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RE: New reissue KT88 versus vintage Genelex/Gec KT88

Posted by Jim McShane (D) on April 29, 2010 at 07:21:54

In Reply to: RE: New reissue KT88 versus vintage Genelex/Gec KT88 posted by Pat S. on April 28, 2010 at 08:51:35:

"I have my quad biased at 50ma and thought that might be pushing it a bit. Won't 80-85ma be pushing this tube too hard,"

It depends. The tube has a maximum current rating WAY over 85 ma., so as long as the dissipation is within reason 80 ma is just fine.

Using a "McShaned" Citation II for example, you'll typically see 455 to 460 volts or so on the plate and just 1 - 1.5 volts on the cathode. So let's say the voltage drop across the tube is 455 volts.

With 80 ma. cathode current we can compute the dissipation wattage using the formula Watts = Current times Voltage. Well, .08 amps (80 ma) times 455 volts equals 36.4 watts. The plate is rated at 42 watts, so we're fine. Also, some of the current you measure at the tube cathode is screen current, not plate current. At idle you can figure on 8 ma. or so as a rule of thumb for screen current. So the plate is dissipating .008 x 455, or 3.6 watts. That's fine too. And if you subtract the screen current from the plate current then the plate dissipation is only 455 volts x (80-8 ma or 72 ma). That computes to just under 33 watts.

"possibly driving it into distortion and lowering it's power?"

Not unless there are other issues with the amp - a tired power supply or ??

" Could that also be putting too much stress on the amp? I love the sound of these tubes at 50ma, but am now curious about adjusting up their bias. But, for me, they are expensive, and I don't want to wear them out prematurely or over-work my amp. Any thoughts?"

There is no hard and fast rule here. As far as tube life goes, there is a finite number of electrons the cathode can emit over its life. So running them harder does wear them a bit sooner. Of course, leaving them in the box reduces wear a lot! So it's a question of what you want - in your car, do you occasionally like/need to accelerate fast? Well, it wears out your car and burns gas faster, so you have to decide if it is worth it.

As far as the amp itself - that's a case by case deal. I think MOST amps work fine with higher bias currents, but it's not a sure thing. Check with the amp maker and ask them about it - you may get a "pat" (no pun intended) answer, but it's worth asking.

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