Altec "coke bottle" issue or maybe general tube mi

[pirate]

hi guys,

i am hoping someone may be able to shed some light on this one...
i am using an altec M11 "coke bottle" on vocals and everything is great! until it started to lightly distort... after changing out my signal chain and seeing no results, i turned the mic off. i turned it back on about 10 min later and it was clear for a little while but sunk back into the distortion pretty quickly, maybe 10 min after power up.

i didnt have my DMM with me, but when i do, any advice as to what to check out first? is it possibly the tube just being old? it uses a 6au6 tube if that helps...
is the power supply getting out of whack?
maybe the capsule?

also as a side bar, since i am going to be digging around in the power supply and the mic i can take and post pictures. the schem for the power supply is inside the chassis too for anyone interested in that...

thanks in advance for any help...

 

[ioaudio]

try to provide more information. schematic?
get yourself a dmm and check b+ and heaters directly after powering up and 15 minutes later.
could be the tube, most likely.

 

[pirate]

thanks,

I will be heading out there with a DMM tonite and will investigate. so far, all signs point to the tube...

 

[ciminosound]

Here's the location of the manual and schematic:

http://www.dvq.com/hifi/images/m11.pdf

:thumb: :sam:

 

[pirate]

thanks, ciminosound!

i actually got the manual from the other M11 thread when i did a search. There isnt a lot of information out there that i could find, this is awesome!

thanks again!

From what i have read here on the board, the 6au6 tube is not consistant as far as noise. any advice on brands or should i just buy a few and swap out until i find the best one?

 

[Brent Casey]

It's very likely that the caps in the PS are quite crusty. You may want to overhaul the supply if it hasn't been done already.

 

[ulysses]

You will find that the tube is not defective. There's not much about a tube that's likely to be okay at power-up and then fade after a few minutes.

There's also the old "moisture on the capsule" problem which sounds similar to your trouble but I've never experienced it and I don't know much about it. That one is solved by storing the mike with a dessicant. But that's not likely to be the problem either.

My guess is that your power supply filter caps are not happy, and they get a lot less happy once they warm up. Could also be the selenium rectifier in there, who knows. Measure your power supply voltages, but be careful. You have to have the mike connected while you measure, since that's how they're specified. But it sounds to me like your supply voltage is fading upon warm-up. Electrolytics are known to fail with age, so look there first. Seleniums aren't famously reliable, so look there 2nd. Tubes are known to remain basically functional for decades, so shouldn't draw immediate suspicion. The tube in the 150 is rather difficult to remove (not as difficult as the 5840 in the later 165 and 175 bases) anyway. Still, if you happen to have a spare around, you can try a swap for your own gratification. But don't go buy a tube until you've ruled out the more likely scenaria.

 

[dale116dot7]

Careful on changing tubes in an Altec Coke Bottle. Only the very best need apply - it's too easy to make that mic noisy. Replace the electrolytic capacitors and check the resistor values with an ohmmeter and get the power supply working properly first. You can heat that capsule up to drive out any moisture if you need to - the instructions on doing this are in the manual. Any leaky electrolytics in that power supply can upset the tube bias.

I rather like the sound of the Coke Bottle mics. It's the one seller's regret that I have... I sold it and I should not have.

 

[deveng]

Recently got one of these beauties but it had a lot of hum on the output. Finally tracked down all the 4 multi-section caps and I recapped the beast. Measured voltages before, during and after each step of component replacement paying special attention to the voltages after replacing the selenium rectifiers. I added a 10 ohm wire-wound in series just after the new silicon bridge rectifier and the loaded filament voltage was decent at 5.3V. No problem there. After replacing the B+ selenium with a 1N4007, the "mic attached" plate voltage went up to 297V. The schematic shows both "mic attached" and "mic detached" voltages. It states the plate voltage should be approximately 270V. At 297V the plate current measured 1.67ma so total B+ current draw is only about 3ma. To drop the plate down anywhere close to 270V would require a 8-10k ohm series dropping resistor following the 1N4007.

The 6AU6 spec sheet states maximum plate voltage for triode connection is 275V but I know tubes are forgiving on exact plate voltages. Is it worth the effort to drop the plate voltage down?

