Load / primary impedance

[webhead]

Hi there,
I'm planning on scavenging my JCM800 OT and build something else with it.
It's a hammond replacement OT and it's rated for a primary impedance of 1700 (that is 4xEL34 at 100W).

I'm planning on something low wattage and maybe using different output tubes so I've been looking at some data sheets. The main concern is getting lower power here so I was planning on using only two EL34's and doubling the secondary output impedance (that is, hooking the 8ohm speaker to the 4ohm output because the primary doubles as well).

So when I was browsing different load impedances for AB1 push pull pairs I've noticed that for some type of operations the output wattage of an EL34 pair is significantly lower then 50W. I've also noticed that the load impedance was about double (+6000 instead of the 3400).
And for triode operation this was even more off.

So what is up with impedance matching ? If I would switch some EL34's to triode like in a JCM900 wouldn't that throw load impedance way off? Even if I pulled two tubes and doubled the secondary connection as well, switching those two to triode would even further mess up the impedance matching.

How flexible is this exactly ? A quad of EL84's would have a load impedance of 4000, if I double my secondary impedance on my OT the primary would be 3400. I'm aware that the OT has a ratio ofcourse instead of a fixed prim impedance but still, I'm confused.

Thanks for the info.

[Robert M. Martinelli]

Hi,
First of all, take a look to this thread :

http://music-electronics-forum.com/s...ead.php?t=9119

As to the various impedance the same output tubes have in different operating conditions, you should check some old datasheets, they were very accurate so chances are you' ll get what you need ( e.g. personally I own an original Philips 1967 datasheet and, if I look in the EL84 pages I find all the curves under various conditions, pentode/triode, class A an AB, and all the above at different plate voltages too ). Once you know the tubes' output impedance at the conditions you will have in your circuit, you can refer to the formulas you' ll find in the above thread to obtain a perfect match.

Hope this helps

Best regards

Bob