Hi Gerry,
I don't know about the GI7B and have never used one; however, the unwanted
reactance of the AL80 bandswitch and tank wiring, and resonances in the
filament choke and HV RF choke, and undesired inductance in the parasitic
suppressor (and other factors) make me think it would be impossible to
simply "add six meters" to the AL80 (even with a vacuum relay to disengage
all original tank components).
I've modified AL80's to 50 MHz, and the mod works very well, but I remove
the bandswitch, all plate tank components except for the PLATE TUNE cap
(which becomes an excellent 6 meter LOAD cap), change the parasitic
suppressor, the filament choke, the HV RFC, etc in order to make it work.
To achieve any sort of reasonable Q, the "new" 6 meter TUNE cap must have a
minimum tuning value of about 3 pF, so I use the Jennings vacuum variable
7500V 2-30pF cap there. Running straps around to "switch" to a different
tank on six would very likely result in tank resonance below 50 MHz, even
without any plate tuning cap at all.
Of course, circuits other than a pi-net will work...one famous circuit for a
3-500Z (and similar tubes) used no plate TUNE cap at all, but a variable
inductance made by rotating a shorted loop inside the plate tank inductor
for tuning. You might make that work with your idea, but I'm not sure where
you'd find the room for that inductor and its rotating shorted loop. You
might be able to wind a 50 MHz RF choke on the same form as the original HV
RFC, and use a floating relay contact set to short out the HF choke winding
when switching to six -- haven't tried that, but it seems plausible.
As you likely know, with the original Ameritron plate tank circuit, when
dipping the tank on the high end of ten meters (29 MHz), the TUNE C is fully
unmeshed and set at its absolute minimum capacitance...
WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Gerry Hull [SMTP:windev@inetmarket.com]
> Sent: Friday, October 03, 2003 12:55 PM
> To: amps@contesting.com
> Subject: [BULK] - [Amps] AL80 GI7B Conversion...
>
> Hi,
>
> I am going to modify my junkbox AL80 to use the Russian GI7B microwave
> triode. I've seen the W4EMF conversion page for the gla-1000. I believe
> the
> AL80 tank components have enough range to cover 160-10, with slight
> modifications. I'll be happy w/500 - 600w out.
>
> I'd like to run this on 6m also. Rather than modify the bandswitch and
> tank, I
> was thinking of putting a vacuum relay on the
> output, and switch in a completely different tank and output connector for
> 6m.
> In addition, I'd put a relay-switched tuned input for 6.
>
> My question is: Is this crazy? Has anyone done it before? Probably my
> biggest challenge will be finding room for the 6m tank. Should I ground
> the
> HF Tank circuit when on 6, or leave it floating?
>
> TIA & 73,
>
> Gerry, W1VE
>
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