A Tube Mic

[analag]

Test the voltages before they pass through the resistors (1Gigaohm and 250Meg). Do not try to test for it at the capsule...your tester will load the circuit at that point.

 

[skipwave]

I just realized I am still wondering what purpose that 1k resistor in series with the transformer winding serves.

It is also in series with the Rk, so that totals 2.82k, and the polarization voltage divider (virtually no effect there), and all those 0.1uF bypass caps around the capsule circuit. Hmmm......

 

[analag]

[quote author="skipwave"]I just realized I am still wondering what purpose that 1k resistor in series with the transformer winding serves.

It is also in series with the Rk, so that totals 2.82k, and the polarization voltage divider (virtually no effect there), and all those 0.1uF bypass caps around the capsule circuit. Hmmm......[/quote]

A quick study of the classics will aid you in deciphering this piece.
The 1K that is floating Rk and the tranformer above ground creates a partial cathode bypass, which drives up the tube gain without the derogatory sound effect of using a cap to achieve this result. 2.82k is supposed to decrease the current flow through the tube to ensure acceptable noise level. And all those 0.1uF...c'mon. Wrap your eyes around the old German stuff and get back to me.

analag

 

[skipwave]

[quote author="analag"] And all those 0.1uF...c'mon. Wrap your eyes around the old German stuff and get back to me.[/quote]

Right, that wasn't meant as a separate question. I know the function of capsule bypass capacitors. I was wondering how it affects them to be attached to a reference above ground.

But I will certainly spend some time contemplating the classics. :green:

 

[analag]

The caps think they are connected to ground, just like the transformer and Rk. Now Rg is almost functioning like it's being supplied by a fixed bias.
BTW a big coupling cap between the tube and transformer don't equate to more low frequency response. Sometimes less is more to those who understand effect of resonance, capacitance and impedance like the old boys did back in the day.

analag

 

[guavatone]

I thought g3 ties to K on pentodes?

 

[analag]

[quote author="guavatone"]I thought g3 ties to K on pentodes?[/quote]

And if it's triode connected? And if it's truely triode connected why would we want a negative potential between the control grid and the plate anyway.

 

[mata_haze]

HI guys,
I am trying to put together the microphone but have practically no experience with vacuum tubes (and as I would like to avoid any damage..) would it be possible to get the static voltages of the circuit above?
just to make sure I don't blow anything up..

Best,
Mattia.

 

[analag]

You mean open up the mic and probe around with a tester....when all you have to do is follow the schemo. Be brave man.

 

[Sorr]

Perhaps you could tell us what brand and type 1G resistors and output cap is used and recommended by Analag?

Sorr

 

[analag]

The 1 Gig resistor came from DigiKey stock # MOX200J-1000ME-ND
output cap is Solen fast cap, if space is at a minimum you can use Xicon metalised polyester. The capsule is Peluso which I had him test for a certain sonic character before shipping out to me. It all worked out.

analag

 

[mata_haze]

hi,
I jus tput together analag's microphone and powered it up yesterday.
everything (apparently) went fine IE: no blown tubes or anything like that,
BUT...
I cannot get any signal at the output, only a 20mVpp 50Hz "sinewave".both with or without the capsule.
I did measured all the static voltages I could (.gif attached) I hope it might reveal something I cannot see.

so I wonder:
is anyway possible to test the amplifier section on the mic by itself, to make sure it does amplify and then connect the capsule?

I am using a Sowter 4383f trafo (which is 7:1 reversed) and a peluso 367 capsule.the B+ is 190V, heater 6.3VDC

I hope someone could give me any hint or suggestion.



Best
Mattia.

 

[analag]

Disconnect the 1Gig resistor and all capsule related connection from the grid. Place a 100K resistor where you have the 1Gig, this will allow the amplifier to work properly outside of the microphone body. Disconnect the transformer also and place a resistance load between the cap and ground(100K will do) you can now test for amplification. Also the Node labelled "P" should be at least twice 72.2V (145-160) Keep me posted.

analag

 

[mata_haze]

Thank you for your quick reply Analag.
is this what you meant:



Best,
Mattia.

 

[analag]

Yes but you should omit the resistor and the cap and bring that end of the signal generator (signal) to ground. Once you verify that the amplifier is working, reconnect the transformer and check the output level. This should be a simply fix. Let me know how it's going.

analag

 

[mata_haze]

just to make sure:
is this right?



All the Best,
Mattia.

 

[analag]

That is correct.

analag

 

[mata_haze]

here it comes.tested with and without transformer.
it works perfectly.
I have then tested the whole mic on the bench, by substituting the 1Gohm grid resistor with a 470Kohm one.
it seems to trasduce but the amount of noise and hum I get it's very high therefore it's quite difficult to tell if it is working correctly or not.

as I am still prototyping the board is there any suggestions on how to lay components down on the pcb in order to minime the noise level?
IE "keep heater supply away from audio" or something like this?

following are 2 links to the static voltages I've got without and with transformer.

 

[analag]

Noise and hum shouldn't be high...did you ground one end of the heater. Is the PSU working correctly? Try to keep the control grid area as isolated and clean as possible also try substituting that tube with another and see if the noise level changes.

analag

 

[mata_haze]

[quote author="analag"].did you ground one end of the heater.[/quote]

no..shall I do that?