MK47 PCB tube mic kit - build thread

Availability and pricing see my WM thread : http://www.groupdiy.com/index.php?topic=35667.0

also check chunger's build, super nice & detailed: http://www.groupdiy.com/index.php?topic=47112.0



Schematic:



PSU Schematic:



Kit:





break away the rounded pcb part, use file for edges

solder all transformer pins

100K Resistor



30K Resistor





dont forget to trim the ends of the transformer pins

Diode

bend for upright position

check orientation - just like this



1µF 160V

goes to this side







27R Resistor



be careful not tuse too much solder here

3Mohm





2Mohm





trim wires flush to pcb

your cutter will not like this but it works

squeeze gently

like this

bend the contacts down a bit

align with silkscreen







use lots of solder - mhm that smells good







the relais

solder two pins while keeping the relais aligned horizontally

use some glue to secure the relais

trim and file the rounded part

bend the sockt´s pins together, gently - UPDATE: insert the tubes before soldering the sockets, twice in and out to relax the contact strength a bit.





check that the socket sits flush before soldering. solder two opposite pins and check orientation again if needed.



fit the small glass isolators with some super glue



try to not let any glue run into the glass tubes

assemble



solder one of the pads



check for proper orientation and angle before soldering the rest of the pads.



all pads must be soldered

cut-offs from resistors

bridge







100Mohm (yeah thats many ohms)

the high impedance part of the circuit is build floating in the air.

the tube

fit the  tubes now

so the grid pin of the socket is straightened out

the other end of the 100Mohm comes here

check that the pin golden pin is centered and does not touch the pcb

take another piece of wire to build the bridge between the two pins





the second 100Mohm resistor



cut the wire short enough before soldering - should not touch the relais



bend the 10Nf like shown

trim and insert





extend the bridge from the two golden socket pins (grid) to the according relais pin



and extend to "front"



bridge the rear



so that it does not touch the cap







the "heater dropper" - it needs to touch the mic housing for heat transfer like the original U47.

it goes here BUT DO NOT SOLDER IT NOW



An example for mounting the Kit and providing the required heat transfer (t-bone sct700 shown)

HOly Cow!  Beautiful.  Excellent photo documentaaaation and illusstratttion....  ugh I can hardly type...  What is this?  What are the two tubes?  That's a CCCP relay?  I thought it was a cap!

Excellent!

EDIT: I'll answer my own question: http://www.groupdiy.com/index.php?topic=35667.0

Somehow this project flew completely by me -- I didn't know it existed.  I even looked in the White Market and couldn't find it, until I did a search.

And I see now that IS a CCCP cap -- the silver thing is the relay.

6028 tube
Eballs seems to have quite a few up for auction .
Anyone want to go in a a case of 100  ;D

Dude! Awesome!!!!

While we are at it, is it appropriate to talk about capsules in this thread?

I'm thinking Thiersch M7. Is that what you'd recommend max?

How much are they?

JD

Matthew Jacobs said:

Dude! Awesome!!!!

While we are at it, is it appropriate to talk about capsules in this thread?

I'm thinking Thiersch M7. Is that what you'd recommend max?

How much are they?

JD

Click to expand...

+1 That's the next thing on my mind in addition to how the hell to make more money to buy all these DIY goodies! And then how to finish the ones I have in the pipeline...

Matthew Jacobs said:

Dude! Awesome!!!!

While we are at it, is it appropriate to talk about capsules in this thread?

I'm thinking Thiersch M7. Is that what you'd recommend max?

How much are they?

JD

Click to expand...

EXPENSIVE, but as usual quality costs, 349.68EUR, including PP fees, Holder and shipping in Europe, and they had a price break of 10% on the Bluline  , 25% on the redline, at that time, IIRC.

Thanks Zayance

I was expecting the €300-€500 price tag. Nice to have a more accurate price.

If these are the real deal and the best, then I think the price is not to bad...

