KT88 - 35 or 42 watts?

T

tonesfoyobones

Well-known member
Hey guys wanted to replace the power tubes in my Ultra Lead. Got some new EH KT88's and wasn't sure as to bias them as 32 or 42 watts. I see different views online. I do have an email out to Fryette to get their recommended bias mA.

Thanks in advance!
 
I would bias them to the power output of the amp as a starting point. So that the OT can handle the power. If it is the other way around (i.e. OT can handle way more than tubes can produce) then the tubes (if biased/setup right) just distort, they don't put out more power or melt. But I think if the PT can put out the voltage and the tubes can amplify it (i.e. KT88s) then you could be stressing the OT if you really crank it. Although I would expect an UltraLead to be overbuilt.

When I built my Mini-Major I bought the PT/OT from Merren and he said I should be able to get 100W out of 2x KT88 (about 505vdc B+).
 
For new production 6550 or KT88 use the lower 35 watt dissaption rate just to ensure your not stressing the tubes. If you have NOS GE6550 or Tung Sol's 42 watts is no problem.

Also 6550 KT88's don't like a hot bias. They lose their definition and clarity. 50 to 60% at most. I usually find 50-55% sounds best.

Try it yourself...bias it hot play it then bias it cooler and you will feel and hear a difference.
 
harddriver said:
For new production 6550 or KT88 use the lower 35 watt dissaption rate just to ensure your not stressing the tubes. If you have NOS GE6550 or Tung Sol's 42 watts is no problem.

Also 6550 KT88's don't like a hot bias. They lose their definition and clarity. 50 to 60% at most. I usually find 50-55% sounds best.

Try it yourself...bias it hot play it then bias it cooler and you will feel and hear a difference.
Click to expand...

Appreciate it!
 
harddriver said:
For new production 6550 or KT88 use the lower 35 watt dissaption rate just to ensure your not stressing the tubes. If you have NOS GE6550 or Tung Sol's 42 watts is no problem.

Also 6550 KT88's don't like a hot bias. They lose their definition and clarity. 50 to 60% at most. I usually find 50-55% sounds best.

Try it yourself...bias it hot play it then bias it cooler and you will feel and hear a difference.
Click to expand...
Good advice.
I'd go with the 35 watt plate dissipation number in the bias calculation as well.
My experience has been that most amps designed around a 6550 / KT88 power section are getting all the drive & distortion from the preamp.
I'm not sure anyone designs a KT88 / 6550 amp with the idea of power section clipping.
Usually the goal is for the power section to cleanly reproduce whatever the preamp is feeding it.
The KT88's or 6550's are used to provide maximum thump and headroom.

I would suggest using the 35 watt plate dissipation number, mark the 70% dissipation point on the bias pot ( so you know what not to exceed ) and then bias by ear.
I bet you end up around 50% - 60% as recommended above.
 
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