Power Supply Alterations - I am safe here?

[leigh]

As previously detailed here (and here), I was trying to refurbish the mic pres from an old Harman Kardon tube mini PA.

I've since decided to redo the whole audio path based on the original Hamptone project (Tape Op #30), keeping mostly just the chassis and the power supply.

The power supply was set up as a voltage doubler, which put out waaayy more voltage than I needed (500+ VDC unloaded). It looked like this circuit, taking the DC from the + and - (the "0" output was not used in this design):

I removed (not bypassed) D2, and as I'd hoped, this cut the voltage down to about 250 VDC.

Q: Is this now safe? Are the caps seeing too much action?

I understand this to be a half-wave rectified circuit, with heavy filtering. I just want to be sure that I'm not now overworking those caps.


Cheers,
Leigh

 

[mcs]

[quote author="leigh"]Q: Is this now safe?[/quote]
Yes, but use a bridge rectifier instead. That way you will get 100/120Hz ripple instead of 50/60Hz. The voltage will stay at approx. 250V.

Best regards,

Mikkel C. Simonsen

 

[CJ]

Since D2 is not there, take out C2 and paralell it with C1, or add a resistor to the cap to form a pie filter.
Measure your unloaded voltage and compare it with the voltage ratings on the caps.
Half wave is ok, you will have some dc on that M6 core but that will help drop your output down perhaps, once you get a load on it.
Put a 220k 2 watt resistor on there from + to 0.
This will be a bleeder and draw some dc at the same time so you can see what your transformer does with the dc.

 

[leigh]

[quote author="cjenrick"]Since D2 is not there, take out C2 and paralell it with C1, or add a resistor to the cap to form a pie filter.[/quote]

I'm not familiar with pie filters. Care to explain what that would mean here?

[quote author="cjenrick"]Put a 220k 2 watt resistor on there from + to 0.
This will be a bleeder and draw some dc at the same time so you can see what your transformer does with the dc.[/quote]

Cool, I'll try that.

Leigh

 

[owel]

Pi filter... Technically, it should be an inductor. High frequencies (example: PS ripple) get shunted by the capacitors, and low frequencies (i.e. DC) are passed by the inductor.

In the diagram, substitute resistors instead of inductors.[/img]

 

[NewYorkDave]

"Pi" just refers to the configuration, since it resembles the Greek letter "pi." It can be built with resistors and caps and still be rightfully called a "pi filter."

 

[leigh]

[quote author="owel"]

In the diagram, substitute resistors instead of inductors.[/img][/quote]

Thanks for explaining that.

The circuit actually already has another stage of capacitor filtering, that looks like the circuit on the right. After the circuit I posted above (minus D2 now), the + side runs through a 27K resistor, then there's another big capacitor (20 uF I think) to ground. That's the B+ supply, which is then distributed to the tubes through further resistors.

Since I've already caulked the C1 and C2 caps into place, I'm gonna leave them there, as long as it's considered safe. (It may be redundant or ineffective, but it's easier to just leave 'em there at this point.)

Leigh