MK47 PCB tube mic kit - build thread

[david-p]

Hey guys, I am having trouble getting 48v, and 105v with the mic hooked up. The most I can get is 45v and 99.8 v with the pot turned fully clockwise. Do I need to lower a resistor value somewhere before the pot? I want to test this myself with trial and error, but I want to use the mic tomorrow, so I'd hate to mess something up before then. It works without any problems and sounds good. I read somewhere back in this thread that the incorrect voltage values could possibly lower the lifetime of the capsule, so I am a bit worried. Any advice will be greatly appreciated.
Thanks

Understood that you will want to eventually establish the design voltages, and others who have faced the same problem will tell you what to do about it; but using the mic with the voltages that you quote will pose no danger to either vacuum tube or capsule.

David

 

[Volume11]

Thanks David, I'll go back and read through the thread when I get the time and post any updates.

 

[diylan]

So I finished building the PSU and in testing I'm getting some very odd voltages...

I'm using the Triad VPT230-110 and I'm getting over 300v off of the secondaries! I'm in the US so this is wired for 115v in (blue to violet, gray to brown) and 230v out (red to orange, black to one side of rectifier, yellow to the other).

Secondary 1: 336v (319v under load)
Secondary 2: 313v (299v under load)
Rectifier out: 805v (737v under load)
B+: 667v (241v under load)

Needless to say, I haven't brought the mic anywhere near this thing yet!

It's probably something obvious but I just can't figure out why the power transformer secondaries are putting out such high voltage. Any help would be much appreciated. Thanks!

 

[kante1603]

So I finished building the PSU and in testing I'm getting some very odd voltages...

I'm using the Triad VPT230-110 and I'm getting over 300v off of the secondaries! I'm in the US so this is wired for 115v in (blue to violet, gray to brown) and 230v out (red to orange, black to one side of rectifier, yellow to the other).

Secondary 1: 336v (319v under load)
Secondary 2: 313v (299v under load)
Rectifier out: 805v (737v under load)
B+: 667v (241v under load)

Needless to say, I haven't brought the mic anywhere near this thing yet!

It's probably something obvious but I just can't figure out why the power transformer secondaries are putting out such high voltage. Any help would be much appreciated. Thanks!

Hello,

have you measured the ac from the wall outlet?Did you check the batteries in your DMM (when batteries go weak DMMs show pretty "funny" things!).
Beside of that I´ve looked at the specs for your transformer (via mouser),your wiring seems to be correct.
But the measurements are completely out of spec:
If wiring is correct you should get something like 230vac on the secondaries if wall outlet is 115vac-under load.
After the rectifier you should get something like 322ish volts dc,again under load.
BTW:How did you load it (you wrote you "....haven´t brought the mic anywhere near this thing yet")?
If your mains ac is correct you should post some pics of the transformer wiring and of the rectifier(is it set up correct etc.).

Hope to have helped,

Udo.

 

[diylan]

Wow, I'm a huge idiot over here... feel embarrassed about even posting, but it's pretty funny. I spent hours looking at the build next to the schematic trying to figure out what was going on (before posting), even swapped out the batteries in my dmm, and then I figured it out:

I accidentally had the dmm set to measure AC instead of DC.  :

Just goes to show that you really need to check every last detail.

 

[kante1603]

Cool to hear ;D
No need to feel bad,we all make our more or less stupid things,but stating it here is what counts!
Have fun with your microphone,you'll love it!

Cheers,

Udo.

 

[cfierik]

Has anyone tried and had success with a U48 style build on these?

 

[micaddict]

Has anyone tried and had success with a U48 style build on these?

In this long thread there were a couple of posts about the U48 variant.

grantlack asked a question about it in reply #504:

as i look at the u48 schematic, it appears that the fig8 pattern was accomplished by simply switching a 1nF cap into the signal path between the membranes.  any reason the same approach couldn't be taken in the mk47?  the execution would be simple enough, it's just a matter of whether it would behave the same.  i tend to find omni to be less useful in my day-to-day work.

to which mad.ax responded in reply #510:

If your plan is to make a MK48 (sacrifice the omni patern to favor a figure 8 ) it's doable.
Pay attention to the U48 schematic. It's not just a matter of adding a cap and a resistor, you also need to modify the voltage divider.
In the U47 the 2M/3M extract 60V from the 105V PSU in order to polarize the front of the capsule. If you'd polarize the back with 105V, you wouldn't get a real figure 8 pattern (cause you'd need 120V). So for the U48 they changed the voltage divider  to 3M/3M. That gives 52.5V to the front, and then 105V at the back is right.
You can do the same with the MK47. Just keep in mind that, just like with a real U48, you'll loose a bit of sensitivity because of the lower polarization voltage.

Axel

And following that logic, I asked this in reply #653:

"If you'd polarize the back with 105V, you wouldn't get a real figure 8 pattern (cause you'd need 120V)."

Does this mean that without the voltage divider modification you'd get either cardioid or hypercardioid?

I never got a reply to that one BTW.

Anyway, as a start, perhaps this helps.

