Altec "coke bottle" issue or maybe general tube mi

[Junction]

Before you go rebuilding the mic, try the "breath test" This is a standard test endorsed by Neumann for their condenser microphones, I dont know this Altec, but I see no reason why the test is not suitable. With your mouth wide open, breathe softly onto the capsule (while it is plugged in and powered up) You are breathing moisture onto the capsule surface. If within a matter of seconds, maybe 5-10 seconds, the audio dies, it might drop 30 or 40dB and then it takes possibly minutes , 3 or 4 minutes before the audio returns to normal level, then you have a dirty capsule. The moisture from your breath and the dirt causes a short across the capsule and hence the dramatic audio level drop. You will then need to clean the capsule surface ... its quite simple, but that's a another story

 

[Junction]

Note you appear to be responding to the first post, from 2006.

Oops, thanks Doug, must have been still half asleep.

 

[deveng]

Well, it does seem that the Altec cable was designed especially for this use. Unfortunately original cables are selling for crazy $. I bought Mogami Neglex 3172 but obviously have not tried it yet. Cable *should* work with just seven conductors. Whether shielding will be sufficient, capacitance will be an issue, etc. I have no idea.

I just received 25' of Redco TGS-M7 I'll let you know how this cable works out. It looks really good, soft flexible jacket, one twisted pair and a couple of larger gauge conductors for the filament. I have purchased the Gotham cable in the past. Its excellent cable but a bit stiff.

 

[deveng]

Wired up the coke bottle mic (cardioid capsule) with Redco TGS-M7 cable. It appears to work well although I have no experience with this particular mic. The Mogami 3172 should be fine too. The chest plate mic works as well but has lower output (omni capsule). The re-cap job solved the hum issue. I only did a quick check for function and noise using a portable Mackie mixer. Some typical tube mic hiss at high gains but nothing out of the ordinary at first listen. Neither of the tubes are microphonic. I'll buy some replacement 6AU6's for spares and try them out soon. Now looking at the Rycote Universal shotgun mic mount. Any recommendations on a suitable shock mount? Mic body is 28mm at the base. I have the original mic stand adapter wired with the Redco and I have an extra set of Amphenol 8 pin connectors which I'll wire up for use with the shock mount option.

 

[deveng]

Does anyone know what the maximum SPL is for the 21b capsule? I'd like to try this mic on loud sound sources and because of its age and rarity I don't want to damage it. I've done searches and downloaded the manual but no documentation on the sound level spec.

 

[mjrippe]

Well, according to the attached Altec doc, the 21D was capable of 150dB SPL.  Other capsules (some designed for NASA) went even higher.  I have a 21-BR-180 which is rated for 170dB and it works great on kick drum.

 

[deveng]

Sir "Rippe",

Thanks for the 21d info! Did some further research and it appears the 21d is very similar to the 21b and the only difference was "they changed the way the sound enters the mike capsule". If they're the same basic design and materials, I would believe they'd be also similar in SPL handling. Since ribbons can be used on high SPL source like a electric guitar cabinet (with precaution and angle of placement) I'd be this mic could be used on guitar cabs as well.

I'm experimenting with some non-typical mics. I recently tracked a timbale/side snare with a Sony C37 FET and it sounded fantastic. The Altec "coke bottle" on a guitar cab will be next.

 

[PRR]

The SPL for "high distortion" will be lower than the SPL for "destroyed".

I don't mean you should close-mike a 1-ton bomb. But these capsules should not be ruined by anything you would want in a studio. If they give over 2% THD on kick-drum, who cares?

 

[deveng]

I tried the Li'l Nessie chest mic on a 2x12 cabinet loaded with WGS Veteran 30's driven by a Plexi amp. Sounded surprisingly good. Volume was actually quite low as the amp was on the Variac mode at 20 watts. Three mics were used, an SM57, a BIV-1 ribbon and the Altec 21b. The SM57 and BIV-1 were no surprises in the sound. The Altec 21b was in the "middle". Not up front like the SM57 or warm and beefy like the ribbon. It was even across the frequency spectrum, sounded quite smooth. Good blended with either the SM57 or the ribbon as well. Best combination was the 21b and the BIV-1.

I like this mic so much, I think I'll build a second power supply so I can use the coke bottle and the chest mic as drum overheads. I highly recommend trying one if you get the chance. Simple and elegant mic.

 

[mjrippe]

Hey Jeff,

Try using the coke bottle as a mono overhead in front of the kit.  Gaff tape Nessie to the drummer's chest.  Tell them not to grunt too much or sing along! 

Mike

 

[soapfoot]

This is an older thread, but rather than start a new one I figured I'd add to the repository of dialogue here.

What type of output level should I expect from an M11 system with 21b strapped for 250 ohms output?

I just got my first M11, restored the power supply, verified that all voltages are in spec, and it sounds great. BUT the output seems a bit on the low side, lower than a KM84, lower than I might expect from other condensers.

Does this seem normal/right?

Before I had the mic, someone had replaced the B+ selenium half-wave rectifier with silicon, and had not adjusted the resistors appropriately. Therefore, it had been running at a rather high plate voltage.

I have a few NOS RCA 6AU6a tubes that I could try. But I'll reiterate that the mic sounds great, just with a bit lower output than I expected (though admittedly I had no idea what to expect).

 

[mjrippe]

Output level should be comparable to other SDCs.  If all the voltages are good, the tube would be next to check.  Let it burn in overnight at least for the noise floor to settle and see if you have stronger output.  There is also a low microphonic variant, the 6136 that I have been using in all my coke bottle restorations with good results.

Mike

 

[soapfoot]

A fresh RCA 6AU6A fixed it.

Normal output now, and sounds gorgeous!

 

[thecr4ne]

Well, according to the attached Altec doc, the 21D was capable of 150dB SPL. Other capsules (some designed for NASA) went even higher. I have a 21-BR-180 which is rated for 170dB and it works great on kick drum.

Thank you for sharing this. I've been trying to find info on the 21BRXXX capsules for a while and this is exactly what I needed to see.