D47 Rookie mistake

[mihi_fuchs]

I brought some mic's to a friend's studio so his wife (Bulgarian Opera singer) could try them out. We did the 251 and C12, then he plugged in the D47. No signal. As it turns out he had left phantom power on. Rookie mistake from a very experienced engineer. B+ is sending signal, 408a's are not lighting up. Can someone give me a starting point? Did he blow up Moby's beautiful tx, or do I get off easy with a blown up cap?
Thank you
Oh, here's the links to the D251, and D12;
https://www.dropbox.com/s/dqj52suw7xlhqoy/ANNA 251 Sample 2.wav?dl=0
https://www.dropbox.com/scl/fi/c73w...le-2.wav?rlkey=xnor0aqhwz4y9s82upybe4fbv&dl=0

hm, as thomas said, i would not be too much afraid of the phantom power. however, as you describe the 408a is not lighting up, you may want to check if you plugged in the mic into the switched on psu?
the heater voltage in that setup is unregulated, if i remember right. if you have the psu switched on without load on the heater side and plug in the mic to that afterpowering up the psu, that can kill your filament. maybe measure the tube filament and h+ in the mic?
hope that‘s not what happened but what you describe make that scenario possible. then your tube filament has gone and you need to change tube.
if find that to be a possible reason just check with another cheap 408a - if it works then, you know what happened.

hope that helps,
best regards,
michael

 

[HerbertR]

HR1 and HR2 are the connections for the "famous" room heater 1.5K, which is mounted directly in the microphone. It conducts from the 105V B+ rail through this large drop resistor the voltage of about 70V down to about 35VDC heater voltage of the two WE 408A tube heaters, which are connected in series.

I think the 78 volts are measured incorrectly. HR1 = B+ and should measure about 105V against ground/microphone housing, frame... The voltage DC measurement between points HR1 and HR2 should show the voltage drop of about 70 volts for the heater voltage.

I think P2 and P3 are the XLR pins 2+3 and the measured resistance should represent the DCR of the (incorrectly connected here) secondary winding. Colors should be white and blue for S1 and S2 (DCR about 20-40 ohms I think, at least much lower than the other winding). Primary P1 and P2 with about 500 ohms DC resistance should be red and black.

 

[Khron]

measure the tube filament and h+ in the mic

Well, this particular circuit uses a pair of 408A tubes (with higher-voltage heaters), powered in a very similar manner as the U47. As per the last round of measurements, the B+ reaching the mic is "only" in the region of 75V or so, for reasons yet unknown, which is quite a bit lower than the ~105V it's supposed to be.

 

[HerbertR]

I'm not sure whether the secondary winding, in this case the winding with the higher DCR and the thinner wire diameter, may have suffered damage from the phantom power. The winding with the lower DCR would probably not suffer any damage due to the thicker wire diameter. At least I don't know of any case where the phantom power damaged the "correct" secondary winding on the XLR side. But I'm anything but a transformer specialist. With the thinner wire diameter on the correct primary side, I think something like that could happen. What do the real specialists say about the BV08 ?

 

[geekmacdaddy]

hm, as thomas said, i would not be too much afraid of the phantom power. however, as you describe the 408a is not lighting up, you may want to check if you plugged in the mic into the switched on psu?
the heater voltage in that setup is unregulated, if i remember right. if you have the psu switched on without load on the heater side and plug in the mic to that afterpowering up the psu, that can kill your filament. maybe measure the tube filament and h+ in the mic?
hope that‘s not what happened but what you describe make that scenario possible. then your tube filament has gone and you need to change tube.
if find that to be a possible reason just check with another cheap 408a - if it works then, you know what happened.

hope that helps,
best regards,
michael

Thank you Michael. I swapped out both tubes (I have a bunch) and that wasn't it.

 

[mihi_fuchs]

Thank you Michael. I swapped out both tubes (I have a bunch) and that wasn't it.

