Behringer B2 Pro to G7

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sbranco

Well-known member
Joined
Jul 25, 2005
Messages
86
Location
Northwest NJ
Hey you guys,

We are finally ready to try modding a B2 pro into a G7. Planning to do it Friday. This is basically going to be on perfboard like Michael Krusch's Rode NT mod to a g7.

However, we are going to use a Cinemag 2480 transformer with a 1:10 ratio. I think this transformer should give us a lower signal than the 1:5 Lundahl 1538. Its a tight fit and the 2480 is small.

I seem to remember a thread that modding the cap and/or resistor between the tube and transformer can better match the two.

We have never had gain problems but I am trying to plan for contingencies.

Any ideas what to try if we are not getting enough gain?
 
Try it in real-life before getting worried..

And btw, perfboard is not really good in microphones where you want high-high-high insulation resistances..

Jakob E.
 
Is it because of all the copper on the board?
I've been doing some good work with these things, I'm also finishing up a G7 look-a-like, with different electronics and a Cinemag CMO-5/200CT which is on the way. I setup the tube circuit frequency response to start diving where the capsule's frequency response spikes up. All resistors to the capsule are 1Gig and coupling cap from the CF to the trannie is 10uF polyprop.
It should be fun to see what I can achieve with a cheesy old 797 capsule.

Analag
 
[quote author="sbranco"]
However, we are going to use a Cinemag 2480 transformer with a 1:10 ratio. I think this transformer should give us a lower signal than the 1:5 Lundahl 1538. Its a tight fit and the 2480 is small.[/quote]

Ok, lower voltage output, lower output impedance.

I seem to remember a thread that modding the cap and/or resistor between the tube and transformer can better match the two.

I'm afraid I don't remember this.

We have never had gain problems but I am trying to plan for contingencies.

Any ideas what to try if we are not getting enough gain?

Hmmm. No idea. Check over the wiring again and again? That's what I usually do. There aren't really any common errors I don't think.

And btw, perfboard is not really good in microphones

Is it because of all the copper on the board?

Answer:
high insulation resistances

Yeah, I suppose the tracks are a little too close, and also quite thin for some applications. Has anybody ever noticed large parasitic capacitance in perf-board?

For high-impedance capsule areas, people sometimes de-flux and clean the board to avoid problems, noise, etc. This wouldn't really be easily possible with perf-board (I often have flux joining tracks together, but it's not usually a problem).


[/quote]
 
OK so we didn't finish but we did get started.

Here are some photos of our progress. Thank you guys for your replies.

I will update more as we go. Using the smaller Cinemag 2480 transformer gave us more room to spread out the components. We are flipping the tube over from Michael Krusch's version to get the tube closer to the capsule.

We are putting the psu in a disconnect box we got from Lowe's. It was about $8 and is a strong steel box that might be used on the outside of a house for an air conditioner electrical hook up. Just about the right size.

BehringerB2toG711-25-05008.jpg


BehringerB2toG711-25-05004.jpg


BehringerB2toG711-25-05011.jpg


BehringerB2toG711-25-05012.jpg


BehringerB2toG711-25-05013.jpg


BehringerB2toG711-25-05002.jpg
 
[quote author="gyraf"]Try it in real-life before getting worried..

And btw, perfboard is not really good in microphones where you want high-high-high insulation resistances..

Jakob E.[/quote]

We should clarify what you mean by perfboard. Are you referring to perfboard with the copper through holes on one side, or even perfboard without copper?

I always use perfboard that's just holes in fiberglass (phenolic, or whatever it is =) ).


agreed, nice work there.
 
[quote author="fum"]
We should clarify what you mean by perfboard. Are you referring to perfboard with the copper through holes on one side, or even perfboard without copper?
[/quote]

Good point. That stuff is track-less. I thought you meant the usual vero-board with copper tracks. So, that stuff is effectively point-to-point then?

Not IC friendly!!

Nice pics - who are those geeks in the last picture though? :razz:
 
Hey its just the standard perfboard without any copper. So it is effectively just fiberglass or whatever the board is made of with holes in it.

I labeled the pics in photobucket but the labels didn't come through.

The last pic is of our engineer (drummer) and electrician (guitarist) from left to right.

Pics include the tube side (no tube mounted yet), component side, inside of the power supply, power supply box and diy geeks. (I am supervising the work and doing important stuff like taking the photos).
 
I use the copper clad perf board and I simply remove the copper from around the sensitive areas...it's really quite simple.
 
Ok we are still working on this project. Finally got a chance to fiddle with the power supply. I am getting really good at discharging caps. Its a little disturbing but preferable to the alternative.

We either had a bad LM317 or the pins were not making contact. Used a dremel to ream out the three holes for it in the pcb and finally got our heater voltage to come through.

We had 17V going in. With a 1.8K resistor, the voltage out was about 12 or 13 V. So we hooked up a little 5K trimpot in place of the 1.8K. The good news is that we were able to adjust the voltage with the trimpot. The bad news is we could still only get it down to about 7.8V.

Any ideas on what to do next? Our regulator is the LM317AEMPCT-ND.
 
Thinking about it, our 17 volts in is almost double what you would expect. Maybe we hooked up the first transformer in series by mistake. We were intent on figuring out how to hook up the second transformer in series to make it a 220 rather than 110. and the voltage we got with the 1.8K resistor was 12 or 13V, again about double what we need. Have to check with my other diyers. Does it make sense that that could be our problem?
 
Was browsing through the Turnkey catalogue (London based cattle market for audio gear) and noticed that the B-ringer C1 costs £25.99. Could this make a cost effective housing for the G7 project? According to the manual available on their product page the dimensions are

shaft: 54mm, length: 139mm

Judging by the scaling of Gyraf's PCB design on this here monitor I reckon that might be just a little too tight to work. Figure that maybe you could squeeze some kind of trackless perfboard construction in there though.

EDIT: I just kinda answered my own question by looking around on the G7 posts. There's a reason why people go for the B2 Pro - it's the right size and you can reuse most of the bits...
 
The Deadringer C1 looks pretty crap. The housing doesn't look great quality and the grill might not sound great.

I'd stick with the B* models.
 
Just after this post was first made I have completed a G7 looking mic with a B2 Pro capsule. I used a modified U48 circuit to bypass the need for a cap between the capsule and tube grid. 200V B+, 160V, 80V to the capsule, 6.33V at the heater pins. How does it sound, in a word excellent, very in your face type sound. I am now building another G7 looking mic with a CEK-367 capsule, a seven pin tube and a Cinemag transformer will do the honors this time. In fact I used this circuit in a cardoid mic which turned out so great I had to used it one more time, considering I never use the same circuit twice. I built a discrete version of the Nady TCM1050 PSU and wired all my tube mics to run off of either one, with my version the self noise from the mics are almost non-existent. Only when using this PSU can I use my JFet/Tube input mic pre and get noise levels comparable to transformered input preamps. In tube mics the circuit does matter contrary to what is often said especially when working with Chinese capsules, that need to be interfaced with a head amp with a curved frequency response.

Analag
 

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