Power transformer question

[AlNewman]

Hi,
So I have a power transformer from an old radio, that I couldn't find a model number or schematic for.  The output transformer is toast.

On wall voltage, it looks like the heaters are running 8v@ 0 current, and the HT is running around 320v/side@ 0 current.

I'm thinking of doing a matchless lightning type build, with a diode rectifier.  I have a Hammond 1751M output transformer, which is 8k primary rated for 15W.

Does this sound feasible?  The power tranny was into a single ended amp originally, and they were all 7 pin sockets.  I don't even know what type of rectifier it was using, all the tubes were gone when I arrived.

[HotBluePlates]

... So I have a power transformer from an old radio, that I couldn't find a model number or schematic for.  ... The power tranny was into a single ended amp originally, and they were all 7 pin sockets. ...

The power transformer can support exactly the type & number of tubes found in that radio.  So I'd only consider re-using it if you also want a single-ended amp of the same power output as the radio was originally.

It's kinda like hearing me say, "I've got some wheels off a Jeep, and want to build a vehicle around them.  Can I build a Wrangler?"  And then you find out I robbed the wheels off this vehicle.

[kagliostro]

To know more about your PT you must load it with resistors

Load the heater wiring till you have 6.3v on it

about HT you must load the winding for a 5/6% drop of voltage, then keep the transformer connected (all winding with a load) and wait,  control the PT temperature and if within an half hour it didn't become too hot measure the current that flows on windings, that is the max current you can take from the PT

When dismounting an old apparatus to save transformers if you want to avoid problems you must register on paper the more info you can, name of the apparatus (hoping to find a schematic as source of info) number and kind of tubes (to know about currents) and if it is an audio amp if it is an SE or PP

Franco

[AlNewman]

That's really valuable info from everybody.

I'll try testing the transformer, I do have other single ended transformers I can mate it with.   It does seem beefy, and I was kind of digging the voltages, so maybe it (could) work.  It is good to know that there's a way to experiment without committing fully.  I'll be able to use that information again and again.  Thanks.

[kagliostro]

One other parameter to keep in consideration for a first orientation, just something to have an idea about the transformer to be tested, is to consider it's weight and compare it with the weight of other known transformers

This apply both to PT and OT (on OT you must obviously consider if it is a PP or SE OT)

Franco

[AlNewman]

Ok, so in order to test, let's say ideally I'd like 75ma on the HT side with a 15v drop, and 2a on the low voltage side with a 2v drop, would I set it up like this?   

[sluckey]

Ok, so in order to test, let's say ideally I'd like 75ma on the HT side with a 15v drop, and 2a on the low voltage side with a 2v drop, would I set it up like this?

No. That setup will quickly burn up that little PT.

[tubeswell]

Ok, so in order to test, … would I set it up like this?

Load the heater wiring till you have 6.3v on it

[AlNewman]

Ok, I was able to draw the 8.16v on the heaters to 6.32v with a 1.5R resistor.  It wasn't easy, and is kind of looking Frankensteinish, but I think it will work to test.  I started with 2 4R 25w resistors in parallel, and added an 8R 25w to bring it to around 1.7R, and I had about 6.7v or so?  Then I tried 6 1ohm 1 watt resistors, 3 each side in parallel to get me to 1.5 ohms, and it worked great until solder connections started melting...  which was cool.  So I added a 6v bulb in parallel to the original heap, and ended up with my 1.5 ohms. 

So that gives me a voltage drop of 1.84v over 1.5 ohms, or 1.23 Amps.  Definitely not enough for 2 el84s, but certainly should work for a single ended 6v6, as far as heater voltages + preamp.

Now what is the best method to load the B+ side of the transformer?  Do I simply try adding resistors from each side of the winding to the center tap?  Or would I build the rectifier first as in my original drawing?  Some help would be greatly appreciated.

[kagliostro]

The current disposable in AC isn't the same disposable in DC

Guessing a DC value we can multiply the AC current by 0.6

But as you can add diodes and a capacitor it will be better to go that way

Remember to cook your transformer for half an hour when you reach the 5 or 6% voltage drop on HT with both windings loaded (HT & heaters)

If after that time the transformer isn't too hot measure the current and that is your PT disposable current

Franco

[sluckey]

Think of what you're doing! You have 6.32V across 1.5Ω. Thats 4.2A!!! Not 1.23A

So that gives me a voltage drop of 1.84v over 1.5 ohms, or 1.23 Amps.

NO. That 1.84V drop is internal to the PT and due to the resistance of the filament winding.

Did you really melt solder? If so, I wonder what happened to that thin coat of varnish on the filament winding wire inside the PT.

[PRR]

> ...power transformer from an old radio..... the HT is running around 320v/side   .... The power tranny was into a single ended amp originally...

Then it suits a Champ-like amp. One '42 (6F6, but I dunno a 7-pin in this family) and a few small tubes. Vacuum rectifier.

While both HBP's "Jeep" and the top-line RAV4 are 4WD, the 19-inch(!) wheels on the RAV don't fit the hubs or fenders on the kiddie-Jeep.

[AlNewman]

Maybe I wasn't explaining well enough.  I'm measuring directly at the transformer.  I left it on for a good 1/2 hour, and voltages crept down to 6.2v.  The resistors got hot as hell, (I believe they're actually 50W), but the transformer didn't even get warm.  The light bulb glowed away happily.

I installed diodes from each side of the b+ tap as well as a smoothing capacitor, and I am reading 434v B+ with diodes installed.  I guess next step is to start dropping voltage at B+ to center tap?

[AlNewman]

> ...power transformer from an old radio..... the HT is running around 320v/side   .... The power tranny was into a single ended amp originally...

Then it suits a Champ-like amp. One '42 (6F6, but I dunno a 7-pin in this family) and a few small tubes. Vacuum rectifier.