Skylar's mic body, MK47 kit, PSU and this capsule, puts this mic in the £1000 price range. That's a 1/5 of the current second hand price for a real U47 right? Not to bad......

Has anyone around here tried Beesneez capsules?  I've tried contacting them on the differences between the K47 variations with no response.

Ioaudio can you comment on the sonic differences between the mk7 and this 47?


Regarding the beesneez capsules I'm going to be putting the k7 capsule in my pair. Ben will tell you his capsules win hands down against any other capsule. He says the same about the mics.

I WANT ONE!
I WANT ONE!
I WANT ONE!
I WANT ONE!
I WANT ONE!
;D ;D ;D ;D ;D

Max, great documentation!! I'm really looking forward to this  ;D

Over in the white market thread, we had been discussing the power supply a bit.  It was rightly noted that that wasn't the place for such a discussion.  Perhaps now is the time to pick it up again since we'll be building soon? 

Need to find suitable transformers - there were some good thoughts on perhaps stringing two back-to-back (thanks Kingston).  Also there are the Hammond transformers, although they are expensive, heavy, and would have a (large) wasted secondary - might make more sense for 110V since it seems to be half the price of the 220V version for some reason.  Don't feel so bad about the waste if it's not so expensive.  But of course I'd still like to find something more appropriate.

Axel made a good point about adding a choke like the original PSU.. Is this worth thinking about?

Curious if anyone has given all this any further thought in the last few weeks.

  Brian

No one asked how the tubes will age?

I would want to know how the tubes age.  I can understand they can be matched at the beginning but how do they track with time?

One thing I would also match is the filaments voltage drop at a set constant currents so both filaments work about the same if they are wired in series.

This all might not matter much and it might be fun to use mismatched tubes but I am suprised no one asked yet.  This was one of the first things I noted when I read about the use of two separate tubes in ||.

I don't think it matters much if the tubes are matched or not. They are paralleled and every mismatch will be averaged. It is not pushpull or something. The filaments do not need a perfect match either, they have quite a large operating range. I guess, as long as the tubes are reasonably quite and within their normal operating range, things will be fine, even with aging.
Tobias

In order to know how the tubes will age, I guess our best bet is to build the thing and see by ourselves...
I tend to believe that the benefits from paralleling should overcome the drawbacks from the tubes aging different (if they do...)

Matching filaments sounds sensible, but I bet that those tubes being military grade and from the same batch, one shouldn't find much difference between samples.

Those tubes being dirt cheap (for now ), experimenting with voluntary mismatched ones could indeed be a fun, and affordable, game.

A question to Max: I wonder about the 27 ohms resistor whose value is different from the 29 ohms of the original U47 schematic?
Did you choose 27 in order to bias the tubes a little higher than 1.1V? Or is it because carbon comp being prone to drift, you found out that 27 ohms actually measure more in the 29 ballpark?

Axel

Unbelievible tutorial! If someone messes this up he must be a wizard of some sort.

What comes to capsules, I'm very happy with my Thiersch red line SWT7. Beezneez capsules are very interesting also. They have been approved by Klaus Heyne himself to be the best K47 copy out there. I think it was the K7 variation he evaluated.

nashkato said:

no big problems  putting the kit together , but one thing . fitting the tubes into the new sockets after assembling  most of the kit .
an advice from what i experienced which hopefully may help a little .
put the tubes into the sockets before soldering the sockets to the pcb (take care not to break the solder pins , though.),

Click to expand...

A very wise tip indeed, it's hard to get the tubes to fit and even harder to get them out.
If you have a 7-pin tube in your junk box, use that instead of risking the 6028s.

thanks, i added the info to the first post.

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.

Aging: we are all getting old  i published the tube type so no prob.

27R - the heater string operates at a slightly higher amperage - around 44mA, tube bias around 1,2V

Maybe someone should put together another Thiersch order ???

FerrariT said:

Maybe someone should put together another Thiersch order ???

Click to expand...

Oh yeah...

FerrariT said:

Maybe someone should put together another Thiersch order ???

Click to expand...

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