MK7 - tube mic project

[zebra50]

Hi!

Did anyone in the UK / EU find any good value alternative chokes to the triad suggested? That one is available from RS but is US stock, so for UK buyers is subject to a £10 surcharge unless I buy £100 worth (plus tax & post). I couldn't find a suitable alternative at farnell.

Another thought - I'd consider buying a load to bring the cost down if enough people want them locally.

 

[Kingston]

again,

it's available at mouser, part number 553-C40X.

Mouser ships free of charge to EU countries for orders over 60-70 euros (or something). It's usually cheaper to order from there than from any EU store (along with digikey).

 

[zebra50]

Really? Sounds good.

I'll have a look but I'm getting stung for import duty @20% on nearly everything from the US these days. Our government needs all the money it can get. 

 

[noulou]

http://www.tube-town.net/ttstore/index.php/cat/c68_Hammond-15x-Series.html

Hammond 159Y looks like it should do the job nicely

 

[Kingston]

import duty @20%

oh damn. I forgot import duties vary a lot between countries. We only have like 2-4% import on everything, Asia, US etc. VAT will be added on everything, obviously, but mouser and digikey are simply so much cheaper compared to anything on farnell etc. EU distributors.

 

[tardishead]

Will the mic work with the Nady TCM 1050 capsule before I go ahead and buy a new K47??
If it is a K67 style copy I assume the mic will sound very bright??

 

[ioaudio]

it will work, and it will be very bright.

 

[rodabod]

Will the mic work with the Nady TCM 1050 capsule before I go ahead and buy a new K47??
If it is a K67 style copy I assume the mic will sound very bright??

And crap in my personal opinion!

 

[zebra50]

Now then Roddy, don't hold back. Say what you really think. 

 

[nashkato]

hi
anyone built the powersupply with max schematic and the kingston PCB from the blackmarket.
i use a 12V /0,5 A 160V/10mA  transformer and don´t seem to come any lower than 14 V on the heater output.
i got 15.9V after the rectifier , but still about 15V after the choke

 

[Kingston]

first obvious question: is the mic & tube connected?

I mean come on, ohms law.

And here's successful example.
http://www.groupdiy.com/index.php?topic=36857.0

I have also built and tested mine and it works fine, just no pictures yet.

 

[nashkato]

  first obvious question: is the mic & tube connected?

  no

ok , then voltage drops with tube heated , thanks .
just i ain´t got any experience with this kind of heating .

 

[0dbfs]

  first obvious question: is the mic & tube connected?

  no

ok , then voltage drops with tube heated , thanks .
just i ain´t got any experience with this kind of heating .

I'm no expert, but I believe that the heater is the same as a low-value/high-wattage resistor. I also believe the resistance changes depending on whether it is heated or not. On a cold PF86 I measure the resistance between heater pins as about 2.7 Ohms.
If I reference the Mk7 schematic (and indicated voltages) and consider how the heater portion of the circuit can be considered a series of voltage dividers to ground, I plug some numbers into various voltage-divider-calcs and parallel-resistance-calcs to see if I can make some sense of it. I think the PF86 heater should have a resistance of about 19 Ohms in a heated state.

Best,
jonathan

EDIT: PF86-Heater calculates to be about 15 Ohms (in-service/powered-on), and the PF86 datasheet confirms that.

 

[ioaudio]

because of the biasing and the fact that the tube is underheated we have to calculate the load resistor a bit different.

thats why i put volts&amps on the psu schematics:
http://www.twin-x.com/groupdiy/albums/userpics/MK7_psu.JPG

if we want to measure the psu output loaded with no mic/tube attached, we need a resistor:
ef86: 7,6V / 0,18A = 42,2 Ohm :: 7,6V * 0,18A = 1,35 W
pf86: 6,4V / 0,27A = 23,7 Ohm :: 6,4V * 0,27A = 1,7 W

-max

 

[kilmister]

Max,
as I told before I'm gonna use EF804S so I have couple perhaps lame questions but here they are:

What is reason for underheath?
Voltage in cathode should be 1,25V? How accurate it have to be and which is better little high or low bias voltage?
If I calculated right R* should be 7R5 to get 6,3V for heater and 1,25V for cathode and voltage from PSU is 7,95V.

Thanks,
Paavo

 

[ioaudio]

paavo,

the reason for lowering the heater´s emmision is noise-related.
most famous tube in this regards is the vf14, specified at 55v 50mA runnning at 36 - 1.1 = 34,9V ~40mA
but every tube is a bit different, and especially the fillament windings differ a lot.

the ef804s is specified 170mA 6,3V+/-5%
i would run it at 6,3v for a few days and go down to 6v finally.
the cathode at 1,25v is probably a good starting point, but play around with a couple of resistors to find the best working point for the tube.
please let us know about your proceedings.

-max

 

[tardishead]

Finished my mic today - thanks Max.
So the only problem I have is the dreaded hum. I have read up on hum issues in this thread especially Jonkan's. Congrats on getting the mic hum free but it seems to me the solution is a little convoluted when the problem could be a lot more simple.
I had a look at possible sources for ground loops and found a few::

On the PCB - the body meets the ground buss on both sides through the screws that secure the PCB. Possible loop?
On the original TBone mic - the body was again connected to the chassis through the xlr. So this is 3 points so far where the body is grounded.

On the TBone mic xlr pin 4 ground is also connected to pin 7 and chassis.
And on the PSU side pins 4 and 7 and chassis are again connected together.


I am not an expert but this seems prone to problems. It would be cool if we can outline a simple grounding path that would minimise hum problems so if people still have issues they can look elsewhere.

A few points - please correct me if I am wrong:

The mic cable shield only needs to be connected at one end.
The mic body should be grounded at one point only.
Chassis and PCB and signal grounds should be connected at one star ground point on the chassis.

 

[tardishead]

OK - just as I thought I now have a hum free mic.
So this is what I did.

Disconnected the cable screen on the PSU side.
Took the links on the PSU 7 pin xlr off between pins 4,7 and chassis. XLRs on PSU have continuity with chassis and therefore the chassis star ground point.
I did not change any ground connections within the mic however.

So this greatly helps the hum - but I do have a lot of noise.
Tube is noisy though but I only tried EF86 that I had already around. I will look into that next.

 

[plumsolly]

It looks like you figured out the ground issues, Emperor Tomato Ketchup also lays out a ground scheme here: http://www.groupdiy.com/index.php?topic=36857.0 Congratulations on your new mic! - Best, Ben

 

[tardishead]

What is the best noise floor that people have recorded on this thread.