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Re: [Amps] Power transformer for 600 watt rig

To: <>, "'Amps'" <>
Subject: Re: [Amps] Power transformer for 600 watt rig
From: "jeremy-ca" <>
Date: Wed, 19 Mar 2008 14:40:20 -0400
List-post: <">>
----- Original Message ----- 
From: "Gary Schafer" <>
To: "'jeremy-ca'" <>; "'Amps'" 
Sent: Wednesday, March 19, 2008 11:50 AM
Subject: RE: [Amps] Power transformer for 600 watt rig

>> -----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.

Incorrect. Long established practice often utilizes seperate supplies as the 
voltages are often different or the total current is too much for a single 
supply. I suggest that you at least research the old magazines and handbooks 
that included building for AM.

>> > 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.

I already have. READ the wisdom of the "ancients". Im certainly not 
discovering anything new; the knowledge has been published since the early 

>> > 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
> working.

Correct, He also stated that he gave up on the idea.

> Collins and Henry radio have successfully used a tuned choke in SSB amps 
> for
> many years.

Also true, However I dont know too many on AMPS with the resources to have a 
custom designed choke and capacitor built for a one-off amp. Its not a 
matter of dragging parts out of the junk box. It actually requires careful 

> 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.

It also makes the transformer and rectifiers very unhappy if they are sized 
just for a properly operating supply.

>> 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
>> SSB
>> > 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.

That is a reasonable value for a 1500W key down amp. A similar rated 5000VCT 
oldie will measure around 70 Ohms for 1/2 the secondary which is the 
measurement to be used. It will also be larger and heavier.

I have one 1948 monster here that is 9000VCT @ 2A CCS and I can barely budge 
it; havent figured a use for it yet.

That 5000VCT in a standard FW will deliver around .9V RMS with a nominal 10% 
of bleeder current and swinging choke input selected to have ~ 2X critical 
inductance. That is with MV rectifiers; use a plug in SS replacement or 
build your own and its a bit more.

The regulation will be at least as good as a single fat capacitor with a 
current design transformer.

BTW, those plug in SS replacements have been running just fine in place of 
866's and 872's since the 60's. In lots of Henry's, Collins and BC 
transmitters. Doesnt bother the choke or the 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.

Yes, but primarily due to the larger filter caps that presented a longer 
charging time to the point of safe current. Today its called an inrush 
current limiter and is in the primary.
That HV resistor was continued into the early SS circuits also when they 
werent as good at handling transients and peak currents. With the advent of 
the 1N5408 they became history.

> 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.

No, but they could operate extremely close to their peak rating when the 
swinging choke was sized correctly or a fixed choke used for the Class C 
amp. The transformer itself limited transients to a safe value plus the use 
of line bypass caps. I build with a lowpass filter in the input for several 

Now with good solid state diodes chokes are a
> thing of the past in power supplies that need good dynamic regulation.

I thought you said a tuned choke was needed for that?

A swinging choke and a low value cap can provide virtually identical 
performance to a fat cap or string. It comes down to a trade off in whats 
available and at what cost as well as size, weight and longevity.

Size and weight arent issues to many builders and they are also discovering 
that the "ancients" werent so dumb after all.


> 73
> 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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