I have a mule chassis that has a PA setup with a PP output section, cathode biased(currently 250R, common, unbypassed), PowerScaling, 400v Va, and a little less on the screens. OT is 8K Primary JTM45 103 clone from Marstran.
With most power tubes, the scaling does as expected in lowering the headroom for the PA. Usually, as you get lower in voltage, the PA starts squashing > sorta fizzy sounding at really low levels > tubes shut off.
KT88's behave very differently in this set up. With the given combination, they're biased in limiting class A (about 45mA per tube at idle). As I turn the voltage down, the output stays clean(but nice warm sound) down almost to the very point where the tubes turn off. There is a very small band where the sound gets a little compressed then fizzy. It really sounds like a big loud amp (it is really loud)right down to that point.
Just an observation if anyone is interested.
Playing with PP KT88's in a PowerScaled PA
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Stevem
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Re: Playing with PP KT88's in a PowerScaled PA
It sounds fizzy ( distorted) because as the power scaling drops the plate voltage the output tubes needs for input signal level drops off in a massive way, so you need a master volume control after the PI section coupling caps.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Playing with PP KT88's in a PowerScaled PA
Yes, I've built 5 powerscaled amps all with a master volume. Even with the PA drive signal scaled down proportionally with the MV, at the lower end of the voltage range right before they stop conducting, most tubes don't sound as good.
The KT88's sound pretty much the same almost right down to when they shut off...even with the MV all the way up.
The KT88's sound pretty much the same almost right down to when they shut off...even with the MV all the way up.
Steve