Ah, that's great news. Yeah, my bet - I was with the DEF-47. Nevertheless, the PSU is probably still unregulated in terms of B+ either. Therefore I'd check the famous 1k5 resistor. It is rated 10W. That corresponds to ca. 120V max voltage. If the PSU charges up unloaded the voltage can go easily far higher than that voltage. Maybe you just grilled your power resistor. That would account for the tubes not lighting up too of course.
The high voltage is also an issue regarding the capsule, as the diaphragm can be sucked in. However that process most of the times is reversible (saved my life a couple of times ;-)) ... The Caps are all 250V if I see right, so I would not expect issues there.
But 250V into the resistor would require it to be 40W rated. That could maybe be an issue. you can easily check wattage here: https://www.omnicalculator.com/physics/resistor-wattage
Regarding the transformer: as your measurements suggest it is still intact that goes in line with my experiences with UTM and 3u audio 47 transformer. I had both in the circuit "front-back" with phantom power and they still work fine. Additionally you would hear that wrong installation and you would not describe a working microphone before the incident. But of course you can just feed it a 1V sine wave at 1kHz and see what comes out on the other hand with an oscilloscope, if you have one at hand.
Supporting that assumption is the fact that the testing voltage is 0.5kV/AC according to the BV08 data sheet for both sides. It is AC, but still a lot higher than the 48V/DC of your phantom.

Hope it is the resistor - easy to measure and easy/cheap to replace.

If you already tried all of that, and it did not help, maybe let us know your findings. Maybe we can help with the new data ... ;-)

Best wishes
Michael

 

[Khron]

But 250V into the resistor would require it to be 40W rated.

But how long would that last? Perhaps only until the last capacitor discharges down to the "under load" voltage, so... probably not all THAT long (imho).

 

[mihi_fuchs]

But how long would that last? Perhaps only until the last capacitor discharges down to the "under load" voltage, so... probably not all THAT long (imho).

no - definitly not long. but 10 to 40w rating is a difference. for other psus it takes about couple of secs. however - checking it costs you 1 sec - even less ;-) …
and it explains the bulbs not lighting up whereas the transformer issue does not interfere with that.
even if unlikely - it’s still worth a shot, as there are not that many components and that resistor is already quite hot under normal conditions.

 

[geekmacdaddy]

Ah, that's great news. Yeah, my bet - I was with the DEF-47. Nevertheless, the PSU is probably still unregulated in terms of B+ either. Therefore I'd check the famous 1k5 resistor. It is rated 10W. That corresponds to ca. 120V max voltage. If the PSU charges up unloaded the voltage can go easily far higher than that voltage. Maybe you just grilled your power resistor. That would account for the tubes not lighting up too of course.
The high voltage is also an issue regarding the capsule, as the diaphragm can be sucked in. However that process most of the times is reversible (saved my life a couple of times ;-)) ... The Caps are all 250V if I see right, so I would not expect issues there.
But 250V into the resistor would require it to be 40W rated. That could maybe be an issue. you can easily check wattage here: https://www.omnicalculator.com/physics/resistor-wattage
Regarding the transformer: as your measurements suggest it is still intact that goes in line with my experiences with UTM and 3u audio 47 transformer. I had both in the circuit "front-back" with phantom power and they still work fine. Additionally you would hear that wrong installation and you would not describe a working microphone before the incident. But of course you can just feed it a 1V sine wave at 1kHz and see what comes out on the other hand with an oscilloscope, if you have one at hand.
Supporting that assumption is the fact that the testing voltage is 0.5kV/AC according to the BV08 data sheet for both sides. It is AC, but still a lot higher than the 48V/DC of your phantom.

Hope it is the resistor - easy to measure and easy/cheap to replace.

If you already tried all of that, and it did not help, maybe let us know your findings. Maybe we can help with the new data ... ;-)

Best wishes
Michael

Michael,
Such great information that I shall copy and paste for reference. This is gold to me. Thanks for all that.
I found that I was getting 105v out of the dropping resistor so I replaced components that could blow right up to the capsule. then swapped the capsule out with a Flat 47 and BAM! The healing qualities of Arienne's capsules magically brought back the voltage. The phantom power blew up my M49 capsule.
Thanks to all who helped. Thanks Herbert once again.

 

[geekmacdaddy]

Addendum: It was one of Arienne's capsules. I emailed her about buying a replacement and she got back to me in minutes "My capsules have a lifetime guarantee. I'll ship one out tomorrow". Does it get any better than that? I don't know her, but I love her.

 

[AdamFrisen]

Addendum: It was one of Arienne's capsules. I emailed her about buying a replacement and she got back to me in minutes "My capsules have a lifetime guarantee. I'll ship one out tomorrow". Does it get any better than that? I don't know her, but I love her.

Don't wait till Valentine's Day.

 

[Khron]

The phantom power blew up my M49 capsule.

I still can't help but wonder just HOW that could have happened, if that is indeed the case... Even if we assume a 48v pulse got to the other side of the transformer, and got boosted up by 6.5x (so up to 300v or so), that would still have had to go through not one, but TWO RC filters (100k plate resistor, 1uF to ground, 30k+2Meg+100Meg and another 10nF to ground), before reaching the capsule.

