Tube mic pissing me off!

[nevetslab]

When I have issues on a condenser mic's capsule electronics, I'll disconnect the capsule all together, and using shielded cable all the way from my generator thru a 1000pF cap tacked into where the capsule attaches to the input stage, leaving as little open wiring between shielded cable to feed side of the cap, and the cap tacked in to the circuit, then you can drive the capsule amp to see what's going on. It will be a bit noisy, but, should allow you to see if the same problem you're fighting with the capsule and voice or external source is still occurring.

[drewl]

Hey, been busy
I can feed the preamp a signal and it amplifies great.

So it must be the element.
Signal starts out strong, fades away with a whooshing noise, then after a few seconds
it pops and
signal is back to full strength.

Took some pics, but phone won't upload them, I'll try tomorrow.

[drewl]

Here are pix of the element.

[vintagekiki]

https://groupdiy.com/index.php?topic=19807.0

http://apexelectronics.com/downloads/servman/sm_apex460.pdf

http://www.angelfire.com/ky3/craigorris74/a460schemCFremoved.JPG

¹⁾ Small correction

[drewl]

Yes, I thought I had fixed that when I drew this version, thanks.

So I have everything hooked up properly.

preamp is operating properly.

Element flakes out with a continued pressure applied.

[nevetslab]

Yes, I thought I had fixed that when I drew this version, thanks.

So I have everything hooked up properly.

preamp is operating properly.

Element flakes out with a continued pressure applied.

I recognize the element & the mechanical support assembly, along with the frame. For a couple years, I was working at MXL Microphones in El Segundo, CA doing some preamp design, PCB layout work and production test procedures/fixtures for their Chinese-sourced capsules like you have here. You can rotate the capsule, and the two yellow wires, making what presently is the back side of the capsule the front side, and see if that restores order. Are you experiencing any substantial 'wind noise' when the mic is powered up, just idling in a quiet room? If so, that is a case of moisture in the capsule assembly, which can be baked out. You'd want a stable oven that doesn't wander in temperature. 150 deg F for 12-24 hrs normally does the trick.

Another thought would be to pick up a used MXL or siimilar mic off of ebay and swap capsules. As you're grounding the frame in this mic, along with the back side of the capsule (I think), a single-sided capsule (other side is clear mylar, not gold-element and no contact screw) can be installed in your mic.

I haven't gone searching for replacement or new capsules like this. They tend to be more in cost than a new cheap mic that uses that Chinese capsule.

[drewl]

Yes, I'm getting "wind noise" real hot distorted signal at first before signal drops.

Baking, eh?

Sounds like fun!

[vintagekiki]

Are electrodes (g1 / k / a) of half unused tube connected to gnd ?
Look angelfire link.

[The Dude]

I'm not sure if it's reflection, or debris, but that 3rd pic of the element sure looks like it's dirty or pitted. You might try cleaning it.

[drewl]

Yes, unused half of tube grounded.

[nevetslab]

That third image showing the other side of the capsule looks like the front side, it having accumulated all the crud on the membrane over time from mouth moisture. Cleaning the Chinese capsules like this seem to stand up to a bit of scrubbing with distilled water (sparingly!) and your sable brush more than the AKG capsule I see in the 'Cleaning a Dirty Microphone Capsule' image.

I would try re-mounting the capsule 180 deg, swapping the wires so the 'clean side' is now the front side (unsolder the wires at the preamp end, and don't remove the screws/terminals from the element, as you can tear the diaphragm. And, DON'T unsolder the wire at that capsule terminal!). In that Apex 460 Schematic, that capsule wiring is what I was referring to, where the output is taken from the frame, also having the capsule bias, and the front side is grounded, with the back side being attached to the pattern switch. In Cardioid mode, that switch would place that connection at the frame potential. If it were grounded, like the front side, you have Omni mode. So, with the wind noise present, cleaning the diaphragm won't eliminate that, while it will reduce the foreign matter adding mass to it. Baking the microphone, or, if you're real careful, you could just put the capsule assembly mounted in it's cradle attached to the nylon base in a small glass baking dish and dial your oven in for 150 deg F, assuming you have an accurate digital thermometer & thermocouple to monitor the temperature cycling. My autoclave oven, while a simple thermal switch used in it's temperature regulation, cycles from 140 deg to 155 deg F during it's long operation. Obviously you assess and tweak the oven for that temperature range before even considering putting it into the oven. I've also used the front dashboard of a car on a really hot day, a couple days running in the summer before I had the autoclave oven.

[drewl]

I have tried swapping the position of the yellow wires which made no difference.

And yes, the element does look dirty/pitted.

Why distilled water? Will denatured alcohol work?

I am going to bake it. Was going to have the wife do it all day when I get around to taking it apart.

[vintagekiki]

- Have you tried replacing tube with a new one (not like new). Best try Boogie low noise tube SPAX7-A

https://www.mesaboogie.com/tubes/pre-amp-tubes/SPAX7.html

- When connects to gnd microphone point (R10 / C4) does preamp still oscillate.

[nevetslab]

I have tried swapping the position of the yellow wires which made no difference.

And yes, the element does look dirty/pitted.

Why distilled water? Will denatured alcohol work?

I am going to bake it. Was going to have the wife do it all day when I get around to taking it apart.

Denatured Alcohol is too aggressive. Distilled water is what I've always been told/have used, apart from Freon.

[drewl]

Freon, like component icer?

I have that.