testing pt for ma

[mark_enger]

ive got a bunch  of pts and ots from old organ and stereos, record player,  and just plain old
mystery transformers,  and id like to test them for current,   the one im testing right now came out of a old wurlitzer organ it ran a pair of 6l6 4 12ax7  and a 5u4.   so i tested the pt  hv windings = 380 unloaded in a chassis im building into a 6l6 deluxe. with my meter on ma setting im reading 37 ma, now i now this pt has to be stronger than that, it also ran about 28 other 9 pin preamp type tubes, so rectofied what can i expect my
current to be?  i hope you guys understand what im askng lol    thanks   mark

[jjasilli]

You have enough info to compute.  If you really mean 6L6, that's 19 watts max plate diss, as opposed to the modern 30 watt 6L6GC's.  Figure 1mA for ea preamp tube.  Though 12au7's can theoretically pull much more current.  Just add up the current draw of all the tubes.

Here's a chart formerly posted by PRR which might also help: 

If that PT was handling all the filaments, then your heater voltage may be high with only a few tubes to draw current.  In that case use resistors in series with ea leg of the supply to knock it down.  Or maybe rectify it to DC, and use a regulator to knock it back down to 6 volts. 

[mark_enger]

     Theres two seperate windings,  one for all the tone bank tubes and the one im using which supplied just the amp.   recto tube 2 6l6gc and 3 12ax7s so im fine there. my concern is if i measure the hv winding and it only measures 37 ma per hv winding, what can i expect that to measure after recto is there a formula? iv got a lot of iron id like to measure.  mark

[jjasilli]

I don't think your static measurement is useful to spec-out the tranny.  I don't think there's any simple way here.  The best way is to know what current draw the tranny was supplying.  If you don't know that, then the old tranny chart is helpful.  Otherwise it was noted somewhere in the forum that you can have the tranny drive a larger and larger load; note when voltage really starts to sag; and measure current at that point. 

[tubesornothing]

jj is right on the nose.  Your mA measurement is not much use without a load on it.  Use the specs that jj suggested.

Gar Gillies has a method for calculating max current of a PT: measure the resistance of one half of the secondary winding (one side to ct). Plug in the tranny and measure the voltage across this one half winding.  Divide the measured voltage by the resistance and then multiply by 25.  Never done it but its worth a try.

[mark_enger]

ok thanks   mark

[jhadhar65]

Here's what you need:

http://www.exphil.com/product_index.php?id=Hipot%20Testers

Scroll down to #3522-50.  It's only $3,000 plus shipping by truck.  And we'll send you all of our unknowns to test for us, too.

Seriously, though, in the grand scheme of things, $3 grand isn't that much money considering what we typically spend over time playing this game.  It might be a worthwhile investment if you find that you use a lot of unknown transformers.  Depending how long you do this, it could realistically pay for itself over time.  Would you pay shipping and a reasonable fee to send your transformers off to be hipot tested?  If so, there are bound to be others out there who would do the same.

[jjasilli]

Well, yes.  Coils, like chokes, inductors and trannies, require specialized test equipment.  The equipment is so expensive as to defeat the benefit of coming into a cheap bunch of unknown units.  For outside testing, round trip shipping of your item alone is prohibitive, not to mention the labor costs of paying the labor for someone else to test it. 

Hell, for the cost of a decent 'scope, you could have a brand new hand-wired Ampeg J-whatever on sale, or an eggnator 4X 12 celestion cab.  So everyone's got to decide for themselves where to draw the line.

In this case, looks like the best game plan is:  Punt!

[tubesornothing]

I am just awaiting for PRR to chime in on how to build one of those HiPot power testers for $1.99 using the center from a bathroom paper roll and a paper clip.

[mark_enger]

well thanks for the replys i dont think i got 3 gees to test these, i know this one will work for me
for the 6l6 deluxe of some sort. with 380-380 i should get a desent b+, my 6v6 sounds great cant wait to hear ths one with the 6l6gc plus these are old old transformers and should be underrated thanks jj for the cool chart        mark

[PRR]

> power testers for $1.99 using the center from a bathroom paper roll and a paper clip

No, you use fence-wire.

[Shrapnel]

I went and did some ohms law stuff to see what was needed to test a 290v winding on a PT, and the power dissipation is the biggest problem, I think it was close to 74W to attempt (with a successful non-overload) 250mA. and 56W? for 160mA.

Wouldn't recommend it unless one has some really hi-power resistors on hand. (and for many different voltages will require many different resistance values and power ratings.)

[tubesornothing]

For electric cars we'd load test the battery pack by  a coil of coat hanger wire stuck in a 5 gal pail of water.  Works great.  I bet it'd work real well with a 500V B+


Hey maybe that's what PRR was alluding to!

[jhadhar65]

I don't know why not.  He used to power test amps the same way with wire wound resistors in a bucket of water.  When the water boiled off, just add a little more.

The big gray can transformer hanging on your front yard power pole does the same thing (assuming you Canada guys are set up like we are further south).  The only difference is it uses oil instead of water.  If your transformer's dripping, don't hang around underneath.  A big surprise is coming soon.

[PRR]

> load test the battery pack by  a coil of coat hanger wire

See? You know this stuff.

[RicharD]

>it ran a pair of 6l6 4 12ax7  and a 5u4........the pt  hv windings = 380 unloaded

That's all the info you need right there.  Obviously it's juicy enough to drive 2 6L6's.  Counting on thumbs, 2 6L6's draw about 150mA.  A few preamp tubes = a few mA.  380V unloaded will most likely be a 350-0-350 loaded.  (350*1.4)-44 = 446VDC....call it 440 give or take.  Looks like a great place to start to me.