MK47 PCB tube mic kit - build thread

[plumsolly]

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

 

[nashkato]

  http://www.tube-town.net/ttstore/product_info.php/info/p2192_Ringkern-Netztrafo-30VA-Preamp.html

 

[pasarski]

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?

 

[Skylar]

how about K7 group buy?

http://www.groupdiy.com/index.php?topic=41451.0

 

[pasarski]

how about K7 group buy?

http://www.groupdiy.com/index.php?topic=41451.0

Thanks! I've totally missed this.

 

[Sredna]

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

 

[ioaudio]

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

 

[Sredna]

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?

 

[ioaudio]

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

 

[Sredna]

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

 

[ioaudio]

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

 

[Sredna]

Ok, thanks! Then I can stop looking for Ra of VF14

Heater dropper is not on the transformer side. Why? (resistor is wirewound=inductor?)

 

[ioaudio]

we do not want to grill the transformer 

 

[Jim50hertz]

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

Still burnin' and noisy, or better? 

 

[Sredna]

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.

 

[Jim50hertz]

Good news Anders. 

Seems like it's definitely worth having a stock of these around.  Thanks.

Jim

 

[Jim50hertz]

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? 

 

[ioaudio]

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

 

[Jim50hertz]

Thanks Max, I will go with that then... Do I simply omit the first two 1K resistors and sub the choke in?

 

[nashkato]

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 ?