BPM CR-76 tube mic

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gec

Well-known member
Joined
Feb 18, 2007
Messages
78
Location
Italy
Hi,
I had this gift from a friend of mine and when I checked it I heard a lot of windy noise.
bpminside4.jpg


So I thought to clean the capsule, but I didn't pay so much attention and some gold is gone.

capsulea.jpg


The microphone is working, but I don't know how it was working before. So it's time to improve it.
Maybe somebody can tell me what to do to mod this mic...

bpminside2.jpg


-First of all the capsule (it's 34mm in diameter): do you think is better to restore or to change it? Is this model like a peluso CEK-367?

-The tube: It's not clear, it seems written 6j33b but i didn't find anything with this code, it's 8 pin. On the BPM site in the forum (in german) it seems that the first batch came with a 6b33 (?) the new ones with the 6ak5w mounted with socket.
Is there any equivalent better tube?

-Should I re-cap it?

Thanks, Gec!!!
 
The body looks a little like a B2 pro, does the top black plastic disconnect from the bottom?

First thing to do is find the tube data sheet. Maybe it is a good tube for the microphone. I would guess it is a pentode wired as a triode I have not found a spec sheet yet on the web.

Is there a transformer on the side not in the top picture? If not is there a output transformer in the power supply?

Next trace the circuit. Is it omni and cardioid? It is hard to tell if there is another pattern like 8 from the pictures.

I see three red 1 gig ohm, 1KM

red, purple, black, orange, gold?
green blue yellow gold?
blue grey black red gold?

What value on the red resistor at the bottom left?

1uf output cap?
green film cap value and voltage?
electrolytic cap value and voltage?
ceramic disk cap value and voltage?
 
The body looks a little like a B2 pro, does the top black plastic disconnect from the bottom?
Yes, it does
First thing to do is find the tube data sheet.
I've found the data sheet only for the second batch tube the 6ak5w
Is there a transformer on the side not in the top picture?
Yes, here it is

bpminside3.jpg


Next trace the circuit. Is it omni and cardioid?
Yes it's omni and cardioid, here is the schema, sorry, it's the first time I use this kind of software. For different components I wrote the codes I read on them, I'm newbie and I'm not sure about the exact meaning of all the codes :oops: I measured some voltages around...the tube is bottom viewed

76schema.jpg


bpminside6.jpg


bpminside5.jpg


Thanks for your help, Gec
 
Interesting

A few GUESSES and some math

6j33 might be a Russian tetrode (I have not found a data sheet yet) with grid 2 connected to the anode. 5.75 VDC fil looks fine. Three grounded pins would make me think one is one lead of the heater and the other two shield connections.

820 ohms looks like it might be the cathode R does it have about 1.2 volts DC across it?. Might be fun to cap bypass.

158VDC - 57VDC =101VDC across 68K is about 1.48ma.

C2 C4 and C5 might be fun to change to types of film caps of the same value and voltage ratings

High pass and pad look to be switched, I would think C5 is the pad and R7 the high pass

C3 is 60VDC rated?

Google ask etc the neumann m49 versions and KM53 54 versions schematics. C4 and R5 look like feedback from anode to grid.
 
[quote author="Gus"]
Google ask etc the neumann m49 versions and KM53 54 versions schematics. C4 and R5 look like feedback from anode to grid.[/quote]

Good one, Gus :wink: Thats what the legend says about those mikes:
The guy who made them was a former Neumann R&D engineer. He left Neuheiser, found BPM and sold some of his mikes for very fair prices (made in Berlin, not China). Then he was forced to quit his business through Neumann/Sennheiser lawyers because he had "stolen" their circuits. :roll:
 
The story I heard is very different. BPM's American distributor was PMI Audio. After a while PMI got inquiries about the true origin of those BPM mics which were advertized as Made in Germany. People who took them apart said they looked pretty much like Chinese mics. And apparently they were. When PMI asked to see the production facilities, BPM finally admitted that the mics were in fact Chinese made. PMI then ended the business relationship and later on produced their own line of mics, which came from the same factory as the former BPM mics, and in order to avoid any confusion, the original B- and C-series mics had "797 Audio" written below the SP logo.

A while ago I found some stuff on rec.audio.pro that seemed to confirm that version. BPM definetely bought mics from 797 and there was some kind of dispute between them. BPM accused them of bad workmanship and 797 claimed BPM didn't pay them for a whole batch of mics. What is open to dispute is the degree to which BPM altered the mics delivered from China. Maybe they used their own circuits - although I see the familiar red 1KM resistors - but I'm pretty sure the capsules were 797 capsules. I wouldn't know who else should have produced them.

If I remember correctly the BPM founder was a musician, not a technician. I may be wrong, though.
 
People who took them apart said they looked pretty much like Chinese mics. And apparently they were.

In fact, looking for an equivalent of the tube 6j33b, I ended my research here
Do you know how to win this auction? :grin:

Ok, my questions are still:

-Which capsule can I switch for? Is the Peluso CEK-367 a way to go for?
-Which tube can I use instead of the chinese toy?

Thanks for your help, Gec.
 
First how does it sound now?

2nd what do you want?

3rd what is the transformer specs?

Do you want to clone something or build something?

If the circuit does not sound they way you want changing the capsule to anything else might be a waste of time and money.
 
Ok, here is my take on this....The capsule is destroyed. You can see the holes in the mylar, so that is most likely the cause of the windy noise. The design is based on some old tube mikes and actually is not bad.

If it were me, I would leave the mic alone, get a replacement capsule....about 100.00 Euro and install it. I bet the mic would sound quite nice then.... You could acheive better performance by changing the transfomer, but you would be spending a good deal of money and time. My suggestion is to replace the capsule and leave it be. I know that mic and it sounded pretty darn good....
 
Gus:First how does it sound now?2nd what do you want?3rd what is the transformer specs?Do you want to clone something or build something?
Ok, it has a very nice top end but no low end, and it's a bit noisy. What I want from this mic is a good working mic (good... not great). I don't want to transform it in something else. To clone something or build something I'll spend my time on a G7 or in a proved mic mod.

I wrote to BPM and they told me that I can send the mic to a BPM service to change the tube (and some parts) for a better low frequency response (and noise reduction), all for 79 euros. I think it's worth it.

Gec.
 

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