> The LDG amp module is easy to install and use in a HB amp.
> The purists will
> all agree against using an L network for cathode matching
> and I tend to
> agree to some extent. I bought an LDG amp module for my
> 3CPX5000A7 project
> and so far have had much success with it.
This is an important point. Tubes with very low cathode
impedances and operated with higher conduction angles have
significantly more stability in input impedance as the drive
level and output loading are varied. They have more power
gain also, and are more immune to what goes on at each end
of the tube.
The 3CPX5000 is an extreme example that way, although I
would still use a good reasonable Q input that produces a
low impedance at harmonics. (Like any tube with a cathode
common to the heater, you have to watch hum from filament
voltage drop across the cfilament choke.)
On the other hand the extreme in being "input circuit
critical" comes from tubes with higher cathode drive
impedances or more prone to harmonic or intermodulation
distortion. Tube with higher grid current as a ratio of
anode current fall into this class.
The reason for this is the input circuit is in series with
the output circuit in a cathode driven PA. When the cathode
has a very low impedance, like a tube in the order of 20-25
ohms, and an output impedance that is very high (several
kilovolts at modest current on the anode) and very low grid
intercept the input is much more isolated from things going
on in the tube.
On the other hand a tube with 100 ohms of driving impedance
and less anode impedance is a different world, especially
when the grid current is several times higher.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|