While both HBP's "Jeep" and the top-line RAV4 are 4WD, the 19-inch(!) wheels on the RAV don't fit the hubs or fenders on the kiddie-Jeep.

Well, I won't be reusing the chassis...  If and when I figure out the HT requirements, I'm also thinking a champ-style build.  This is all just a lot of fun and learning for me.  I haven't done the deep dive into power sections yet.  Thanks for everyone's help.

[sluckey]

The 7-pin 6AQ5 is very similar to a 6V6 but needs a lower B+. Since you have the chassis compare the wiring of the output tube to the pin-out of a 6AQ5.

[AlNewman]

The 7-pin 6AQ5 is very similar to a 6V6 but needs a lower B+. Since you have the chassis compare the wiring of the output tube to the pin-out of a 6AQ5.

Yes, looks like that may be the culprit.

So with a rectifier, I'm assuming a 6x4, the power tube, and 4 unknowns.  I'm assuming a couple 6ba6/6be6, and maybe a 6c4.  It also was running 2 lamps.

So say 1 amp draw for the power tube and rectifier, say another amp for the 4 remaining tubes, and another .5 amp for the bulbs.  That's still only 2.5 amps.

As Sluckey pointed out, I was doing my math wrong, although I tested it with my multimeter and got 3.6 Amps in the heater string to get it to 6.3 V.  Maybe just a difference in wall voltage compared to the 50's, maybe they didn't really care if they weren't bang on 6.3 volts?  Maybe a combination of both?  Anyways, i did cook it for a good 30 min twice, and didn't notice any real overheating of the transformer.  The resistors certainly did cook, enough so that I wouldn't want to leave it unsupervised for any length of time.

As for the high voltage wiring, I used 55k resistance to reduce the voltage from 437V Dc to around 415 when it warmed up.  But that over the resistance, and also tested with my multimeter only gave me .007 mA.  Am I doing this wrong?  Should I be loading it to 5% less than the original voltage of 320V instead?

Edit:  Or should I be measuring the voltage drop across the transformer windings which read as 440 Ohms 1 side, and 404 ohms on the other.  With a 21 voltish drop.  The math is kind of confusing me here...

[sluckey]

As for the high voltage wiring, I used 55k resistance to reduce the voltage from 437V Dc to around 415 when it warmed up.  But that over the resistance, and also tested with my multimeter only gave me .007 mA.  Am I doing this wrong?

No. You're finally doing it right. 415V across 55K calculates to 7.5mA. Very close to your meter measurement. You need to be working with resistors in the 3K to 7K range. Connect a resistor across the power supply and measure the voltage. Divide the measured voltage by the resistor value to get the current flowing through the resistor.

[tubeswell]

So with a rectifier, I'm assuming a 6x4, the power tube, and 4 unknowns.  I'm assuming a couple 6ba6/6be6, and maybe a 6c4.  It also was running 2 lamps...

Even if you knew the original tubes, you might not get 6.3VAC on the heater winding with them in. Some old radio PTs ran the heaters at slightly less than 6.3v e.g.,


https://www.vintageradio.co.nz/static/images/models/philips/594/Philips_NZ_model_594_schematic.png


So figure out the resistance you need to get a 2A draw across 6ish volts (and make sure the resistor can dissipate a lot of heat)

[AlNewman]

Ok, so I tried a 3k resistor, (or rather 3 1k 5w resistors) on the high voltage side, and that brought me down to 295v.  Sooo, from what I've learned, that would equal around 98mA.  I just tested voltage, because I highly doubt those resistors could handle a stress test on the transformer.

So, considering the original output tube, if it was a 6aq5, (should have) ran at less than 300v, or ideally less than 250v, I am thinking 300 voltish should be well within a safe range?  And at 98 mA, I still may be able to consider a push/pull el84 build?

[sluckey]

Sounds like that PT has more juice in it than I first thought. My Dual Lite P/P EL84s only draw 67mA. See if you can rig up a 5K load resistor and check that.

[AlNewman]

That puts it at 329v.

It is a pretty beefy transformer...  It seems like they did funny things back in the day, there's always some huge carbon comp dropping resistors that are usually fried, but in the 100R to 1k value.

Every one of the power transformers are like boat anchors.

[sluckey]

329V and 5K means 66mA. Let that run for an hour and see how hot the PT gets. If it doesn't burn your fingerprints off I'd say it's probably good for a pair of P/P EL84s.

[AlNewman]

I'll give it a try.

The resistors are rated for 5w @ 1000r.  I'm not sure if we are testing the transformer or the resistors at this point.

I'll put em on a plate and see..

[sluckey]

They'll be dissipating 21 watts. I'd hang them in the air. Maybe point a fan at them.

[kagliostro]

To me is important that you load at the same time also the Heater winding

about the resistors .... put them in a glass can (1/2 liter or more) and fill with oil

Franco

[AlNewman]

Nah, it's all good.  We're at 1/2 hr, and it's dropped 12v to 317.
Transformer is just getting (barely) warm to touch.

[kagliostro]

Nah, it's all good.

if you refer to

To me is important that you load at the same time also the Heater winding

I don't agree, you must load at the same time the Heater and HT windings, both loads contribute to the heating of the Transformer

Franco

[AlNewman]

Don't worry, the heater is loaded as well.

[mozz]

If you want to check the filament windings, hook up the actual tubes you plan to use, don't fiddle with resistors for that part. Chances are it was meant for 110v-117v, so load it down till 6.5-6.7vac. If it doesn't get warm you will be fine. Any pictures of donor radio?

[AlNewman]

So I think we're pushing an hour at this point.

Reading 313.8v at high voltage winding

There's a nice smell of a transformer at work, but the transformer is just nice and warm.