6V6 output transformer

[Paul P]

I would appreciate some help understanding my output transformer's specifications, just to make sure I've wired things correctly in my amp whose OT output is switchable for 1 or 2 6V6's (SE).
I have trouble getting my head around what happens to the primary impedance when going from one 6V6 to two in parallel.

The OT is an Allen Amplification TO11C (9960737) with the following description :

Heavy-duty upgrade output transformer for single-ended amps using a 6V6, 6L6, EL34 or dual 6V6 power tubes is custom wound in the USA by Heyboer to stringent Allen specifications.

8,000 ohm primary to 8 or 16 ohm secondary for use with a single 6V6.
4,000 ohm to 4 or 8 ohm for use with a single 6L6, EL34 or two 6V6s in parallel.

Rated 95ma continuous! Wound on a paper tube with paper-layered and specially interleaved windings and featuring a special grain-oriented lamination steel for maximum output and clarity. Excellent choice for 5F1, 5F2A and similar amps with more commonly available 8 ohm speakers. Improved performance over stock output transformers.

The OT is no longer available and I can't find a schematic for it. Going solely from the above description, I've drawn up my own schematic, but I'm really not sure if what I've drawn makes sense.

I have a feeling the output taps (on the drawing) may need switching to reflect the way the secondary is wound (?) The amp itself is wired like the drawing wire-colour-wise (so I switch between green and yellow to go from one tube to two).

[Dave H]

Looks OK to me

[g1]

I have trouble getting my head around what happens to the primary impedance when going from one 6V6 to two in parallel.

The 2 tubes paralleled have half the impedance of a single tube. Same as what happens with 2 speakers or resistors in parallel (when they are both the same value).
Nothing happens to the OT primary, except that it needs to be half of what it is for a single tube.

The way this is accomplished is by changing the loading on the OT secondary. Loading with half the impedance at the secondary reflects back half the impedance at the primary. The turns ratio remains the same.
So an OT that has 8K primary, that is only true for specified load. In this case, pretend the green wire is not there. 8K primary with 16 ohm secondary. Now you put 8 ohms on the secondary instead of 16 ohms and the primary impedance becomes 4K instead of 8K.

[mozz]

It will work both ways, just put 2 output jacks on it and use whichever sound better. It's not going to blow up your amp or increase or decrease your wattage any appreciable amount.

[Paul P]

Thanks for the replies. So to get the benefit of twice the power at the speaker we halve the impedance on that side of the OT and supply the power tube side with twice the power (same voltage, twice the current).

I believe my confusion comes from thinking of the secondary as a resistor and that more windings in the output of the OT would reduce the power at the speaker. It's really the other way around, more windings, more power.

[Helmholtz]

An OT acts as an impedance multiplier with fixed conversion ratios between secondaries and primary.
In this case any load impedance connected to the "8 Ohm" secondary appears multiplied by a factor of 1000 at the primary and any impedance connected to the "16 Ohm" secondary gets multiplied by a factor of 500.