Help with dummy load for PS testing?

[kiira]

Hi!

I have put together a new PS for my homemade tube pre ( http://www.2tough.com/~kiira/rec/micpre.jpg ) and the schematic is at http://www.2tough.com/~kiira/rec/micpre-ps.jpg . It is mostly made out of parts from my junkbox and I would like to test it out with a dummy load but alas, I don't know how to figure out what kind of dummy load to use across the outputs. Could someone point me in the right direction?

Oh yes... is there any software aound for designing power supplies? It seems like it would be fairly easy.

Thanks!

Kiira

 

[PRR]

The easiest dummy load for the heaters is a heater. First check the no-load voltage: it will be around 18V if all is well. If much different, you have a wiring error. Then aligator-clip your cheapest 12A?7 tube's heater to it and check that the voltage drops a little. I suspect that just one tube will bring it down to 15V, so it will go down to 12V-13V with two or three tubes. This isn't very critical, certainly not for initial smoke-test.

It looks like the high voltage should be around 460V no-load. The 450V caps will stand that for many minutes, long enough to check. The loaded voltage is supposed to be 320V at 13mA. 320V/0.013A is a 23K 5W resistor. However personally, if the no-load voltage was about right and I triple checked all the resistors and capacitors, I would not bother with a dummy load test. I would fire-up with a voltmeter on the B+, check that it went up to almost 460V on cold-start, then sagged to about 300V-350V as the tubes warmed.

Be very sure that your capacitor and rectifier polarity is correct before plugging in. Connect your voltmeter red and black the "right" way and be sure it reads the right polarity. (That was one advantage of the old needle meters: with digital display it is easy to miss the little "-" sign, but you can't miss the sound of the needle slamming backward.)

 

[NewYorkDave]

Just use Ohm's law:

R = V / I

and make sure the resistors have an adequate power rating:

P = VI

A couple of quick observations about the circuit. You used varactor symbols for the rectifiers. Use the regular diode symbol (a single line indicating the cathode) instead. Also, I suggest that a better way to implement the heater supply would be to use a fullwave rectifier and a filter cap on the output of the 12.6V transformer, followed by a 12V regulator (e.g., 7812) and the heaters wired in parallel instead of in series. You can insert a regular rectifier diode in series with the regulator's "ground" terminal (anode to regulator, cathode to ground) to give 12.6VDC output.

 

[NewYorkDave]

Here's what I'm talking about, in outline form:
http://electronicdave.myhosting.net/miscimages/heatersupply.gif

 

[kiira]

[quote author="PRR"]The easiest dummy load for the heaters is a heater. First check the no-load voltage: it will be around 18V if all is well. If much different, you have a wiring error. Then aligator-clip your cheapest 12A?7 tube's heater to it and check that the voltage drops a little. I suspect that just one tube will bring it down to 15V, so it will go down to 12V-13V with two or three tubes. This isn't very critical, certainly not for initial smoke-test. [/quote]

Ok... got it.

It looks like the high voltage should be around 460V no-load. The 450V caps will stand that for many minutes, long enough to check. The loaded voltage is supposed to be 320V at 13mA. 320V/0.013A is a 23K 5W resistor. However personally, if the no-load voltage was about right and I triple checked all the resistors and capacitors, I would not bother with a dummy load test. I would fire-up with a voltmeter on the B+, check that it went up to almost 460V on cold-start, then sagged to about 300V-350V as the tubes warmed.

Be very sure that your capacitor and rectifier polarity is correct before plugging in. Connect your voltmeter red and black the "right" way and be sure it reads the right polarity. (That was one advantage of the old needle meters: with digital display it is easy to miss the little "-" sign, but you can't miss the sound of the needle slamming backward.)

Cool coo. I wasn't sure how much harm I could cuse the caps an such with no load on them. I think I'll put a resister in there anyway though just in case I fall asleep while I am measuring the voltage. I actually have an old VOM with a meter in addition to my Beckman.

 

[kiira]

[quote author="NewYorkDave"]Just use Ohm's law:

R = V / I

and make sure the resistors have an adequate power rating:

P = VI[/quote]

meep. That looks suspiciously like mathematics to me.

A couple of quick observations about the circuit. You used varactor symbols for the rectifiers. Use the regular diode symbol (a single line indicating the cathode) instead. Also, I suggest that a better way to implement the heater supply would be to use a fullwave rectifier and a filter cap on the output of the 12.6V transformer, followed by a 12V regulator (e.g., 7812) and the heaters wired in parallel instead of in series. You can insert a regular rectifier diode in series with the regulator's "ground" terminal (anode to regulator, cathode to ground) to give 12.6VDC output.

Hey... couldn't I just use an LM317T after the caps instead? That would regulate the the supply and let me do away with the 2 watt resister yes? I mean with the two resisters and cap it would need acourse.

Here's what I'm talking about, in outline form:
http://electronicdave.myhosting.net/miscimages/heatersupply.gif

RAWK!!! Thank you all. I kinda gotta keep the heater scheme the way it is though because I made the circuit boards so they're wired in series. and all like that and I don't want to have to make another board for the filiment supply.

 

[NewYorkDave]

Arrgh... The polarity of D2 in my little sketch was wrong. Just goes to show the dumb mistakes you can make when you're trying to work fast.

The sketch has been corrected...

 

[rafafredd]

Hey dave...

You´ve grounded pin nine on the last heater... shouldn´t we ground on the four pin 5´s and let all pin 9´s float?

Maybe I´m wrong, but can you please, have another look at the drawing?

 

[NewYorkDave]

You could do it either way. The way I've drawn it, the heater supply is balanced with respect to ground. If you prefer, you can remove the connection to V1/pin 9 and ground the low side of C2.

 

[rafafredd]

But if I ground pin 9, only, shouldn´t I ground all the pin9s in all tubes?

 

[NewYorkDave]

No, the heater supply should be referenced to B- at one point only.

 

[rafafredd]

Thanks. Sorry for all the confusion.

I´ll have another look at the circuit later, because I´m really not understanding it yet...

 

[rafafredd]

ok, now I understand. :?