[AMRadio] Re: Pw Supply

ne1s ne1s at neandertech.com
Thu Dec 7 13:20:01 EST 2006

Hi Jack, 

See comments/answers below. 


Jack Schmidling writes: 

> MFJ has a 900v one for about $100.  Is there any reason why I can not use 
> two of these and tie one side of the secondaries together and call that 
> the center tap and have an 1800v ct transformer/s? 

Jim Candela has already given the correct answer ("no"), and why. 

> I am using 866's so I must have a center tap.

Not necessarily! Use 4 866's in a full-wave bridge. But this will require a 
minimum of 3 filament transformers for the 866's. 

> Second question is: 
> Mine has the HV going to the choke and then to two 4 mf 3kv caps in 
> parallel.  Is it not more conventional to put a cap on each side of the 
> choke or two chokes for a choke input filter? 

Choke-input and capacitor-input filters have very different voltage outputs, 
not to mention dynamic voltage-regulation. Also, mercury-vapor rectifiers 
get VERY UNHAPPY looking into capacitor-input filters. If you use MV 
rectifiers, you MUST use a choke-input filter. With a choke input filter, 
you need to be cognizant of the minimum current being sourced from the 
supply and the required critical (minimumum) inductance of the choke. Do you 
have a period (1930's - 1950's) handbook you can review? 

> What would I gain or lose by moving one of the caps to the HV side? 

If you use cap-input with MV rectifiers, you'll at least lose the 
rectifiers! Otherwise, you'll gain unloaded output voltage, at the expense 
of poorer dynamic voltage regulation, & put more stress on  *(from higher 
peak currents in) the (SS diode or vacuum) rectifiers and plate transformer. 

I thought this has mostly already been covered in this thread? 

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