OTL for 40-80ohm speakers
 

I have a fresh plan making a little different otl, for driving speakers having multiple 8ohm series connected speakers.
At first, it sounds like quite easy load for otl.

Ok, first of all I want 50watts/channel for 80 ohm impedance.
There should be option for half impedance, like 40 ohms.
4 x EL509 / channel should do this job easily?

powersupply/power transformer will be 2 x 110VAC. 400VA/ch, should be enough..

what do you think about this idea? :)

50W/ch into four (presumably series connected full range) drivers = a sh*tload of db, even from moderately efficient drivers... Are you SURE you need that much drive?

50w is quite a lot for an OTL !
Just do some math:
A EL509 as cathode follower has an R-out of 1/(0.018ma/V) = 55Ohms. As circlotron it's a bit lower and as futterman its half the cathode follower R-out. So even without further global feedback this isn't bad.

Power:
P=(I^2)*R, for P=50W and R=80Ohms, this yields: ~0.8A-rms and ~1.1A-peak and for 40Ohms: ~1.1A-rms and ~1.6A-peak.

EL509 max. Ik is 500mA. So you can calc. for yourself how much tubes you need for that much power you want.

I have never built an OTL amp, so I can't tell you about some good designs. But I have some names in my head. Just google for: "Rosenblit OTL", "Sandman OTL", "Pickie Makovski OTL", "Murray OTL"

there will be 12 x 4" full range drivers (smaller model has only 6 drivers).

But yes, 50W is good to have in reserve.

So as I did calc that amount of tubes required, I got 4 tubes / ch. 2 for negative and 2 for positive side.

I just looked into Bruce Rosenblit's book and on page 74 he says: "The maximum current that a 509 can pass is just over one amp."
This disagrees with the datasheet value of 500mA and seems to be the solution why bruce gets 80W into 8Ohms with "only" 8 tubes.

According to this, 4 tubes per channel should be enough. But as I said, have neither used EL509 for audio nor built an OTL

I say go for it!
Here is some more info about OTL and calculations with EL509:
Ultra-Linear OTL Power Amplifiers

Thank you for info, as this is my very first OTL. I have made tens of traditional tube amps tho..

You should consider the schematic on post 25 on this thread:
http://www.diyaudio.com/forums/tubes...ube-amp-3.html

I think the circuit is very elegant in its approach and the output tubes does not cost that much.

If you put a lot of drivers in series then they will get litttle or no electrical damping, unless they happen to have identical bass resonances. You will need to provide mechanical damping instead.

Well, there has been OTL amps driving 800ohm speakers.. and those had very clean sound!

DF96, what do you mean about mechanical damping?