MK7 - tube mic project

[pH]

I've been using my MK7 with the stock capsule that came in the Tbone and it's been working without problems after sorting out a hum issue. Just got one of Dale's M7 capsules and did the swap. Nothing but hardship and head scratching since. 

I'm getting either nearly no sound or loud rumbling until the voltage drains. I assumed I had a problem with the position voltages, but they check out correctly on my meter. I have a few questions that hopefully you more knowledgeable mic folk can answer:
1) does an M7 capsule have a polarity, ie. front and back? It looks pretty identical, and Dale uses the same color wire for both sides.
2) shouldn't the front diaphragm work in cardioid by just leaving the rear diaphragm disconnected?
3) can you damage a capsule by applying the voltages in the wrong polarity?

Sorry so daft, but it's my first mic, hopefully something simple I'm overlooking...

-paul

 

[rodabod]

The capsule should operate exactly the same as the crap Chinese one. Remember you need some sort of contact in touch with the M7 (say a metal capsule mount) in order to connect the backplate (whereas the Chinese model had a wire screwed on directly). Make sure your mount has very high insulation from ground.

 

[plumsolly]

As Rodabod says: make sure you have ALL 3 connections to the capsule made: backplate, front, and rear diaphragms. If your old capsule was cardioid only, then it would only have 2 connections (backplate and front diaphragm) Sorry if this is obvious, Ben

 

[pH]

Yep, used one of Mark's really nice mounts and soldered the base wire to the backplate connection. I even took electrical tape and jammed it on to the bottom of the rubber mount after Mark mentioned that the rubber in these made a poor insulator.

Is that loud thundering sound the diaphragm being pinned to the backplate? Is this harmful? I'm really paranoid about ruining these beautiful new capsules.

Problem seems to be with the front diaphragm, as cardioid doesn't work but the back diaphragm works in 8 until it starts making the holy racket...

 

[rodabod]

If you haven't already, check your polarisation voltage before the large value polarising resistors, and check there isn't any flux/dirt/possibilities for leakage where you soldered your capsule (and any other area before the grid).

 

[bradb]

Darn....

I bought and wired up the Loralin CK1589 Mouser: 105-13571....

which is make before break...

so my parts list needs to be changed... perhaps we need the Loralin: CK1458 Mouser: 105-14571

YET....

Looking at the schematic, why is the "break before make" necessary?  I'm looking at what would happen as we switch with make before break.  We'd short some resistors and the current would increase from 0.6mA to 1mA briefly, but where's the harm...?

 

[bradb]

Lemme show you guys my tube sockets..

really its quite ridiculous... opening up the hole in the pcb is not an option, it would seriously weaken the PCB.

sorry for the huge and poor quality pics...

I saw max has a nice small one... can these be sourced in the US?

 

[ioaudio]

brad,

very good solution. check if you still have enough space for the output & filter cap.

Does it matter if the pattern selection switch is make before break or break before make?

i need a break-before-post. i was wrong, it doesnt matter which switch you take - sry for the confusion.

-max

 

[Jonkan]

Ok, so ill go for 16v 10000uf caps then, and hope i can fit them in the generic psu.

Is panasonic NHG series an ok alternative here, or should i go for something like panasonic FC or similar with lower esr?

Max, do you have any pics of your modded generic psu innards? Would be nice to have a look at it. Is it really cramped in there, and hard to fit the mods?

/J

 

[ioaudio]

in the lower half of this page: http://www.groupdiy.com/index.php?topic=30124.220

 

[e.oelberg]

I'm getting either nearly no sound or loud rumbling until the voltage drains. I assumed I had a problem with the position voltages, but they check out correctly

is the mount insulated from the mic body ? I don't know the mount, but for instance the FLEA capsule mounts need to be insulated from ground. What I did is,
I swapped the metal plate you mount the capsule holder on with a Plexiglas one, this gives me know a very good insulation

 

[bradb]

This may be a better 9 pin socket for NA builders.

http://thetubestore.com/sovt9st2.html

Requires a 3/4 in hole

the hole in the MK7 pcb is 11/16 in.

 

[MATTI]

I´m very happy now that I got mine, in a 6 months or so I´ll have a special mic up and hopefully working.
I have an extra Dale M7 but it might be I´ll try Australian.
No hurry now that I against all the odds had the money for this moment, last time all were sold in hours... 

Matti

 

[pH]

Finally solved the problems I've been having with the non-functioning position switch and excessive rumbling noise. The rubber mount on these chinese mics might look nice and fit well but it can't be trusted as an insulator (it doesn't have to be, with the stock capsule sitting on a delrin saddle). I even had isolated it with electrical tape but apparently this isn't suitable for high impedance isolation either. Eventually took teflon thread tape for plumbing and wrapped both of the metal couplings until they looked like a mummy before jamming the rubber mount back on. Also put several layers under the brass capsule mount and on the threads of the screw itself. Now everything works like it should!

Thanks fellas for talking me through this 

 

[MATTI]

I use LeWilson copy of original mount and Tiercsh has one also, they work and look good also for the extra pling at the unsecure artists eyes

Matti

 

[bradb]

Guys,

What is everyone using for their tube sockets? ??? ??? ???

I don't think the smaller one I found and linked to above is going to work.  http://thetubestore.com/sovt9st2.html  if the socket is inserted from capsule side, and the metal retainer is mounted on the other side, whats going to hold the socket down?


Any help is appreciated!!!

I know at least 2 people have finished their mics...  Max what is this socket: http://www.twin-x.com/groupdiy/albums/userpics/13~0.jpg

 

[ioaudio]

thats a old "preh" socket, made in germany.
i had them in stock, but run out, so i will try to find a source.

 

[bradb]

Thanks Max...

If anyone else has found a good solution too, pipe up...

 

[Jonkan]

So im sitting here trying to order the parts needed for this build and have another question. Is it better to order a new ef86 tube, or to try and find a nos ef86/pf86 ?

Any good european sources besides ebay for nos tubes?

I recently bougth a box of 4 ef86 for my g7, and only 1 of them had an acceptable noiselevel. So quality seems inconsistent for sure.

I have no clue how newer tubes compare to older ones.

Any help is gretly appreciated.
/J

 

[Jonkan]

Im guessing this isnt an easy question to answer. Ill think ill try to find some nice nos ones, in general everything was better before, right?

Has anyone ordered the sc700t mic from thomann yet?

Im wondering if the t.bone sc700t logo is engraved into the mic body, or just painted on. If possible it would be nice to get rid of it so that people wont question me all the time how good these mics really are (hopefully). You know what i mean...

/J