KT88 push pul using mismatched OT

[Tage]

SO I've been thinking of a power amp design were I would like to use 2 KT88/6550 to make around 70 watts, nice and clean(ish). But looking through data sheets they recommend using a load of ~4k - 4.4k Ohms. Can I use a 100 watt power transformer and hook up my secondaries differently to change my primary from 2k to 4k?

I'm sure this is possible just having trouble figuring wich way to go...

[defaced]

Yes you can do this. Since you want to double the primary impedance, you have to double the secondary impedance. So in your application, the 4 ohm tap becomes an 8 ohm tap, and the 8 ohm tap becomes a 16 ohm tap. Now if you're using a cab with a 4 ohm rating, you're screwed unless your OT has a 2 ohm tap or you rewire the cab or buy a different OT.

[Tage]

Ok that is what I figured. I have 3 16 ohm cabs, and a switchable one 4-16. I just read up some more on it and came to the same conclusion, I just had to think it through, because if 4 is reflected as 2k, then 8 would be reflected as 4k.

Thans for the help Mike?

[jamesmafyew]

I've actually done this very thing with some 2xEL34 50W guitar amps (typically 3.4k to 4k primary). Changed out the OT for a 100W (1.7k primary usually) unit, then hooked up the secondary taps to double the primary Z. Usually gets about 75W clean out of a pair of KT88s. Not a huge volume increase, but lots more bandwidth & clarity, and faster transient response.

[Steve Conner]

When you mismatch like this, the bass rolloff frequency goes up, because the OT magnetizing inductance is half what it would be if you used a properly matched OT.

It probably won't be an issue. It might even give the illusion of more bass.

[Tage]

yeah I figured there may be some frequency response change. I just thought that this would be an easy way to get a transformer that can handle the power, without getting something specifically for the task. ALso I have a transformer on hand that I figured would be good to use.