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Old 3rd Jan 2012, 10:59 pm   #21
mrmagnetophon
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Default Re: AKG C60 valve mic

I tried the mic today with a 5840 tube, and wouldn't you believe it, 60 cycle hum, turn off the mic and really low frequency hum. What is going on? I'm trying everything and all i get is noise, its really getting discouraging.

-Chris

I have just connected the Negative supply to the GROUND of the amplifier, and removed the ground of earth from the ground shield (so no interference or shorts would occur). The improvement was night and day, The mic still had hum, but now it was picking up the signal of my voice. The hum was 60 cycle.

-Chris

Last edited by mrmagnetophon; 3rd Jan 2012 at 11:14 pm. Reason: Added content that was required for solving issue
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Old 3rd Jan 2012, 11:19 pm   #22
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Default Re: AKG C60 valve mic

Hi Chris, I'm struggling to keep track of this post.
Which circuit are you using?
How are you turning it off?
What happens when you remove the valve or disconnect the LT supply (with everything else connected and "on".
Rob.
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Old 3rd Jan 2012, 11:27 pm   #23
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Default Re: AKG C60 valve mic

Hi Rob,
Sorry for the confusion.
I have attempted to build a new circuit, i will post that here.
I heard that a 5840 valve might solve the problem with a different circuit.
I am turning the mic off via a power strip (like a switched outlet)
When i disconnect the LT supply, (after the mic has heated) the mic does not hum as loudly, and you can start to hear a little music, than the valve cools off too much and no sound. No hum either.
-Chris

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Old 4th Jan 2012, 12:01 am   #24
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Default Re: AKG C60 valve mic

Schematic here:
Also i amp playing music, from a speaker, and have the mic infront of it, thats why i say you can hear a little music. Problem either must be coming from the HT supply, or something with the capsule. I have tried 2 capsules.
-Chris
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Old 4th Jan 2012, 12:25 am   #25
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Default Re: AKG C60 valve mic

Ok Chris, thanks that's useful.
Have you tried running the heater from a battery?
Am I right in thinking you've tried several different circuits, but get the same fault with all of them?
I was wondering earlier if somehow the amplifier is self oscillating somehow due to leakage currents, what happens if you replace the mic with a small capacitor 10pF perhaps?
Rob.
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Old 4th Jan 2012, 12:30 am   #26
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Default Re: AKG C60 valve mic

Hi Rob,
I will try the battery soon, Yes i have several capsules, and the same problem with all of them.I'll try the capacitor idea.
I'll come back to you on those tests.

Ok now we're making progress, i've done a few things that may help show the problem.
When i remove R4, and turn the mic off, i get sound with no hum, as the mic is slowly draining. Now if i replace R4 and turn the mic off, it drains instantly and no sound when i turn the mic off.
-Chris
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Old 4th Jan 2012, 12:44 am   #27
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Default Re: AKG C60 valve mic

Hmmm, curious.
Can you test the valve for heater-cathode leakage resistance (hot).
It will be interesting to learn what happens when you run the heater from a battery, with the rest of the LT supply disconnected.
Good night, catch up tomorrow.
rob.
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Old 4th Jan 2012, 12:46 am   #28
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Default Re: AKG C60 valve mic

Rob: i tried the battery idea, well its not LT hum, although this is what happens.
The caps are able to store a huge amount of power, with continuous heating by battery, i am able to listen for 5 minutes of music coming through the microphone. And it doesn't sound to bad. With no HT i can still here a little bass coming through the speaker.

-Chris
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Old 4th Jan 2012, 12:56 am   #29
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Default Re: AKG C60 valve mic

With R4 removed from the Altec Lansing circuit, no DC current will flow from the cathode through the valve; the valve will not operate correctly at all.
This circuit is rather odd in that there appears to be no grid leak resistor from g1 of the valve to ground. Perhaps it relies on whatever insulation leakage resistance exists in the capsule to perform this role, this doesn't seem like a great idea to me. Other valve mics including the AKG circuit you showed earlier use a very high value resistor as the grid leak; 10s or 100s of megohms.

