Tube mic circuit please help

[dax]

Hi All,
I have a older u87(non ai)  which has been gutted and a tube circuit installed. (I received the mic with no prior knowledge of who or what may have happened)
The modified circuit was  basically  installed in the space under the capsule's "socket" and now the plastic has fallen apart leaving  me to reverse engineer / repair the circuit.  I have all ready purchased a new socket from Sennheiser and plan to bread-board the components and  mount the tube similar as before BUT I am having trouble with the circuit as it fell apart before i could accurately trace it.  I have done the drawing below to the best of my ability but it is not working as it should.  can anyone  help or see where I may have gone wrong

 

[RuudNL]

I would check the schematic again, because there are errors.
For example: the 1 uF capacitor should be connected to the anode of the tube.

 

[panman]

I would check the schematic again, because there are errors.
For example: the 1 uF capacitor should be connected to the anode of the tube.

No, that 1 uF is correct. But the heater is missing the other leg, so it doesn`t work.

 

[dax]

Thanks, I am mostly unsure of the polarization voltages.  Specifically that thete is 120vdc on diaphragm.  I am not designing this circuit but rebuilding it.  But I'm not certain I have re drawn it correctly

heater pin 9 is grounded 

 

[gyraf]

120V polarisation looks right - if you have +60 fixed on backplate, then  0-120V polarisation on back membrane gets you +/-60V relative. See e.g. the G7-page for description.

You need a ground/0V at front membrane circuit, giving you a permanent -60V relative to backplate.

I don't like the 0.01uF from tube anode resistor to grid. But it may or may not be right anyway (order of magnitude?).

The output transformer driving capacitor is probably shown connected to the wrong end of the 100K anode resistor. Otherwise you are adding these 100K to tube's internal resistance. Why would you?

Are you sure the circuit actually worked well at any point - that it's not someone's mod-attempt gone horribly wrong?

Jakob E.

 

[RuudNL]

The output transformer driving capacitor is probably shown connected to the wrong end of the 100K anode resistor.

That's what I said...

 

[panman]

The output transformer driving capacitor is probably shown connected to the wrong end of the 100K anode resistor.

That's what I said...

Oh yes, the pic only shows a small portion of the schematic an I missed the resistor.  I only see that now when I copied the whole picture. My bad!

 

[dax]

Thanks for help so far everyone.  I am a little unsure of the proper terms for the capsule components.  I believe it is an early U87 capsule and the drawing is correct as I measured the pins to the lead wires for continuity.  The numbers i used are the pin numbers from the U87socket

120V polarization looks right - if you have +60 fixed on back plate, then  0-120V polarization on back membrane gets you +/-60V relative. See e.g. the G7-page for description.
You need a ground/0V at front membrane circuit, giving you a permanent -60V relative to back plate.

Ok yes that sounds right now finding that voltage is my task.  The back plates are connected together is this correct? 
there is currently (not shown on my drawing ) ground connection that I am unsre where to connect to the circuit

I don't like the 0.01uF from tube anode resistor to grid. But it may or may not be right anyway (order of magnitude?).

I believe this is correct to the original

The output transformer driving capacitor is probably shown connected to the wrong end of the 100K anode resistor.

I most likely drew this incorrectly and it is wired as it should be

Are you sure the circuit actually worked well at any point - that it's not someone's mod-attempt gone horribly wrong?

  Yes I am sure it worked previously. this is my attempt to rebuild someone else's circuit gone horribly wrong 
I am trying to draw the circuit but wasnever 100% on how it was configured as the whole thing fell apart- old age plastic was brittle.  It was working well until the old "plastic socket"  crumbled upon disassemble and  therefore the circuit built using those pins fell apart.  I have been reading as much as I can especially the G7 circuit .  I even am tempted to rebuild the G7 circuit but not sure it will fit in the U87 body with the tube hanging below the capsule socket.
Can I add any pictures to help?  thanks again

 

[ruairioflaherty]

Why not build a circuit that is known to perform well instead of rehabilitating a circuit of unknown provenance?

 

[dax]

I think  that is the solution as gyraf also touched on using the G7 circuit on a perfboard.  And mounting somehow