[Amps] Tuned input board on SB-220

[Bill Fuqua]

       If the LDG tuner can match a capacitive load simply having a 
capacitor from cathode to ground should do. It should switch to an L 
network with a capacitor input and inductor coupled to the cathode and the 
cathode shunt capacitance. Then you will get the Q needed to achieve 
sufficient flywheel effect. However, you still have a problem with having 
to switch cathode capacitors when changing bands.

73
Bill wa4lav


At 03:35 PM 3/24/2006 -0500, Roger D Johnson wrote:
>Gary Schafer wrote:
>
> > You can look in Bill Orr's handbook and he says you need 13 pf/meter of
> > capacitance at the cathode for proper operation. That's 1040pf on 80 
> meters.
> >
> > Then figure out what L network values are needed to match the particular
> > tubes input impedance to your 50 ohm driver. In most cases you will 
> find the
> > capacitance will fall short of the 13 pf/ meter value.
> >
> > Here is a handy calculator to do it.
> >
> > 
> http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.h
> > tml
> >
> > 73
> > Gary  K4FMX
>
>I just typed a msg to George, K6GT, and it occurred to me that an L net
>will not match 50 ohms to 50 ohms. It would seem to me that, if at some
>frequency/loading combination your amp looked like 50 ohms, the LDG
>tuner would go into bypass mode and you would have nothing on the
>cathode side.
>
>Don't get me wrong, I think that this is a terrific idea but LDG needs
>to go one step further and use the freq determining circuit to switch
>in the proper input capacitors to give an appropriate Q.
>
>73, Roger
>
>
>--
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