I would worry about a fuse in series with the plate supply. If it blows and if
you
continue to apply drive, wouldn't there be a possibility of toasting the grids?
On 8/14/2011 2:24 AM, Mike Tubby wrote:
> 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
>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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