OT Impedance Question

[Furtz]

I’ve been lurking on this site for a few months, and am really impressed with the knowledge and the willingness to share that knowledge that you people have. It really is a treat.
So, I have this friend (really, it isn’t me) whose next build will be a single-ended 6V6 cathode biased with three 6V6s in parallel. The idea is to run it with one, two or three tubes. For OT impedance matching, my thought was to find a transformer with 4/8/16ohm taps that would reflect the proper impedance for two tubes with an 8ohm speaker on the 8ohm tap. Here’s the question. If running with one tube, should the 8ohm speaker be switched to the 16ohm tap, and if running three tubes, should it be switched to the 4ohm tap?

[tubeswell]

Yep OTs are all about reflected load, so changing the load on the secondary affects the load on the primary.

A single 6V6 runs happily between 5k and 10k with anything from 6k6 to 8k2 being 'optimal' (depending on the plate voltage) for most situations. Doubling the number of tubes in parallel halves the load, so you hit 2k5 - 5k with 3k2 in between. Three in parallel would be 1/3 of what one tube is (so 1k7 - 3k2, with about 2k5 being ideal).

Therefore if your OT was (say) 4k Pr Z with 4R, 8R and 16R secondary taps, then you could run your 8R worth of total speaker impedance off the 16R tap to get a reflected load of 2k for the three 6V6s in parallel.

Alternatively if your OT was 6k6 Pr Z with 4R, 8R and 16R secondaries, then running 8R worth of speaker impedance off the 16R tap would give you 3k3.

Similarly, if your OT was 5k Pr Z with 4R, 8R and 16R taps, then hooking an 8R speaker load to the 16R tap would give you 2k5, which would be ideal.

[Cygnus X1]

Not to hijack but would using alternate setups as described affect the power rating(s)?

I have a similar conundrum-I want a 5K primary on a transformer that is listed as 10K with a 4 and 8 ohm tap.

So if I run an 16 ohm speaker load on the 8 ohm tap then the primary is 5K.
Or 8 ohm load on the 4 ohm tap.
(Right)?

But does that turn the 50 watt rating to 25 watts?

Thanks

[Andy Le Blanc]

I believe the watt rating stays the same. thats a good question.

Aikens amp has a good practical explanation.

Its a ratio, 5k to 4, or... 1250 to 1. 5k to 8, or .... 625 to 1.
You use the load to reflect the plate impeadance.

625:1 with a 16r load will reflect 10k

3 6v6.... 1/ the sum of 1/r1 + 1/r2 + 1/r3..... may be.

RCA mentions a 8.5k load class A, 1/ (1/8.5k x 3) or 1/ .000352941 or 2.8K

I bet 2.5k would work but with self bias I'd go higher, even a 3 k opt
It works well to reflect up, tone wise, a 1.5k SE with the right load to get 3k
would be very nice.

hammond has a 30w that you can get 2.5 k and will handle 200ma
triodestore has one thats rated to 225ma

[martin manning]

I want a 5K primary on a transformer that is listed as 10K with a 4 and 8 ohm tap.

So if I run an 16 ohm speaker load on the 8 ohm tap then the primary is 5K.
Or 8 ohm load on the 4 ohm tap.
(Right)?

But does that turn the 50 watt rating to 25 watts?

If you have a 10k primary with 4 or 8 ohms on the secondary, you need to use a 2 or 4 ohm speaker load to show 5k on the primary. You have two Z ratios available, 10k/4 =2500 and 10k/8=1250.

As for the power rating, I think the primary current might be the limiting factor. If you get the same 50W at the output at half the design impedance, the primary current will be increased by 1.41 times, and the primary voltage will be reduced by the same factor. At 25W output, the primary current would be the same as originally, but the primary voltage would have to be reduced by half. Bottom line is it would be better to use an OT designed for 50W at 5k if you want 50W at 5k.

[Furtz]

Thanks guys. I think we're on the right track.