Question About Output Transformers

[RightLurker]

A Fender 125A13A output transformer, such as is found in a 6G6A Bassman, has a primary impedance of 4200 ohms (correct for two 5881/6L6s), and a secondary impedance of 4 ohms. I figure the turns ratio to be about 32.4 to 1.

A Fender 45548 output transformer, such as is found in a 6G8 Twin, has a primary impedance of 2000 ohms (correct for four 5881/6L6s) and an output impedance of 4 ohms, and I figure the turns ratio to be about 22.36 to 1.

Question 1: If you place a 45548 OPT between two 5881/6L6s and a 4 ohm load, what happens to the tubes, the speakers, and the sound of the amp?

Question 2: If you place a 45548 OPT between two 5881/6L6s and an 8 ohm load, is there a proper impedance match between the power tubes and the speakers? In other words, would doing this be the equivalent of putting a 45548 OPT between four 5881/6L6s and a 4 ohm load?

This is a subject I've been struggling with for some time - I have a tube- rectified, blonde Fender Bassman with a factory-original 45548 OPT and a 2 12, 4 ohm speaker cabinet. I know a bunch of "transitional" 6G6/6G6A Bassmans left the factory with 45548 OPT like mine. I'm debating replacing the 45548 OPT with a 125A13A replacement.

Thanks in advance, and sorry for the headache!

RightLurker

[Phil_S]

You can't change the turns ratio (obviously) but you can fiddle with the secondary impedance.

You cite one transformer as 4200:4 and another as 2000:4.

When you pull two power tubes from the 2000 ohm primary (one from each side), then it becomes 4000:8. That should work fine.

On the 4200:4, you really can't "double" it by driving it with 4 tubes because it probably isn't beefy enough for twice the output. You can, however, use it with a pair of 6V6 or EL84, functioning as 8400:8 and it will work just fine. You can also use it with a quad of 6V6 or EL84 at 4200:4 and that will work, too, because the output will be in the 30-40W range, roughly equivalent to the pair of 5881/6L6 in power.

OK?

[quayhog]

I have a blonde Bandmaster head that a previous owner installed a Twin Reverb output transformer. The two 6L6 amp now wants to see an 8 ohm load.

I plan to install it in a combo cabinet with either a single 8 ohm 15" or two 10" 16 ohms speakers wired in parallel.

[RightLurker]

Thanks for the replies, but I guess I did a poor job of asking my question. Let me try again:

My amp has an OPT designed to couple 4 6L6 tubes to a 4 ohm speaker load. Would it be electrically and sonically equivalent to use the same OPT to couple 2 6L6s to an 8 ohm speaker load?

Thanks again.

[ampgeek]

Electrically, yes. Sonically, maybe better because you will be running at ~1/2 of it's design current and it is higly unlikely that you will saturate the core. That often leads to a significant loss in fidelity.
Cheers,
Dave O.

[Andy Le Blanc]

With correct loading there is a small range where you get the most Pout
with the least distortion, for any given tube type.

4k load for two 6l6 is the commonly accepted fender loading for a fixed bias and the amount of feed back,
doesn't matter how you get it. For cathode bias the loading is a bit greater, around 5K,
I've seen some examples of either 2 6l6 or 4 6v6 with a 5.6k load that sounded very nice.

I like the the tone when the impedance ratio is used to step up the load to where the tube likes it.
Its has been stated in the forum that that is one of those tone tricks.

You can use it to your advantage, IE, you can use twin iron and 6550, KT88/90 to push current,
keeping the fender voltages, to explore different tube types.

Mix and match opt and pt to get the loading for the tube type you want.
but keep in mind the opt current limits, the heavier the tranny the more current you can ask of it.