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Old 3rd October 2011, 03:49 AM   #1
yero is offline yero  United States
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Default Tv power transformers for audio use?

Is it possible to use tube TV's power transformers for tube or solid state audio power amps.
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Old 3rd October 2011, 05:10 AM   #2
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In general, yes.

Of course "attention to details" is an important part of ANY kind of design or engineering. Some details you need to be concerned about include:
  • Are the voltage and current ratings suitable?
    • The typical TV power transformer from the 1940's thru the early 1960's had a 5 VAC - perhaps (though rarely) center-tapped - winding to power the rectifier filament. If you are using a tube-type rectifier, this winding can be used as-is.
    • There was also at least one 6.3 VAC winding for other filaments. This winding almost certainly has the capacity to light the filaments on a single-channel power amp, with or without a typical preamp using 3 or 4 tubes. The single 6.3V winding may not have the capacity for a two-channel power amp.
    • There may be a second 6.3V winding, though usually at a lower capacity than the first. This may be capable of lighting the filaments in the second power amp channel, or could be wired in series with the first 6.3V winding to supply filaments requiring 12.6 volts.
    • If you don't have the second 6.3V winding, but must have 12 volts for filaments, the 5.0 VAC winding can be placed in series with the first 6.3V winding to get almost 12V for the filaments IF you don't need the 5.0 VAC for rectifier filaments (i.e., you use a solid-state rectifier). It's a usable solution, though not really desirable in my mind.
    • The high-voltage (B+ ) windings were typically rated for somewhere between 100 and 200 watts, more or less. The voltage ratings varied quite a bit, perhaps as low as 250 VCT up to 600 VCT. Of course, depending on factors such as rectifier drop, and choke- versus capacitor-input filter, the DC plate voltages you get may be as low as 150V or as high as 400V.
  • The voltages obtained from a tube-type TV power transformer probably aren't suitable for solid-state circuits.
  • The main limitation on transformer power is the magnetic core. Consequently, the rated power of unused windings (such as 10 or 15 watts from the 5 VAC winding) can, as a first approximation, be redistributed to other windings.
  • You can get a good estimate of a transformer's total power rating (all windings, added together) by comparing its weight and dimensions to catalog descriptions of other power transformers.
  • Transformer insulation tends to deteriorate over time, and the deterioration is aggravated by heat. A marginal design that was run "long and hard" may fail soon after installation in a new amplifier. In many cases the insulation failure is in the leads connected to the windings, rather than the windings themselves (look where many leads are bunched together to pass through a grommet in the transformer's shell casing) and can be easily repaired. Shorted turns in a winding itself, or winding-to-winding, will require removal and replacement of the affected winding(s) even if the defective windings aren't being used. Open windings can usually be ignored, as long as they aren't required.
  • It's possible to change a winding's voltage by adding or removing turns. This is usually most practical with the filament windings, since they are usually the outermost layer. When the 5.0 VAC winding isn't used (because your design uses a solid-state rectifier) it's often useful to convert it to a 6.3 VAC winding. There are web pages showing you how to determine the turns-per-volt factor for your particular transformer, add the necessary windings, and restore the insulation.
  • It's possible, though rather tedious, for mere mortals to remove entire windings and re-wind the transformer for different voltages (e.g., the lower voltages and higher currents required by solid-state amps). It's easier if you can leave the original primary winding intact, but that may not be possible. As I said earlier, the main controlling factor is the power rating of the laminated steel core itself.
Dale
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Old 3rd October 2011, 06:14 AM   #3
yero is offline yero  United States
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Dale thanks, Can't you also lower the b+ with zener shunt regulation? What about the power that supplied the CRT. Was that a seperate transformer or was there a voltage doubling circuit used. I thought that the picture tube used very high voltages.
Roy
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Old 3rd October 2011, 06:37 AM   #4
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yes , Dchisholm has made a very accurate description !
My one ( and only ) tube preamp that I've done , it was with two ECC 82 in SRPP , and ... a transformer from a Dumont Tv found ...in the meadows !
It was giantly overkill for the purpose , but the two 6.3 V secondaries worked well for that circuit . HT was regulated with a mosfet ( IRF 820 ); I don't think shunt regulation would work for high current demanding applications ,such power amplifiers .
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Old 3rd October 2011, 10:21 PM   #5
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Quote:
Originally Posted by yero View Post
Can't you also lower the b+ with zener shunt regulation?
Regulation can be done with either series or shunt circuits, though the designs are complicated by the high voltages. I seem to recall a series of articles in "Audio Amateur", circa early 1980's, that developed some solid-state series regulators for B+ supplies.

Quote:
What about the power that supplied the CRT. Was that a seperate transformer . . .
CRT anode voltage (typically several kilovolts) was created in a flyback step-up circuit driven by the horizontal deflection oscillator. The horizontal/flyback section of the TV had a reputation for being rather cantankerous and failure prone. If you REALLY need those kilovolt voltages (WHY????), you'd probably do better to look for an old oscilloscope chassis - they often had a very high voltage winding, either as part of the main power transformer, or on a separate transformer.

