I've always used:
TUNE = 2-30 pF vacuum variable (actual range needed c. 2-10 pF)
LOAD = 200 pF air variable, similar to the TUNE cap often used in HF
amplifiers
L = 3 to 4 turns 1/4" copper tubing, 1.50" inside diameter, spaced same as
tubing diameter, e.g., 1/4" spacing between turns; adjust spacing and turns
for dip at 50 MHz.
To make the L, I usually start out with a couple of feet of 1/4" diameter
copper tubing, pinch one end shut and fill it full of dry sand, pinch the
other end shut and wind it on a 1-1/2" diameter pipe, then form each end to
mate with the TUNE and LOAD cap terminals; then, cut to appropriate final
length and form (sand will pour out), flatten both ends in correct plane and
position for attachment to the two capacitors, and attach. Then, adjust
TUNE for a couple of turns in from minimum capacitance, adjust LOAD for
about 1/3 mesh, apply GDO and adjust inductor for dip at 50 MHz. Do this
with the tube in its socket and connected to the tank, as it adds a little
bit of C. *No voltage required* of course.
Somewhere between 3 and 4 turns always nails the six meter tank circuit...3
turns if there's more stray C, 4 turns if there's less. If you see you need
less than 3 turns, chances are there's too much stray C and you need to
neaten things up and make all connections shorter.
WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Erwin Cremers [SMTP:pe2six@xs4all.nl]
> Sent: Sunday, May 04, 2003 1:18 PM
> To: amps@contesting.com
> Subject: [Amps] qb3.5/750 on six tank circuit question
>
> Dear readers,
>
> I've been trying to get my qb3.5/750 or 4-250B amp to run on Six meter but
> keep having trouble with the tank circuit.
>
> Has anyone an idear of the values that i should use to get the thing to
> run on six/50Mhz. I'm mainly interested in the Tank circuit. So the tune C
> value, the L (if posible with turns and diameter) and the Load C value.
>
> Va = 1900 volt
> Ia = max 450 mA
>
> I hope you can be of assistance.
>
> Regards,
>
> Erwin Cremers
> PE2SIX
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