 

[emrr]

I would add the dropping resistance, after all selenium has a dropping loss which you've gotten rid of.  The existing dropping R's would have been different had it been a silicon rectifier, likewise again different were it tube. 

 

[deveng]

Doug,

Thanks for the input. The dropping resistors are cheap, and it only needs to dissipate less than 100mW. I'll give it a shot.

By the way, has anyone ever seen the "desktop" accessory mic? I got that one with the same 6Au6 tube. It has a movable gooseneck about 6" long and the omni capsule. I can't find any information or pictures of one of anywhere! There is a table adapter the will hold the "coke bottle mic" but this one is a self contained mic. Very unusual.

 

[emrr]

I feel like somewhere I've seen an ad with that desktop version in it. 

Since the B+ also defines the capsule polarization, I'd be concerned about that too.  Unless I'm seeing that incorrectly. 

 

[mjrippe]

By the way, has anyone ever seen the "desktop" accessory mic?  I got that one with the same 6Au6 tube.  It has a movable gooseneck about 6" long and the omni capsule.  I can't find any information or pictures of one of anywhere!  There is a table adapter the will hold the "coke bottle mic" but this one is a self contained mic.  Very unusual.

Do you mean the one in the photo below?  This is a pic of an Altec 21B "chestplate" mic that I serviced for a client.  The string goes around your neck and you can angle the curved piece.  I have an Altec ad that mentions the existence of such a thing but this is the only one I have ever seen.

 

[deveng]

Mj,

That's the mic! Its quite the "lapel" mic right! You also have the omni capsule fitted on it. Does your omni thread onto the coke bottle body?

 

[deveng]

Expected around 10k or so dropping resistor for the plate but by experimentation found a 2.7k was good. Now the plate voltage with either microphone attached is 267v, close enough for me. I'm working on a 10 song album now and will be doing another starting in January, will be fun to try this mic on something!

 

[mjrippe]

Mj,

That's the mic!  Its quite the "lapel" mic right!  You also have the omni capsule fitted on it.  Does your omni thread onto the coke bottle body?

Regards,
Jeff

Yes, the capsule is the 21B omni that fits the coke bottle.  The "lavalier" mic I renamed Lil' Nessie because it looks like a baby Loch Ness Monster!

I also wanted to say thanks for your posts because I am going to be building a 518 power supply from scratch soon 

 

[deveng]

Hah! Loch Ness is a great description of it. I'll need to have a look at those threads again. I didn't want to force it or cross-thread.

Let me know if you need any close up pictures, voltages, etc. The P518A is easy to open up and easy to work on.

 

[mjrippe]

Hah! Loch Ness is a great description of it.  I'll need to have a look at those threads again.  I didn't want to force it or cross-thread. 

Let me know if you need any close up pictures, voltages, etc.  The P518A is easy to open up and easy to work on.

Regards,
Jeff

Yup, I've restored a few of them but never built one.  Scored a coke bottle with a few spare capsules but no PSU, so now I gotta DIM (Do It Myself)!  Finally gathered all the Cannon P8 connectors but could not afford a TB-103 or 4665 transformer so I'm going to attempt it with a 4722 wired in reverse.

 

[deveng]

I need to replace the cable for the coke bottle mic. I have the original mic stand adapter (to Cannon P8) and strain-relief. The original is a 14 conductor, with 3 conductors for balanced audio in the central core. Since I don't think you can get this cable any longer, I'm looking at the typical Redco, Gotham, Mogami tube mic cables. I bought the Gotham for my MK47 but found it to be very stiff.

Have you replaced the cables, if so what do you recommend?

 

[mjrippe]

I need to replace the cable for the coke bottle mic.  I have the original mic stand adapter (to Cannon P8) and strain-relief.    The original is a 14 conductor, with 3 conductors for balanced audio in the central core.  Since I don't think you can get this cable any longer, I'm looking at the typical Redco,  Gotham, Mogami tube mic cables.  I bought the Gotham for my MK47 but found it to be very stiff.

Have you replaced the cables, if so what do you recommend? 

Regards,
Jeff

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.