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Old 5th Jan 2012, 11:09 pm   #41
mrmagnetophon
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Default Re: AKG C60 valve mic

Well i've stuffed it back into the case. And testing it again, its not humming, but the valve just blew from a lose connection. Will fix it up and let you guys know.
-Chris
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Old 6th Jan 2012, 7:17 am   #42
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Default Re: AKG C60 valve mic

Tried again, with new valve. still hums, anyone have ideas?
I've done all logical trouble shooting to no use.
-Chris
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Old 6th Jan 2012, 11:57 am   #43
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Default Re: AKG C60 valve mic

Box it up, and hide it under the bench.
Or admit defeat use an FET.
Rob.
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Old 6th Jan 2012, 8:46 pm   #44
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Default Re: AKG C60 valve mic

Can't do that Rob, its got to work. I've became quite annoyed, therefore im installing a new ground buss bar through the studio and test area, to eliminate the problem hopefully.

-Chris
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Old 6th Jan 2012, 9:14 pm   #45
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Default Re: AKG C60 valve mic

When you remove the valve the hum stops, there has to be a problem with the gain of these valves, but i have tried many of them.

-Chris
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Old 6th Jan 2012, 10:15 pm   #46
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Default Re: AKG C60 valve mic

Chris

I wouldn't immediately condemn the valves, after all you are attempting to reproduce commercial circuits, many examples of which must have been sold and performed as designed.

I think that your problem is caused by any one or combination of these issues

1. A poor or non-existent earth connection e.g. to the fixed part of the mic capsule.
2. Unfavourable layout, causing hum pickup or instability.
3. An earth loop.
4. Excessive ripple on the HT supply.

Unfortunately, it is very difficult to fault find such issues from afar.

Do you have access to an oscilloscope?

John
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Old 6th Jan 2012, 10:47 pm   #47
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Default Re: AKG C60 valve mic

I have access to my very old 2, no grids (graticules) are on the screen, but they are usable to rough probe yes.
-Chris
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Old 7th Jan 2012, 2:49 am   #48
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Default Re: AKG C60 valve mic

I am building a totally new power supply. Now the transformer has a ground pin on the plug, my mixer amp is grounded. Will that cause hum?

-Chris
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Old 7th Jan 2012, 3:48 am   #49
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Default Re: AKG C60 valve mic

Also, refer to the schematic, the connection between pins 3 and 6. Is that a .25 or 25 mfd capacitor?
Thank you,
-Chris
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Old 7th Jan 2012, 12:57 pm   #50
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Default Re: AKG C60 valve mic

If you're referring to C1 in the Altec Lansing circuit, it states 25 MFD.

I have some reservations about this circuit, namely :-

1. A pentode is used. This is likely to be noisier than the triode used in the AKG circuit.

2. There is no grid leak resistor. As a result, the g1 potential and hence bias could vary widely from valve to valve.

3. Related to 2. A true condensor mic capsule i.e. not electret requires a DC polarising potential. In this circuit, this will be obtained from the uncontrolled g1 potential due to the lack of grid leak resistor. Performance is likely to vary widely from valve to valve.

4. A DC heater supply is used but this is unregulated. The heater voltage will depend strongly on transformer regulation and on the forward voltage drop of the rectifier diodes. If the original circuit used selenium types then use of silicon rectifiers is likely to result in a higher heater voltage which would over run the valve heater.

The AKG circuit employs a triode valve, employs a grid leak resistor and has a sensible means of providing the poalrising potential. It also provides a regulated DC supply for the valve heater. I think you would be much better off considering the AKG circuit instead.

John
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Old 7th Jan 2012, 3:21 pm   #51
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Default Re: AKG C60 valve mic

Hi,
having a power supply with an earth connection will probably cause less hum, but it is possible that you may get an earth loop if the mixer amp is earthed by another method ( for example the audio leads to the power amp ).
as to the fault with the Microphone, one thing to consider is that when the valve is in place the circuit will be sensitive to coupling or interference, as the grid on the valve is a high impedance point. as removing the valve stops the hum this doesn't rule out that the valve may be ok and there is another fault some where.
regards Peter

Quote:
Originally Posted by mrmagnetophon View Post
I am building a totally new power supply. Now the transformer has a ground pin on the plug, my mixer amp is grounded. Will that cause hum?

-Chris
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Old 7th Jan 2012, 8:24 pm   #52
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Default Re: AKG C60 valve mic

John the problem is with the AKG circuit is I can't get any sound out of it.
They say that the 5703 valve is a replacement for the AC701, if you boost up the heater voltage, but it just seems not to work at all.
-Chris
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Old 7th Jan 2012, 10:33 pm   #53
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Default Re: AKG C60 valve mic

How are you boosting up the heater voltage when using a 5703?

Are you using any of the original AKG components or layout?

John
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Old 7th Jan 2012, 11:10 pm   #54
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Default Re: AKG C60 valve mic

The original layout, not the original components.
-Chris
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Old 8th Jan 2012, 2:34 am   #55
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Default Re: AKG C60 valve mic

Well imagine this. I have 2 schematics and 2 circuits set up on a bench, each at different ends.

I will now jump back to the 5703 valve, do not confuse yourselves here.

Here is my layout: A capsule connected to 3 feet of low impedance microphone cable running down into the bottle with the 5703 valve connected. The valve runs into a connecter which then is attached to 20 odd feet of microphone cable 5 wire.

The wire runs into the power supply and is being fed:
A 5 volt heater feed +
A 150 volts B+ feed
A common negative
A signal output

The signal output runs into the schematic circuit, through the transformer and capacitor back to common negative.

-Chris
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Old 8th Jan 2012, 3:03 am   #56
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Default Re: AKG C60 valve mic

OK, well with either of these circuits, if there is more than a couple of inches of any sort of cable between the capsule and the valve, you can expect big problems.

Neither of the 2 circuits you've shown here require a 5V heater supply. One requires 4V, the other requires 6.3V.

Please realise that, if you wish to reproduce a microphone circuit such as this and expect it to perform in the same way as the original then every aspect of the circuit and its layout is absolutely critical.

John
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Old 8th Jan 2012, 4:54 am   #57
mrmagnetophon
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Default Re: AKG C60 valve mic

Well the AKG mics used extension poles that were about 40 inches long, that would go between the capsule and the mic body-valve.
The 5703 valve uses 6.3 volts for heater, I am under powering it with 5 volts to save the heater, pulls a little more current I know, but still heats well.
-Chris
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Old 8th Jan 2012, 10:23 am   #58
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Default Re: AKG C60 valve mic

You should not under power a valve heater, it leads to cathode poisoning
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Old 8th Jan 2012, 8:29 pm   #59
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Default Re: AKG C60 valve mic

Can we assume that the microphone cable you are using is twisted pair with an earthed braid?

Ordinary coax might cause problems.
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Old 8th Jan 2012, 8:44 pm   #60
mrmagnetophon
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Default Re: AKG C60 valve mic

Well I'll correct that and put the 6 volt supply on it. (might be causing more problems)
The cable that I am using is called audio-pipe, 2 wires surrounded by a very thick braided shield earth.

-Chris
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