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Power supply
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6923
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Author:  marty [ 19 Oct 2020, 21:26 ]
Post subject:  Power supply

Hello!

Quick question,

I'm building the power supply for a KT66 driven, single ended stereo. I'm attempting to build one power supply to drive both left and right outputs & the pre-amp. What's the harm in that?? Cross talk perhaps? I've never heard it, will it cripple my stereo image? Lmk

I have selected a power transformer with the proper current rating for the 5U4 @ 3amps and proper current rating for two KT66s and a 6N1P for the pre-amp. The heater current for the 6.3v puts me at 3.8amps.

My question is, how do I find the proper high voltage B+ for the secondaries that supply the plates? KT66 plate voltage is 550v max and the plate current is 200mA. I was naively looking at a transformer with 600v CT @230mA secondaries. Will this suffice?

Here's my reference schematic obivously with some critiques for a kt66 https://diyaudioprojects.com/Schematics/Mikael-Abdellah-SE-KT88-Amplifier.htm

thx bb

Marty :lildevil:

Author:  Suncalc [ 20 Oct 2020, 19:03 ]
Post subject:  Re: Power supply

To answer your question you really need to know how you will be biasing the output tube. The transformer you reference will certainly supply all the current you need, but the voltage may or may not suffice.

In the KT88 project you reference, the B+ is 400v and the bias is 75ma. With the 470Ω cathode bias resistor this sets grid bias voltage at 0.075A*470Ω=35.25v. So total plate dissipation is ≈ (400v-35.25v)*75mA = 27W. This is too hot to run a KT66.

I used the only KT66 UL curve I had to plot a rough load line. For the KT66 in SEUL I would suggest a 3.5kΩ load and a bias point of Vp=325v, Ip=75ma, Pd=24.4W, Eg=-30v, & Rk=400Ω. This would require a B+ of approximately 325v+30=355v plus maybe 5 to 10 volts for the drop in the output transformer primary.

If I were building this power supply I would want a high voltage secondary in the neighborhood of 650 vct (i.e. 325-0-325) to 700 vct (i.e. 350-0-350) with an output current in excess of 150mA.

But, in my opinion, you should probably finalize your amplifier design before you begin to worry about the power supply.

Hope this helps.

Author:  gofar99 [ 20 Oct 2020, 21:38 ]
Post subject:  Re: Power supply

Hi, I agree. I probably would use KT88s instead of 66s. Easier to source and lots to choose from. They will handle the power level you mention.

Good listening
Bruce

Author:  thatguy [ 21 Oct 2020, 14:56 ]
Post subject:  Re: Power supply

Suncalc wrote:
In the KT88 project you reference, the B+ is 400v and the bias is 75ma. With the 470Ω cathode bias resistor this sets grid bias voltage at 0.075A*470Ω=35.25v. So total plate dissipation is ≈ (400v-35.25v)*75mA = 27W. This is too hot to run a KT66.


I've read a lot on this subject and for some reason the way you worded it there it finally clicked with me.

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