M49 build - changing tube type

[kpearsall]

I've had wind-like noise in mics like that before. In one case it was a bad cap, though I can't remember what that cap was doing right now. In the second, which is an SM81, it's a grounding issue. Still comes and goes. That mic's been torn apart so much over the years that those little pressure connections are getting worse and worse.

 

[rstreber]

Thanks for this!  Heater voltage is right at 6.3v.  I've been thinking that this is a tube issue because the noise generally goes away for anywhere from a week to a month after installing a new tube, it has always come back either intermittently or consistently after a while. 

However, it's certainly possible that I've been looking at the wrong thing, so I can try isolating the noise to another element.  I'll swap out the capsule with another good M7 that I have to see if by chance it's the capsule, but this is a new Thiersch Blue-line M7 which (I would hope) isn't failing already.  But I guess it's certainly possible. 

The coupling cap is a Mundorf Mcap, and all other caps are new, high-quality film types.  For the moment, I'm going to undo what I did a couple days ago (putting in the bias trim pot and the 600pf cap from plate to ground), try to get the plate voltage to spec, and see where that gets me.

Thanks again for all of the suggestions!

 

[riggler]

Have you checked out 5703 tubes?

 

[0dbfs]

re: wind noise.

Also try out the breath test on the suspect capsule. I recently had one (it was an MK7 w/PF86) start making some wind noises on some recordings and was able to reproduce the discharge sounds with the breath test. Swapping the tube (not the capsule as you would expect) made the sounds go away and I haven't been able to reproduce since. On another DIY mic the breath-test revealed a definite capsule issue and resulted in quite loud discharge noises and even cut-outs. More thunder and lightening than mere wind.

I have also had discharge noises in tube mic's which went away reliably after cleaning the hi-z section with a stiff "acid-brush" and fresh 99.9% alcohol. One time I had to swap the cap between front/back capsule to make the noises go away which was a styro and another time the hi-z backplate resistor (which was a NOS 100M TRW) required swapping. Maybe it was the rear diaphragm polarization resistor. I don't remember but these "simple circuits" can be a bit of a bitch at times when these noises creep in.... Especially when they are sporadic.

Cheers,
jb

 

[rstreber]

I have some 5703s lying around but haven't tried them out yet. Any particular advantage to them over other candidates? 

Actually, the latest with this mic is that I replaced the 100k plate resistor with a 56k to get the plate voltage up to 41.5v.  I also spent some time going through and doing a lot of cleanup work, including de- and re-soldering all of the ground and hi-z junctions to ensure that everything was solid.  I also replaced parallel the 5k trim pot / 5k cathode resistor with a 1.5k resistor and removed the 600pf cap from plate to ground.  Finally, for the hell of it I decided to reinstall one of the EC1000s that was previously installed (but which I thought had gone noisy.)

Well, for the moment it seems to be nice and quiet (and sounding great.)  I'm not sure if the noise was coming from one or more factors that were address by the things I mentioned, but I'm starting to suspect that this wasn't a tube issue after all.  I'm going to try the breath test to see what happens, but I'm pretty sure the capsule is good.  Anyway, thanks again for all of the input on this. I'm hoping (knock on wood, fingers crossed, etc.) that this guy is in good shape for at least a little while!   

 

[rstreber]

Once again, I've got a noisy mic on my hands, although this time the noise is much lower-level (for the moment.) Kind of a low-level dull crackling sound which occasionally ramps up and then dies down again to a low level. I had it powered up for the last three days; it was perfect when I left the studio yesterday, noisy when I came in this morning.  This is with an EC1000 that had been in the mic previously for a few weeks before things got noisy. When I installed it the other day, I thought that it was a confirmation that my noise problems may not have been tube-related, and that may still be the case.

I'm going to try swapping out some other components to see if any progress can be made.  It's just maddening though - it seems like any time I even open the mic and close it up again, the noise "goes away" for a little while.  So, it's incredibly difficult to figure out where it's coming from. Any change I make seems to fix it temporarily!  I'm starting to think that there has to be another weak solder joint somewhere which is being agitated when the mic is moved or something.  Hmm... 

Whatever it may be, I am determined to fix it. This mic is just too good-sounding to not get it right!

 

[riggler]

I have an old Altec condenser that does this. I can fix it for a day by loosening and tightening the capsule. It can be fine for weeks, then all of a sudden the intermittent noise appears. Are your capsule leads crimped? If you can leave the mic opened up and running, then wait for the noise. Then VERY gently tap around with the eraser end of a pencil, checking capsule leads, anything else in a Hi-Z area.

And are you sure it's not a change in humidity causing it?

 

[rstreber]

Interesting that you should mention the capsule leads, because I think I located the source of the problem: the wire from front membrane to grid.  I re-stripped and re-soldered it to the turret where it meets the grid wire, and it has been dead-quiet (relatively speaking) for about 48 hours now. 

I guess it's the nature of the beast with this mic that these kinds of problems can easily be masked.  Since you have to remove (i.e. un-solder) the front / back membranes and backplate wires to remove the capsule deck any time that you have to work inside the mic and then resolder everything when it goes back together, it's easy for a problem like that compromised wire to be temporarily "fixed" upon reassembly only to weaken after the mic is jostled around a bit. 

Question: is it a bad idea to use heavier guage wire for hi-z connections?  I'm using 28AWG silver teflon wire, and it's quite fragile (at least in my clumsy hands.)  If I used something that is mechanically more robust, are there potential problems? 

 

[PRR]

> at least in my clumsy hands

If 16 gauge suits your hands, it's sure less "problem" than a mike that's crapped/hissed-out most days.

Fat stuff off the capsule adds capacitance, which lowers output; also "shades" sound from the diaphragm. All of these ill effects are nearly un-measurable and surely inaudible, not near as bad as a hissy-fit.