The AM Forum

I had taken my Valiant apart several years ago to do a thorough cleaning and rebuild.  It is on the bench for the summer as I slowly put it all back together.  Anyhow, as I attempted to remount T1 (the plate transformer) I notice (and then remembered) that it is not the stock iron.   This is a Geo. Sturley 118:1800vct 0.5A, 0.4kva unit.

I am guessing around 800vdc after 3B28's and choke input supply.  WAY more than the 600vdc specified.

Does anyone have the specs on the stock Johnson transformer for the Valiant? 

Tnx, Steve KK7UV

Plug-r-in. More HV more RF and modulator action!!!!!!!!

Thought about doing so, but......isn't the modulation tranformer opertaing at its limit in the Valiant anyway?

Or, instead of having to find a stock Valiant plate transformer I could look for a bigger mod. iron.

Steve

You could use a "bucking" transformer to lower the primary voltage to 90VAC. It may be easier to find a low voltage transformer with a suitable power rating (around 100VA) then it is to find a different plate transformer.

For example: http://www.antekinc.com/details.php?p=664 (28VAC 100VA) or http://www.antekinc.com/details.php?p=680 (28VAC 200VA)

Stu

The stock Valiant HV transformer is 760 volts either side of center.   If you reduce the final and modulator current a bit to stay at the rated input power the mod transformer (and other stuff in the RF chain) should be OK.  BUT the new transformer with the HV supply under light load may produce a DC voltage that doesn't leave much safety margin for the filter caps - especially if your line voltage is already on the high side.

I would follow the earlier suggestion to buck the primary a bit to get the voltage closer to standard.

I'll give the bucking transformer a try.  Checking my math here...

118:1800vct will be 1830vct with 120 line voltage.  The ratio is 15.25
I need 1520vct so I need to buck the primary down 20v to 100v.

So I need a 120v:20v transformer with the 20v secondary rated for about the same current as what will flow in the primary of the big transformer.   With the big transformer running (max) 760v at 400mA or about 300VA, I come up with 2.5A on the primary.

How did I do?

You did fine.

But take note,  if you see smoke, you made a mistake. ;D

Fred

A heavy 12 volt filament transformer will work just fine for bucking.
Regards,
Gary