[Hallicrafters] Rectifier replacement

[Richard Knoppow]

----- Original Message ----- 
From: "Joseph Pinner" <jpinner33 at att.net>
To: <hallicrafters at mailman.qth.net>
Sent: Saturday, November 15, 2014 10:23 AM
Subject: Re: [Hallicrafters] Rectifier replacement


>I do the same here. My boatanchors run fron a variac at 
>110V.
>
> I do believe it helps extend the life of tubes and 
> components.
>
> On 11/15/2014 11:57 AM, Greg Gore via Hallicrafters wrote:
>> The Weber tech is probably right when you add together 
>> the higher input line voltage of today and the higher B+ 
>> the Weber solid state rectifier produces. I've had good 
>> results running old equipment on 110V through a variac 
>> which reduces power transformer stress and a great deal 
>> of excess heat which would otherwise be produced from the 
>> 120 and higher input volts.
>
> -- 
> Joseph Pinner
> Kingston, TN
>

     The difference in voltage when using a solid state 
rectifier in place of a vacuum tube one is considerable. Its 
difficult to state a definite value because vacuum tube 
diodes are not linear resistors but for argument's sake lets 
say you have a normal B+ of about 300 volts you may find it 
as high as 350 or 400 volts. Where the current demand is 
fairly constant you can correct for this with a fixed power 
resistor in series with the rectifier. Since the tube is a 
non-linear resistor a regular resistor will correct the 
voltage for only one value of current but will be close 
enough for most cases.
      Where one is replacing a mercury vapor or gas 
rectifier the problem is somewhat simpler because mercury, 
gas, and SS rectifiers have a constant voltage drop. For a 
mercury vapor rectifier its about 15 volts regardless of 
current demand and I think the gas tubes are about the same. 
Since silicon diodes have something on the order of one volt 
drop one can string a series of diodes to get the same drop 
as the tube.  Not a problem because the diodes are cheap and 
you need the high voltage capability anyway.
     Probably the most practical method of correcting for 
high line voltage is to use a bucking transformer.  There 
are several articles on the web on how to do this. Most ham 
equipment was designed when standard supply voltage was 
about 115 volts, at least in the U.S. Currently (no pun 
intended) line voltage in many places is as high as 125 
volts. Depending on the average line voltage a 6 or 12 volt 
filament transformer will work well as a bucking 
transformer. Ideally, one could use a line voltage 
regulator. There have been two types in general use, the 
first is the mechanical type made by General Radio among 
others. This is a motorized Variac with a control circuit. 
The response time is very fast and the output has no 
distortion and has approximately the overall efficiency of 
the Variac. The other type is the Ferro-resonant type such 
as the well-known Sola constant voltage transformer. Sola 
type transformers work very well and have the advantage of 
being very good line transient filters. They also shut down 
if the secondary is shorted so provide some degree of 
protection.  The older ones produced a lot of distortion of 
the output wave but newer ones put out a reasonably pure 
sine wave. The other problem is that they tend to be noisy, 
run hot, and are not terribly efficient especially when 
running at minimum load.  They are best when they run under 
at least half load all the time. Nonetheless, they turn up 
on the used market fairly often at reasonable prices and 
work very well. A word of caution: test any Sola or other 
brand of CVT before buying it. Sometimes the capacitors go 
bad and I have also had Solas that did not regulate very 
well despite replacing the caps. I don't know what the other 
problem is but suspect it may have something to do with the 
laminations being displaced.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com