Re-using Output transformer from Hammond Model M organ.

[pullshocks]

 I want to do a build with the OT that came from a Model M organ, with the AO number of AO-21007-0. The PT was fried, but the OT is not shorted or open.
The schematic shows a push/pull 6V6 amp with a plate voltage of 300 V, and an 8 ohm speaker.   Does anyone know the primary impedence of this transformer?  Would it be OK to use it in a 6v6 power amp with a plate voltage of 400 V?
This may have been addressed in past forum threads, but I tried searching the forum but could not locate this information.
Thank you

[sluckey]

I think it will be fine. Read this short thread to see how to check the impedance ratio of that OT...

http://el34world.com/Forum/index.php?topic=23499.0

[pullshocks]

Thanks Sluckey.


Unfortunately, I do not have a bench power supply and no more room in the shop for any more equipment. 


I seem to recall reading elsewhere that the test can be done with a fixed voltage, with more math.  From watching the video of testing the Princeton transformer I notice that he needed 30 volts primary to get 1 volt on the secondary.  So I am guessing this would be in the ball park of a useful test voltage.


Putting on my "knows just enough to be dangerous" hat, I have a multi tap PT that can give 40 V, and maybe a couple other PTs with outputs in the tens of volts. And of course there are  the 5 and 6.3 volt taps. 


If it would be safe to take the test voltage straight off the transformer,  I can probably handle the extra math.  And I can verify by also testing a transformer of known impedence.

[92Volts]

Voltage straight from the transformer is fine. And 5v is an OK voltage to test with. The easiest way to get a measurement is to apply that voltage to the secondary (speaker) side. If you apply it to the primary (tube) side the output would be quite low and your voltmeter may not offer enough precision.

This will produce a high, potentially dangerous voltage. Use the same caution you would whenever you work with high voltage.

Let's sanity-check this for safety and to avoid damaging the transformer.

It's likely 10k impedance at most and the speaker is 4 ohms at least (unless you know otherwise)

Impedance ratio "should" be 2500.

Voltage ratio is the square root of 2500, that's 50.

If we apply 5v to the secondary (speaker) side we expect around 250v to appear on the primary (tube) side.

That's high, it's as much or slightly more voltage than it saw in actual operation! But it's not crazy and shouldn't fry the OPT.

You can see why people prefer to apply a medium voltage to the primary side, you'd get a lower and non-scary voltage on the secondary. But this is do-able with a 5v source.

Remember to apply and measure AC!

[sluckey]

Unfortunately, I do not have a bench power supply and no more room in the shop for any more equipment. 

There was a lot more info in that thread besides that video. You already have everything you need. Filament transformer and a voltmeter. A variac makes it easy to adjust the voltage to 1 volt but it ain't necessary.

[2deaf]

I seem to recall reading elsewhere that the test can be done with a fixed voltage, with more math.  From watching the video of testing the Princeton transformer I notice that he needed 30 volts primary to get 1 volt on the secondary.  So I am guessing this would be in the ball park of a useful test voltage.

Putting on my "knows just enough to be dangerous" hat, I have a multi tap PT that can give 40 V . . .

That video started off with some helpful hints, but then descended into an attempt to sell you an overpriced piece of equipment that you don't need.

That 40V sounds good to me and a lot safer.  With 40V connected across the full primary, I would expect something like 1.5V across the 8 Ohm secondary.  I like that better than 5V on the secondary and 150V on the primary.  If you post your voltage readings, folks will immediately come back with your primary impedance. 

[PRR]

A classic 2-6V6 OT will be so close to 8KCT that you can just assume 8KCT.

EL84 will always work with the same OT, just change bias. (And ponder the 330V rating.) Or 6L6/EL34, but working <400V and giving 6V6-like power.

[Ed_Chambley]

From Hammond Tone Cabinets, the small JR20 uses 2, 6V6 and it is 10K/8ohm.


Here is a good link for the schematics.  Don't know if it will help, but I really like to use Hammond Organ parts to use.\


http://archive.org/details/HammondOr...delsMM2M3M-100