Hi there,

I have two altec 150 microphones with new tubes. Their PSU has been rebuilt. According to the original schematics, all the voltages are about 10% higher but the microphones works fine.

My only concern is their output gain. It is pretty low and I was wondering if that was normal. if I plug them straight into MOTU 828 sound card built-in mic preamp, I have to put the trim all the way up to get proper gain, even then shouting at more than 30cm away from the capsule does not overload the preamp.

What do you think ?

Alex

Did you measure the voltages whilst the PSU was`under load (ie. with the microphone connected? The voltage will probably "sag" down. 10% doesn't sound too bad anyway.

Remember you can't measure the capsule voltage with a DMM since they are very high impedance.

Perhaps someone who owns an Altec 150 can tell you about the output level (or look at a spec. sheet).

Idoia wrote:I have two altec 150 microphones with new tubes. Their PSU has been rebuilt. According to the original schematics, all the voltages are about 10% higher but the microphones works fine.

Well I did both types of measurements as the schematics also provide voltages without load. And yes 10% is the maximum of excess of voltages that I have.
A DMM ? What is it ?
It may also be useful (maybe not) to precise that my caps are 21B (closed and with holes)

Idoia wrote:A DMM ? What is it ?
It may also be useful (maybe not) to precise that my caps are 21B (closed and with holes)

DMM is short for digital multimeter.

What are 21B caps?

Maybe you could put this post in Klaus Heyne's mic lab.... Be careful though, there is a fascist regime going on over there...

21B are the capsule of the mic.
They are omni and have been made right before the 21D.

DMM, ...of course...

Klaus Heyne's mic lab: url ?

Idoia wrote:21B are the capsule of the mic.
They are omni and have been made right before the 21D.

Ahh.

Klaus Heyne's mic lab: url ?

http://recforums.prosoundweb.com/index.php/f/33/0/