The Dumpster Transformer Topic

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The Dumpster Transformer Topic

Postby wiredbecker » Fri Apr 06, 2007 1:11 pm

I find transformers all the time but never pick any up because I have no idea what I'm looking for. The local junk shop almost always has no name amps that have been stripped of their tubes and left to die. How do I safely measure these found transformers? Are there any tricks to knowing their output before I spend those precious couple bucks? What are some names to look for?
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Postby erichayes » Sun Apr 08, 2007 9:36 pm

Hi All,

Let's start with power transformers. Since around 1950, power transformers have had a color code for their leads that was developed by the Radio Television Manufacturers Assn. (RTMA or "retma").

Primary: black, or black and black/color tracer. Multitapped primaries used multicolored tracers.

Hi voltage: red; center tap: red/yellow; bias tap: blue or slate

6.3v heater: green; center tap: green/yellow

6.3v heater #2: brown; center tap: brown/yellow

5v rect.: yellow; center tap: yellow/unspecified color tracer.

Bias: blue or slate

There might be other windings, but the above are the major ones.

AS far as names go, most transformers were made by Original Equipment Manufacturers (OEMs) and will have either the finished product's name (Zenith, Admiral, RCA etc) on it, or nothing except maybe a part number. There were some notable exceptions in the hi fi field, where a manufacturer would point out that his transformers were made by Acro or Peerless, but this was generally only the case for output transformers. Nobody really cares where a PT comes from, but you might find an occaisional Stancor, Triad/Utrad or Thordarson label.

reset

To check the actual voltages on the windings, it is HIGHLY recommended that a variac be used. If you're gonna be messing around with high voltages and dried out electrolytics, you'll need one anyway and--who knows?--the money you spend on the variac could be recouped after one or two wise transformer buys.

If you just can't come up with (or justify/rationalize) the funds for a variac, you can use a 12.6 volt (or any other stepdown voltage, for that matter) filament transformer; you'll have to do a little more computing to get the correct voltages, though.

Set the variac to 11.7~12.0 volts and hook up the transformer primary. Measure the various secondary voltages and multiply by 10. These are the NO LOAD voltages of the windings, and probably should be reduced by around 10% to get a more accurate feel of what the transformer's actually going to do.

As far as probable voltages and currents go, look at the output tube complement and see what the data sheets tell you about maximum ratings for those tubes. This is a no-cost substitute for the above, but not very accurate.

More tomorrow
Last edited by erichayes on Mon Apr 09, 2007 8:26 pm, edited 1 time in total.
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Postby wiredbecker » Mon Apr 09, 2007 2:20 am

you RULE
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Postby erichayes » Mon Apr 09, 2007 10:00 pm

Okay, on to output transformers . . .

Rule of thumb #1: High quality OTs are relatively heavy compared to most low to medium quality OTs. BUT, low to medium quality OTs are not necessarily lighter than high quality OTs. Good OTs need lots of iron to keep the core from saturating at high power/low frequency operation, but they also need to be wound a certain way, called interleaving, in order to flatten the frequency response out. It's much more time consuming than conventional winding (remember, a power transformer only has to operate at one frequency: 50 or 60~), so it's going to cost more to build. Just as a f'rinstance, my Eico HF-89 OT clones (as well as the originals) have 16 different wire gauges in the primary winding, alone. Most console makers figured their customers wouldn't hear the difference between a low or medium quality OT and a high quality one--especially given the crappy speakers they were using.

Therefore, when harvesting OTs from old chassis, be cynical and skeptical of their quality (unless you know for a fact that the chassis was built by a bona fide hi fi manufacturer). Then allow yourself to be pleasantly surprised if you should happen to find a sleeper.

Rule of thumb #2: If the OT has screen grid taps, regardless of manufacturer, it's probably at least medium quality. Look for leads going to pin 4 on most octal output tubes and pin 9 on 6BQ5s. The ECL8*s are usually either 7, 9, or 3. Consult your tube manuals for details on other tubes.

