All,
The grid earthing arrangements in the TL922 are a little odd and
superfluous [goes back to an arguement about splitting the anode-grid
and grid-cathode capacitances, etc]... I would go with what Jim says -
direct grounding of the grids in several places with wide copper strap
and do away with the caps and resistors altogether.
If you want grid protection [a good idea my my book] then like Jim says
a 250mA fuse will do the trick.
There's a lot of useful engineering tips and mods for the TL922 over here:
http://www.xs4all.nl/~pa0fri/Lineairs/TL922/tl-922eng.htm
I have grounded the grids, fitted a grid fuse and replaced the anode
parasitics as described on this site.
My TL922 appears to produce 50-100W more out on each band (especially
10m) with a corresponding 10-20W reduction in drive and is
unconditionally stable.
Mike G8TIC
On 06/08/2010 13:27, Jim Thomson wrote:
> Date: Wed, 4 Aug 2010 10:34:40 -0500
> From: rcm@mail.icnet.net (Randall Mullinax)
> Subject: [Amps] TL-922 grid chokes
>
> The grid chokes in my TL-922 were replaced with 10 ohm resistors
> by the previous owner. The amp works fine except on 10 M where the
> resistors go up in smoke within a few seconds.
>
> This bring up several questions.
>
> 1. Is the amplifier oscillating on 10 M?
>
> 2. Should I replace the resistors with the factory 470 uH chokes
> or directly ground the grids?
>
> 3. Where can I get replacement grid chokes?
>
> 4. I?ve read about adding/increasing negative feedback. Does anyone
> have suggestions?
>
> Thanks for any and all comments.
>
> Randy
> AD5RM
>
> ## I have worked on a couple of TL-922's yrs ago. I remove the 6 x grid caps
> and 2 x chokes..toss em.... and strap all 6 x grid pins to the chassis, with
> copper strap,as
> wide as practical.
>
> ## your drive requirements will drop by 20-25 watts on ALL bands.
> The amp will be stable as a rock. Since your xcvr is now operating with
> 20-25 w less drive.. the xcvr's TX IMD will be much better.
>
> ## Overall signal quality will be better. IF you want a real grid fuse,
> use a
> 3agc type.. and insert it in series with the existing grid shunt. [either
> side]
>
> ## I installed a 3agc fuse holder on the rear of my 4 x L4B's... with a FAST
> rated 250-300 ma rated fuse. If the grid fuse blows open... there is now
> no path for dc grid current. Power output drops to zero. Input swr
> will rise to infinity. Xcvr will reduce it's PO..down to nothing. Takes
> 4 x seconds to change the fuse. I have blown one fuse since 1990.
>
> ## If ur hell bent to keep using the 10 ohm grid fuses... then you will have
> to
> increase the value of the stock grid caps. Stock..you have a total of 600
> pf.
> This has to be increased to a total of 1800 pf. [ per socket] .
>
> ## you can do this by adding a pair of 560 pf caps to the existing caps.
> [per socket]
>
> later... Jim VE7RF
>
> later....... Jim VE7RF
>
>
>
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