MK7 - tube mic project

[nhaudio]

Martin K,

thanks for that page!
After listening, I think I´ll go with the mylar.

cheers

nhaudio

 

[585musicman]

When is the next batch for these kits?i want one bad!

 

[pH]

Hey Max, I'm trying to cobble together your PSU with stuff I have in my shop and I don't have the same values of capacitors on hand, and I'm wondering how much can I deviate from the uF values on the heater circuit? Could I get away with 3000uf and 22000uf caps in place of the 4700uf and 10000uf? I'd like to understand how this would affect the circuit.

Thanks,
Paul

 

[ioaudio]

hi paul,

yes that should work - bigger is better - less hum.

-max

 

[mmmyesm]

Are there any of these kits left? I see on the previous page a link to follow for the new batch but it is either missing or off limits... If you're out will you be doing more?

 

[ioaudio]

there will be more sets in the future, i will post the link to the white market thread/feeler soon.

-max

 

[ioaudio]

update:
check the white market for a feeler for new sets. i think about making a set with the tube included, and probably including other parts as well.

http://www.groupdiy.com/index.php?topic=35667.0

-max

 

[Gus]

Question

How many buyers have built a microphone with the kit and have no hum and/or noise problems?

 

[pH]

I changed mine to use the self-bias method after I got hum in the original bias using the modded Thomann psu. Max mentioned that that cheap power tranny isn't up to the demands of this circuit, so I've got some Edcors ordered to eventually build a proper psu - I'll report back soon.

Using self-bias it's dead quiet, a stellar mic that I've used non-stop since. Wiring it this way doesn't feel like a compromise, I could leave it this way and be perfectly happy (but I'm a tinkerer!). I think my pal imo finished the 'real' psu with the Edcor and a choke and resolved the same hum problem that I had using the original wiring.


-Paul

 

[Jonkan]

I changed the grounding in the thomann psu and got my mics totally humfree. Cant remember exactly what i did, but i think i described it in a post in this thread.

/J

 

[imo]

I did make the PSU with the choke and it sounds great. I'm still waiting on getting a new mount to solve the small amount of leakage from the capsule, but the mic is hum free. The problem was in the original PSU design
Ian

 

[EEMO1]

help me to understand...

any pros / cons between the two different approaches to the biasing?

 

[ioaudio]

david bock summed it up:

reasons for fixed bias:
1) no overshoot
2) no capacitor dry out
3) no bias perambulation
reason against it:
1) tube variance

 

[EEMO1]

ok, fixed bias it is. 

 

[ioaudio]

it´s not necessarily better - there are many nice sounding self bias mics around.

 

[Jonkan]

My first impression, although very unscientific and probably alot of placebo due to expectations etc is that the fixed bias mic sounds a little "older" if you know what i mean. A bit more like you would expect an old tube mic to sound, where the self bias sounds a bit more transparent...

But Im not willing to bet my money on it being true either, so take it for what its worth (not much really  )

/J

 

[3nity]

I wonder why no one has finished a mic??
Never seen pictures anyway!

 

[bradb]

A few have, if I remember right, search this post.

i'm waiting on the U47 clone bodies to finish up......


looong project...

 

[seavote]

david bock summed it up:

reasons for fixed bias:
1) no overshoot
2) no capacitor dry out
3) no bias perambulation
reason against it:
1) tube variance

tube varience.? can someone explain? from 1 tube to another used in the mic? or is this something akin to "sag" in a tube rectified tube amp? still asking "newbie" questions after 3 years at this.

 

[rodabod]

tube varience.? can someone explain?

With self-bias (cathode bias), the bias/operating point is self-regulating because as more current flows, the greater the magnitude of the bias voltage formed across the cathode resistor. This pushes the valve closer towards cut-off and so it keeps operation in check.

You may find with fixed-bias that you have to adjust the bias a little to achieve a good idle point for a given valve (of a given health and quality). This valve's characteristics will change with age and you may find the bias needs to be changed to compensate. Same could apply if you changed the valve. Some valves will be fussy and might not like being operating at a given bias... This is where sometimes self-bias can save problems.