[AMRadio] Re: Amplifier with input drive to screen grid

ne1s ne1s at neandertech.com
Tue May 23 13:23:34 EDT 2006

It sounds like they were connecting the 813s as zero-bias triodes, more 
often done in modulator service. Don't know why they'd do this instead of 
run them GG in this application, though, unless there was an appreciable 
difference in gain. Maybe the guy just wanted to experiment with something 

I don't see how the grid current meter connected as you described would 
register any grid current at all, 'cuz I can't see that the control grid 
would go positive wrt the cathode in that configuration. Does it move off 
the peg at all?? 


Bill Fondren writes:
> I recently traded for a linear that has a strange input circuit, at least to me.  The amp is built in a 18 " Bud rack mount cabinet weighting about 150 lbs.  To heavy for 1 person to handle so I deceided to split the RF deck from the power supply and put it in a taller cabinet along with the exciter.  No info was with the unit.  I have used it a couple of times and it works fine .  It uses a pair of 813's in parallel.  I started to draw out the circuit finding that the drive goes to a tuned circuit and then to the sceen grids of the 813's.  The control grids are connected together and go to a 10K resistor, the grid current meter and then to grd.  No connection to the drive circuit at all other than the interelectrode cap. I guess.  The plate circuit seems to be normal.  Just wondering if any one has seen something like this before.      Bill     K5PML

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