Tube condenser microphone help!
 

Hello, I am very new to this site. I hope this is in the correct place.

Recently i built a AKGc60 tube microphone, but all it does is hum, i've been trying to locate the problem but no luck, if i replace the capsule with a capacitor it still hums. The only time it doesn't hum as much is when i remove the resistor running from the plate to the cathode.

You definitely need to ask some tech to look at your microphone. It may be hard though to convince some tech to try to help you after you found there a resistor running from plate to cathode. ;)

If you look at the schematic of the AKG c60, its supposed to be there.
Also, i have been a tech for 10 years speciality being tubes, and this mic is just fighting me the whole way.

Can't you measure voltages on anode and cathode? Ripples of B+? Does Selenium rectifier awfully smell? Does current source supply 4V to the filament?

And what purpose serves your resistor from plate to cathode?

Which resistor are you referring to? I don't see any resistor going straight from the anode to the cathode.

If you mean the bleeder R7 in the N60A unit which goes from anode to ground, then you may have to check whether all your supply electrolytic capacitors are OK.

...and check layout. Minus from rectifier can be connected to the ground in wrong place.

I think he's talking about the 180 Meg. grid resistor from grid to bottom of the 1.5K cathode bias resistor. (180 Meg is pretty large for a grid resistor isn't it?)

True, but in a condenser microphone the grid or gate resistor must be very large, otherwise you get too much current noise and low-frequency roll-off. In solid-state condenser microphones 1000 Mohm is not unusual.

These things are very fussy regarding layout and construction. They make noise if the layout is wrong. They make noise if you use the wrong type of PCB material. They make noise if the lead dress is slightly wrong. They make noise if you don't clean every trace of flux of the board, or if you use the wrong type of chemical to clean the board. They even make noise if you use the wrong type of PCB standoff. We don't even touch them when they come in the shop, just send them off to a guy who's been fixing them for 30 odd years...

You might try taking this to the prodigy pro DIY forum. There are a number of mic experts over there.

I see no resistor from plate to cathode. Which resistor are you referring to?

The circuit is a bootstrapped cathode follower that has an input impedance much higher than the 180M resistor would suggest.

I assume since you are familiar with tube circuits that you have already checked the voltages and everything is normal. You also have no way of measuring the grid voltage with common equipment, so one remaining possibility is that your tube is a little gassy and won't work in a mic circuit.

Also you say you built the mic. Are you using an AC701k tube or something else?

Maybe try connecting a low impedance generator as input just to see if signal gets through. If everything seems OK then I would try a new tube.