fine tuning amp pi-network

Lane Zeitler zeitler at IBM.NET
Mon Jun 28 01:26:37 EDT 1999

The single 3-500Z amp project is going rather well but a little slower than
planned. So far the amp is around 80% done. Only thing left is mounting the
coils in the output tank and still need some doorknobs but no hurry on
those. Have enough C in the tune and load air variables to get me all the
way down to 40 and maybe 80 with no problem.

It actually started out as a modification to a Heathkit SB-1000. The only
original parts left are the front panel, the plate tuning air variable,
filament choke, tube socket, TR relay, and some hardware. Eventually will
install Rich's QSK. Did his version of QSK to a couple of L4Bs and it really
performs great.

Replaced the original multimeter with a multimeter from an Ameritron AL-82
because they measure up to 4000 volts dc which is the normal anode voltage
for this amp now. The gird current meter has been replaced with the original
multimeter so now I have 100% full time monitoring of Ep on the left meter
and Ip on the right meter. Eventually will incorporate monitoring the
filament E also but no hurry. With the Dahl plate and filament xmfr my
filament E is 5.22 volts as measured at the filament pins with the tube

Anyway, in optimizing the L for the output pi-net what are the recommended
methods? I see not much difference btwn 1.8 and 1 uH of L on the output when
tuning the 10 meter band. I did use a roller inductor to determine what the
optimized L was but that was with full out of the deck on each band and I
just know there must be a better way. I have the coils right now for 10
through 40 meters including 17 and 12 meters having their own switch
positions. The value of L does not seem to be really critical in the
resultant output power. Also does not seem to vary the Ip all that much

Anyway, just wondered how some of you HB'ers have optimized your tank
circuits for each desired band.

Sorry I am not much of an engineer but I am learning a lot as I go. I
homebrewed a quad 572B but it was rather poorly built so I thought I would
do things right this time.

BTW I will have the front panel completely taken down to parade rest and
re-silkscreened. I will snail mail pictures to Larry W7IUV for his amps
photos web page when it is all done.

So how do we optimize the tank guys? I do have a lab quality grid dipper as
well as the MFJ 259 SWR Analyzer.

San Diego

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