[Amps] Need for input tuning circuit
craxd
craxd1 at ezwv.com
Sun Oct 10 09:57:34 EDT 2004
Vic, Peter, and others,
I think what Vic is talking about is using a "grid leak" bias circuit.
The tube actually has bias this way by charging the capacitor up which
is parallel with a resistor. This charging happens quick each time the
tube is operated. Vic mentions using a cap in series with this for grid
driven or in parallel for cathode driven. However on the grid driven,
both can be used, the C block and the one in parallel along with added
bias voltage trough a choke to G1. Years ago, a lot of the so-called
grounded grid, sweep tube amps used this method to cut the tubes off.
There's an electronics education manual from the US Armed Forces
Institute named "Understanding Radio" That shows the usage all through
the book with very good explanations of how to use it. This books last
copyright I had was 1951 so were talking old school (used a lot in
triodes). You hardly ever see grid leak bias today though, but I built a
two tube 6KG6A grid driven amp for six meters about one year ago using
it. It used as above, both grid leak, and a bias supply. In Vics case,
he was running both grid leak bias, and killing the screen voltage with
a clamp tube if I read that right, on idle. If so, it would have most
likely killed the tube as he said.
Will Matney
How was full class-C maintained on the opposite cycle, when no grid
> current flows?
> For full class-C the 813 has to be biased with -155V at 2250V EHT
There needs to be a capacitor across the grid resistor that is charged during
the period that grid current is flowing. If you have a parallel tuned input
circuit from grid to ground, the blocking capacitor that keeps the bias from
being shorted out by the coil performs this function. Or you can have an RF
choke connected to the grid in series with the grid resistor and a bypass
capacitor connected between the junction between the choke and resistor and ground.
-- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco
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