( ONE MORE!!!!! ) OT question

[thorens]

@gary
The primary of the OT is fix. It's just depending on the type of tube (or how much tubes you use in the power stage). It's in the data sheet called load resistance necessary between Anode and Anode. The impedance is a more complex thing. It's the resistance depending on the frequence (including turn ratio, thickness and material of the tranny steel, speaker impedance ...)
Tom

[gary sanders]

OK say I want to use a quad on an OT originally for two tubes?Does that do the opposite to the speaker load like half the impedance?(referring to earlier when I used an OT made for a quad and used a pair of 6L6 and it doubled the speaker load)

[Firestorm]

Strictly from an impedance perspective, yes, exactly that: one half the original load. BUT: the OT primary was not likely wound for that much current (wire gauge too small) so it will run hot; and you will likely lose half the rated bandwidth, which might not matter for guitar, but could for hi-fi, bass, keyboards or pedal steel.

[cxx]

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.

+1 on this.

I think this is the way to think about it. The transformer is ideally transparent, making one side's impedance look like a fixed multiple on the other side. So you hook up an 8 ohm speaker on the above transformer and it looks to the tubes as 4K ohms. The ability to handle the power is a independent of the impedance multiplier of the transformer.

[gary sanders]

Thanks guys.That helps a lot.Thanks to all for all of my questions answered.I have only 100 more to ask in due time.