Rule of thumb #3: There is no rule of thumb for color coding. For single ended transformers made in the USA during the '50s and '60s, blue was usually plate and red was usually B+. Push-pull OTs usually went with blue as the start plate, red as B+, and brown as the finish plate. Screen grid taps, if present, were usually blue and brown with a yellow or white tracer. Again, if it goes to the pins mentioned above, it's a SG tap.

Dynaco transformers are a notable exception to the above.

Virtually all console "hi-fi"s had dedicated secondaries, usually 8Ω, and the wires were generally just enamelled wire. In the case of component amps, you can pretty much count on black being 0Ω. Brown was 4Ω a lot of times, as was green for 8Ω. Yellow was popular for 16Ω, but so was orange, in which case yellow might be 4Ω instead of brown.

Another couple of ROTs before I turn the podium over to fr?re Brown for his snappy method of determining primary impedance using nothing more than smoke and mirrors: DC resistance can tell you a lot about the windings, especially the primary, in a push-pull OT. Measure from the center tap (red) to either plate lead and record the resistance. Measure from the CT to the other plate lead and note its resistance. The CT->plate resistance that's lower will be the start side. Same holds true for the screen taps. Hey SWJ, weird how things happen in bunches, isn't it?

Finally, if you should happen across a carcass made by Voice of Music (VM), kindly discard most of the rules of thumb listed above (the DC resistance one will work, but you might not know how to apply the results). VM circuits could be the electronic equivalent of the Kraft Food recipes you used to find in TV Guide back in the '70s and '80s, i.e., totally, certifiably off the wall. All bets are off concerning VM output transformers.

Okay, Ed, you're on . . .
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Postby wiredbecker » Tue Apr 10, 2007 12:36 am

I'm gonna print this thread out and add it to the toolbox I carry in the trunk of my car for easy reference out in the field.
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Postby EWBrown » Tue Apr 10, 2007 6:59 am

Those VM OPTs were really teensy on the 6V6 console chassis that I stripped and rebuilt as 6EM7PP. (Shannon has those and a couple other small OPTs of indeterminite origin). The only original VM parts in that amp are teh chassis and the power trannie, all else is new.
That power trannie was huge and hefty, and had enough power reserve to feed an ST70, almost, so it just loafs along in 6EM7PP service.


Comparing VM's "imaginative" circuitry to Kraft food recipies is definitely doing a disservice to the packaged / processed food industry Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_08 Even Kraft instant mac ahd cheese mixed with cubed spam is more scientifically sound, though watch out for those unpredictable "after-effects) :parp:

My "smoke and mirrors" method of roughtly determining an OPTs primary impedance is fairly simple and straightforward. All one needs to know is that transformers impedances are a squared function of the turns ratio.
So, if a tranny has a 20:1 turns ratio, the impedance will be 400:1, etc.

I use a moderately low voltage AC "wall wart" transformer as my test voltage source, the two I use are 16VAC and 24VAC, (approximately).

Commect the low AC voltage to the OPT's primary, power it up, and measure the actual AC voltage. Then measure the secondary voltage, I usually use the 8 ohm tap, or otherwise assume 8 ohms for a single output. All that remains is to take the ratio of the AC voltage on the primary, to the secondary voltage, calculate the result, and then square that, and multiply by the known or assumed secondary impedance.

For instance (using nice rounded numbers), I have 16.0 VAC across the primary a-a leads, and 0.5VAC across the 8 ohms secondary, the primary impedance will be 8 * 32^2, or 8X 1024, or 8192 ohms. Call it 8K for convenience. Close enough for horse-shoes, hand grenades or government work...

Take a mystery SE OPT, same process, assume 16.0VAC across the primary, and 0.8V across the secondary. 20:1 ratio, 400:1 after squaring, so the primary is 8 * 400 or 3200 ohms. Just what the doctor ordered for 6B4Gs or 2A3s. ..

I've seen the opposite approach take, 6or 12VAC to the secondary and measure the high voltage across the primary. Don't do it this way! Definite Shock hazard, and this can fry a trannie in short order...

