23 May 2020

6AK5 Microphone build thread

I'm not a professional blogger, doing some organizing posts here. I think maybe somebody might find this helpful.

MXL V89MBP donor body for RoyerMod/BigUgly/G7'sh mic update 21-July

Picked up one of these that was not working on battery. Had the hole already cut and a switch so thought to make cardioid/omni version of the Schoeps circuit. (I am not interested in doing any metal work)  Upon a brief inspection, some of the circuitry is on the main board on a smd circuit board, and some on the board containing the switch. Seemed too complicated now.

But there is about the same amount of space as found in the MXL 9000/76t; so I thought I could just gut the thing and do a point to point SELA/Royer'sh with a 6AK5 instead of a 5840 and a dual sided capsule. I have some generic K47 clones so will use that. Will post some pics of the internals soon. One more project on the table. This should be relatively simple though.

Have taken apart mic partially, removed XLR plug (which was in pretty tight) remembered this time that the screw is threaded counter-clockwise and tight is up not down. The switchcraft I have will go in much easier. Need to solder the wires to it next. I've sketched out a schematic and a parts placement guide. Plan to leave the switch board in place as it already is lined up, and just remove all the other parts on it. Here is my schematic, the circuit is pretty much the Royer/SELA with an added grid resistor (I have a 2G to try out, with longer leads than the 1G's from the original mic) also an omni/card pattern switch. I want to keep my PSU's consistent (for no good reason, I'm not going to have that many....) so will use the  100/60 voltage divider as in the Royer rather than the  120/60.

 Here is my proposed layout, basically following the schematic:



The transformers are noname appr 11.5:1 or so. I used one in my other RoyerMod 2001, and in the "U87 Front End" mod I did to another 2001. I got several of these off Ebay and in a trade.



UPDATE: 21 July

Got the sig lines figured out. I'll post some pictures soon. Got the perf board laid out. Getting the tube to fit is going to be a bit of a challenge, I may use a chassis mount socket and remove the bracket, then zip tie it to the perf board.

UPDATE: 5 August

Here are some in progress photos


Here is the original guts:


Some of the new parts, a five pin switchcraft xlr, and the noname transformer




The original PCB, removed the teflon standoffs.

The new circuit board, pretty much soldered.


Removed the parts from the switch PCB, drilled some holes to mount a tube socket; it's a chassis mount tube socket,I need some screws for the standoffs


saved the teflon standoffs for the omni switch



The circuit is soldered, mostly everything is all ready to go, just need the screws for the standoffs. These need to be about 25mm or so.

UPDATE: 17 August

17 August, 2014 22:21I looked over some of the work, and mis-soldered some of the connections on the tube socket. Pay attention the first time.

UPDATE: 19 August

Now I've decided to go ahead and raise the B+ for the tube (why not, makes it more complex doesn't it.) That will mean either my PSU's aren't interchangeable, or I'll need to modify my other BigUglyRoyer.  ~160v seems like a good compromise if I want to try other tube circuits. The PSU PCB will accommodate a simple voltage divider, and I can use a 2-3M resistor in parallel with the 3M on the perfboard to take the 160v down to 60v for the capsule, and that will preclude having to rip apart my circuit board.  (at least that's the theory.)


13 August

 http://www.filmsoundsweden.se/backspegel/sela.html

 The Sela 12 uses a 60v B+, so no voltage divider for the capsule. Pretty simple circuit. I am thinking about bumping up my B+ a bit higher on this one though. The 6AK5 looks like it can take a bit more than the 5840. I'm not sure that would make that much difference though. I also have an MXL 9000 that I've been looking at. There was a discussion on that mic at GroupDIY. Although looking at it, I had a bright idea to put in an EF86 maybe and swap a few parts. The tube is wired in parallel so comparing the pinouts for a 12A*7 and the EF86, it's only a matter of swapping a couple of wires for the tube. Then look at the G7 vs the MXL9000?

21 August

Changed my mind about upping the B+ don't see where that is worth the trouble. Also, the кондензатор turned out to be too big, so in goes a smaller 2.2/250v.  

27 August

All the soldering is done, now just putting it back together. Took some pictures, but a little embarrassed; some of the worst soldering I've ever done...

7 September

-Smoke test sat

-Optest sat, although I may have wired the omni/card switch the other direction. Will need to actually use next to see how it sounds,just did a quick check out








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