A quick simulation shows that gross overvoltage would have had to be present for about half a second(!!!) until the capsule voltage got to its nominal 55-60v, if we assume starting from zero.

(Edit: Which it couldn't have done, because transformers don't pass DC)

A 100ms long overvoltage would have only raised the voltage at the capsule to (or by) 13v or so. But that's ignoring the quiescent plate current draw of the tubes...

Since that capsule is now removed from the microphone, can you measure the resistance between the backplate and the front / back diaphragm?

 

[geekmacdaddy]

I still can't help but wonder just HOW that could have happened, if that is indeed the case... Even if we assume a 48v pulse got to the other side of the transformer, and got boosted up by 6.5x (so up to 300v or so), that would still have had to go through not one, but TWO RC filters (100k plate resistor, 1uF to ground, 30k+2Meg+100Meg and another 10nF to ground), before reaching the capsule.

A quick simulation shows that gross overvoltage would have had to be present for about half a second(!!!) until the capsule voltage got to its nominal 55-60v, if we assume starting from zero.

A 100ms long overvoltage would have only raised the voltage at the capsule to (or by) 13v or so. But that's ignoring the quiescent plate current draw of the tubes...

Since that capsule is now removed from the microphone, can you measure the resistance between the backplate and the front / back diaphragm?

Both read as offline. I can tell you that I recorded with the mic the day before, all went well.

 

[Khron]

"OL" does not mean "offline", but "over limit". Where did you place each probe, though?

But assuming there's no continuity between them, what about capacitance? You'll need something that can measure down to some tens of pF...

 

[geekmacdaddy]

re-measured bearing in mind probe placement. (mounting hole, and screw in the middle of capsule. one side OL, one side 14.5Ω. I don't have anything

"OL" does not mean "offline", but "over limit". Where did you place each probe, though?

But assuming there's no continuity between them, what about capacitance? You'll need something that can measure down to some tens of pF...

I placed probes on screw in the middle capsule, and in mounting hole on the ring. Also tried with this using wire ends. Capacitance was 72pf and 7.48Ω.

 

[Khron]

re-measured bearing in mind probe placement. (mounting hole, and screw in the middle of capsule. one side OL, one side 14.5Ω. I don't have anything

I placed probes on screw in the middle capsule, and in mounting hole on the ring. Also tried with this using wire ends. Capacitance was 72pf and 7.48Ω.

So one side of it did indeed get zapped - really odd. I'm assuming no visible damage anywhere?

The good side could still be used for a cardioid-only mic though...

 

[geekmacdaddy]

So one side of it did indeed get zapped - really odd. I'm assuming no visible damage anywhere?

The good side could still be used for a cardioid-only mic though...

not after I poked it full of holes with my probe . Actually, the good side is clean

 

[geekmacdaddy]

Now the original capsule (M49, Arienne) is back in and the mic is cardioid only and that's ok with me. I really love the sound of the capsule and I'm happy I didn't lose it completely.

 

[JeffKall]

Now the original capsule (M49, Arienne) is back in and the mic is cardioid only and that's ok with me. I really love the sound of the capsule and I'm happy I didn't lose it completely.

I have two of Dany’s D-EF47’s with Guosheng’s M7’s … but I am using Matador’s Universal Tube PSU’s … so I have been locked into Cardiod only because I don’t have a way to supply voltage to the 48V RS reed relay to switch pattern …

But like you … having Cardiod only is ok with me, too …

One day I would like to ultimately build Dany’s PSU’s (it was just a budget thing at the time) … just to have the D-EF47’s work as Dany intended !

In the meantime … I received my pair of Ari’s Flat 47’s … and I am looking forward to installing them in the D-EF47’s … but I may only do one just to give me a chance to do some A/B testing between the 3U M7 and the Arienne Flat 47 (but based on the consensus here I have pretty much decided on the Flat 47’s)…

But regardless of capsule … I love the sound of Dany’s D-EF47’s !!!

 

[Khron]

so I have been locked into Cardiod only because I don’t have a way to supply voltage to the 48V RS reed relay to switch pattern …

---

One day I would like to ultimately build Dany’s PSU’s (it was just a budget thing at the time) … just to have the D-EF47’s work

Surely a resistor and a switch is not that complicated? I'm assuming the coil resistance of that relay is known..?