Metropoulos Forum

Im trying to work out what impedance the primary of an OT should be for a single ended EL34.
I cant see anything on the datasheets ive got listed for singles ended.
I was told its around 7.5k, however, alot of schematics have 3.5k and lower impedances, ive even seen one circuit using a transformer as low as 1.5k, so im totally confused.

Any help would be great thanks.

I figured this out once, I believe it's actually 1.75k. (assuming Marshallesque values which are actually kinda low) As an experiment I tried using a 100watt OT on a single ended 1 tube el34 amp. (just ignoring the primary center tap) It worked, however the OT isn't specifically built to deal with a single ended output section as there will always be a magnetic bias in a SE OT which is cancelled out in a push pull configured OT. This could lead to saturation quicker than one would want. This is why SE OTs have a gap in the laminates to keep the magnetic flux under control. Having said all that, since you're only running one tube where 4 could be, you're probably not going to blow anything up with excess currents, so if you are experimenting (and you promise not to blame me if anything goes wrong), go for it but don't necessarily expect it to sound right.

Well this is the thing, im building this amp on a design of an old friend who has since deceased, i have his working model to test and his amp using the same impedance OT sounds fine, infact the amp sounds awesome!
Its just that ive got so many sources telling me so many different impedances, i dont know whats right anymore! lol

5k is a good starting point for SE EL34 amps.