One of the approaches I've used in the past to adjust or tune PA output
networks is the "reverse approach" use of a product like the MFJ Antenna
Analyzer.
Look at it this way. The PA output networks' purpose is to transform the
anode value of Z to something near 50 ohms. In reality, the network really
doesn't care which way the signal flows. Therefore, using the basic
calculation to determine the anode Z, or from various charts or otherwise,
based on DC voltage and current, with the tube and components in place but
without any power applied, temporarily place a 1/2 watt or so carbon
resistor, equal to the anode Z, from anode to ground. Then using the
analyzer at the output of the amp and at the frequency desired, adjust the
components of the network for something near 50 ohms as shown on the
analyzer.
I've used this method to move my broadcast transmitter from the BC band to
160M, to other bands such as 75M and 40M. Works swell every time and saves
lots of cut and try with power applied, not to mention stress on components
in an "untuned" condition.
73
Bob, K4TAX
----- Original Message -----
From: "Michael J Tubby BSc Hons G8TIC" <mike.tubby@thorcom.co.uk>
To: "2" <2@vc.net>; "AMPS" <amps@contesting.com>
Sent: Monday, April 21, 2003 3:54 PM
Subject: Re: [Amps] 6M 8171
> >
> >> Sometime ago, I bought an 8171 with socket, chimney and filament
> transformer
> >> with the idea of building a 6M amplifier. This project is not intended
> to
> >> be for the greatest power output, but for an amplifier that would
> withstand
> >> the strain of considerable EME operation. and be easily cooled with a
> QUIET
> >> blower.
> >>
> >> I've looked at the specs on the tube, and at various tetrode designs
for
> 6
> >> meter operation, and I'm completely stumped.
> >>
> >> To start with (while I enjoy drilling, hammering, filing and soldering)
> I'm
> >> the furthest thing from an amplifier designer.
> >>
> >> The most significant problem is that the large input and output
> capacitances
> >> are hard to deal with at 6M. At least that is my conclusion.
> >>
> >> A while ago, on this reflector, someone was dealing with a similar
> problem
> >> (although not with the 8171), and suggested using the W6PO circuit,
> scaled
> >> to 6M.
> >>
> >> Could that be an option? What are my options?
> >>
> >> Thanks, Barrie, W7ALW
> >>
> >
> >Barrie,
> >
> >I don't know the specifics of the 8171 (4CX10000 ?) but I have played
with
> >8877, 3-500 and 3CX800 amps on 6m.
> >
> ? The RL of an 8171 is c. 1200?, so, at 50MHz, a total C-Tune of roughly
> 80pF is needed for a Q of 15. Since the anode-screen C, stray C, and the
> neutralizing bridge total c. 30pF, a 50pF Tune-C should do the job.
>
>
> Like I said - didn't know thge specifics of the 8171... with RL down at
> circa 1200R the transform's not that great then... hence the standard
> pi-tank
> works. If the tube used 20Kv HT @ 0.5A then it would be a different
> thing to tame...
>
>
> >The relatively large anode capacitance makes the conventional pi-tank
> rather
> >"touchy" up at 50MHz - my 8877 amp copied directly from the well known
> >GJ4ICD/G3WOS design would tune but wouldn't load at about about 1100W
> >output until we had it on a network analyser to find out what was going
on
> had
> >to change the L by about 5-10% to fix it.
> >
> >You might find a better approach would be to use the L/L-C tank circuit
wit
> h
> >a rotating shorted-turn loop to vary the L - my 3-500 6m amp (copy of the
> >1971 QST design) uses this technique to great effect.
>
> ? An excellent solution for higher RL tubes, Mike.
>
> Certainly found it to work well. The amp is stable, efficient and remains
> tuned
> well even when thrashed in contests. It should be noted that if you change
> the
> 4.5 turns to 1.5 turns and reduce the diameter of the loading coil by
1/3rd
> the
> amp works equally well on 70MHz (UK 4M band).
>
> On both bands it will do > 850W carrier with remarkable aparrent anode
> efficiency using a modest power supply. Just goes to show that 30+ year
old
> designs are still hard to beat today.
>
>
>
> >I got the tank circuit to
> >work first time by putting a 3K3 resistor across the tube and sweeping
the
> >output socket and adusting (bending) the coil around until it covered
> >48-52MHz when rotating the shorted-turn loop.
> >
> >Something like this should 'scale up' for the 8171...
> >
> >Mike G8TIC/M0VHF
>
> - R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
> www.vcnet.com/measures.
> end
>
>
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