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Old 31st Dec 2011, 11:44 pm   #1
mrmagnetophon
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Default AKG C60 valve mic

I've been working on my AKG C60 microphone. Everytime I attempt to use it it just hums badly, all you can hear is oscillation, I've been over the schematic 1000 times and everything is there. The preamp is fine in the piece I am using it with. Any one know of this issue?

Chris
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Old 1st Jan 2012, 12:40 am   #2
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Default Re: Akg c60 valve mic

Turn the amp off and no hum, turn it on and a full on mid frequency roar of hum! Even with a little volume I get a full on roar!

-Chris
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Old 1st Jan 2012, 10:18 am   #3
Michael Maurice
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Default Re: Akg c60 valve mic

Is it mains hum, ie 60Hz or 120Hz or a higher frequency?

I dont know this particular mic but assume (maybe wrongly) that heaters are DC, if not then there could be some heater/cathode leakage.

If mains hum then try smoothing capacitor.

If not then check all other capacitors especially paper ones.
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Old 1st Jan 2012, 3:47 pm   #4
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Default Re: AKG C60 valve mic

Hi
I assume your C60 has N60A power supply. The valve heater is powered from a regulated 4volt power supply. As mentioned in the previous post, and given the age of this unit there are quite a few electrolytics that may need replacement. Attached circuit for reference. I hope it is legible.

A later AKG SolidTube microphone and power supply came in for repair recently with very severe hum problems. In this instance both HT electrolytics had not only failed but also oozed electrolyte over the PCB.

Cheers

Rich
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Old 1st Jan 2012, 3:48 pm   #5
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Default Re: AKG C60 valve mic

I assume you're using the AKG N60 PSU with it ?
Is yours the version using the AC701K valve ?
If so, do you have the correct +4 v DC on pin 2 of the PSU output. ?
The heater supply to this rather rare valve is provided by a regulated PSU which could
be the cause of the problem if unsmoothed DC is getting to the valve heater.
The 'HT' supply has 3 R/C sections for smoothing, so if faulty is less likely to cause hum, but one or more caps could be tired.
Time to get the 'scope out ....


Andy
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Old 1st Jan 2012, 7:05 pm   #6
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Default Re: AKG C60 valve mic

Here's the funny thing though, all caps are brand new and seem to be fine when tested on my testers. The hum is a very low pitched loud hum, say around 40 Hertz it seems. I've got 2 of these mics, the other one is missing the valve and power supply, I was told to drop a 5703 valve in, I don't know if that's the correct fit?

-Chris
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Old 1st Jan 2012, 7:52 pm   #7
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Default Re: AKG C60 valve mic

Chris

The 5703 heater voltage is 6.3V as opposed to the 4V of the AC701K so would not be a good choice unless the power supply was modified to suit.

John
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Old 1st Jan 2012, 7:58 pm   #8
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Default Re: AKG C60 valve mic

Hi Chris,

If the hum was nearer 60Hz then maybe look for one open circuit diode in one of the bridge rectifiers or an earth loop with connected equipment?

Now, if hum is really 40Hz then maybe some form of instability is occurring, however the valve is just configured as an impedance match so would not normally expect this type of problem.

Does the hum change when the LF gain switch is moved to each position?

Is there a switch on the PSU to select local ground (earth) or equipment earth?

Cheers

Rich
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Old 1st Jan 2012, 11:13 pm   #9
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Default Re: AKG C60 valve mic

Hi Thank you John, yes no problem if modification needs to happy, really not a huge deal to bump it up just a bit.

Rich, The hum is coming out of the woofer section of the monitor, its really low, i can hear by ear that its lower than 60 hertz.

I have not noticed a difference, i will clip a grnd lead on and see if i can isolate any ground problems, i prefer usually to use equipment ground and keep everything on the same ground in those terms.
Also the hum does not change, even if the capacitor is swapped out in the lf gain.
-Chris
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Old 2nd Jan 2012, 5:33 am   #10
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Default Re: AKG C60 valve mic

Its being ground with the EARTH and the hum is just awful, try the negative and still awful.