John
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Old 4th Jan 2012, 1:01 am   #30
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Default Re: AKG C60 valve mic

The heater cathode voltage is 4 volts.
The resistance is immeasurable.
The problem with the AKG circuit was it hummed to.
If we could just get a circuit together, be it AKG or altec, that would not hum it would not matter to me which was which.
Thank you,
-Chris
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Old 4th Jan 2012, 1:36 am   #31
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Default Re: AKG C60 valve mic

Putting a .001 cap in place of the capsule resulted in slightly less hum, but still quite a lot at louder volumes.
Does that help?

-chris
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Old 4th Jan 2012, 10:50 am   #32
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Default Re: AKG C60 valve mic

Quote:
Originally Posted by mrmagnetophon View Post
Does that help?

-chris
Only in as much as it suggests it's the amplifier rather than the capsule.
Ok, lets try something else. Can you connect a switch in the HT line, after the rectifier but before the smoothing caps. As you said in an earlier post, the capacitors hold enough charge to run the amp for a while. This may help to locate where the hum is coming from.
Rob.
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Old 4th Jan 2012, 2:00 pm   #33
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Default Re: AKG C60 valve mic

Out of necessity, in order to match the characteristics of a condensor mic capsule, the input impedance of these amplifier circuits is extremely high i.e. as close to infinite as is possible.

This means that any wiring connected to g1 of the valve in these circuits will easily pick up hum and other interference if it is not well shielded and / or is very long. The wiring connecting g1 to the capsule should be as short as possible and should be routed as far away as possible from that to all other valve electrodes in order to avoid the possibility of oscillation and / or hum pickup.

Chris, perhaps you could post a photo of the arrangement of the valve and capsule and associated wiring within the mic body.

John
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Old 4th Jan 2012, 9:08 pm   #34
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Default Re: AKG C60 valve mic

Rob, by unplugging the microphone, and leaving the battery connected, aren't i doing the same thing, as no more electricity is flowing into the caps from the rectifier.

John: the wire from the capsule is shielded and about 4 inches long in this configuration.

Hope this info helps.
-Chris
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Old 4th Jan 2012, 11:47 pm   #35
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Default Re: AKG C60 valve mic

Could i just run the output of the mic to a quarter inch plug and run it into a direct box? My mic transformers are 1:5 ratio and 1:8 ratio.
-Chris
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Old 5th Jan 2012, 12:08 am   #36
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Default Re: AKG C60 valve mic

Chris, re. posts #23and #28, you say when you switch off the mains supply the hum goes away with the heater running from a battery. What I am trying to prove (or disprove) is whether the hum is due to lack of smoothing on the HT line, or direct pick up from the mains / transformer somehow.
So no, they're not the same thing, in one case the mains is on, in the other case it's off. Please don't think I'm claiming to be an expert on these mics, I'm just trying to apply a logical fault finding approach.
The variation in capacitance of the capsule is very small, to add a length of (capacitive) coax would attenuate the signal too much. That is why the amplifier has to be at the microphone end.
Have you considered using an FET?
Rob.
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Old 5th Jan 2012, 12:22 am   #37
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Default Re: AKG C60 valve mic

Hi Rob, i did exactly as you mentioned. Switched the HT, with the same results as post #28. As in i could hear music for a few minutes, until the caps drained off.

-Chris
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Old 5th Jan 2012, 12:34 am   #38
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Default Re: AKG C60 valve mic

Ok, well it's either components or layout.
Are you using brand new components or ones out of the junk box?
Although they're holding their charge, they may have a high internal resistance if they're any more than 10 - 20 years old.
Rob.
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Old 5th Jan 2012, 12:42 am   #39
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Default Re: AKG C60 valve mic

All brand new components, No older than 1 year since manufacture, all components have been tested for leakage and resistance on my heathkit condenser tester, which has been properly maintained. I have rechecked them with my ideal, radioshack, and fluke meters.
I am using a rather fail safe testing system.


Will this help? I tested the capsules, all test for .140- .130 to .147 when talking close to it. NF is the scale for reading.
-Chris
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Old 5th Jan 2012, 12:57 am   #40
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Default Re: AKG C60 valve mic

Is shorting negative and ground together dangerous?
-Chris
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