[AMRadio] Grounded grid amp

[Larry Szendrei]

On 1/27/2011 10:54 AM, John Coleman wrote:
> I have heard of guys doing that suppressor grid mod. For GG ops. But for
> class B zero bias audio I drove the screens direct and had an 18K res from
> screen to grid.  I thought that was a little weird but it was recommended by
> RCA on the 807s so I did it on the 1625s.  There was a paper published about
> this type of class B work with the 807s. and it was in the operation
> descriptions in the ARRL handbook for 807s or 1625s.  For whatever reason
> RCA did some extensive testing for this type of service.  I think it was
> because some folks had very good luck with the 6L6s grid and screen tied
> together and where not have good luck with the 807 that way.  There seemed
> to be a distortion anomaly and putting the 20K resistor between the screen
> and grid then driving the screen made it smooth.
>
> John
>
John and list,

I had always thought a good reason for the resistor in series with the 
control grids when using 807s this way was to limit the grid #1 current, 
which would almost certainly be excessive and damage the tubes without 
such current limiting (because you really need to goose the screen 
voltage on peaks when driving them like this).

Also, I don't know why a 6L6 would be any different in this connection 
than an 807 for the same operating parameters - electrically they are 
identical. Maybe the problem showed up because fone can apply higher 
plate voltage to the 807?

I've never tried running this connection on 807's but heard Steve WA1QIX 
using them as modulators in this mode not too long ago - but he was 
using a solid-state circuit to proportion the signal between the screen 
and control grid; he said using a linear element there was a horrible 
idea! I want to get more details on his circuit; he certainly knows his 
stuff and he sounded great (as he always does no matter what creation of 
his he's running).

73,
-Larry/NE1S