[AMRadio] Grounded grid amp

[Nigel Holmes]

This is how you build a G-G amp. with 572B. http://www.youtube.com/watch?v=zB8cVwZyVmc 

Probably a good time to mention VK has an annual "Ross Hull Memorial VHF/UHF" contest. Ross VK3JU was a former asst. tech editor of QST who plated himself on the 6 kV CRT supply of a TV he'd built.

Nigel VK3DZ

-----Original Message-----
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 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
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