Audio Technica AT 4060 Noisy tube or capsule

[nevetslab]

I have an Audio Technica AT4060 Tube Condenser mic in the shop that's noisy. May be tube noise, though may also be moisture in the air condenser capsule. The tube used is a 6922 dual medium gain Triode, though unlike 6072's, 12AY7's, which have pin 9 at the center of the heaters between pins 4 & 5, this one has Pin 9 connected to a shield between the two triode structures, and the heaters run on 6.3VDC.

I have a couple 6922's on order to check out...an Electro-Harmonic 6922, hand selected is what's installed at present. I have a standard production EH 6922 and a J/J 6922 coming in to give a try. Meanwhile, I have the mic in my temp controlled lab oven cycling at 150 deg C to bake out any moisture that may have settled in there.

The schematic I was able to download from DIY Forum, though not very clear. It's an odd tube configuration, one that I haven't seen before, and am curious to see if anyone can shed some light on the lower Triode in this circuit. The trim pot appears to be capsule bias adjustment.



AT8560_PSU schematic.pdf
at4060 Mic Schematic.pdf
at4060.pdf
6922-Raytheon.pdf

[nevetslab]

Unusual Dual Triode circuit

Has anyone seen a dual triode circuit like this? Used in the AT 4060 Tube Mic.

[Leo_Gnardo]

Has anyone seen a dual triode circuit like this? Used in the AT 4060 Tube Mic.

I had an idea or 2 but rather than conjecture I'd say ping gingertube, somehow I'll bet he has an answer. It does look like there's some half clever feedback system going on there.

[nevetslab]

The EH 6922 tube and the J/J 6922 tube came in today, so I'm just now checking out the J/J, having listened to the mic with the EH 6922 for a while. No more LF random noise as I was hearing last week, not having another 6922 to listen to. Baking the mic in the lab oven at 150 deg for 6 hrs didn't yield any change in the offending random noise, so that pointed to a tube problem. Or, after putting a new tube in, maybe still needing more baking time to clear the moisture in the capsule, if there was any contributing to the problem.

When I swapped to the J/J 6922, moving the mic yielded instant clipping from air movement on the 1" diaphragm, I guess. Or handling noise? I let it warm up a good 15 minutes, then tried moving the mic again, and got the same results. Shut it down, put the Electro Harmonix 6922 back in, and none of that behavior is found. The hand-selected tube that was installed from the factory was an EH 6922. The other they use is a Sovtek.

At any rate, it appears that the tube was indeed the issue, and what I don't have here in the shop is an acoustic isolation box for dealing with microphone capsule/tube noise. I had one when I was working at MXL doing microphone development work in the mid-90's. At Westlake Recording Studios, we just made use of one of the studios for a while in having a quiet room. With this location, I can hear the LF rumble of the commercial jets across the boulevard at Burbank Airport, as I'm in slingshot distance of the runway. And, with clients in the rehearsal studios down the hall, one of which I share a wall with, Kick and bass come thru just fine!

[Leo_Gnardo]

The EH 6922 tube and the J/J 6922 tube came in today, so I'm just now checking out the J/J, having listened to the mic with the EH 6922 for a while. No more LF random noise as I was hearing last week, not having another 6922 to listen to. Baking the mic in the lab oven at 150 deg for 6 hrs didn't yield any change in the offending random noise, so that pointed to a tube problem. Or, after putting a new tube in, maybe still needing more baking time to clear the moisture in the capsule, if there was any contributing to the problem.

When I swapped to the J/J 6922, moving the mic yielded instant clipping from air movement on the 1" diaphragm, I guess. Or handling noise? I let it warm up a good 15 minutes, then tried moving the mic again, and got the same results. Shut it down, put the Electro Harmonix 6922 back in, and none of that behavior is found. The hand-selected tube that was installed from the factory was an EH 6922. The other they use is a Sovtek.

At any rate, it appears that the tube was indeed the issue

I find that this tube type is highly variable when it comes to noise. Some are OK for use in a guitar amp but few qualify for a super sensitive mic amp. About your bake-out box, 150C, is that really the case? I'd expect 150F in a dry atmosphere would be enough, and fear 150C would bring on damage. I could be wrong. But it would be good to have either confirmed, thanks!

[nevetslab]

That was 150deg F, NOT 150 deg C. I had to look back at my posts, saw the typo. Thanks.....yes, that WOULD do some damage. I haven't used this 6922 tube before, and missed the fact that it's heater between pins 4 & 5 is 6.3V, not 12.6V. Odd behavior with the J/J tube. That one is going back to Antique Electronic Supply.

[Leo_Gnardo]

That was 150deg F, NOT 150 deg C. I had to look back at my posts, saw the typo. Thanks.....yes, that WOULD do some damage.

Whew, thanks for clearing that up. Was worried if I or anyone else tried that, we would wind up with a tasty microfono al forno. Goes well with a nice chianti!

[nevetslab]

For sure. Don't want to bake an AKG C24 or Neumann U67 at 150 deg C, or any other mic for that matter. But, on the other hand.....what's for supper?

[mozz]

Is that a cascode for the 6dj8? Not too familiar but i know they are set up somehow like that.

[nevetslab]

I guess you could call it that. I'm used to seeing cascoded transistors above a diff pair in the front end of a discrete SS amp, though the bases of the cascode are usually connected to a DC voltage source. In this instance, the mic capsule-coupled stage is working as a cathode follower, but has signal from it's plate feeding the triode below, which has as it's plate load that cathode follower, and I'd presume voltage gain to boot. Clever circuit....one I haven't encountered before.

[nevetslab]

After contacting Audio Technica and pricing their hand-selected/aged 6922 preamp tube at $50.41 + shpg, it was affordable. Far more so than for the Sony C800G (a hand-slected 6AU6A for around $200). After listening to this AT 6040 mic with the EH 6922 tube I had bought a week ago, I shut it down and swapped it out for the new AT-selected 6922 tube. Also an Electro-Harmonix 6922. It's residual noise was better than the non-selected tube (I'd hope so). But, I also found in moving the mic, I got similar handling noise out of the capsule like I had with the J/J tube. Odd that I didn't get any of that with the non-selected EH 6922 tube I just bought.

But, of course, this normally sits in the shock mount, and once set into place, no need to be moving it in a fashion as I was, trying to find a hum-null zone in this shop, where I have a 5KVA Topaz isolation xfmr 'singing' and resonanting on my bench. Power up a good condenser mic in a room, turn the gain up, and with headphones on, you hear every bit of environmental noise locally and in the building. I'm only 20 feet from the power room, where we step down the power off the utility pole outside.

Prior to having looked closely at the 6922 data sheet, other than seeing it 'looked' like the same pin pattern as a 6072/12AY7 tube, I had swapped one in. I got a lot of hum, and shut it down. That's when I looked further in a different forum where I had found the schematics, and was clued in on the 6922's heater is 6.3VAC, AND pin 9 isn't a centertap to those heaters. It was a shield between the two triode structures in the tube. No indication of that pin being grounded, but, I was relieved to hear the mic work again with the 6922 tube installed. The new tubes installed yielded the absence of the circuit noise that it came in for, as well as low level 60Hz hum.

I checked this morning with a different condenser mic (running off Phantom Power), and heard the same low level 60Hz hum in the room, so it's NOT anything that could have been damaged by substituting the wrong tube. Whew!