( ONE MORE!!!!! ) OT question

[gary sanders]

Is it ok to use an OT out of a 4x 6L6 in a 2x 6L6 amp?

[selloutrr]

yes

[Phil_S]

Adjust for the impedance. If it was 2K primary in the 4x amp, it is 4K primary in the 2x amp. That means the secondary is also doubled. If the secondary was 16 ohm, you are looking at a 32 ohm load requirement.

[gary sanders]

Adjust for the impedance. If it was 2K primary in the 4x amp, it is 4K primary in the 2x amp. That means the secondary is also doubled. If the secondary was 16 ohm, you are looking at a 32 ohm load requirement.

I almost guessed that! Thanks!

[gary sanders]

So how does half the output double the impedance?

[David Root]

Has nothing to do with power output per se, that's merely the result of going from four tubes to two, or two tubes to four.

Two tubes instead of four. Two tubes in push pull has an optimal output impedance, usually, but not always, for maximum power output, and it is related to the output tubes' plate resistance. Four tubes is push-pull with two tubes in parallel on each side. Effective plate resistance of two tubes in parallel, as seen by the OT primary, is halved, this translates to halving the primary impedance. Or the opposite going the other way, doubling the effective impedance from four tubes to two.

[Phil_S]

David, I agree with what you said 100%. Sometimes a concrete example helps those who are learning. He asked a good question, how is it that half is double? I'll try not to make this worse.

Resistance or impedance (there's a difference, but it's all Ohms) is halved [note 1] when resistors (or tube plates) are wired in parallel. So, a single pair of 6L6 might be paired with a 4K primary. The 8 Ohm secondary on that OT is simply a function of the turns ratio of primary to secondary. It is actually a function of the square of the turns ratio. So 4000:8 = 500:1. Sqrt (500) = about 22:1 turns ratio. Since the turns ratio in the transformer is fixed, the relationship between the primary and secondary ratings is also fixed.

If you go from 2 tubes to 4 tubes, you are generally working with 2 pairs wired parallel into each end of the primary. That means your primary load is now 2K (double the tubes, half the load, see the note). It's a strictly linear relationship for the transformer. That 8 Ohm secondary at 4K primary is now "worth" only 4 Ohms with 2K primary load. Do the math. 2000/4=500 (square of the turns ratio), same as above and the sqrt is still ~22.

Tubes will function over a range of primary impedances. This is a whole other topic.

Note 1: Half works nicely for two in parallel, but it is actually more complex when several items are involved. It is based on the sum of the reciprocals. I suggest you look it up if you aren't familiar.

[gary sanders]

Tubes will function over a range of primary impedances. This is a whole other topic.

Go ahead.I'll be here all week.I never get tired of useful information!

[Phil_S]

Tubes will function over a range of primary impedances. This is a whole other topic.

Go ahead.I'll be here all week.I never get tired of useful information!

Sorry, I didn't mean to bait the topic. I'm not technically proficient enough to do justice to the topic. Maybe you'll find an article on the Valve Wizard's website for drawing load lines. Load lines is the relevant topic. Much has been written by people who know this stuff inside and out.

[gary sanders]

cool.will look it up.

[Phil_S]

You can probably find some Excel based load line calculators, too. Maybe search this forum. I don't remember where I found them.

[thorens]

Phil is right. The word is load resistance.
In the data sheet of the 6L6 exists a value: load resistance (AB1, fixed bias, tetrode ...) that the tubes expecting when they (sorry) look at the ot. Remember all power tubes are parallel. If the ot was build for 4 6L6 parallel for a 4Ohms speaker, 2 6L6 need the double load resistance. You can't change the ratio inside the ot so you have to double the speaker impedance (8 Ohms).
Tom

[gary sanders]

OK just because I want to know,what if you did the opposite and put a smaller OT on a bigger load?Say you had an 8k primary and put a 2k primary OT on it?Like the opposite of my original question.Would this make the impedance less?or just blow up....theres got to be a pow/sizzle somewhere in this topic.( Knowing what NOT to do is good also!)

[Phil_S]

The question doesn't fit nicely into a static model. There are all sorts of parameters: primary inductance, current capacity, etc. Many of the tubes for which a 2K primary might be suitable might be way to powerful for your typical 8K primary.

To answer your question directly though, your 8K:8z transformer will also function as a 2K:2z transformer. It's just basic math that depends on the turns ratio.

I'm thinking, your typical 8K primary is for a pair EL84 or 6V6; that's about 15W-20W. A 2K primary might be for a quad of EL34 and that's probably 80-100W. So, you see, you can't just make a random switch based on the turns ratio.

[David Root]

Generally, lower primary into higher secondary is OK, if we're talking about the same power tubes, like Phil said. Higher primary into lower secondary, not so good, also with the same power tubes.

One caveat--one level up or down only, ie 4 ohm primary into 8 ohm sec is OK with any but el cheepo OTs. 16 into 8 likewise but less so. 4 into 16, or (especially) 16 into 4, no way. And not anything but 2 ohms into a 2 ohm load.

Lastly, if you crank a pair of 6L6GC or EL34 into an OT designed for a pair of 6V6 or EL84 you're likely looking for trouble, even if you match the load impedances. Yes I know Neil Young does it but his techs know what they're doing.