trinityamps.com

So he just needs to shave 2.5W off the dissipation to preserve the life of his KT66. So the recommendation for the increase in R15 is my guestimate value for what would reduce the dissipation by around that amount.

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Great tones come in small glass jars!

Thank's for the information guys

Stephen, can you show me how you got 27.5 watts since my calculation give me 30.23 watts. I use 342 volts x .0884 Amps.

Can you tell me if max plate dissipation of the JJ Kt66 is really 25 watts? Or is it an average suitable plate dissipation?

Thank's again

I've just rechecked the calculation, and it is indeed 30.23W being dissipated - and not good for your KT66!

This is assuming that you have a total cathode resistance of 318 ohms, as you stated. 750 ohms in parallel with 570 ohms actually gives 324 ohms, so I guess your resistors are slightly below their official value, which is pretty normal. Even if they had been 100% correct in value, the dissipation would still have been around 29.7W.

In this case I would suggest trying something like 680 ohms for R15, giving a total of 357 ohms. Like I said it's a bit of a trial and error exercise.

Stephen, I wonder if in your calculations, you accidentally subtracted 0.025A screen current, rather than 0.0025A.....

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Great tones come in small glass jars!

Yes, 30W is correct. I had used 342 volts as plate voltage but 342 was the voltage between the cathode and the plate. So when I added back in the cathode voltage of 28.9, I got 30.2 Watts.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks again guys,

Will try to find a 680 ohms 5 watts and will keep you informed of the new dissipation.

I can't find a 680 ohms 5 watts resistor so I used a 984 ohms (1K) instead. Here is what I got so far.

Voltage between the cathode resistors ( 410 ohms when combined) is 31.25V so cathode current is .0762A
Voltage between the screen resistor (1K) is 2.05V so screen current is .0020A
Plate current is .0762A - .0020A for a total of .0742A
Voltage between cathode and plate is 350V
So I got 350V x .0742A = 25.97 Watts. A little over i guess. Maybe around 1200 ohms will be ok?


Wall is still 117.5VAC
So is B+ at 435VDC

I would estimate you need combined cathode resistance of 395 ohms to get about 25 watts, but 26 is close enough IMO.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Is that the combined resistance when you measured with a digital meter? 984 in parallel with 750 ohms should give 425 ohms. So it sounds like your 750 resistors are probably a bit lower than their marked value.


Serge's KT66 is dissipating slightly too high, apparently with a combined resistance of 410 ohms. So I reckon around 430 to 440 ohms *combined) would hit it right on the nail.

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Great tones come in small glass jars!

Yes the value was measured with a digital meter but I made a little error measuring the combined cathode resistors value. The correct value is 423 ohms after multiple verifications.
So 31.25V / 423 gives .0738A
Screen current is still .0020A
So Plate current is .0718A times 350V i got 25.15 Watts of dissipation now.

Do you think it is safe or is it preferable to shoot for 24.something watts?

0.15W doesn't make any difference worth caring about. Also, note that when you put a signal through a Class A amplifier, the averege dissipation actually decreases.

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Great tones come in small glass jars!

Thank you very much Zaphod.

So in retrospec, it seems that a 1K ohms 5 watts resistor in parallel with the 750 ohms will do the job nicely for a KT66.

Good work Sergelac, I'll actually add this to the manual. Now how about some comments on the Tone with that Tube!!

Still love that Tramp Logo BTW!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps