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My TRAMP project is nearly finished; I just need to add the appropriate Cathode Bias resistor for future KT66/KT88 operation. I am hopeful that someone can help me understand which resistor I need.

On Page 50 of the TRAMP Amp Builder's Guide (ver 1.96), it says: "Replace the stock 560 Ohm resistor with a 1K resistor and it is in parallel with the existing 750 Ohms. This will do the job nicely for a KT66, KT88 or the EL34".

So, seemingly, the guidance is to install a parallel 1K cathode resistor. If I understand correctly, this 1K Ohm resistor (In parallel with the fixed 750 Ohm bias resistor) sets the total cathode resistance at 429 Ohms. But, I don't understand "why" this value works. And the more I think about the biasing situation, the more I realized that I am not sure how a KT66 or KT88 works in this amplifier at all. Both tube types seemingly have rated maximum power dissipations well beyond the 15W capacity of the TRAMP's output transformer. It must be that the biasing resistor (in combination with the plate voltage???) derates the tubes enough so as not to damage the OT.

Can someone explain the math to me? With 429 Ohms of cathode resistance in this single-ended amp, how much power does a KT66 or KT88 produce?

Any and all insights are appreciated!

S_Two

True Class A amplifiers will *at best* produce slightly under 50% of the output tube's power dissipation, assuming all conditions are optimum. We are basically running these bigger tubes at around 25W to 30W dissipation, at the most, so that around 15W max is produced in the OT. I forget the details of the exact calculations we did, but in reality we're probably talking more like around 13W output. But 13 tube watts is pretty darn loud!

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Great tones come in small glass jars!

This is the way mine is done:


In my Tramp the 560 was left in and not a 1k. Tramp was built by Coco.
Works great and as Zaphod says "13 tube watts is pretty darn loud!"

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Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

Thanks for the replies!

I have ordered a variety of 5W ceramic wire-wound resistors, with values ranging in small increments from 600 ohms up to 2K ohms. My back-of-the-envelope calculations suggest that 700 ohms should be about ideal for the particular KT88 datasheet I used, but then again I don't have that much trust in my figures yet I will try several of the resistors in the circuit (with a current probe on the cathode), and let you know how it goes.

Again, thanks for the input.

I have continued to experiment with the TRAMP.

I installed a selectable 5W, 1000 ohm ceramic wire-wound resistor, in parallel with the stock 750 ohm cathode bias resistor. The combination measures out at 423 ohms, well within the 5% nominal tolerance of the components.

With a JJ KT66 tube, I measure the following voltage/current relationships:



My question is, Do these curves look "correct"? I was unable to take voltages with the Power Level > 9. I though I heard the beginning of "crackling" sounds from the speaker as I slowly advanced the Power Level knob, so I immediately discontinued the experiment. Does the onset of crackling suggest that the bias level is too high?

MANY THANKS for any input,

S_Two