OT - Determining SE Amp Wattage

[Paul Marossy]

How do you determine the wattage rating of a single ended tube amp circuit? I'm just about finished with my Octal Fatness, and I'm trying to determine if my output transformer is adequate for the power tube I want to use...

Anyone?

[R.G.]

Question simple.

Answer verrry hard.

The simple answer is:
For a single ended Class A circuit with a perfect pentode, the power out is
P = 1/2 (Vb*Ib)
where Vb is the plate supply and Ib is the static bias current through the tube. This applies where the Ib is chosen so that the transformer does not saturate to either side of the signal swing.

Good transformers have insertion losses of 1db or so in well designed systems, so there's not much power loss there.

However
The power handling of a transformer is determined to a first order by the lowest working frequency, then by the exact working conditions. You can't say P=Vb*Ib/2 in all cases, because that assumes the transformer is well designed to the frequencies involved and does not saturate or distort.

So the size of transformer you use has to be specified not only for the power to be transferred, it must also be specified for the value of the static DC bias on the output tube(s). Otherwise, it distorts and you don't get nearly the expected power out at the lowest frequency of interest.

That usually means that you have to gap the core, then add more core metal and primary windings to get the primary inductance back up. A SE output transformer tends to be four to eight times the mass of a P-P output transformer for the same output power.

So I'd say that the best way is to look up your output tube in a tube manual, get the recommended Vb and Ib bias point specs for it in a single ended hookup, then consult your transformer specs for whether it can withstand at least that much DC and as low a frequency as you want to put through.

There is a whole blather of other stuff, but that's the most important bits.

[Paul Marossy]

Thanks RG.

Aside from factoring in the performance of the output transformer itself, I think I actually did do a (very) rough calc. correctly, based upon what you have written. That confirms my suspiscion that my current output transformer might be a little underrated for a 6L6 or a EL34 power tube...