[Amps] 3CX3000 input impedence

[Conrad_G0RUZ]

I think that keeping the loaded Q to between 12 and 15 will be extremely
difficult with a 3CX3000A7 at 50 MHz due to stray C.

There is a program called T.A.P by KD9JQ which will help with this design. I
think that obtaining a loaded Q as low as 12-15 will be very difficult at 50
MHz with this tube due to the stray Cs. The key is to use as little C1 as
possible and a large surface area on L1 to keep losses down.

GM3SEK also has a spreadsheet on his website that will be helpful for this
process, allowing sensible values for C1 and L1 to be chosen. This will give
a feel for the what the design Q will needed to be practical.

73

Conrad G0RUZ

> -----Original Message-----
> From: amps-bounces at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Tony King - W4ZT
> Sent: 27 February 2006 17:57
> To: Barrie Smith
> Cc: amps at contesting.com
> Subject: Re: [Amps] 3CX3000 input impedence
> 
> Barrie,
> 
> The first rule of amplifier building after keeping your hands 
> in your pockets is to know what your plate load resistance 
> is.  Without it you can't know the proper values for C1, C2 
> and L to maintain the proper Q.
> 
> Your friend must first decide what OUTPUT power his amp is 
> going to be designed for.  Then what the input power is going 
> to be with some reasonable efficiency (usually around 55%) 
> (Pin=Pout/.55).  Then, knowing what his INPUT power will be, 
> use the plate voltage (under load) to determine the plate 
> current at that voltage (I=P/E).  Then use the formula 
> (RL=Ep/(1.6xIp)) to approximate his plate load resistance. 
> That will be the value of non inductive resistance to place 
> between the plate and ground for testing.
> 
> Don't fudge on the value of C1 and L. There are several on 
> line tools to calculate the values of the tank circuit.  Stay 
> within reasonable limits to keep the Q of the circuit between 
> about 12 and 15.  Get the Q too high and tuning will be 
> extremely sharp and things will begin to melt down because of 
> the high circulating currents.  Too low and tuning will be 
> too broad and you'll have practically no harmonic suppression.
> 
> Good luck with your project!
> 
> 73, Tony W4ZT
> 
> 
> Barrie Smith wrote:
> > A friend is building a 3CX3000 amp for 6M.
> > 
> > He wishes to ballpark the tuned input circuit by installing
> a resistor between the tube input and ground.
> > 
> > What would be the proper value for this resistor?
> > 
> > TIA, Barrie, W7ALW
> > _______________________________________________
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