/ed B in NH
Last edited by EWBrown on Mon Jun 03, 2013 7:54 am, edited 1 time in total.
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Postby TomMcNally » Tue Apr 10, 2007 8:11 am

Hah ... every time I see VM mentioned, it brings back a funny memory of this little radio station in Atlantic City owned by an 80 some year old woman when I was a kid. I'd help her out with technical problems after her engineer husband passed away. She had a home type tape recorder she called "The VIM" made by Voice Of Music ... they played all of their commercials from little reels of tape. When the real Magnecord tape deck would break, they would put a microphone up to the speaker of "The VIM" and play the commercials back into the microphone. Very hi tech ! Oh ... I'll go post an almost shocking story involving her.
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Postby wiredbecker » Fri May 18, 2007 1:12 am

Found this great transformer training this evening

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Postby EWBrown » Wed Feb 03, 2010 8:45 am

The VM power trannie had a "mystery" 120VAC secondary which was wired to an "auxillary" octal socket. A little research turned this up:

Turns out that the VM console's AM/FM stereo tuner used series-strung filaments in the usual cheap "AC/DC" format, and the 120VAC winding was simply a built-in isolation trannie for the tuner.
So the power trannie had the usual primary, HV CT secondary, FOUR 6.3VAC filament windings, 5VAC rectifier winding and the 120VAC "isolation" winding. Needless to say, it had a plethora of leads, and they didn't necessarily follow any normal color coding scheme. For the 6EM7 PP amp, I just wired all four 6.3V windings in parallel, so I had more than anough current available for the 4 tubes @ 925 mA each, or 3.7A total. I figured that VM wasn't going to engineer in any unnecessary extra current, so I used all that it could offer, just to be safe.

Weird, indeed.... VM and "Mad Man Muntz" both used some rather innovative "value engineering" in their products. In other words, they made them as cheaply as humanly possible, which could still (supposedly) work well.

/ed B
Last edited by EWBrown on Mon Jun 03, 2013 7:56 am, edited 1 time in total.
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Postby MashBill » Wed Feb 03, 2010 7:25 pm

This thread should be "stickied". Great info!
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Postby LinuxGuru » Thu Feb 04, 2010 3:37 pm

I see no reasons to pick up dumpster transformer, new one custom ordered from factory should cost in USA around $100.
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Postby EWBrown » Thu Feb 04, 2010 4:45 pm

For me, half the fun is using free / junk / salvaged / cheap parts in my designs and builds It helps to have hamfests and swapmeets in your area, or at least good active local flea markets. Don't overlook the local dump (recycling center / transfer station) for a swap table or a scrap metal bin, if they will let you "dumpster dive".

Yard sales can be fruitful, best to llook for sales at old houses, especially if they have barns or large sheds. Newer houses and condos yield mostly used baby clothes and toys, or broken VCRs and TVs, and are generally sparse for good tube stuff ;-)

/ed B
Last edited by EWBrown on Mon Jun 03, 2013 7:56 am, edited 1 time in total.
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Postby nyazzip » Thu Feb 04, 2010 6:14 pm

a hundred bucks is a lot of cheap beer.
i still recall how me and a buddy furnished an entire "party room" in his mom's garage using stuff people set out on the curb for garbage pickup. you should have seen us move a couch on top of a '79 Trans Am. i STILL have the Pioneer SS receiver from those exploits
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Re: The Dumpster Transformer Topic

Postby mr2racer » Sun Jun 02, 2013 7:01 am

Wiredbecker,

There used to be a link to a photographed article here. It told you how to test a power transformer to get its output and approximate current rating. Do you have a copy of it? Or can you tell me how to get one? What the hell is CJB.com anyway"

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Re: The Dumpster Transformer Topic

Postby Geek » Sun Jun 02, 2013 11:45 pm

mr2racer wrote:What the hell is CJB.com anyway


Just a hotlink-blocker.

If he can see it here, it's in his cache.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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