50 watt fender OPT reflected Z

[sloblues]

after reading up on how to determine an unknown OPT's reflected Z to the tubes from a link posted by a fellow ampager I am baffled for this reason:
this is a NSC018343 - reportedly schumacher - 50 watt universal fender amp opt with 2,4,and 8 ohm taps. I have no hookup diagram so I do not know the what the secondary taps are. This is what I did:
I applied AC thru my variac to the Primary (white Ac wire to the Center Tap (red/yellow) and Black AC wire to the blue and or brown Primary tap) until I had 1 VAC on the secondary measured from black (ground) to each of the three taps: Results: It took 11.5VAC to get 1VAC on the Green tap (secondary wire), 16.2VAC to get 1VAC on the Green/Yellow tap, and 23VAC to get 1VAC on the White tap.
Having established what I believe is the 'turns ratio' I squared the Primary VAC to get the Impedance Ratio which yielded:132.25, 262,44, and 529 respectively.
This gives me a Primary Reflected IMpedance range of 264Ohms to 4234 Ohms (multiplied each Z ratio by 2, 4, and then 8 ohms)

The amp I want to try this OPT in - exact duplicate Fender AB763 - Super or VLR style with GZ34 and Two 6L6GC's.

I think this topology requires something around 4K - 7K for the Primary on the OPT ? If this is so, how can this NSC opt I have be intended for Fender amps when it appears the highest Pri Z it can have is 4.2K - and I don't even know what the secondary Z is that yields the 4.2K Primary, but the 4.2K comes from the Ratio achieved from the secondary White tap.

Did I do this correctly? Is this OPT not usable in this situation?

[MWJB]

18343 PT is typically 4K:2/4/8ohms. It is the right OT for the task you describe.

The wire colours you quote are not familiar to me, So I'm not sure where you have been applying your AC.

Your black secondary is most likely the common (or the secondaries that read the highest dc resistance between them), apply 1VAC to each of the other secondaries in turn, CONFIRMING ACTUAL VOLTAGE with one meter, whilst measuring the voltage on the primaries with another meter (do not rely on a dial on your voltage source).

40-45v on the primary will be 2ohm tap

30-32v on the primary will be the 4ohm tap

20-23v on the primary will be the 8ohm tap.

...you could just try measuring dc resistance from the common to all the other secondaries (biggest DCR = highest ohm tap) but the resistances here will be very low, don' know if it will be conclusive?

[MWJB]

Ooops! "Fat fingers"...obviously meant 18343 "OT".

[Steve Conner]

An OT's rated impedance is measured plate-to-plate. If you measure from one plate to the center tap, you'll see one-quarter of the rated impedance.

Not one-half, as impedance goes with the square of the number of turns, and a half squared is a quarter.

It all works fine, because when a tube datasheet gives a recommended load impedance for a pair of tubes in push-pull, they quote the plate-to-plate figure, too. Each tube actually works into a quarter of that, in class-B at least.

[MWJB]

Re-reading your original post, it loks as though you have only applied the AC to half the primary (one leg to CT)? That would mean that you need to double the voltages you read.

E.g. 11.5VAC becomes 23VAC = 8ohm tap (green).

16.2VAC becomes 32.4 VAC = 4ohm (grn/yel).

23VAC becomes 46VAC = 2ohm tap (white)...FYI these secondary colours follow with Fender's Deville OT and equate to 4.23K primary Z in all cases.

Double check as I describe above to be sure.

[MWJB]

But you will see half the AC voltage at the primary when determining turns ratio.

[sloblues]

now it makes sense thanx folks!

now that I have applied AC to the entire primary, across the blue and brown wires I can get the turns ratio, square it and multiply times the secondary of 2,4, or 8 and that shows this OPT is a 4.2K primary as was suggesed. Thanks all for clearing this up!