recommendation for a QUIET 6au6/ef94 for Schoeps tube mic?

[strangeandbouncy]

Hi,


I have been looking for ef94/6au6 for my Schoeps CM61. I had a handful of part used Sylvania/rca etc usa made tubes in my spares. ALL are noisy, some less so, some very noisy! I am assuming that it is the tube at fault each time, since the noise varies in volume and frequency. What brand should I be looking for? I cant find any telefunken (the obvious choice) anywhere at the moment. What about GE 5 star? does the 6072 5 star magic apply to 6au6 too?

If I find Telefunken, any idea how many to buy(roughy) to select a good 'un? And how about GE 5 star?


I love this mic, but it is un-usable due to the sound of the wind howling through the track at the moment!


Kindest regards,


ANdyP

 

[zebra50]

Hi Andy,

since the noise varies in volume and frequency

I've not had much trouble using 6AU6As in some DIY mics, so I wondered if you have another problem - capsule or hi-Z cap, or even the power supply - that is exacerbated by the tube selection.

The changing frequency seems odd, as though you have an oscillator! Faulty capacitor?

You'll no doubt have thought of this, but do you have another mic you can try the tubes in, to eliminate or confirm their 'badness'?

 

[strangeandbouncy]

Hi,


  i wish i did have another CM61 to try the tube in! I am pretty sure it is the tube and not the mic. the tube I had been using was quiet as a mouse for a couple of weeks, then went very windy on start-up and settled down after 1/2 an hour or so. gradually, it started not settling down! Each tube has a sighly different character of noise. Some have more hf, some less. Some are like a howling gale, some a gentle breeze . . . None are acceptable for recording an acoustic, no matter how amazing it sounds . . .


  I remember reading somewhere that you have to select from a bunch of bottles for 6au6, (or was that 6ak5)? for an early Sela mic . . . .


        Kindest regards,


          ANdyP

 

[gridcurrent]

What brand should I be looking for? I cant find any telefunken (the obvious choice) anywhere at the moment. What about GE 5 star? does the 6072 5 star magic apply to 6au6 too?

If I find Telefunken, any idea how many to buy(roughy) to select a good 'un? And how about GE 5 star?

7543 ?

 

[zebra50]

Hi Andy,

I'll have a look in the spares box when I get to work - I had some GE and RCA 6AU6a, which worked OK at least in a DIY sony C800 style circuit. I'd be happy to post you a couple to try.

I don't recall the Sela discussion, but I remembered that there are some suggestions in the discussion of analag's 6AU6 tube mic...

http://www.groupdiy.com/index.php?topic=22825.msg435513#msg435513

The final suggestion seems to be the 6AH6. I just checked the data sheets and at a glance they look similar. However, the 6AH6 has a 450mA heater, so whether it would work for you would depend on your power supply.

http://tubedata.itchurch.org/sheets/093/6/6AH6.pdf
http://tubedata.itchurch.org/sheets/020/6/6AU6.pdf

Stewart

 

[strangeandbouncy]

Hi Stuart,



    if you happened to stumble across a tube you have, that would be most kind indeed!



      Many many thanks!


      ANdyP

 

[Gus]

I bought a number of 6au6 and 6au6 like tubes when I built the C800g type circuit.
I read what I could find, and built a burn in circuit for 4 tubes at a time.
100K plate, 1K cathode, 10 meg grid to ground for each tube.
140VDC IIRC B+

Next I made the heaters DC and switchable between around 6.4 VDC and 5.7 VDC.  7805 with 2 diodes in the ground leg and 1 diode in the ground leg.

I would power the tubes at 6.5VDVC for about 4 hour then power them at 5.7VDC for 4 hours
did this about 4 times for each group of 4
IIRC I found the two heater voltage burn in at Steve Bench's site

I found the russian 6au6 type to often be less noisy maybe something to do with the bigger bottle.  The 1 mullard  I bought was a noise generator(not a good sample size)  some GEs were not that noisy.  I will look for my notes

If I was to do this again I might use a variac for the heater and use higher AC for the upper voltage and lower AC for the lower voltage

 

[strangeandbouncy]

HI Gus,



    many thanks for the fascinating info. Seems a tad involved, but all in a good cause! Any explanation for the procedure? I'd love to know.

    I am limited to slender varieties of 6au6/ef94, due to the very narrow body of the mic. Hard to get a cigarette paper round a tube, so no Russian tubes, I fear!


  Kindest regards,



      ANdyP

 

[emrr]

the tube I had been using was quiet as a mouse for a couple of weeks, then went very windy on start-up and settled down after 1/2 an hour or so. gradually, it started not settling down! Each tube has a sighly different character of noise. Some have more hf, some less. Some are like a howling gale, some a gentle breeze . . . None are acceptable

That doesn't sound like any tube noise I've ever experienced.  I don't know this mic; can you pull the capsule and ground the grid for a listen?  I'd wonder about almost every other component BUT the tube, if I had this experience.  In experimenting with a DIY tube Oktava preamp conversion, I found my breadboarded experimental PSU to be the chief source of noise.

Good luck.

 

[strangeandbouncy]

Hi Doug,


    this model has a detachable capsule/head. I am pretty sure it is not the capsule. It has recently been re-skinned by Schoeps. It does not "collapse" with breathing.

  Kindest regards,


          ANdyP

 

[emrr]

Okay.  Short the grid anyway and have a listen.  Exercise the tube in the socket, have a listen.  Etc. 

 

[Gus]

What is not clear about the burn in circuit?

 

[riggler]

Gus, why start with a higher grid voltage?

6.5VDVC for about 4 hour then power them at 5.7VDC for 4 hours

Or wait, let me try to answer this myself... you start with a higher grid voltage not to "break in" the grid, but to provide more resistance to current flowing from cathode to plate... So you're essentially breaking in the plate, right?

Would the same break in procedure be advisable for the submini tubes like 5703, etc??

I want to build this device. What did you mean here?:

7805 with 2 diodes in the ground leg and 1 diode in the ground leg.

So ok, a 7805 regulator, I get that, but the # of diodes? I'm gonna go grab the datasheet...

 

[Gus]

Grid voltage?

Whats so hard to understand?

A cathode biased circuit fragment for each tube (100K plate,G2,G3 to B+)   (1K cathode to ground)   what ever grid to ground resistor you want.

Apply B+ and cycle between heater voltages for hours to days

Test tubes in the microphone

oxides

 

[rodabod]

Andy, what dimension are you looking for? I could measure a new spare I have here.

 

[strangeandbouncy]

Hi Roddy,



  normal Sylvania, RCA, Westinghouse, and I am sure Telefunken, fit snuggly, but I could not insert a Russian one. It was too fat and too long!


    Kindest regards,


      ANdyP

 

[zebra50]

but I could not insert a Russian one. It was too fat and too long!

Hi Andy,

I haven't forgotten this, just took me a while to dig out the right box of junk. I found a GE which I recall was OK, and another unbranded one.

I'll pop them in the post tomorrow.

Stew

 

[strangeandbouncy]

Hi Stewart,


    No need to hurry! Whenever suits. I am afterall, in your debt . . .


      kindest regards,


        ANdyP

 

[zebra50]

Not at all - it's just DIY karma 

 

[strangeandbouncy]

  diy Karma is just about the best there is . . .