[ARC5] Smart People: 60Hz Transformers Faster?

[Glen Zook]

As for using a power transformer for audio, it really depends on the frequency response that is desired.  Old TV power transformers have been used very successfully as plate modulation transformers for  transmitters in the 100-watt output range for decades.  Of course, for that purpose, the frequency response doesn't have to be any more than around 3000 Hz.

Many moons ago, like in 1960, not that long after upgrading to General Class from Novice Class, I built a home brew plate modulator to replace the screen modulator that I had been using with my WRL Globe Chief 90A.  That modulator used a pair of 807 tubes.  Not knowing any better, I just used the power transformer from an old TV set for the modulation transformer.  I did use an audio transformer for the input to the 807 tubes.  I used the HV secondary, which was center tapped, for the plates of the push pull 807 tubes and the 117 VAC primary winding with the HV to the 807 final amplifier tubes in the Globe Chief 90A.  

Frankly, I didn't have a clue as to things like "proper" impedance and I just saw that the power transformer had windings that "matched" the modulation transformer in the diagrams for modulators in the old copy of the ARRL Handbook that I had.  There was a "garage shop" TV repair shop about a block from my parent's house and the owner kept me in as many old TV chassis, to strip for parts, that I could haul off.  The driver transformer came from one of these chassis and the power transformer came from another.  The B+ for the modulator came from another chassis and the entire modulator was built on a chassis from still another TV.  Looking back, the modulator wasn't all that "pretty"!  It didn't even have a cabinet.  But, it worked and, even better, I got "rave" comments, from other operators, on the quality of the modulation.

The 807 tubes, in the modulator, were not gassy but did get the blue glow that happens with some tubes.  As I talked, this blue glow would vary with the modulation.  I had a couple of other teenage amateur radio operator friends who like to come over and watch this glow. 
Glen, K9STH 

 Website: http://k9sth.net
      From: Brian <brianclarke01 at optusnet.com.au>
 To: Spr Imop <spr at earthlink.net>; David Stinson <arc5 at ix.netcom.com> 
Cc: ARC-5 List <arc5 at mailman.qth.net>
 Sent: Monday, November 23, 2015 12:43 AM
 Subject: Re: [ARC5] Smart People: 60Hz Transformers Faster?
   
Hello Dave,

In general, you can use a transformer at a higher frequency than that for 
which it was designed. But there is a limit. It is to do with the quality of 
the iron. As the frequency gets higher, some transformer-specified irons / 
steels will start absorbing much more energy than at the rated operating 
frequency. Running a transformer designed for 60 Hz at 800 Hz should be OK - 
but you will be carrying more iron than you need.

People who claim that they have used power transformers as audio 
transformers are often old folks for whom presbycusis has set in and they 
can't hear that the high frequencies are severely distorted and / or 
attenuated. While the transformation ratio may be OK, the impedance matching 
will be all over the place. Another problem with using power transformers 
for audio work is that there is usually DC flowing, which alters the 
behaviour of the iron - the whole hysteresis curve gets shifted up, one side 
of the audio wave flat-tops and the transformer heats up. If there is enough 
heating the iron can be permanently damaged.

You can also stuff a transformer by having net DC flowing when you use 
half-wave rectification - doesn't matter if it's at 60 or 800 Hz. Most 
computer SMPSUs rely on half-wave rectification - and electricity supply 
authorities get upset because of the net DC flow in their distribution 
transformers.

73 de Brian, VK2GCE.

On Monday, November 23, 2015 4:47 AM, Scott opined:

I used to use power transformers for pp output transformers and they worked 
fine. I expect 400 Hz to work fine, too.

/Scott Robinson

> On Nov 22, 2015, at 7:59 AM, David Stinson <arc5 at ix.netcom.com> wrote:
>
<snip>>
> But what about running a 60 Hz transformer at 400?
> Saturation shouldn't be a problem, but what about hyste...hister... yeah 
> those loses.
>
<snip>>
> 73 Dave S.

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