When I play with this design, it appears that 5 uH for the inductor at
3.8M is just too low.
Rp 1000
Cstrays 20 p
Ctune 410 pF
L 5 uH
Cload 1660 pF
Those values seem to agree with what you described.
Raising the Inductance to 7 uH
Ctune 267 pF
Cload 990 pF
Which seems more like a typical 75M tank.
None of this varies very much with Rp. The tune cap and tank inductor
mostly define the resonant frequency, the load cap defines the match
between antenna and Rp.
I'm using a graphical smith chart program for this analysis...much
easier than any of the other things I've tried. You don't even need to
understand smith charts, just move the component values around until the
line hits center of the chart.
jeff, wa1hco
ka1mwp wrote:
> GU-84b based amp -- tetrode boards for control.
>
> OK so, working again on my amp, 2.3kv on plate, 350v (when ON) to G2 and
> G1 has -65v. I have well over 1 amp available for plate current if
> needed.
>
> Used several tank calculators, Jim Hawkins, PI network v1.0, G4FGQ's
> calculator..whatever
> Else I could find out there to do comparisons. Found out that the W4ZT
> calculator was pretty close.
>
> I used about 940ohm for my plate impedance, I can run 1, 1.5, 2kw
> calculations but I started with 1.8kw. assumed that the tank Q
> Was 12 -- 12.5, Rp made 968 ohms.
>
> I set this all up with my Boonton inductance meter, cap meter on the C1
> and C2 (all out of circuit) and assembled it. I have done this
> Several times. I used my MFJ analyzer and setup the circuit, tube in
> place connected just swept the tank and looked OK.
>
> Power up -
>
> 3.8mhz,
> C1 -- variable to 0 to 500pf, vacuum cap. Set as needed
> C2 -- variable to 1000pf vacuum cap PLUS THREE 470pf Russian door knob
> caps. 1400- 2400pf total available. (I checked these with cap meter all
> ok)
> Set that as needed.
> Inductance pretty close to 5uh a little below, I had to short one coil
> section to get it there as I am using the whole coil I wound
> Out of ¼ copper tube -- wound over a Krylon spray can.
>
> Now then...
>
> Problem I am CONSTANTLY having is to have tons of C2, like the vacuum
> cap has no more, seems like both 80m (and 40m
> I tapped the coil there for 40 also). I can pull easily 1.5kw out on
> both these bands but its with the vacuum cap out all the way (max
> capacitance) and
> When I am turning the cap in (decrease pf) the power decreases. And I do
> see the grid current start to drop also.
> I switched from a 600pf to 1000pf vacuum cap to see what that would do
> and it still wanted more. I have checked the total C2 pf readings they
> look OK.
> What's wrong with this picture??
>
> Now what direction should I go in here with this? Assume different Rp
> and see what I can poke and hope with?
>
> Thanks --Chuck ~ka1mwp
>
>
>
>
>
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