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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.
Follow Ups:
Jim
Your calculations on plate dissipation are straight forward.
My PP Cary is modified to triode with a 100-ohm connected across the pins 3 & 4 and the transformer connection to pin 4 removed. So grid 2 is "tied" to the plate via the added R. My concern is whether or not g2 will be able to withstand the high bias discussed herein ?
If not, what would be the "lowered" limit for plate dissipation?
Calculate the current passing through it (G2). It's the same calculation as I just mentioned above.
You can measure the voltage across the 100 Ohm resistor and use Ohm's Law to calculate the current. For instance, you might find .6 volts drop across the 100 Ohm resistor. If you do, then you know the current flow is .6/100, or 6 ma. If the screen is at 450 volts, then you know the screen dissipation is 450 x .006, or 2.7 watts. That's within spec. So is 450 volts - but often tubes have different limits if triode connected so check to be sure. IIRC it's 600 volts on KT-88s that are triode connected.
There is no change to the plate dissipation - calculate it the same way as above.
Okay,
So last night I upped the bias from 50ma to 60ma in my amp. It could be my imagination, but I think it sounds better than before. My amp is a Cayin TA-30 35WPC ultralinear integrated amp. The curcuit is identical to Cayin's current production AT-50. I did a search to determine the plate voltage for this amp and came up with nothing. The stock amp is supplied with a quad of EL34's, but you can also use 6L6 and 6550/KT-88 types, as long as you adjust the bias accordingly. This amp uses very robust power and output transformers and it and the Gold Lions are showing no signs of strain at 60ma. Still, I wonder if you or anyone else here might know what the plate voltage rating is for these amps and what the optimal bias setting would be.
Emailed Paul from Tube Audio Design, who I originally purchased the amp from. He recommended that I don't exceed 50ma in this particular amp as the higher idle current might stress the output transformers.
10 ma per tube could overstress the OPTs?? Wow.
It's their amp and I would hope they know it better than anyone. But boy, that strikes me as saying that the OPTs are not particularly - well, let's just say "robust".
That's my opinion, not a verified fact.
He decribed them as under-spec'd. The amp is supplied with EL-34's running at 35ma. It is capable of running KT88's, but apparently within a lower bias range, with 50ma the recommended max.
Okay, be sure to pay attention to that then!
Will do. This amp has a lot of power, headroom and a wide frequency response for its power rating. I'm thinking it could handle 60ma. But then again, it is ideally designed for EL34's, with the ability to run KT-88's, but within certain parameters. At least that is my understanding. So back down to 50ma. Why invite trouble?
Best Regards,
Pat
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