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Need help on this biasing riddle, question of KT88 wattage?!

DonaldDemon

Member
Messages
9,557
Sometimes I really hate owning a KT88 amp. Tubes are more expensive and info out there is questionable b/c they are not as common.

I've biased my amp (Splawn Pro Mod) several times now and I always second guessed this one thing every single time because I find conflicting information.

WHAT IS THE MAX DISSIPATION OF KT88's???

Bias calculators always seem to use 35 watts as their number but several datasheets say 42 watts for KT88s!

This makes a big difference in trying to achieve 60% dissipation. My amp has a plate voltage of around 485 mV. at 60% for 35 watts that's around 43mA bias point. At 60% for 42 watts that's around 52 mA. Which one is correct? This is for Ruby KT88STR and in the past I have used Penta Labs KT88.
 
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sled

Member
Messages
732
Here's how I bias my amps...

I'm very patient.

I select a conservative setting and play it for a month or so. Seldom do I go back and bias again. It's just not a night and day thing.
 

DonaldDemon

Member
Messages
9,557
Here's how I bias my amps...

I'm very patient.

I select a conservative setting and play it for a month or so. Seldom do I go back and bias again. It's just not a night and day thing.
Right, and that's how I do it as well. That doesn't answer my question though.
 

sled

Member
Messages
732
Yes it does. You have the conservative number.

There's nothing so magical about 60%. Set it, play the amp. Tweak it later but the difference will probably be so marginal it's not worth the effort.
 

DonaldDemon

Member
Messages
9,557
Yes it does. You have the conservative number.

There's nothing so magical about 60%. Set it, play the amp. Tweak it later but the difference will probably be so marginal it's not worth the effort.
Ok thanks for your contribution. There is an optimal range for tone and tube life and that would vary significantly depending on which max dissipation rating is used.
 

DonaldDemon

Member
Messages
9,557
Well here is what I have to add, hope it helps.

KT88 info. http://en.wikipedia.org/wiki/KT88

Weber Bias Calculator. http://www.webervst.com/tubes1/calcbias.htm

Today I received my new bias meter. It is the Compu-Bias. It is a wonderful tool.
FYI... the tube bias info on my meter says the wattage range for KT88's is 21-29 Watts.

I used it on my Splawn Streetrod today.
That is a good example of the contradiction that I am trying to clear up. the wiki page (and several others) have 42 watts stated for max dissipation. The Weber calculator is using 35 watts max diss for their calculations. Why is that?

Sounds like the Compu-Bias is using 35 watts as well. 60% of 35 watts is 21 watts.
 

DonaldDemon

Member
Messages
9,557
Still no clear answer for this? I bought an Amp-Head probe which has made things much easier. I would really like to know which number for max wattage KT88 users are using for their calculations
 

DonaldDemon

Member
Messages
9,557
I 've wondered the same thing, 35 or 42??
You make a hell of a Caucasian, Jackie. :D

Yes, there is no consensus it seems. I tend to go by the manufacturer spec sheet if I can find it. I see words like "rated for xx watts" used in the same sentence for the calculating "max dissipation" which is unlcear to me. Rated for 35 watts means that is safe operating zone or maximum?

I try to get a range of say 55%-70% dissipation and go by ear from there but the calculations vary greatly (about 10 ma) by using 35 or 42 watts.
 

drbob1

Gold Supporting Member
Messages
29,047
One of the problems is the fact that the original KT88s were real and dissipated 42w. Some of the new KT88s are just 6550s in a new bottle and are only worth 35w. The problem is to figure out which is which. I'd email the Tubestore or KCANOS and see what they have to say. It WILL depend on the brand.
 

DonaldDemon

Member
Messages
9,557
One of the problems is the fact that the original KT88s were real and dissipated 42w. Some of the new KT88s are just 6550s in a new bottle and are only worth 35w. The problem is to figure out which is which. I'd email the Tubestore or KCANOS and see what they have to say. It WILL depend on the brand.
Yeah I had a suspicion that was the case. I wouldn't know how to tell other than the KT88s having a bigger bottle than most 6550s
 






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