 

[cfierik]

I really appreciate the work quoting the text , and alas I have read the whole thread. Mostly wondering if anyone has had it work as a 48.
I will be building one regardless, but figure 8 would be far more useful to me.

 

[mad.ax]

And following that logic, I asked this in reply #653:

"If you'd polarize the back with 105V, you wouldn't get a real figure 8 pattern (cause you'd need 120V)."

Does this mean that without the voltage divider modification you'd get either cardioid or hypercardioid?

I never got a reply to that one BTW.

Kinda...
If you keep the 3M/2M voltage divider which gives 60V for the front, then 105V means a difference of 45V (105-60) to polarize the back. The sensivity of the membranes would then be different... The end result would be somewhere between cardioid and Fig8... Super or hyper cardioid...

Axel

 

[micaddict]

Thanks, Axel!


Henk

 

[Studio Mollan]

Hi guys!
Does anyone know if one PSU can power two microphones? Possible drawbacks on this?
It could be a money saver if it's possible.
Great work on this thread!!
/
Emil

 

[nashkato]

i guess so , if you account for the higher power needed.
but , if i understand it correctly , you could not power only one mic at this PS.
less current draw from one mic => less voltage drop across resistors => higher voltage at output.
Also you would have to use quite higher wattage resistors than already used .
current drawn from two mics is doubled => P=I to the square * R => P= over 8 Watts ! at the 1K resistors.  Thats quite some heat.

so i guess building one PS for two mics would be siginificantly more expensive than building 2 PS , one for each.

i hope i´m right with my calculations and assumptions

 

[zayance]

Hi folks,

I received the PCB's for the PSU, i remember having some folks that wanted me to help them by etching some,
i accidently erased PM's so feel free to contact me, not much was ordered so....

Thanks

PS: Thanks to Volker (not Silent Arts) for helping me clarify some obscure points for sending the boards to fab.

 

[Studio Mollan]

i guess so , if you account for the higher power needed.
but , if i understand it correctly , you could not power only one mic at this PS.
less current draw from one mic => less voltage drop across resistors => higher voltage at output.
Also you would have to use quite higher wattage resistors than already used .
current drawn from two mics is doubled => P=I to the square * R => P= over 8 Watts ! at the 1K resistors.  Thats quite some heat.

so i guess building one PS for two mics would be siginificantly more expensive than building 2 PS , one for each.

i hope i´m right with my calculations and assumptions

You've got a point there. That's ruled out then. Thanks!
/
Emil

 

[muffy1975]

Hi team,

people have mentioned connecting a mica capacitor in place of the capsule so to be able to test the microphone circuit.

How many mica capaciotors will i need to do the job and where should i connect them?

I presume i will need two 39pf caps.One cap will connect between the front tab and the back plate tab and the other will connect between the back plate tab and the rear tab.

is this correct?

michael

 

[david-p]

Hi team,

people have mentioned connecting a mica capacitor in place of the capsule so to be able to test the microphone circuit.

How many mica capaciotors will i need to do the job and where should i connect them?

I presume i will need two 39pf caps.One cap will connect between the front tab and the back plate tab and the other will connect between the back plate tab and the rear tab.

is this correct?

michael

Presumably you want to measure the gain of the circuit, or just to see that it is working properly.  This means that instead of connecting the capsule to the vacuum tube, you attach a sine wave from a generator THROUGH a capacitor, which can be as large as 80 pF, to the grid of the valve.  Then you measure the output of the amplifier and vary the frequency to see that it is all working and not distorting.  You will have to have the body of the mic closed in order not to pick up hum.

David

 

[muffy1975]

hey there,

yeah i want to turn on the microphone and 'burn it in' and test the voltages before connecting the capsule.

i want to be able to check if the circuit is all fine so that i'm not panicking and desoldering stuff once the capsule is connected etc

do i need one or two capacitors and where do i connect them? i won't be using a sine wave generator, just listening to the circuitry.

where is the valve grid? do i  need to know where this is????

can i turn on the microphone without the capsule or the capacitors in it's place?

thanks in advance

michael

 

[david-p]

hey there,

yeah i want to turn on the microphone and 'burn it in' and test the voltages before connecting the capsule.

where is the valve grid? do i  need to know where this is????

can i turn on the microphone without the capsule or the capacitors in it's place?

You can certainly turn the mic on with impunity without a capsule connected.  You probably cannot measure all the voltages, but the anode and cathode voltages should be measurable.

If you do a google search for the data on the valve you use, it will tell you which pin is which electrode.  The grid is the electrode to which the capsule is connected.

I cannot say more, as I dont have the circuit schematic in front of me.

 

[wave]

So... My wall voltage is a little high and because of that, I'm measuring 280 VAC off the transformer sec. I have the 230 VAC Triad.
My circuit is working correctly and I'm able to dial in 105 VDC with the test load engaged.
My question is, since my wall voltage is high and conversely my trafo secondary is high, will that make my resistors work harder?
I'm asking because I used 7 watt resistors in the circuit (10 watt in the test load portion) and man they are getting very hot. So hot I can't touch them for more than a half second.
IS it normal for them to get pretty hot?

Dave