Official Universal Passive Tube PSU Thread

[Guff]

I just noticed that I've been referencing R1/R2  left to right when looking down on the PCB but looking at the photo of the PCB in this thread R2 is on the left (looking down)

 

[Matador]

I just noticed that I've been referencing R1/R2  left to right when looking down on the PCB but looking at the photo of the PCB in this thread R2 is on the left (looking down)

These voltages are with the mike? Or a dummy load?

 

[Matador]

So those voltages show you are pulling 6.5mA through the mike.  This sounds quite high.

Please test with a dummy load between B= and ground, inserted right into the 7-pin XLR output on the PSU.  Based on my reading of the U67, the tube draw should be about 0.9mA.  So the test load should be a 240K resistor of at least 1/4W rating (220K would work too).

U67 - 210V, 0.9mA = 240K @ 0.25W

 

[Guff]

The voltages I posted earlier were with no load at all. I just tried again with a 220k dummy load at the 7pin jack. I get approximately the same readings R2- 36.9/157 and R1- 157/276.  I've been reluctant to hook up the mic without being able to get in the ballpark with B+

 

[Matador]

Ok, that's even stranger then. 

Something is pulling a hefty current through the B+ filter resistors.  Try this:

1) Remove the test load, and disconnect the pattern switch:  just remove the three wires from the top-leftmost terminal block (when viewed from the top, with the AC power connector on the back facing you).  See if the B+ recovers.
2) Check the value of the bleeder resistor:  I think it's labeled R6.  It should be 499K.
3) Make sure that you grounded the negative lead of your volt-meter to circuit ground (there are several round pads visible for this, or you can use the appropriate ground-labeled green terminal blocks).

 

[Guff]

removed the test load and the pattern switch wires-no change to B+
removed R6 and put in another 499K - old one tested at 499K
had the negative DMM lead on a ground pad- still no change to the readings already posted. Heater still looks good

 

[Matador]

Can you confirm all of the caps are in the right place?  Like no 4700uF/25V cap was put in place of one of the 350V or 250V ones (C1 through C4 are all rated 350V or 250V)?

Otherwise, one of the main filter caps must be leaking.  Remove C1, then retest, then C2, etc, through C4.  Eventually the voltage should recover and then you'll have found the culprit.

 

[Guff]

Matador, I want to apologize for wasting your time. I had the values for C3 and C10 switched so indeed I had a 4700uf 25 V cap at C3 and the 100uf 250V and C10.  I thought I was being so careful on checking values before installing but made a stupid mistake anyway. Should I replace these caps with new ones? Will they be damaged? I'll have to order the 100uf ones.Thanks for all your help.

 

[Matador]

No worries, I've done that more than once myself.

Obviously, the 250V cap at 6V isn't a problem, but I would replace the 4700uF one that was run at high voltage.  Generally, these punch through (which is why it dragged down the B+ line, as it essentially became a partial short) and can't be trusted over the long haul after over-voltage stress.

Glad you got it nailed down.

 

[Guff]

re-used the 250 volt cap and replaced the 25 volt  with a new one. B+ adjusts perfectly to 210 volts. Plugged the mic in and trimmed just fine to 210 for B+ and heater was right on -6.3 volts and the mic works so that's a big plus. Thanks again for your help and patience Matador. Anybody considering this PSU for Dany's DU67, it works great provided you build it correctly! 

 

[kmozart]

Will 160K work as a test load for C12?  That is the one I have at the moment.... 

 

[Matador]

Will 160K work as a test load for C12?  That is the one I have at the moment.... 

Close enough.   You'll end up re-adjusting with a real microphone in any case.

 

[seancook]

if you arent using the bias supply, say in a c12a psu, would you leave it floating/not connected or connect it to ground? and if so, is there a difference with connecting it to ground in the psu or at the mic end (if i where to use the psu for a c12 as well)

 

[Matador]

Just dial the bias down to 0V via the trimmer resistor, then just leave it unconnected.  You don't want to hard ground it, as it will make using it with a C12 in the future difficult.

 

[Potato Cakes]

For checking the polarity voltages with this circuit, should I be getting the following?

Omni = B+ voltage
Cardiod = 1/2 B+
Figure 8 = 0

Thanks!

Paul

 

[Matador]

For checking the polarity voltages with this circuit, should I be getting the following?

Omni = B+ voltage
Cardiod = 1/2 B+
Figure 8 = 0

Thanks!

Paul

Which microphone is it powering?

 

[Potato Cakes]

This will be for a 251 ptp without the relay switching scheme.

Thanks!

Paul

 

[Matador]

If you follow the basic polarization scheme where:

1) Front diaphragm is at 0V
2) Backplates are at +60V

Then, on the rear diaphragm:

1) 0V polarization = omni
2) 60V polarization = cardioid
3) 120V = figure 8

 

[Potato Cakes]

Gotcha.

Thanks for the info!

Paul

 

[Phrazemaster]

Weren't you guys working on a regulated version? I can't seem to find the thread...