Neumann U67 circuit experiments knowledge & experiences needed .......

[gary o]

Hi Im experimenting with this circuit to learn & make a nice mic ..... I know there are boards & a build thread already but was hoping to learn a little more about the circuit....

I have made a point to point version of the circuit minus correct transformer & feeback parts R18 R19 R20 C3 & C18 not using a K67 at moment... Im getting a cheap K67 to play with & may buy maxes BV12 later just wanted to get the circuit running & see how she works..... so ANY info would be great..... heres a schematic I followed for my experiment .....

http://www.danalexanderaudio.com/Micpics/Neumann/U67Schematic.jpg

I have read some interesting things so far like C17 may have been after thought & different 1 uf value for C9 but any ifo will be great Im not too hot on some basic electronics tho ....

As for my circuit it works as is sounds very bassy but the low cut works as does -10db....

Not sure whats goin on with my voltages at the moment my B+ is 215V at the moment the anode on schemtic says +75V Im sure I had 80V there but now theres 183V seem a lot ? but then theres only one 150K R14 between B+ & anode ?

At R12 R13 node my meter read 43V ..... tried another meter it read 48V.... both meters are fairly cheap but I would have thought I could measure accurately at this point in the circuit ?
Any way I lowered value of R15 from 3M3 to 2M2 & meters measure 59V & 64V now ...

I have several used EF86 tubes but cant seem to find one without at least a little white noise.... still wondering if its something else tho one tube seemed the best but now sounds terrible with white noise.....Im worring about the anode voltage now ..... I have made many many point to point experiment mics never had noise issues with 6072s & 5840s 6AK5s & so on , I have read EF86s seem to get noisey.....

Im A & B ing my U67 mic with Violet Vin67 (flat) capsule & BV08 tran for now  against a Ela 6072A circuit with a 1 gig grid resistor that has Tim Cambel capsule & Haufe T14/1..... the U67 has more level quite a bit more bass prob too much sounds a bit ribbon like but with more top its fun but the white noise is annoying tho not terrible.... under haeting tube a little 5.7V as schematic...

Ant thoughts observations more than welcome..... big thanks

 

[gary o]

Update I had bad connection plate voltage is better 72V in ball park I guess... cathode R10 R11 node is 1.75V good I think

Re the noise its not terrible now but still there I think its from the filiment power supply.... Im using a LM317 at moment its interesting that the ground is the heaters + I never seen a mic like this before , I tried swapping polarity no better .....

I have now changed the circuit to a M269 but using a 6AK5 (like 5840) in place of the AC701...... it has lower 120V B+ and a few componet changes & filiment power supply in more the usual BUT this mic has more noise still .........I worked out its coming from filiment power supply I guess because its connected to cathode...... So I powered filiment with a battery....... silence......I then tried to reverse the filiment supply as per U67 heater supply + connected to ground of mic not silent but much much better......should i leave it reversed, how do I make a silent power supply, I see that supply for similar M49B has a choke ?

What I dont understand is the noise being added bt supply is constant white noise mush sound not hummmmm I wonder why this is and what I can do about it ..... the battery sound great but not practical.....