible the smoke came from some of the undersize wire the builder used in 
the primary.  I sure hope so, but it was bedtime when I got the output over 
600W and I thought I had pressed my luck far enough!

Oh yes, I found a 15% error in the meter I was using in the cathode.  It's 
now changed AND the new meter is plugged into a new jack wired into the 
negative lead of the PS, so it's the real thing, plate current.

Thanks for the suggestions and the correspondence.  Making things work is 
the best part of hamming, for me.


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