MK47 PCB tube mic kit - build thread

[gary o]

Quick question my power supply voltage seems to have crept up to 109 VDC only 4 volts but is this 4 volts too much what effect will this have......shorter tube life .....change to sound.....both ....

Thanks ....by the way touch wood no popping still ......

 

[zayance]

Quick question my power supply voltage seems to have crept up to 109 VDC only 4 volts but is this 4 volts too much what effect will this have......shorter tube life .....change to sound.....both ....

Thanks ....by the way touch wood no popping still ......

Don't you have a HT pot for tweaking?

 

[nashkato]

  Also, should I be able to measure 60V across front capsule to rear? Or front to backplate?
I am only able to measure around 7V

a usual Multimeter (10MΩ) won´t be able to measure accurately in the HiZ area.

 

[gary o]

Zayance.....I have no trimmer pot....it will mean changing fixed resitor values in my case....id like to learn what would higher or even lower voltage would do in this mic circuit , wondering if critical to the U47 ish sound

thanks

 

[jordan s]

i finished my PSU tonight. All seems well. I'm able to easily get 105V, however, when I have the test switch on, the 2.3K resistor gets awfully hot. I can feel the heat and smell it and after being on for a few minutes, it starts to smoke a little. I'm using this part from mouser:
http://www.mouser.com/ProductDetail/Vishay/AC05000002401JAC00/?qs=R4%2fAj8xQbdXdY28FMFLRcw%3d%3d

It's actually 2.4k because I couldn't find a 2.3k but it's rated at 5W and the stats say that it has a operating voltage up to 275C which is pretty hot. you guys think that maybe because this is only in circuit for a short time while I check the voltage, it shouldn't be a problem?

Also, I wanted to make sure I wired the power transformer correctly. I'm using the triad VPT230-110. On the primaries, I have blue and violet tied to neutral and brown and grey on hot. The secondaries are in series with red and orange tied together and capped off, yellow and black individually are going to the opposite ends of the rectifier diodes. On one end I measure 147V and the other is 133V while the transformer is in the circuit. Seems odd to me, is this correct?

 

[nashkato]

well , as far as i can see from the datasheet of the triad PT , the wiring seems to be ok  living in a 115V country .

 

[madreza]

Hi Guys,

I have 105V for the pattern !!! it's weird !
I Also get the same voltage for the light !

when Mic isn't plugged I have more than 300V everywhere !
what should I do ?

 

[zayance]

Hi Guys,

I have 105V for the pattern !!! it's weird !
I Also get the same voltage for the light !

when Mic isn't plugged I have more than 300V everywhere !
what should I do ?

Haven't built mine yet, but 300V without the Mic plugged it's tottaly normal,
even if 300V is maybe a little High, but you could have a high Mains voltage so...
on Ioaudio's schematic, the Test circuit, has a 2.4K test load, wich is the Mic's
Equivalent resistance, and that's when you have 105V, when the PSU is loaded
with the Mic, not without.......

105V at the pattern, well i had same doubt when doing the PSU,
but don't forget that the relay inside the Mic has a resistance too,
and so it must be connected also.

105 at the light? LED you mean?? Normal to have 105V going there....
If it's jewel light, aren't you using a 6.3VAC line for it?

 

[madreza]

Thanks Zayance
even with the mic connected, I have 105 V on the pattern. A managed to have 53V trying different resistor combination . is that too high or is that ok ?

 

[zayance]

Thanks Zayance
even with the mic connected, I have 105 V on the pattern. A managed to have 53V trying different resistor combination . is that too high or is that ok ?

hmmm, well like said haven't build mine yet, but for checking pattern voltage,
shouldn't you be checking on the Relay?

 

[bobine]

Look at the schematic again.  You should have a 14K resistor in the circuit when the relay is switched on. You should have a 12K resistor as a dummy load when the relay is switched off.  You want 48v at the relay.

Maybe a photo?


(Also note the 100K resistor in the LED circuit)

 

[madreza]

So, there's no risk to switch on the relay ? even if I have 105V inside the PSU ?

