Re: [Amps] Need for input tuning circuit

[craxd <craxd1@ezwv.com>]

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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