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In Reply to: RE: I was thinking of trying this. posted by DAK on October 12, 2016 at 10:32:17
This is another example of a Net-borne design that isn't fully vetted. I only had time for a quick scan of the article, but that's all it took to realize the author may not understand this topic sufficiently to rank as an authoritative source. Here's what I'm referring to:
"In most applications, the pass transistor will not require a heat sink. For example, in a circuit with 50 volts differential across the TIP50 and 100 mA load, the pass transistor is only dissipating 5 watts. The TIP50 is rated 40 watts dissipation."
This statement tells me the author doesn't understand semiconductor ratings, and that throws the validity of the entire article into question. The 40W rating of the TIP50 is based on maintaining the collector at a temperature of 25C. That requires a big heat sink. The datasheet further confirms this error with its 25C ambient (no heat sink) rating of 2W dissipation. Hook it up as the author suggests - 5W and no heat sink - and the part will fry.
That aside, I'm not saying the circuit won't work, or that it's not effective for screen regulation (although it's much more complex and costly than I would find appropriate). It's just worth pointing out that articles and projects like this need to be approached with your eyes open. It's not a commercial design, and it may be burdened with design parameters or advice/technical opinions that will cause failure in your specific application.
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Buy Chinese. Bury freedom.
Follow Ups:
And obviously very familiar with tube circuits. So other than the faux pas with the heat sink, somewhat forgivable since he is a tube guy, the circuit should be good for a job like G2 supply voltage?
It seems that having an independent G2 supply would be beneficial to a power tube's performance. Dak
" the circuit should be good for a job like G2 supply voltage? "
Well, the concept seems OK in theory. Whether it will function as intended in practice won't be known unless you build it. If I were attempting to use it, I would first need to study the datasheets for the semiconductors to determine whether the design as presented is fundamentally correct. I would also need to consider the usual "what-ifs," such as a shorted output and other issues, then decide whether to include protection for those things. After that, I'd have to build it, attach it to the power supply I intend to use and search for any undesired behavior such as oscillations and noise. The final electrical test would be monitoring of the output with a scope while the amplifier is in use to be sure the circuit is regulating correctly and doesn't misbehave in some unforeseen way. Once all that was done, I would collect a group of people to listen to the amplifier, preferably with a second amp using a "standard" screen feed for comparison.
I know this isn't what you want to hear, but it's the general process that I find necessary in order to ensure my work is good. FWIW, you'll probably find numerous people around who will tell you that mixing SS with HV tube circuitry is asking for trouble. I share that perspective, especially when the circuit isn't a commercial design and the DIY user isn't a design engineer. Some very fine equipment has used nothing more than a resistor and cap to feed the screens. You should also consider VR tubes for this. In addition to everything else, they glow. :)
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Buy Chinese. Bury freedom.
take a look at pages 3-56 to 3-58
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Have Fun and Enjoy the Music
"Still Working the Problem"
The LR8 with a pass transistor is a great option, though it may be overkill, and the zeners are probably better than you think. This amp uses an LR8 with a pass FET. But yes, a heatsink is necessary.
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