The Right OT for the right valve

[Leif_Bodnarchuk]

Does anyone do an at-a-glance chart for output transformer primaries?
Impedances, that is.


single ended (SE) EL84..... p-p EL84
SE EL34.... p-p EL34
SE 6V6.... p-p 6V6
6L6
KT66
KT77
KT88 and so on and so on

Or am i missing something from data sheets?

[Roe]

you may want to take voltages into account as well

[Leif_Bodnarchuk]

But voltages are more valve/tube specific, aren't they?
There are current limits within primaries too.

i'm a little dumb when it comes to impedance, i don't quite 'get' it yet. As far as i know, it's basically AC resistance, but after that...?

[snoof]

An OT has no "impedance", it has a ratio. It matches the impedance of the tubes to the impedance of the speaker. So if your OT has a turns ratio of 20:1 the impedance ratio is 400:1 (20*20=400). So you could use that OT as 3.2k pri and 8ohm sec (400*8=3200). Or 1.6k pri and 4ohm sec, or 6.4k pri and 16ohm sec. Assuming that the OT can handle the current and wattage needed by your design.

http://www.radioremembered.org/outimp.htm

As for the data sheets, the plate-to-plate impedance info is there for select voltages. If you use other voltages, extrapolation is needed,(ie. gotta do the math) or call it close enough for gov't work and go with what ya got.

http://www.mif.pg.gda.pl/homepages/fran ... 6L6WGB.pdf

for instance, with this data sheet, read the chart for class AB, plate voltage, grid #2 voltage and load resistance and that will tell you what you need to know.

[Roe]

the 3k4 (or 1k7 for 100w) primary marshall used only works for lower voltage, around 425v if I remember correctly.
the 8k primary on the kt66 amp is way high