Dale
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Old 5th October 2011, 09:08 PM   #6
adamamp is offline adamamp  Canada
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Quote:
Originally Posted by dchisholm View Post
I seem to recall a series of articles in "Audio Amateur", circa early 1980's, that developed some solid-state series regulators for B+ supplies.
Yes: they were so reliable, had such a low parts count, proved very effective, easy to mod for any B+ voltage up to 1 full amp and cheap to make, I have no idea why so many folks have strived to reinvent the wheel using FETs and a comparably large host of parts
Z Power to the people.
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Old 5th October 2011, 09:33 PM   #7
adamamp is offline adamamp  Canada
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PS: for more detail see:
1, Audio Amateur 4/1979, pg. 7
2, Audio Update #6 9/1977
and the final say for high current up to and including Futterman needs:
3, Midwest Audio; Power to the People 3rd ed., 1982
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Old 6th October 2011, 01:54 AM   #8
yero is offline yero  United States
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adamamp; I thought that we are talking about b+ power supplys. Audio Amateur 4/79 pg.7 is the 40/40 power amp by may he rest in peace Reg Williamson. Can you check that reference or provide schematics thanks, Roy
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Old 6th October 2011, 02:56 AM   #9
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I learned electronics by taking TV sets apart as a kid. In my early teens I was making guitar and HiFi amps from old TV sets. It was the late 60's and discarded TV sets were free by the hundreds at the local trash dump. The power transformer from a late 50's or 60's COLOR TV was a most excellent choice for building a tube amp. Color TV's were still expensive and there weren't too many scrap ones, but they were there and I took them ALL apart.

The old Zeniths were the best. You couldn't kill those transformers. I still have one running a tube amp. A transformer from a 21 inch or larger color TV will run a HiFi amp in the 50 WPC range at full volume forever! Remember in that era most households had ONE TV and it was turned on whenever someone was home. Many houses didn't have air conditioning yet, even in Miami where I grew up. Poor designs didn't live long. Heat, humidity, dust and constant use killed them quickly. The old Zeniths, Philco, Sylvania, and RCA's were good choices. The GE's had issues with the plastics used in their high voltage sections breaking down, but we are after the transformers.

The B+ voltage used in most TV sets were in the 300 to 400 volt range. This is about right for a 2 channel amp that makes 35 to 50 WPC. It also works for a wicked guitar amp.

Black and White TV's had enough transformer for a 50 watt guitar amp or a 25 WPC Stereo.

As mentioned before, you can rip all the windings off a transformer except the primary and wind a custom secondary for use with solid state amps.

When gutting a TV save the power transformer, the vertical OPT (it makes a nice SE OPT good for about 5 watts). Save the sweep tubes since the right ones can make stupid power.....like 250 watts!
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Too much power is almost enough! Turn it up till it explodes - then back up just a little.
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Old 6th October 2011, 04:22 AM   #10
adamamp is offline adamamp  Canada
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Quote:
Originally Posted by yero View Post
adamamp; I thought that we are talking about b+ power supplys. Audio Amateur 4/79 pg.7 is the 40/40 power amp by may he rest in peace Reg Williamson. Can you check that reference or provide schematics thanks, Roy
Hi Roy,
According to my source(I don't have the original, just an article with references to it), Reg suggested in that issue to use a capacitance multiplier(the same design used in the other references) for active B+ regulation.
Bruce Zedike had provided the idea originally(AFAIK) for audio use in 1977, for the "Ultimate Preamp Power Supply" in Audio Update.

This design could easilly be adapted for higher current by properly selecting the semiconductors as demostrated in the Midwest Audio publication(again designed by Bruce Zediker). He had developed these designs over many years in an industrial setting for use with instrumentation power supplies and HV test gear. They have truly stunning results when used in audio applications.

Note: Audio Update (USA)
"A Magazine published in the mid to late 70’s by Audio Dimensions and edited by Henry L. "Ike" Eisenson. Audio Dimensions, Inc. was a company in San Francisco, California, USA that was run by Ike, Roger Stovold (who also did the layout and graphics for the mag) and Ike's mum. It specialised in selling audio kits, upgrades and electronic parts via mail order and to keep their punters updated started publishing a small newsletter/fanzine called Audio Directions that featured news, circuit and upgrade ideas etc. In 1976 the company opened up a that was a mid/high end hi-fi store that they called Audio Directions and then changed the name of the newsletter to “Audio Update” and added more generic hi-fi news and editorial material to the newsletter in which meant an increase size to about 20+ pages.

What set AU/AD apart from other such publications was that the electronic circuits and discussions had an emphasis on valve technology which was very unusual in the timeframe and it was for a time the only publication really dealing with such matters in the USA. Mr. Eisenson later compiled the most popular circuits and modifications in a book called “Tu-Be or not Tu-Be” (sigh). The mag in the end only amounted to 22 or so issues under the Audio update name with an unknown numbers published under the Audio Directions name..."

As for posting a schematic, I have a monster scanner about half the size of my living room(not counting the space lost to my ~900 watt sound system) I could hook-up if you could explain how to post attachments here. You'll have to excuse me, I'm an old fart and not at all IT savy
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