OT impedence question

[alien_audio]

OK, so if you're using standard tubes for an amp, like EL84's, the datasheet tells you what the impedence of the OT should be. All nice and easy.

Thing is, I'm looking to use something a little more esoteric (I've come across some Russian submini pentodes), and have no idea what parameters relate to output impedence.

Can anybody help?

Thanks

Andy

[Rockn]

Most of the sub-mini amps I have seen have used Fender Reverb transformers. I would look up the datasheets because they are around and it is better to use the correct specs for impedance matching.

[loudthud]

Maximum Plate dissipation, Maximum Cathode current and Maximum Plate voltage define a window for the load impedance. For the most part, the numbers you get from a data sheet are long term ratings and you can safely consider twice the rating as an instantaneous limit in push pull service. B+ is probably near the Maximum Plate Voltage but remember plate voltage swings above B+ half the time.

You have to juggle the numbers in a What If game to see what's reasonable. Start with 2X the plate dissipation rating for one tube, pick a B+ and calculate the peak plate current. Is that a reasonable current? What load impedance does that imply? You could go the other way, start with the power and peak plate current and see what B+ would need to be. 2X the plate dissipation rating is an optimistic number on the high side. You'll probably want to go down from there.

As things take shape, the screen ratings come into play. How much screen voltage is required to get the plate near the required peak current? Do the screens draw excessive current when the plate voltage is clipping? Beam power tubes tolerate high screen voltages but many pentodes want a screen voltage about half the B+ and will burn up if the screens are run at the same B+ as the plates. Maybe you should consider a lower B+.

[Rockn]

Here are some subminiature datasheets

Adam's Amplifiers: Subminiature