45 DHT HT Supply oscillating - help!

[Rod Coleman]

Removed C4 and added 47uF capacitor in the 350V HT rail and it now works like a charm!
Thanks for the help...
Cheers,
Ale

Ciao Ale, Glad to hear it!

If you get a chance, please do try the LCLC version, without a regulator, to compare. Even if you only have a cheap 220u/400V in the first C position, and 1 henry choke, temporarily dangling from the side of the amp.....

Without full optimisation for regulator transient response, the sound may still be far behind its ultimate capability.

And, I am pleased that my Filament Regulator kits are working well, and keeping the noise & hum out of the circuit! More & more constructors are trying these out, to good effect.

 

[mogliaa]

Ciao Ale, Glad to hear it!

If you get a chance, please do try the LCLC version, without a regulator, to compare. Even if you only have a cheap 220u/400V in the first C position, and 1 henry choke, temporarily dangling from the side of the amp.....

Without full optimisation for regulator transient response, the sound may still be far behind its ultimate capability.

And, I am pleased that my Filament Regulator kits are working well, and keeping the noise & hum out of the circuit! More & more constructors are trying these out, to good effect.

Hi Rod,
Will do the test probably, but now will try to enjoy a bit of the amplifier after a long time!

With the current HT supply, the HT rail ripple is less than 20mVpp!

Thanks for all the help!

Cheers,
Ale

 

[mogliaa]

Rod is correct. Now the hard ones and Glad to see some RF gain element mentioned.
The Learning curve is that the Mosfet and transistor have fantastic FT's into the MHz regions that have to be tamed right down to avoid them oscillating with the forward shifting poles these types of close loop feedback circuits produce. It's right into Bode plot physics ballpark.....not conversant ? read up ! Unfortunately Morgan Jones valve amps 3rd edition deliberately wiggles out of the physics..but a good switchmode power handbook (Unitrode power supply and design) will have an excellent description and theory that one requires in closed loop design.
The approach in my circuit out of the junk box is pretty similiar. Note ferrites in each mos gate and drain and snubber circuit. The current limiting is only for start up...(reality no HV current limiting supply is fully bomb proof on a 500V to zero s/c test, so don't try it).
Remember whatever spice simulation circuit one uses, it WON'T be perfect...so much depends on layout and parasitics. The bugbear in all HV stuff is reduce the excess gains one doesnt' need, hence a higher reference voltage is a good start. Hivolt Zeners are impedance rubbish....again I use 431's in series (superb example of current sourcing) with a much lower forward Z but make sure no transient spike reaches them otherwise one or all the series chain is easily destroyed. Notice the feedback pole capacitance has a resistor in series...spike/transient damping directly in the TL431 feedpath either from pole feedback from the output or input..The working result is close sand regulation and hum performance. The "Zero" from the output cap is required, all these circuits have an undesirable gain/response pole that has to be neutralised and is often better to design a stable circuit with a bit of transient slowness and use a slightly higher output capacitor value. Study the circuit, and one can simulate it. It's stable on a good pcb layout.
Typical tests that one should do: AC ripple performance close to regulator drop out voltage with maximum current, (illustrates end of loop gain) also with overvoltage conditions with min loop gain.

Remember You are taking on quite a task:
HV stabilised power supplies are notorious for mysterious and unaccounted blow-up's....til one learns about them and there aren't many around that specify complete s/c protection. Prototype them on the soft side, add carbon /HV resistors where necessary and cut them down later on optimisation.
I am not sure if Bode gain/phase plots is part of the standard technical curriculum..so much time has passed since I did it on a slide rule, (1967)

richy

Hi Richy,
Very interesting, I know I still have a lot to improve. Luckily I managed to stabilise my regulator, so won't be changing it for the time being...
thanks for the tips and help
Cheers,
Ale