Tube mic pissing me off!

[drewl]

After fixing some intermittent connections, I have weird issue with an ADK TC-MK9.

Mic works, but with a long sustained note the volume drops down slowly to almost nothing as the cathode voltage of the 12AX7 rises.

I drew out a schematic, a basic one stage amp.

Wasn't sure of the wiring, as it has a front plate and back plate yellow wires, one was disconnected, with a black wire to outside ring of element.

[drewl]

It seems to work best with one yellow wire connected to tube input, and the other yellow and black wires connected together at that 62 volt spot.

When mic volume cuts out, tapping pin 2 input to tube still yields noise like the circuit is amplifying.

I'm dizzy from yelling into this for hours.

[g1]

Have you tried scoping the output for HF oscillation?

[drewl]

Why yes, yes I have.
I have scoped and measured every inch of this bastard.

It's probably a bad element.
But still pissing me off.

[Chuck H]

I can't help but wonder if the high-ish value caps are giving the power supply some trouble keeping up with their charge.?. Between the high value series resistance and the large cap values I think that could happen on sustained notes with any amount of LF in play. Though I can't see the B+ supply circuit. WTH is the 62V circuit doing in that schematic?

[Mick Bailey]

The first tube condenser mic I ever repaired did this and out of embarrassment I never told the owner how many hours I spent on it. My problem was I was testing it without a filter and breathing on the capsule. Because the mic was out of its housing there wasn't sufficient rising warmth to help things either. So, it would start off fine but as the breath invisibly condensed the charge was leaking across the mylar film and the volume gradually dropped. If I left it for an hour and came back it would be fine again.

The problem is worsened if the capsule surface is dusty, as the particles act to cause microscopic droplets to form (seeding). I've cleaned capsules, but it's involved and risky and IMHO a last resort.

YMMV

[Chuck H]

The problem is worsened if the capsule surface is dusty, as the particles act to cause microscopic droplets to form (seeding).

Nucleation! The dust is forming nucleation sites?

Wow condenser mics can be tender. That's great insight on the matter. And I'm almost glad I don't own one of these things.

I wonder if it's safe to place the mic near one of those negative ion air cleaners to try and pull fine particulate from the structure non abrasively?

[Mick Bailey]

They're too tender for my liking. I'd only attempt cleaning if the capsule is so poor that its going to be thrown out. That gold layer is removed so easily and the rest so easily contaminated that improper cleaning is guaranteed to destroy it. The surface contamination always seems to cling to the surface so I'm not sure whether any non-contact cleaning would be effective. There are tried and tested methods. I use a brand-new sable brush of the softest grade possible, rinsed in distilled water from a lab chemical supplier and allowed to dry dust-free. Then moisten it with steam from a kettle and allow it to cool briefly, which also dries it out to the correct moisture level.

If its a valuable mic then I wouldn't touch it at all. I don't want the responsibility of killing a U47 or having to buy a replacement capsule.

[Leo_Gnardo]

I don't want the responsibility of killing a U47 or having to buy a replacement capsule.

Current offering price for a new U47 is US$ 9000. OW! Wonder what's the price of a capsule? Sure would ruin your day to have to buy one.

[Mick Bailey]

I saw a pair of NOS capsules for $17,000 a while back. Incredibly they sold. The VF14M tubes are really expensive - budget $2500 upwards for NOS unless you buy a SS plug-in replacement. Big money stuff I don't want to handle.

[dmeek]

They're too tender for my liking. I'd only attempt cleaning if the capsule is so poor that its going to be thrown out. That gold layer is removed so easily and the rest so easily contaminated that improper cleaning is guaranteed to destroy it. The surface contamination always seems to cling to the surface so I'm not sure whether any non-contact cleaning would be effective. There are tried and tested methods. I use a brand-new sable brush of the softest grade possible, rinsed in distilled water from a lab chemical supplier and allowed to dry dust-free. Then moisten it with steam from a kettle and allow it to cool briefly, which also dries it out to the correct moisture level.

A friend brought me a non-working U87 a while back. Like Mick I didn't want to touch the capsule. It's always been the point where I would give up on a mike. This one was visibly grimey and I figured there was nothing to loose so I tried the
distilled water and brush method after watching a youtube video. And it worked! Not so nervous about it now.

[nevetslab]

Most of my microphone schematics are still in copies of printed documents that I added to my files while working at Westlake Recording Studios in West LA, CA. I haven't yet scanned them to add to my computer database, so I"m somewhat limited in what I have here on the computer.

I've attached two schematics, an AKG C12/N12K variable pattern dual element capsule & it's power supply, along with The Tube mic, which is fixed as a Cardioid pattern, though it too has a dual element capsule. AKG and Neumann normally took the capsule's output from the housing, with the forward-facing capsule element grounded, and the bias voltage applied to the rear element. The logic being to face the unbiased element toward the sound source (moist breath from singer/speaker in vocal micing). I've seen it done many different ways. As you aren't dealing with various pickup patterns (Cardioid, Omni, Figure-8, Hyper-Cardoid), you're not altering the potential between the two capsule elements, as is done on the C12 mic.

You might want to try connecting the tube grid to the frame, grounding the forward-facing element, and applying the bias voltage to the rear element.

I've cleaned many capsules over the years. I do have a coveted stash of Freon, though normally work with distilled water and fine sabel brushes. As has been stated, and know from experience, the deposited gold onto the mic elements membrane is very fragile, and can be wiped off all too easily. I only resort to applying the cleaning water when I can't simply brush off the accumulation of dust/dirt from years of use. The Freon works really good on stubborn deposits, as long as I don't see the gold disappearing.

I also have a small industrial oven....actually an Autoclave oven (powered from a Topaz AC Mains Regulator), which I will use when there's evidence of accumulated moisture in the capsule. 150 Deg F for around 24 hrs will generally bake out the moisture while not damaging any of the components. I'll always try swapping the low noise tube (if it's a tube mic) first, just to be sure its not tube noise.

AKG The Tube mic.pdf
AKG C12 and N12K.pdf

[drewl]

Okay, that cleaning bit sounds insane.

Saw a video on replacing an element.

When it cuts out, that 62v on the black wire was showing up on the yellow wire front plate.

So that seems like some type of dirt or contamination from what you guys described?

[Chuck H]

When it cuts out, that 62v on the black wire was showing up on the yellow wire front plate.

So that seems like some type of dirt or contamination from what you guys described?

I dunno.?. I doubt the full voltage would present across a semi conductive path like grime. Maybe a stray wire strand or some sort of static element (in construction or circuit) could be charging to the presented voltage?

[Mick Bailey]

Can you post a picture of the mic internals? It may be similar or the same as a Nady, or Apex 460. A lot of Chinese mics share the same internals and have roughly the same circuit but usually both triode halves are used.