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Re: [Amps] Power transformer for 600 watt rig

To: "'jeremy-ca'" <>, "'Amps'" <>
Subject: Re: [Amps] Power transformer for 600 watt rig
From: "Gary Schafer" <>
Date: Wed, 19 Mar 2008 11:50:07 -0400
List-post: <">>

> -----Original Message-----
> From: [] On
> Behalf Of jeremy-ca
> Sent: Tuesday, March 18, 2008 10:13 PM
> To:; 'Amps'
> Subject: Re: [Amps] Power transformer for 600 watt rig
> ----- Original Message -----
> From: "Gary Schafer" <>
> To: "'jeremy-ca'" <>; "'Amps'"
> <>
> Sent: Tuesday, March 18, 2008 8:21 PM
> Subject: RE: [Amps] Power transformer for 600 watt rig
> >
> > Filtering isn't the problem. Dynamic regulation is a problem with a
> choke.
> > You get severe voltage drops when current is drawn at an audio rate from
> > the
> > power supply.
> That is in complete disagreement with reality.
> A swinging choke is even worse than a regular choke. They
> > should never be used in an SSB power supply.
> Tell me the difference between a Class B amp and a Class B modulator
> requirements.

The same problems exist with either. However with a modulator, often it is
powered from the same supply as the finals are and there is a heavy static
load on the power supply. The percentage of current change is smaller with
modulation so I suspect the voltage drop problem is not as severe in this
type of service compared to SSB amplifiers.

> > Put a scope on the high voltage output of the supply with a choke (thru
> > appropriate divider resistors) and you will see some pretty nasty
> looking
> > voltage drops with modulation.
> I have many times over the past 4+  decades. A well designed swinging
> choke
> input PS has none of those problems. I stress "well designed".

Maybe you could tell us what a "well designed" power supply looks like with
a choke for an SSB power supply.

> > Only a tuned choke is desirable as it does not have the problems of a
> > regular choke.
> W8JI thru a lot of monkey wrenches into that myth on here over the years.
> Putting into practice what looks good on paper isnt that easy.

I don't know what myth you are referring to but W8JI did state that HE had
problems getting a tuned choke to work without a lot of fireworks.
He did not, to my knowledge, say that a tuned choke was not capable of
Collins and Henry radio have successfully used a tuned choke in SSB amps for
many years.

My understanding of a successful tuned choke is to have it tuned slightly to
the high frequency side of resonance. As current is drawn thru the choke the
inductance drops a little and further raises the resonant frequency. If the
choke is originally tuned right at resonance or below it will be constantly
swinging thru resonance as the load varies causing severe transients which
tend to pop the tuning cap and arc the choke windings.

> A real problem with a swinging choke is that there are very few people
> still
> alive that know how to design one. Peter Dahl built one for me in 1983
> that
> worked just great with a pair of 4-1000A modulators. Last I heard that
> complete amp, modulator and both PS were still on the air.
> >
> > Series resistance of the transformer is the second problem for voltage
> > regulation in a power supply used for SSB. We want as low a resistance
> as
> > we
> > can get. Most of the older power transformers are not well suited for
> > operation because of their rather high series resistance.
> Very true but only when someone tries to get the full secondary voltage in
> a
> bridge and also expects the full wave CT config current using SS and a cap
> input.

Most all of the older transformers have quite high winding resistance
compared to modern transformers.
I have several older transformers that have around 200 ohms secondary
resistance. I have a 1200 vac transformer that gives around 3000 vdc and it
has an 18 ohm secondary. Quite a difference and voltage regulation is far
better than a full wave supply with the high resistance transformer.

In the older handbooks you will also see 25 to 50 ohm resistors in series
with the diodes to limit peak current in the diodes. Today you don't see
those resistors used.

In the old power supply designs there was always a choke or two. But back
then all most of the rectifiers were mercury vapor rectifiers that could not
handle high peak current. Now with good solid state diodes chokes are a
thing of the past in power supplies that need good dynamic regulation.

Gary  K4FMX

> If a builder actually understands the manufacturers transformer specs and
> doesnt apply some halfwit ham super optimistic rating to it AND designs
> accordingly it will perform just fine. I burnt up a few learning that a
> long
> time ago.
> Carl
> KM1H

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