A little G7 Assistance Please.

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Ethan

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I can't seem to find the root of this problem. I've got all the correct voltages before the 1 gig resistors. I have 176V at XLR pin 2 which is a bit high but should be ok temporarily, I was just trying to give an intial test.

The Problem: No Sound (tiny static at high gain)
Heaters are at 6.3V (but with barely any glow, NOS Tesla EF-86)

Any suggestions?
Thanks
 
Hi Ethan,
Sounds odd
Take some measurements at the PSU with and without the mic connected & report back.
Stewart
 
Thanks Stewart,

Here's what I got:

Unloaded at the pins:
1-0V
2-178V
3-0V
4-0V
5-76
6-6.6V
7-0

Loaded:
1-0V
2-177V
3-0V
4-0V
5-75.5
6-6.35V
7-0

That's all within working normal though as far as I can see...
Any help much appreciated.
 
OK.
Check the connections inside the mic that the 170V supply goes to. I found with my mic that the B+ drops considerably with the mic connected:

http://www.omnipressor.com/Other/G7mics/Comments.html

Trace inside the mic and measure the voltages in different points. Hopefully the fault will become obvious.
Good luck!
stewart
 
It HAS to be this tube, unless I keep overlooking the same thing repeatedly. Although, I'm always in denial that it could be the tube...more likely a mistake that I made. But in such a simple circuit like this I can't find any mistakes on my part. I guess I'll have to wait until I get another EF-86 to try :cry: Aghhhh, so frustrating.
 
[quote author="Ethan"]It HAS to be this tube, unless I keep overlooking the same thing repeatedly. Although, I'm always in denial that it could be the tube...more likely a mistake that I made. But in such a simple circuit like this I can't find any mistakes on my part. I guess I'll have to wait until I get another EF-86 to try :cry: Aghhhh, so frustrating.[/quote] I´ve been building and reparing tube guitar amps for ten tears or so, and I always suspect the tube before I suspect anything else. Maybe You could find a simple guitarampgrade EF-86 to at least check if the tube is the problem. This might not be the best sounding/performing tube in a microphone but it will at least give You an indication if the tube is the problem.
 
[quote author="Ethan"]Could I wire up a 12AX7 (correcting for different pinout)[/quote]

That may be more trouble than it's worth, since the 12AX7 is a dual triode and the EF86 in the G9 is a triode-connected pentode... It would involve modifying the circuit somewhat - certainly more than just correcting for the 12AX7's pinout.

I may be mistaken (don't have the schem in front of me and I'm lazy), but with such high impedances in there, it's kinda weird that your PSU voltages barely change when you load it with the mic + the multimeter... I'd look around the very-high-impedance part of the circuit.

Then again, I could be completely off, who knows :?

Peace,
Al.
 
[quote author="Ethan"]Could I wire up a 12AX7 (correcting for different pinout) just to test if I get sound? Could this possibly kill anything?[/quote]The 12AX7 is a twin triode and the EF86 is a pentode. I´m not very experienced using the EF86 simply because it´s not a very common tube in guitar amps.But I do know that the EF86 has more gain than all twintriodes. I think it would be alot easier to find a EF86 and use that instead. Maybe You could even unplug one from a guitaramp and try it out. I know that Matchless and Vox use this tube in some of their amps. BTW are You SURE You got the pin numbers on the tube socket right? I know it sounds stupid but I´ve actually done this myself. If it still is not working, even though all the voltages look right, bring out the solderingpen and heat all the solderingpoints once more. This have performed magic in the past. :wink:
 
Yeah I quickly realized why I wouldn't be able to use the 12AX7 to test soon after I posted :oops: Oh Well... I'll give it another look and if not find another tube.
 
Looked at the schematic and realized strapping a VOM across the XLR pins won't load anything down, so your voltages seem fine.

Is it possible you fudged the 1nF polysty cap going to the grid? those things don't like heat at all... Why don't you arm yourself with an EF86 datasheet and check the voltages around the tube to make sure they make sense? That will tell you right away if it's the tube...

Keep us posted and good luck!

Peace,
Al.
 
OK... Uhh... Problem...

I'm reading 9.7 MICROamps (yeah not mA, but uA) of current draw by the filament. I've checked the connection from the 6.3V rail to the socket to the tube, All good there. So what could this mean? A poped Filament in the tube? Or...?
 
Darn it, why didn't I think of that, they're just like light bulb filaments...

Unfortunately (or fortunately) I've got a hair over 5ohms on the filament pins.
 
[quote author="Ethan"]barely any glow[/quote]

If there is ANY glow at all, I'd say that's not your problem.

Are you sure you're not measuring AC current? And not to insult your inteligence in any way, but you are putting your meter in series with the heater and not in parallel, right?

What (DC) voltage do you read across the 100K plate resistor?

Peace,
Al.
 
Yes I'm sure I'm measuring current correctly (I thought I was seeing things too) :grin: I've got 171V @ plate.
 
5 ohms sounds good. At least you know the filament isn't open.

I think what Al was asking you is, what is the voltage ACROSS the plate resistor. If there's no voltage dropped across the plate resistor, that means the tube isn't drawing plate current.

Check the contacts on your tube socket. Sometimes these become "spread" and fail to make good contact with the tube pins. With the power off and all voltages bled off, you can very carefully re-tension the contacts with a dental pick.

With no power applied, take a DC resistance measurement at the socket between the cathode pin and ground. Again, if this path is open, no current will pass through the tube and you'll get no amplification.

Make sure both sides of your heater supply show good continuity to the socket pins.
 
another tip, might be stupid, might me good, try cleaning the pins on the tube itself with contact cleaner. sometimes NOS tubes have been stored 40 years or more and I´ve seen tubes having almost darkbrown pins from oxidation. since we´re only using 160V here You don´t need to be afraid of using contact cleaner on a piece of paper end cleaning the pins with it. :thumb:
 
Something else to look for is there a white film on the glass inside if so the vacuum could be bad.

from the posts about you have little to no emission from the cathode
 
Cleaned the contacts. Re-Tensioned the socket teeth. I have continuity between Cathode pin and ground. I have virtually no voltage drop across the 100K plate resistor (about a volt difference).

Where to next?

(As frustrating as it is when things don't work right away, I learn the most from you guys when I have to troubleshoot with guidance... Thanks for hanging in there with me!)
 
If the above doesn't blatantly indicate anything, is there a practical way to test if this tube is ok?
 
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