How to calculate: Plate Dissapation, Output Power, Etc

[RJ Guitars]

Hey guys,

I've had a hand-full of fundamental questions that evolve from some of those essential pieces of amp building... after I kinda stumble through things I figure it out, but I thought it might be nice to just write things down in a simple cookbook, then maybe even grow that to an Excel Spreadsheet where you just plug in values and answers are auto calculated. Or maybe some of those simple Visual Basic executables with a couple inputs and answer pops out.

Anyway, calculating plate dissipation seems to come up all the time. But what are the critical elements? Here is a formula I use (I borrowed it from another forum), but I always have to sort a few things out before I believe my answers, but still wonder if I got that right.

For a KT-66 tube - what is my plate dissipation?

I base the following assumptions on the formula below, but are they correct?
1) This is for an amp running in class A/B - thus the 60% duty cycle
2) 30 Watts is the max output I should expect from a KT-66
3) Using "The Power Formula" P=I*E, we derive the plate dissipation for one tube.

For 60% of max dissipation at idle:

30 watts X .60 = 18 watts

18 watts/435 = .0414 Amps = 41.4 mA


There are many other questions, but small bites will help prevent choking on my part...

rj

[Phil_S]

I like this for push pull amps. I don't remember where I swiped it from. Maybe Duncan Munro?

[RJ Guitars]

Phil,

That is excellent, thank you very much. I'm really looking for that exact kind of tool, plus you can always go in and reverse engineer things to get a grasp of what the math is doing.

rj

[RJ Guitars]

I ran some voltage measurements... I posted an extended chart of them on a thread about one of my amp builds over on the Trainwreck pages. I was hoping to draw a little bit of technical discussion here to help me sort out where I want to be as far as plate dissipation goes.

https://tubeamparchive.com/viewtopic.php?t=7300

One thing I should mention is that my amp is using a pair of KT-66's with cathode biasing. It's a relatively low B+ voltage on the plates around 340 volts and I got 30 volts on the 300 Ohm cathode resistor. This gave me 100mA for the pair of tubes, so that makes it right at 50mA of bias current for each tube.

Using the chart that Phil provided that seems plenty safe for a KT-66.

Later I added a couple resistors in parallel with that cathode resistor to add a couple more values and data points.

264 ohms gives 109mA / 2 = ~55 mA per tube
235 ohms gives 115mA / 2 = ~57mA per tube

I read in Gerald Webers first book that in cathode biased tube amps you typically need a higher bias current than using the same tube in fixed bias mode. This would imply that I might still be a bit cold. Does anyone understand/believe what Gerald Weber is saying here? Have I read that correctly and how much more juice can I throw at these tubes?

rj