MK47 PCB tube mic kit - build thread

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I think a Thiersch group buy was attempted/discussed a couple of years ago and if I recall correctly the price break was not enough to make it worthwhile.

Best, Ben
 
fazeka said:
Yeah, I was wondering what kind of price break (similar to Ben's) we could get from Thiersch, en masse...  ???

I didn't find any info on BeezNeez capsules price break. But if there's a appreciable volume discount how about K7 group buy?
 
It's working!  ;D


After initial problems with the 1uF MKT capacitor, it sounds quite nice also with a chinese K47 style
capsule.

The MKT had almost a total short, maybe 160V is underrated? Had some peaks of 120V during the finetunig of the PSU.

The PSU is very simple: 160VAC transformer, CRCRCRCRCR C=68uF R=500R 10W

The resistors are getting to hot to touch but not to melt plastic, the mic is warm and cosy...

So now waiting for a Beesneez capsule, Skylars housing, burning in the tubes, testing, testing, testing....

Thanks Max for this project!

Best,

Anders
 
Sredna said:
The MKT had almost a total short, maybe 160V is underrated? Had some peaks of 120V during the finetunig of the PSU.

oh wow - sorry - i can send a replacement, let me know.
160 vdc should be all right (like the original), just bad luck with the cap.

-max
 
Now continuing with burning in of the tubes.

From the beginning they were quiet with occational pops.
Now, after maybe 6 hours, the pops are still there and are
most frequent at startup time.

Max, what are your experiences with these tubes?
 
let them burn in for at least 24 hours.
how did you solve the heater-droper *s heat flow to the housing?
which housing is in use? can you give some voltage readings?

-max
 
Burnin, burnin...  ;D
Still some snap, crackle and pops but getting better...

Im currently using a SE Electronics Z3300A housing, it's just temporary in the waiting for Skylar's housing.
The heater dropper was soldered with the legs slightly bent as a bow acting as a spring, it's
not that much pressure applied but the housing gets warm and cosy so it works.

Im also thinking about soldering a clip inside the housing tube or maybe install screw connectors on the
PCB and mount the resistor permanent inside the housing.

My readings:

Supply: 105V +/- 0.2V (Variations of +/- 1Volt over several hours, maybe house power variations?)
Heater : 34.8V (Between heater and dropping resistor)
Cathode: 1.3V (Between 27R resistor and pin2/7)
Plate: 43.6V (Obviously too high)
Capsule: 56.3V (At the voltage divider 2M/3M, cant get readings at the backplate)


The plate voltage is about 10V too high, checked the 100k and 30k resistors, they're ok.
Changed to a fresh set of tubes, getting about the same readings but plate voltage is now 50V!

What are your readings Max?

Best,

Anders



 
your voltages are ok, including the plate.
do not mount the heater dropper on the transformer*s side.
two solutions: either drill&tap the base and mount the resistor or use some heat conductive glue.
-max

 
Its much better but still some occasional pops now and then.

I also changed tubes and the new ones were considerably quieter
from the start and after a shorter burn time almost pop free.

Am now mounting the PSU in a proper enclosure so burn in is on halt but I feel confident
that out of the 6 pairs of tubes I have, Ill find at least one or two working pairs.
 
ioaudio said:
Psu - i am currently experimenting with choke / no choke.
The original NG psu was made in two versions - with and without the choke.
Triad 553-C-3X should be nice and cheap.

Hi max, any conclusions r.e. the choke/no choke experiments? 
 
yes:
1. the choke helps filtering crap from the mains and is a better solution for "poluted" places.
2. unloaded voltage does'nt rise as high as with the resistor-only circuit.

-max

 
the triad inductor series offers different types.
max suggested the 10H , 500DCR , 50mA .
there are 7H / 10H , 240 / 270 DCR , 75 /90 mA inductors , too.

any comments on using another one than the 10H/50mA inductor ?






 
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