[Amps] TL-922 --> 21st century

[Mike Tubby]

On 13/08/2011 22:22, Vic K2VCO wrote:
> I have a stock TL-922 that I want to update. I'm going to do the following:
>
> -- step start
> -- vacuum relay QSK
> -- automatic bias switching to cut off tubes when not keyed
> -- check / adjust filament voltage
> -- replace VHF choke in plate circuit with glitch resistor
> -- improve bypassing of HV and filament power leads
> -- no, I'm not doing anything to the parasitic suppressors except checking the resistors!

I assume that you've seen the excellent TL-922 engineering information, 
here:

http://pa0fri.home.xs4all.nl/Lineairs/TL922/tl-922eng.htm


> A question:
>
> The grids are grounded via a parallel combination of 3 220 pf capacitors, a 4.7k resistor
> and a 470 uh choke. I don't understand this. I am tempted to just strap the grids to
> ground. What were they thinking?
>

They were thinking of "super cathode" driven design... discussed in 
detail by W8JI, here:

http://www.w8ji.com/vhf_stability.htm

and these days considered a "bad idea"(tm).



My TL-922 has the following mods:

1. 10R 17W vitreous enamel glitch resistor in series with existing choke
2. 1A fast blow 1 1/4" fuse in series with HT
3. hair-pin parasitic suppressors
4. directly grounded grids

I haven't found the need for any of the other mods (yet).



Mike G8TIC