How much capacitance - continued part III

[Kenneth G. Gordon]

Heathkit was one of the first, if not the very first, 
commercial company to use a voltage-doubler power 
supply for the HV supply in its linear amps, although I do 
know that the ARRL had similar power supplies shown in 
some of its handbooks of the period.

Heathkit engineers did this for several reasons, not the 
least of which was because they were very well aware of 
the in-rush current problem.

This problem, especially acute in straight (non-voltage-
doubling) power supplies wherein the transformer voltage is 
near the DC voltage output, is much less acute in voltage-
doubling supplies since the voltage takes several cycles to 
build up to the designed DC output voltage.

In fact, and this is important, it can be easily shown 
mathematically that if the total filter capacitance in a 
voltage-doubling power supply is LESS THAN 30 MFD, 
there will significant voltage sag at rated load.

Do you want to read that again?

I repeat 30 MFD.

What is the total capacitance in the SB-200 with original 
caps?

21 MFD at rated capacitance of 125 MFD each.

I am not sure what that is in the SB-220, but I think it is 
about the same.

Secondly (and this is almost as important) in a GE Ham 
News article written about the time that SSB started to 
become common in the amateur ranks, it was clearly 
shown by experiment, accompanied by many scope 
photos, that especially in SSB/CW service, capacitance 
below a certain level resulted in really terribly poor dynamic 
regulation, which resulted in very much poorer IMD in SSB, 
and other distressing spurious emissions in CW.

I can dig out the GE Ham News article and send a copy to 
anyone who wants it for patience and an SASE.

The article discusses and suggests a minimum value for 
the OUTPUT capacitance in a choke-input HV filter which 
would pretty much eliminate poor dynamic regulation.

Which brings up another point that those who are worried 
about in-rush current are forgetting:

The reason that choke-input filters were used in the HV 
power supplies of the old days is not because of a worry 
about in-rush current:

The main source of concern was the PEAK current that 
occurs at every 1/2 cycle in a full-wave center-tap power 
supply of "normal" (non-voltage-doubling) design with a 
capacitor input. Choke-input eliminated or greatly reduced 
this problem.

More later...

Ken W7EKB

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