Why on the schematic does it appear that the heater is not really connected to anything? I drew an arrow.

Really thinking the output should be ran from the plate and the cathode. All the schematics i can find do it different than this.

-Chris
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Old 2nd Jan 2012, 9:26 am   #11
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Default Re: AKG C60 valve mic

Quote:
Originally Posted by mrmagnetophon View Post

Why on the schematic does it appear that the heater is not really connected to anything? I drew an arrow.
I think you'll find that it goes to ground, ie pin 3 of the connector.

Quote:
Originally Posted by mrmagnetophon View Post

Really thinking the output should be ran from the plate and the cathode. All the schematics i can find do it different than this.

-Chris
You can't run it from the plate and the cathode it has to be one or the other. In this configuration the valve is used as a cathode follower, in other words changing from high impedance to low impedance before the balancing output transfomer.
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Old 2nd Jan 2012, 6:52 pm   #12
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Default Re: AKG C60 valve mic

Why is that? Look at this schematic.
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Old 2nd Jan 2012, 8:26 pm   #13
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Default Re: AKG C60 valve mic

Would anyone object to using this schematic? I built a new power supply for my mic, put the whole mic in a metal case, properly shielded and grounded it, and still had the problem.
I think it's best to try a new schematic.

-Chris
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Old 2nd Jan 2012, 9:08 pm   #14
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Default Re: AKG C60 valve mic

You can use whatever diagram you like, it's your microphone.

That circuit must have worked at some stage, prestigeous manufacturers like AKG would not go into production if it had the problems you are experiencing.
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Old 2nd Jan 2012, 10:36 pm   #15
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Default Re: AKG C60 valve mic

I meant in terms of quality if that schematic looked like a fairly well put together one.
I read however that there were quite a few problems with this microphone schematic, even in the golden years of its usage.
I did something odd, came up with the idea to inject a cd player output into the microphone transformer, run it just like the valve output, (with the mic off) and what did i hear? less of a bass roar, but now a 80 hertz, maybe 60 hertz, hum!
I inject the signal after the massive 180M resistor and it sounds fine, a little hot sounding, but alright.

Anyone have an idea?

-Chris
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Old 2nd Jan 2012, 10:44 pm   #16
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Default Re: AKG C60 valve mic

I think maybe that resistor is just way too big, once again tried another one, you go through the resistor and too much hum, remove it and no hum. Not tried with the mic on yet of course, but I think maybe the value is too high?
-Chris
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Old 2nd Jan 2012, 10:58 pm   #17
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Default Re: AKG C60 valve mic

Quote:
Originally Posted by mrmagnetophon View Post
Would anyone object to using this schematic? I built a new power supply for my mic, put the whole mic in a metal case, properly shielded and grounded it, and still had the problem.
I think it's best to try a new schematic.

-Chris
Hi Chris,
you seem to be missing a cathode bias resistor, or is it a zero bias triode?
That other circuit is an interesting design, one way of getting the maximum possible gain.
Rob.
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Old 2nd Jan 2012, 11:04 pm   #18
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Default Re: AKG C60 valve mic

Well, i don't think the 5703 is a zero bias valve. I think the schematic is flawed.
-Chris
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Old 3rd Jan 2012, 12:31 am   #19
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Default Re: AKG C60 valve mic

Quote:
Originally Posted by mrmagnetophon View Post
I think maybe that resistor is just way to big,one again tried another one, you go through the resistor and too much hum, remove it and no hum. Not tried with the mic on yet of course, but i think maybe the value is too high?
Which circuit are you referring to here Chris? Whichever it is, I would expect AKG to have placed the 180M resistor and the impedance matchimg valve within the body of the microphone, very close to the capsule. Is your arrangement different?

John
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Old 3rd Jan 2012, 12:47 am   #20
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Default Re: AKG C60 valve mic

The Akg circuit John. Yes the valve and electronics are inside the body of the mic.
The power supply then includes the balancing transformer.
Thank you,
-Chris
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