Tube Specs

[C Moore]

I have a real hard time trying to figure out the reflected primary impedance requirements for output transformers. The tube data sheets are very confusing to me. It seems like the numbers are all over the place, but that can't be....
For example, looking at the EL34, I could not find anything that made sense when looking at the data sheets from Mullard, Phillips, Svetlana, etc. So looking at the Duncan sight below, what output transformer would you buy for a pair of EL34's with a plate voltage of 450 Volts, operating AB1 PP? Is the 5k ohms for just one tube???
Thank You

http://tdsl.duncanamps.com/show.php?des=EL34

[David Root]

Depends on the screen voltage at that Vp. You need to learn how to use a pentode/beam tetrode plate curve to calculate primary impedance. In the example you cite, that is for a pair of EL34s in push pull. Typically a pair of EL 34s is used Marshall style with the screen voltage slightly less than the plate voltage (choke connection) which results in the standard primary impedance of 3K4 for a pair or 1K7 to 1K9 for a quad, at 450 or so Vp and Vs.

The classic method is to pick your plate and screen voltage, find the plate curve for that screen voltage (G2) and measure Vp minus the plate voltage at the mid point of the knee of the curve, subtract that from Vp, and divide that number by the plate current at the mid point of the knee. That is the plate resistance of a single tube. Multiply that number by four for a push-pull pair of tubes or two for four tubes (two p-p in parallel). That gives you the reflected impedance or Zaa of the OT primary in that application.

Theoretically that primary impedance gives you maximum output power at those plate and screen voltages.

In practice it's a bit looser than that because impedance varies with frequency, and the max power point may not give you the tone you are looking for because that's subjective. Also it's not generally a good idea to pick a point much below the knee ie to the left of the knee because you can run into screen current issues. This varies with the tube type.

[tubeswell]

From OTM

"'general' rule-of-thumb with power beam tetrodes and pentodes is that Z.pri should be between 1/6th and 1/8th of the TUBE's dynamic plate resistance (rp) value, with the LOWER Z.pri value yielding lower %THD and the HIGHER Z.pri value yielding higher output POWER...and, anything between those two values is acceptable, depending upon the "goal." "

[C Moore]

From OTM

"'general' rule-of-thumb with power beam tetrodes and pentodes is that Z.pri should be between 1/6th and 1/8th of the TUBE's dynamic plate resistance (rp) value, with the LOWER Z.pri value yielding lower %THD and the HIGHER Z.pri value yielding higher output POWER...and, anything between those two values is acceptable, depending upon the "goal." "

Thank You -
I also appreciate the effort it took for Mr. Root to give me the full mathematical answer.
In my link to to Duncan's sight, how can I tell the data is for TWO tubes? Does it say plainly two, and I am just not seeing it?
Thanks Again

[David Root]

When you look at those tube data sheets, each set of typical operating conditions is identified as a class of service. A1 is for a single tube. AB1 is always for a pair of tubes. Usually AB1 is also specifically marked as "values for two tubes". I use www.tubedata.org because it gives you the manufacturers' actual pubished datsheets, which almost always include plate curves.

Those conditions as described were intended for hi fi music reproduction applications, not musical instrument amps, and are not all that useful to us.

Much more useful are the plate curves, where you can pick your plate voltage, your screen voltage up to a point, and figure out where you want to be yourself.

The rule of thumb is useful too, I had forgotten about that.

[C Moore]

10-4
and once again.....Thank You