Telefunken AR-51 mod question

[M.Brew]

I was comparing the AR-51 to the ELA M 251 and was wondering if it was possible to mod the AR-51 with a ck-12 capsule and ac701 tube. Then it seems the only difference would be that the 251 is handmade while the AR-51 uses a circuit board.

[R-AP.SCI]

Good day...

Far be it from me to state that you will achieve the lucid heights of an Elam251 by just a few simple changes...however...you can come a little closer (whatever that means) by changing the bias resistor to 1.8k and lowering the plate voltage from the power supply to 120v. Yes, this would need to be done in conjunction with your aforementioned changes. I should point out that there is a bit of a forced demand for ac701's (by some collectors and other sellers withholding whatever small stocks of decent ac701's they have) in order to drive up pricing...(at least this has been my experience in trying to acquire them). So I would suggest using the 6072, which is the "E" version tube, while they are in short order as well they are not as rare as good 701's...plus you can use the current tube socket already in the AR-51 while the 701 requires direct soldering.

Please keep in mind that the headgrill is still different (about 1/4 inch to small in the AR-51 in all dimensions...and the support struts and mesh are in a different configuration....all of this makes an audible difference (to some degree or another). Please also obtain the proper mounting hardware for the CK12 and make sure it fits within the head form factor (this is appropriate for accurate reflection profile...even though the grill is already not correct...might as well make it accurate wherever one can). The power supply and connectors are quite different as well and this will account for a further deviation from accuracy, as well as the plastic sub-assemblies in the 251...this will present a different resistance characteristic opposed to the board used in the AR-51 (this might or might not make some audible difference depending upon its reacting in the high impedance area). Lets not also forget the types of capacitors and resistors will not be the same as the Elam...although at least they are in value.

In short, the 251 (as is with many vintage, high end condenser mics) is a sum of its design, parts, part material, construction technique and more...all of these things interact dynamically and in many cases are highly sensitive to change. Please do not take my specificity as an attempt to dissuade your efforts as I would love to hear you post some results after said modifications! I am merely stating that the differences are quite large (at the very least, quite larger than your post implies) and for the most part all have an audible affect. I fully encourage this type of endeavor and wish you the best of luck!

Just as a quick recap...
1) CK12 with proper mounting hardware.
2) 6072 nos
3) change bias resistor
4) lower plate voltage from power supply

Whilst I cant state with impunity that this will get you to Shangri-251-La, I can state that this might put you somewhere closer in the ballpark...but with the aforementioned accumulating deviations from accuracy I wouldn't expect it sound too much like an Elam. But at least the circuit and capsule will be there...albeit different in implementation.

I hope this was of some help.

[M.Brew]

Thanks for the detailed information, it's definitely not as simple as swapping out the capsule and tube. Sounds like I would at the least have to have the power supply and the grill modded as well.

[Fletcher]

Most of what Dimitri said was spot on... I have a little to add, but not all that much. 1st off, the architecture of the grill / body assembly is one of the signature aspects of the tone / texture of the ELA M 25x series of microphones. A 251 E for example is very close to the electronics of a C-12 in terms of capsule, valve, output transformer, etc... but the two sound rather different in application. The remote pattern selection on the C-12 adds a couple db more noise... but that's not from where the main difference stem... the main differences are mainly caused by the acoustic signature of the grill / body assembly.

As for the 6072 thing... 6072 is pretty much a 12ay7. There are lots of tubes in this family which all have different gains [and tonal textures - mainly depending on the circuit / tube manufacturer... which is a doctoral thesis I won't so much as begin to approach]. If you're curious about this stuff here is a quick list of the approximate gain you can expect from several members of this family of "dual triode" tubes:

12AX7: 100/100

5751: 70/100

12AT7: 60/100

12AY7: 45/100

12AV7: 41/100

12AU7: 19/100

Final thought... if you're seriously thinking of going the AC-701 route - you might look at the AK-47 mkII rather than the AR-51. The tube in the AK-47 mkII is a TELEFUNKEN EF-732 which is rather similar in form to the AC-701 [meaning fewer circuit board modifications]. If I'm not mistaken [which I could be] the two tubes run different voltages so that information should be ascertained and double checked before you install an AC-701 [which will run you several hundred dollars!!!].

All in all... the R-F-T series of microphones is designed to get you most of the way to the promised land. While it might be a thought to experiment with things like capsules and tubes its probably not going to be the best of ideas until / unless you know a whole lot about such things - and more importantly, what you're trying to achieve -- BEFORE you start messing about with these things.

T-funk does have a "custom shop" and can fit a lot of these things together for you [should you have the burning desire]. I'd recommend you give Alan Venicofsky a shout at the T-funk office and toss around some ideas. He's really very helpful [and knowledgeable] about these kinds of things and will most likely be of great help with your quest.

I hope this is of some assistance.

Peace

[M.Brew]

Thanks, I guess I'll have to save up for the real thing or maybe get a clone by Bock. I appreciate the detailed responses.

[tornveil]

you could have Alan from telefunken modify an AR-51 through the custom shop. he has already buit me a AR-513 with the ck 13 capsule and ge jan 6072 tube it is a pre-production model, so just ask they can also use the ck-12 capsule for less than the ck-13. the circuit board looks different than the AR-51 also. hope this helps... you will love it. I have compared mine against a tele 251e, AR-51, and, copperheadat at pro audio toys in oregon. very close in sound to the 251e side note. the mic is truly great for everything in my opinion. I use it on vocals everytime without hesitation through a BAE 1073mpf then Retro 176 magic to my ears

also realy like the coperhead for the price its a steal.
later