G7 noise troubleshooting session. [Solved]

[Tergo247]

Yes. Sorry, I misread the schematic while I was measuring. I definitely have 470Ks in there.

 

[gary o]

Im currently messing with a G7 P2P in a chocolate tin with a lollipop mic head Iv had it working good but am now getting some meshy kinda white noise ...... noise goes away when I disconnect B+ before mic completely fades out so iv tried another B+ source lower at 120V noise goes away....plan to investigate more ......

good luck.....

 

[Tergo247]

That sounds interesting Gary, keep me posted!

 

[gary o]

I tried some different EF86 ...3rd one the noise disappeared tho there is some white noise mush still there much lower level.... running on 170V again with the better EF86....

I didnt now how to connect my single sided cardiod capsule ...I had one side to grounded, was working ok ....
I since read Zebras link in this thread thanks for this Zebra

http://www.xaudia.com/xaudia/Documents/Entries/2008/3/5_G7_observations.html

I have wired my capsule up as in the link......& now testing....

 

[zebra50]

I had one side to grounded, was working ok ....

You can do it that way too, and rely on grid-leak to polarise the capsule. The method I show polarises the back plate and grounds the membrane & grid through the 1 gig.

Some more thoughts & ideas here at my blog...

http://xaudiaelektrik.blogspot.com/2010_08_01_archive.html



Let us know how you get on.

 

[gary o]

Great website Zebra thanks Im learning ......My G7 sounds good but I cant shift this white noise mush....I dont trust the EF86s I have they from old tape recorders will get and try NOS one soon....

hope yr getting yr G7 sorted Tergo....

 

[Kingston]

These days finding a low noise EF86 can be a bit difficult and perhaps expensive, and modern remakes are generally Sh*te. That's where your white noise is coming from. Having just 3 to choose from is not even close enough. you need like 10.

Sorting out a low noise tube from a batch of 6BA6, 5879 or a PF86 might be a better idea.

 

[Tergo247]

About that... I'm not very excited to buy 10 EF86s to maybe get one that works. So two things, is there a brand that does them best? and does using a different low noise tube require modification to the circuit?

 

[Kingston]

So two things, is there a brand that does them best? and does using a different low noise tube require modification to the circuit?

No, at this point there are no brands that can be trusted. The new remakes are generally not up to the par of the tubes of the old days, and the NOS stuff you see on ebay is pretty much guaranteed to be harvested for low noise tubes already. So you pay exorbitant prices for crap tubes, unless you get really lucky.

That is the unfortunate reality of using tube gear today.

Regarding alternative tubes, yes they do require modifications. See datasheets.

 

[gary o]

Mine are not new as I said they from old tape recorders I have about 10 some seem to induce a rumble sound others maybe ok if my white noise is coming fron somewhere else.......

How aboat old octal tubes EF37a Ef36 cant remember the number did i read EF86 replaced one of those.....

I have some knew 12BA6 ....saving for vari mu tho......got loads nos 5879s tho.....
illave a word with my tube supply
man about EF86......will get a price for nos....watch this space

 

[Gus]

What I would do is wire a 100pf cap grid to ground disconnect the capsule from the grid, set the heater voltage to 6.3VDC to 6.5VDC install a tube and screw the body on and run the microphone for a day or two check for noise every couple hours.   Then turn the heater down to the voltage you want and test the microphone if still lower noise connect the capsule.

It is a pain.  I have made burn in setups to run/burn in 4 tubes at a time at different heater voltages in circuit fragments plate,cathode bias and grid to ground resistors and a B+ supply.  After hours of burn in I then will try the tube in a microphone and they can still just go noisy after being low noise for may hours.

Tube noise is one reason I am working on solid state circuits and staying away from tubes.  Solid state can sound very good.

 

[Tergo247]

That sounds like good advice Gus, I didn't know burning in could take a day or two. One of the posts I read was of a guy who's mic was fine after an hour of burn in time. I'm looking forward to what your tube guy has to say Gary.

In the meantime, I need to figure out why only one side of my capsule works. That much doesn't seem to me like a bad tube problem. I'll retrace my pin 5 voltages as far as I can at some point today and report back. I know I'm getting 82V coming into the mic (cardioid) but I'll do a sanity check anyway.

 

[tazwolf]

Sound like a dry joint/dirty pcb around either the high resistance capacitors or the wires from
the capsule - i had this problem as well. Check the soldered joints around the arwas mentioned.

 

[riggler]

Could flux be an issue here?

 

[Tergo247]

When my electronics buddy mentioned that I had cold solder joints, I hit everything with the iron again. There were a couple places that needed the flux burned away, but those are fixed now and it helped a lot. As soon as Christmas is over with I'll get back to finding my problem. Merry Christmas everybody!! For your listening pleasure, www.terrygowing.com/work/audio/carol_of_the_bells.mp3

 

[Tergo247]

I hope everyone had a joyous holiday season. I'm finally back on the bench. I did find a couple more cold solder joints and am currently burning in said tube at 6.4 volts starting now. I soldered in the 10pF cap in place of the capsule and the noise didn't diminish at all. I noticed that there is a small hissing sound when I first start up the mic, and after about 2 minutes of the mic being on it gets supplemented with a loud rushing sound that causes the present audio to be intermittent...

 

[Tergo247]

After burning in the tube all day yesterday with little result, I turned on the mic to check it today and the noise coming from the mic was sufficient to clip my pre. Yesterday the noise level peaked at -24dbFS with my particular gain setting, and I'm sure it didn't change. I left it on for 10 minutes and the noise didn't diminish. I've ordered two more tubes, NOS this time. We'll see if they're quieter.

 

[Tergo247]

So the two NOS tubes fell through, but I did snag an Amperex Bugle Boy. Swapping and burning in the Bugle Boy for 24hrs has eliminated most of the intense rushing sound I was hearing before. I'm going to put the capsule back in when I get a chance to see what my S/N ratio is like now. I'm still skeptical, the hiss I hear now is pretty present. No one feel shy about chiming in. Feel like I'm in a rubber room over here.

 

[pasarski]

Did you try the mic without the capsule as Gus suggested couple of posts above?

 

[Tergo247]

Yeah as I mentioned above, I soldered in the cap in place of the front side of the capsule. The noise I'm hearing now is sans-capsule noise. The current tube is way quieter than the previous one, but there's still some hiss. I'm not sure if the noise is coming from the tube or somewhere else, or if it's acceptable. I need the capsule back in to figure it out.. I'll post when I do.