MK47 PCB tube mic kit - build thread

[bobine]

the capsule.

I left the leads sticking out of the glass tubes about a quarter inch or more. I passed each of the three wires from the capsule through the three holes in the top plate. I decided which side was to be the front and attached that wire to the lead marked "front." Ditto with Rear and the Center plate.  I just placed the wire against the lead and quickly soldered them. I may swap front and back soon to hear the difference. There may be a better or recommended way of doing this... but mine works beautifully. Dead quiet.

 

[kante1603]

I'm not finding the 14K 1-watt resistors at Mouser.  Are 1-watt really necessary in this position? 

Thanks,
--
Don

Hi Don,

had the same issue while ordering at Mouser-they have 28k 1 Watt 1% metalfilms so use  two in parallel-I will do so

Best,

Udo.

 

[Winetree]

Finished my 2, now they are resting in their cases
until I can build the power supplies.

 

[idylldon]

Hi Don,

had the same issue while ordering at Mouser-they have 28k 1 Watt 1% metalfilms so use  two in parallel-I will do so

Best,

Udo.

Thanks, Udo.  I thought about paralleling a couple of resistors but thought I'd ask to see if I had overlooked something in the Mouser catalog.  I'll go ahead and do as you suggest.

Cheers,
--
Don

 

[JamesW]

What voltage range can the pattern relay tolerate?

Thanks

 

[JW]

I'd like to know too.

With the p.s. loaded (with a 2k4 resistor, closest I had) and the pattern switch thrown to break the 12k/14k network, the B+ voltage goes from what was calibrated at 105V (when the switch was throw the other way) up to 111V. Just checking to see if this is normal.

 

[Sredna]

You simulated the mic with 2,4 kOhm right? Then you also have to simulate the relay, maybe with 12kOhm? 

 

[Steph.B]

I just received my kit today, thanks Max !

One step closer from the finish line...

 

[androidtube]

Hello guys, greetings to all from a newbie like me!
Someone can tell me if this kit is still available? Thanks.

 

[JW]

I don't understand what's up with my mic.

I had thought the 2.4K test resistor was an either/or thing to simulate the mic. I had thought it was supposed to be switched out of the circuit when the actual mic was loading the power supply.

However, I'm getting 290 volts or something at the B+ in the mic when the 2.4K resistor is switched out of the power supply circuit. Switch it back in and I get 105V at B+ in the mic.

Second problem is, the 1.5K heater resistor measures fine at 1.5K ohm, but it doesn't drop the voltage at all. I even tested this out of the circuit. I'm guessing I probably have a couple dead tubes now too because they were getting upwards of 290V at the heater. I've yet to see them light up though.

Anybody have an idea what's going on?

 

[Sredna]

Why do you use such high voltage rated transformer?

What´s the voltage (without mic but with 2.4K resistor) after the rectifier?

 

[bobine]

Can you take photos of your connections at the mic PCB?

 

[kante1603]

However, I'm getting 290 volts or something at the B+ in the mic when the 2.4K resistor is switched out of the power supply circuit.

Don´t know what transformer you have used,but in case of having 200 Vs on the secondary it looks a bit like it is the (DC?)-voltage right after the rectifier.200Vac x 1,4= 280 Vdc after the rectifier,meaning "290 Vs or something at the  B+ "...definetely something strange going on.
Maybe you should check your psu first (did you use max´s schemo?).

Best,

Udo.

 

[JW]

I used the Triad VPT230-110. http://www.mouser.com/ProductDetail/Triad-Magnetics/VPT230-110/?qs=wkKrz7WmEgNNNgyT8w4YqA%3d%3d
I believe the problem is that I hooked up the secondaries in series, rather than parallel. Big dumbSh*t thing to do if that's the issue. I'll have to change the p.s. resistors. Who knows what else.

After the diode rectification (p.s. loaded with 2.4K resistor) I'm getting 327V.

To get the 105V (loaded) (secondaries wired in series) I needed a total of 3.63K ohms resistance, plus the 500R pot to fine tune.

A paralleled secondary on the power transformer is rated to yield 115VAC @ .22A. I'm guessing that would have been the way to go.

What I don't understand is, if I was able to reduce the voltage to 105V with the 2.4K test load (regardless of wiring the secondaries wrong?) , then why don't I get something close to that at the mic, which is supposed to represent a 2.3K load?

 

[bobine]

I just took my PSU apart to check the voltages.

I have the same Triad transformer. I have the secondary wired in series.

With test load: 103vdc
Without test load: 285vdc

With test load and pattern switch: 108vdv

With mic (no test load): 106vdc  (the 500R pot is not quite enough!)

Pattern switch makes no difference with mic in circuit.

--- where are you measuring voltage with the mic in the circuit?
--- check your cable wiring to see if the B+ is actually getting to the mic PCB

 

[JW]

Hey Bobine, thanks for checking that.

The B+ voltage is definitely getting to the mic pcb. That was one of the first things I checked. I'm checking at the B+/1.5K resistor tabs in the mic.
Actually (duh), disregard my previous post about being worried about the transformer secondaries. The schematic calls for a 200V secondary, so what I did was correct (series wiring)

But does it make sense that I'm getting 327Vdc exiting the diode bridge?

 

[bobine]

The PSU doesn't seem to be the problem since you have the right voltages with (105v) and without (290v) the load. The mic isn't acting as a load, apparently, so the problem seems to be there. The 1K5 heater resistor doesn't seem to be in the circuit. Bad solder joint? 

 

[kante1603]

But does it make sense that I'm getting 327Vdc exiting the diode bridge?

Yes,absolutely!

Looking at your transformer it is 115V x2(in series)=230V.
230V x 1,4=322 V (after the rectifier).So that´s correct.
Very kind of bobine of comparing his psu to yours!
So the psu seems to be fine.

Checking the solder points arround the 1,5k is a good idea as a next step.

Best,

Udo.

 

[JW]

Okay, so I've checked and rechecked the 1.5K resistor. I verified that the solder joints are fine by checking downstream on both sides. Continuity is fine, and it's measuring fine at 1.48K ohms.

I also took a couple resistance measurements at other spots. Not sure if these are the right places to be looking yet, but here goes:

1. I'm getting 32K ohms between the 1.5K resistor and where it connects again (after coming out of the tube) at the 27R resistor.
2. I'm getting 11.5K ohms between B+ and ground
3. I'm getting 107K ohms between B+ and where the 1uF output cap connects to the input of the BV8 transformer.

 

[bobine]

Was the mic disconnected from the PSU for those measurements?

Check to see that the B+ is reaching the heater pins on the tubes.
Check those right angle solder joints and the tube socket joints.