Low Output From Tube Mic

[terrapin]

It is a pleasure to become part of this forum!  So much great information to be learned.

I'm in the process of building C12 type microphone-Peluso CEK-12 capsule, 6072 tube, and T/14 type trasformer.  I've got it assembled, but I'm getting very low output with little top end-quiet and muffled.  

I've cleaned the board, and double checked all my wiring (except for the transformer-its a bit tougher to get to) and everything looks pretty good.  I've tried a couple of different tubes with the exact same result too.  Voltages appear to be about right going to the tube.

Does this sound like a problem associated with a certain error?  I've used the search function and looked in the META's, but couldn't find a similar situation.

Thanks,

Ron

 

[bdubya]

Get in there and get a good look at what's going on at your output transformer, it seems like a likely culprit, if all else looks good.

 

[pucho812]

post pictures so we can help you...  also double check all your solder connections/wiring it's possible you missed something.

 

[rodabod]

Standard fault finding process.

PSU voltages good? Reasonable DC readings at the valve pins?

If you have the ability to 'scope or listen using a probe, then AC-couple the output from the plate (note it will be ~20dB higher than the mic output, unbalanced, and high impedance to the tune of say, ~100K).

Injecting tone can be handy. The valve stage plus transformer step-down isn't far off unity-gain, so drive the grid with low level tone (say -10dBu) and listen to the mic's output. Also drive into the capsule's coupling cap (the usually ~1nF guy).

That, plus a bit of logic wil give you a bit of a start.

 

[terrapin]

I apprecieate all the tips-this will give me a place to start.  I'll report my findings.

Thanks,
Ron

 

[terrapin]

Had a chance to test out my faulty mic.

Voltages were all in the ballpark of the C12 schematic.

Injected a tone (measured -11 dB when put back into the DAW) and got:

-22 dB at the plate output
-33 dB when feeding the signal into the grid.

Does this give any clues?

Thanks
Ron

 

[terrapin]

Just bumping this to see if anyone has any suggestions.
Ron

 

[pucho812]

what schematic did you use. are your PSu voltages in the ball park? heaters? plates? how are the solder connections?

 

[rodabod]

Injected a tone (measured -11 dB when put back into the DAW) and got:

-22 dB at the plate output
-33 dB when feeding the signal into the grid.

Does this give any clues?

Were you injecting before the 1nF cap which connects the capsule to the valve grid?

How did you make your output measurements?

 

[terrapin]

Were you injecting before the 1nF cap which connects the capsule to the valve grid?

How did you make your output measurements?

After tinkering with the mic a bit more a took more measurements.  I put the tone into the pre and my DAW registered -25 dB
Tone before the 1nF cap: -27 dB
After the 1nF cap (directly to grid): -18.5 dB
Before plate/transformer coupling cap:  -8.9 dB
After plate/transformer coupling cap:  -6.9 dB

The signal was going out of the mic, through the power supply, through a mic pre, to my DAW.

 

[rodabod]

After tinkering with the mic a bit more a took more measurements.  I put the tone into the pre and my DAW registered -25 dB
Tone before the 1nF cap: -27 dB
After the 1nF cap (directly to grid): -18.5 dB

I'm still having trouble visualising this. Are the two dB values above describing the output of the mic at the transformer secondary?

It doesn't make sense that the level at the grid would look greater than what it was before the 1nF cap; there's no gain there.

 

[terrapin]

Yes the measurements reflect the mic output-the -25dB measurement is the tone injected straight into the mic pre I was using.  All other measurements should be based on this.

I'll take another measurement around the 1n capacitor and report back.
Ron

 

[terrapin]

All voltages are very close to the C-12 schematic.
Did more measurments-these seem more accurate:

Signal fed direct to DAW:  -25.8 dB
Before 1nF cap:  -29.6
After 1nF cap:  -29.6
Before plate/primary coupling cap:  -20.9
After plate/primary coupling cap:  -17.5

Ron

 

[rodabod]

Ok, now that seems clear to me.

So you've got ~12dB gain, end to end. Considering the step-down output transformer (~ -20dB), that gain sounds fairly reasonable to me.

So, next thing I'd try is to sub the original capsule in, after making sure that the new capsule is well insulated and not getting shorted to ground in any way (this includes making sure the Hi-Z area of the PCB, which is anything prior to the grid connection, is clean of any dirt or flux).

 

[terrapin]

Great, will give this a try.

Ron

 

[lordscruffo]

Hi,

  Did you ever resolve this issue?  I just built the c12 clone as well and have run into the same low output issue!  What did you do to fix it?


-Daniel