>Am I able to use 4-400 tubes in my TL922 amp...I have just been given 10
>of these tubes NIB and was wondering what I had to do to run these in
>place of the 3-500's if the need arose where I had to change them out...
** The problem is that 3-500Zs don't idle the same as 4-400As, so a few
manoeuvres are in order. The battle plan:
1. Short the bias zener.
2. Increase the anode PS potential by c. 500v in order raise the ZSAC to
the needed 75mA/tube in order to achieve SSB linearity. To do this, add
a 200Vrms or so 1A tranformer secondary winding, in-phase, in series with
the stock HV winding. [Peter Dahl can make the boost xfmr]
3. Add a 10-ohm, 3w resistor between pin2 (g1) and pin 3 (g2) of each
tube socket in order to automatically provide the triode connection for
tetrodes.
Since peak emission is the same, P-out with either 3-500s or 4-400s
should be similar. -- note - If I lived in a 50Hz region, I would
replace the stock AC-type fan with a 12Vdc brushless fan operated by a
FWB/C-filter powered from the 11Vac, 15a filament winding.
>It would really save me a lot of dollars to be able to do this..Can somebody
>advise what is involved with modifying or are they just "plug-in and
>go"..The 3-500's are the originals and are well over 20yrs old...nothing
>wrong with them...I am just curious for the future..
> Regards Rick
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Rich, AG6K, 805-386-3734, www.vcnet.com/measures
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