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[terranigma]

What about resistor power dissipations? Has it required a heatsink for BD139 and BC337?

[Elvee]

Originally Posted by terranigma

What about resistor power dissipations? Has it required a heatsink for BD139 and BC337?

When used under sensible conditions, (i.e., ΔVin-out reasonable), 1/4W resistors are sufficient, 500mW (or 1W3 at most) zeners suffice, and the BD139 requires no more than a small cooling clip, and up to 75W output, nothing at all.
The BC327 needs no heatsink.

[Elvee]

Originally Posted by danielwritesbac

What happens if input voltage is lower than the zener?
Thanks!

An afterthought: if you're prepared to jettison the protection completely, you can improve the performances, particularly when operated in "degraded" (electronic filter only) mode.

Life is a succession of hard choices.

[coolet]

which are the correct connection of R18 and C12

[Elvee]

Originally Posted by coolet

which are the correct connection of R18 and C12

Both are correct: the first is suitable for zener level shifting, and the second for a cascode configuration.

[danielwritesbac]

Originally Posted by Elvee

[40v?]Yes, any zener or combination of zeners giving that voltage is suitable: 39V if 1V is not a problem, or two 20V, or 18V+22V, etc.

So, on selecting some devices that can literally weather a storm, the following are questions of suitability:

Q3? BC639/BCP56, 2SD669, 2N3501? a similar selection to the amplifier's phase inverter? What percentage of input voltage is seen by Q3?

Q2 surge voltage tolerance options? 2SD669, MJE344, 2SC3421, 2SC5171? Q2 sees full voltage, doesn't it?

Q1 sturdy options that may withstand a surge?
mjl21193 (16a, hfe25~75, 4mhz, 200w)
mj15004 (20a, hfe20~140, 2mhz, 250w)
mj15023 (16a, hfe15~60, 5mhz, 250w)

I tried to narrow down possible options, but I'm not knowledgeable enough to narrow it down further. We do get some huge power surges and an MOV on transformer primary doesn't seem like a complete solution.

Which resistors need to be more sturdy to withstand power surge conditions?

Approximately how much capacitance smoothing should be provided directly at each rectifier?

P.S.
It seems no problem to provide 2.5v more input than zener, because most unregulated power supplies have a sort of weak peak, charged up on noise (diode, static, line noise) of insufficient current that wasn't going to be useful to power the amp at high output levels anyway. And there is also the matter of transformer pulldown. With 2.5v margin (and 300va transformer), full output doesn't have any more than 0.2v margin remaining. Should it have more margin at full output?

[Elvee]

Originally Posted by danielwritesbac

So, on selecting some devices that can literally weather a storm, the following are questions of suitability:

Q3? BC639/BCP56, 2SD669, 2N3501? a similar selection to the amplifier's phase inverter? What percentage of input voltage is seen by Q3?

No, it's a completely different function. Vceo>1.5V and Ic max>200mA is enough.

Q2 surge voltage tolerance options? 2SD669, MJE344, 2SC3421, 2SC5171? Q2 sees full voltage, doesn't it?

During normal operation, Q1 and Q2 see the in-out difference.
When the protection is active, they can see up to the full input voltage, if the output is shorted.

Which resistors need to be more sturdy to withstand power surge conditions?

R5 and R6 can be upgraded to 1/2W

Approximately how much capacitance smoothing should be provided directly at each rectifier?

Something like 4700µ per 50W output is sufficient

And there is also the matter of transformer pulldown. With 2.5v margin (and 300va transformer), full output doesn't have any more than 0.2v margin remaining. Should it have more margin at full output?

What do you mean exactly?

[terranigma]

I modified powerflux's (lost?) pcb respecting CFP implementation as much as I can do. I built and tested its an earlier version with BD249C output transistors without any observable (eye for smoke,, ear for sound, fingers for heat) problem. This layout hasn't built yet.

I want to share printable pdf's after some reviews and suggestions.

Thanks.

[terranigma]

I could switch 15K B-E resistors and compensation network just below of them..Mislabeled resistor is 27R.

[Elvee]

Originally Posted by terranigma

I modified powerflux's (lost?) pcb respecting CFP implementation as much as I can do. I built and tested its an earlier version with BD249C output transistors without any observable (eye for smoke,, ear for sound, fingers for heat) problem. This layout hasn't built yet.

I want to share printable pdf's after some reviews and suggestions.

Thanks.

That looks fine, and anyway, since it is signal-level circuitry, the order and placement of components matters very little, and you can choose freely the most convenient configuration.