Understanding the Output transformer.

[pleasantbullet]

I have some transformers I pulled from a vintage Tube PA.
The PA had two EL34'S. And was rated at 60 watts.

My thinking was, harvest transformers, use them for a marshall / trainwreck-ish amp.

My question is regarding Plate voltage, how it affects Plate to Plate impedance of EL34's.


The RCA Receiving tube manual says (for EL34s) that the plate to plate resistance is 6500 ohm. Where as on Wikipedia, it says Plate to Plate is 3400 ohm.

Similarly, a Trainwreck uses and Transformer that is 6600 ohm primary.
But a JCM800, or a number of 50watt marshalls can be built with an OT with 3400 ohm primary.

Back to my transformers.

Last night, I put 15vac on the primary. and got 0.71vac on the 8 ohm secondary.
Thats 21.125 : 1 turn ratio , or a 446:1 impedance ratio.
So, 8ohm x 446 = 3570ohm , is the primary impedance of my output transformer.


Whats the next step for me, as to how to utilize this transformer?

could it be used in an express? if not, why?

[rooster]

I think typically, a P/P pair of EL34s wants to see 3600 ohms, that higher number may have been a single tube in Class A, dunno. Anyway, if the physical size of the iron looks right it would work in an Express. Regarding the 6600 ohm Ken choice, it has been said that mating the tube - any tube - to a higher primary than suggested, will influence its harmonic character. This seems logical, but I have no way of testing this thought short of trying different OTs in the circuit.

BTW, the downside to using a 3600 ohm primary in the Express is that the 6V6 is not going to be as happy in the amp. It wants to see 6.6K - 8K in a P/P pair. Eh, which may be - in part - why Ken chose a 6.6K OT.

[pleasantbullet]

thanks rooster, much appreciated. That information helps a lot.


Would i be correct by saying, that the plate voltage applied to the power tubes, does not effect the impedance of a tube, but effects how much power is emitted. Which would in turn govern how many watts the OT is rated for?

[rooster]

I couldn't have said it better.