calculating primary impedance on a Hammond organ OT

[alange5]

I'm reading up on output transformers and trying to better understand utilizing "mystery" unlabeled OT's harvested from old equipment.


The transformer in questions is a Hammond model AO-20936-2 pulled from an old chord organ.  The organ ran a pair of 6V6's through two 8 ohm speakers in parallel (nice Jensen P10R's - one smooth and one ribbed).  I could safely assume it would drop into a 6V6 push-pull circuit, but when determining the winding ratio, the numbers aren't adding up.


I hooked up a variac and AC meter to the primary plate leads, and hooked another AC meter across the secondary leads.  At 62V on the primary, the secondary shows 1V, which would mean a turns ratio of 62:1.  62^2 = 3,844:1 for the impedance ratio.  That would mean a primary impedance of 15,376 ohms with a 4 ohm load on the secondary.


What am I missing?  Shouldn't it be in the neighborhood of 6k-8k?

[shooter]

What am I missing?  Shouldn't it be in the neighborhood of 6k-8k?

not sure where things went sideways, but, 8k is "normal" 6V6 PP.  It was configured in the organ, with the tubes and speakers, I would guess Hammond is correct and build accordingly
learn that "why" can cause unwanted ulcers  and sleep depravation

[alange5]

not sure where things went sideways, but, 8k is "normal" 6V6 PP.  It was configured in the organ, with the tubes and speakers, I would guess Hammond is correct and build accordingly
learn that "why" can cause unwanted ulcers  and sleep depravation

It just seems so far off - nearly twice the "normal" impedance at the designated plate voltage (275V per the Hammond schematic).  A 2 ohm load would seem more appropriate, but I still own the speakers that were paired with this OT and they are indeed 8 ohms each.


I don't know if the organ was operational before it was parted out.  Is there any risk of damage to the tubes/OT with 6V6's at 4 ohms?

[PRR]

WHICH "chord organ"?

I can look at the original intent, but the OT number alone is not finding it.

[sluckey]

I hooked up a variac and AC meter to the primary plate leads, and hooked another AC meter across the secondary leads.

Use the same meter to measure the primary and secondary voltages.

[alange5]

WHICH "chord organ"?

Model S-4. Output section schematic:

[DummyLoad]

are you measuring the secondary across leads/pins 1D & 2D? you are may be measuring one end to the CT of the secondary.


--pete

[alange5]

are you measuring the secondary across leads/pins 1D & 2D? you are may be measuring one end to the CT of the secondary.
--pete

Yes, I'm measuring across both leads.  CT to 1D or 2D (I tried it) gives me roughly double the reading. (124:1-ish).


Tomorrow I will try sluckey's advice (one meter for both measurements) and report back

[92Volts]

I'd agree to use the same meter, but depending on your DMM (basically: if you don't have an expensive Fluke) I'd be suspicious of the meter's AC voltage accuracy/consistency even compared to itself. And in terms of precision you might be dealing with 1 decimal point. At a higher AC voltage range, my cheapo meter only reads increments of entire volts and its lowest range is 1 decimal point.

Larger values (within safe limits) might be easier to read. At the very least, you could check whether 93v corresponds to 1.5v on the secondary. If not you'll know to worry about your meter's accuracy/precision.

[PRR]

Or maybe it is 16K because Hammond didn't want to give you all 14 Watts.

[alange5]

Or maybe it is 16K because Hammond didn't want to give you all 14 Watts.

Must be the case...  I measured both primary and secondary with an accurate analog meter and narrowed it down to a 60:1 ratio. 

[92Volts]

Maybe they expected/planned 2x4ohm speakers in parallel? I'm sure it will work fine, with reduced power.

I'm not sure what you wanted to build with this but 14k would be great for one of those low-watt 12AU7 or 6SN7 push-pull designs.