 

[zayance]

Well the idea behind it if i get it correctly is that when pattern is on one of the position,
you should not have 105V on Pattern OUT (normal), and on the other, you feed 105 volts
across a 14K and the Relay, wich has also a resistance that should be
about 12K i believe, and so having about 48V at it.
So for knowing that you should switch the pattern that sends the 105V to the relay,
or just check the relay you'll get the point, if you check the Voltage across the 12K resistor when in off position,
then you have about half voltage , get it?

But if you made some tweaks as you said before than this would be off i guess.

 

[bobine]

At the wires that leave the power supply going to the relay, you should have 48v. 

I certainly would NOT want to send 105v to the relay.

 

[madreza]

That's what I thought
Thanks Zayance for your mail
I gotta find the right combination ! thanks Bobine for your help

 

[jordan s]

I think I'm experiencing the same thing. I followed the polarity switch wiring diagram that was supplied earlier in this thread. When the switch is in one position, the relay is getting the 105V going through the 14K resistor, which doesn't really drop it much, in my case, it's still about 105V. With the switch in the other position, the relay is sitting between the 14k resistor connected to 105V and the 12k resistor at 0V which rounds out to about 48V.

It makes sense to me that wired in that way, the relay would see 105V but it makes me wonder if that diagram is correct. What I'm thinking is maybe in one switch position it should sit between the 14k and 12k resistors to get 48v and in the other position, it should only see the 12k resistor to ground. Does this sound right?

Also, I'm completely unable to get my voltage at 105V now when last night it was no problem? It's at 111V with the pot turned all the way. Maybe the voltage in my building is different? Perhaps I need to up the value of the resistor feeding the pot. It's 1.5k right now (due to using the triad choke.)

 

[bobine]

I think I'm experiencing the same thing. I followed the polarity switch wiring diagram that was supplied earlier in this thread.

In the ON position, the relay is in the circuit with current flowing from 105V, through the 14K resistor, through the relay, then to ground.

In the OFF position, the relay is out of the circuit.  Current will flow through the 14K resistor, through the 12K resistor, then to ground.

Also, I'm completely unable to get my voltage at 105V now when last night it was no problem? It's at 111V with the pot turned all the way. Maybe the voltage in my building is different? Perhaps I need to up the value of the resistor feeding the pot. It's 1.5k right now (due to using the triad choke.)

I had that problem, too. I had to increase the value of one of the resistors before the pot so I could get 105V somewhere in the middle of its range.  My voltage drifts around by a few volts over time, but not by that much.  It does depend on what's coming in from the street.

 

[jordan s]

Ok, thanks. That's how I currently have it configured. It just so happens that 105V going through the 14k resistor is still about 105V. When the switch is off, the relay terminal is essentially floating, but there is 48V between the 14k and 12k resistors.

 

[bobine]

Okay... i just opened mine up to measure the voltages

Measured at the center pole of the pattern switch (between center pole and ground).
no mic attached:
  mic dummy load switched in, pattern relay switch off:  49V
  mic dummy load switched in, pattern relay switch on (omni): 113V
  mic dummy load switched out, pattern relay switch off: 138V
  mic dummy load switched out, pattern relay switch on: 330V

with MK47 mic attached:
  dummy load switched out, pattern relay switch off: 53V
  dummy load switched out, pattern relay switch on (omni): 51V

AND, with the mic attached, I measured the B+ as 111V with the pot at maximum R.  I got 105V with the dummy load.  I have two power supplies, so I need to modify this one, too.  This is a new MK47, so good thing I checked!
   

 

[jordan s]

Thanks for the info. Looks like I have things set up properly, I just don't have the mic to check with yet but I'm getting pretty much the same readings as you with the dummy switch. I still can't figure out why my 2.3k dummy resistor is burning up but since I only need it for a moment while adjusting voltage, I'm not going to worry about it too much. Can't wait to get that MK47 kit in the mail.