Official C12 Clone - Build and Support Thread

[mica]

Crap just realized the audio clips where blank. Fixed! Should i contact BeesKneez about it? I can see in other posts that the muffled sound have been a problem for others..

/M

 

[tonycamp]

hey fella's, i didn't see a link to this shootout(loose shootout), there are some cool comparisons including a couple C12's with Tim C, and BN capsules
http://realgearonline.com/post/50912/thread

 

[MicDaddy]

My Beesneez K12 diaphragms both were collapsing..

please note that this was after I was able to get some good use out of it or otherwise it had been working great. 

Tim mentioned about hot plugging the mic could be problematic. 

I am now wondering if I might have done this... is the unloaded HT too much for the diaphragms, maybe it stresses and messes with the tension of the diaphragm which later exhibit a loss of tension and collapsing... just grabbing at straws.

Contacted Veronica at Beesneez, sent photos of the collapsing (all capsules I put in circuit after this discovery do not collapse, voltages are correct etc)

She recommend I send it back.

I sent the capsule back to Australia in January... it has since gone missing in transit  :'(

 

[mica]

Ok, i have contacted Veronica. If my voltages are correct, and the patterns are working, what else than the capsule could make it sound so dull?

 

[tonycamp]

Ok, i have contacted Veronica. If my voltages are correct, and the patterns are working, what else than the capsule could make it sound so dull?

look 4 posts  : , and more importantly... listen

 

[mica]

Well i cant find anything that indicates thats there's an error in the build.. The mic works as it should, but just sounds muffled and non-3D..  Still suspecting the capsule..

 

[goneglimmering]

Hey friends, I'm in need of a little assistance. I completed my C12 build and am running checks with a cheep capsul and tube in place of the ones I will use after initial set up. For the power supply, I used an old altcron power supply that I had on hand. I am unable to adjust the B+ voltage below  149v checked at the psu headers. The power transformer in my psu is out putting about 240v out of the 200v secondary. I was wondering if this transformer being out of spec can be the reason for my voltage problem or if I'm barking up the wrong tree? I should also note that the capsul that I'm using for testing does not have a rear diaphragm, also, my bias and heater voltages seem correct, sitting at -1v and 6.3 v. Any ideas would be a great help to me.

Thanks

 

[Category 5]

240V out of the transformer is normal.  Have you checked vintage with a dummy load to rule out the mic?  Which resistors did you use for r1, and r2 in the psu?  Which for R17 in the mic?  What tube are you using?

 

[goneglimmering]

Thanks for the fast reply. For r1 and r2 I used 68k and I used 100k for r17 because the final tube in the mic will be a 6072a but currently have a 12AX7 for testing and initial setup. I didn't have a 180k for a dummy load so I put two 100k in series to give me a 200k dummy load, I figured that would be close enough for a quick test. With my dummy load in place I had no problem getting the B+ to 120v. Is it possible that the difference between a 12ax7 and a 12ay7(6072a) could cause the B+ To be so far off? Is it more likely that I have a bad component in the mic somewhere?

Thanks again.

 

[Matador]

68K for R1 and R2 was the best balance for most common tube types  intended for this circuit (notably 6072A and 12AT7), when factoring the B+ pot set to mid-value (e.g. 50K).  If you can't get it lower than 150V it means the tube isn't idling at high enough quiescent current.  You can increase the current by lowering the bias voltage (e.g., move it from -1V to -0.5V), but that will diminish headroom and increase distortion somewhat.

Given the passive nature of the PSU, it is completely reliant on the tube idling somewhere near the middle of the design point in order to establish the proper working voltage.  Tubes in generally aren't exactly 'tight tolerance', so some fudging may be in order.  If you say it trims right with 200K resistive load (or 120V/200K = 0.6mA), that's pretty close to the 0.75mA that a typical 6072A draws at -1V bias.

If I were you, I'd swap in the intended tube, but leave the beater capsule in there.  You should be able to trim all of the correct voltages, and the capsule (being a capacitor) doesn't change any of the working DC voltages in this circuit (unless it's defective).

If you really want to make it trim out correctly with this particular tube, you'll need to increase the value of R1, R2, or both, to lower B+ down, but keep in mind that you may have to re-adjust them again when the final tube is installed.

 

[Category 5]

With that transformer I think you're going to need 91k in both r1 and r2 even with the 6072a. 

I'm with Matador on getting the real tube in there too.  I'm not a fan of subbing different tube types during setup. 

 

[Matador]

With that transformer I think you're going to need 91k in both r1 and r2 even with the 6072a. 

I'm with Matador on getting the real tube in there too.  I'm not a fan of subbing different tube types during setup.

Hard to say without seeing how the measurement is done:  peak output voltage on the secondary should read 280-290V, down near 270V after rectifier diodes.

If it's truly putting out 240VAC at the secondary, then indeed that's way overwound from standard.  I haven't seen on that high if that's what's really happening here.

 

[goneglimmering]

Thank you category 5 and matador. All is adjusting properly with the 6072 tube in place. Didn't have to change r1 and r2 to a higher resistance. One last question if you don't mind. Should you adjust the heater voltage to 6.3vdc at pin 4 of the tube socket or is it fine to leave it set at the terminal block inside the PSU? How critical is heater voltage in a tube? Are you adding more stress to a tube if you under heat it?

You guys are the best. This was a super fun project!

 

[Category 5]

Awesome news!  You can measure in PSU.  Some find under heating can extend tube life but the jury is out on how it affects sound, at least for me.

You should be able To dial in 6.3 exactly and it hold pretty solid though since that part of the PSU is regulated.  B+ and bias will fluctuate a little and it's normal. 

 

[trans4funks1]

I was looking at the studio 939 store today and noticed this note in the description of the vacuum tubes offered:

"Currently, we are offering new old stock Mullard 12AT7's as an option for our C12 and ELA M251 kits...

...All currently shipping C12 and ELA M251 electronic parts kits contain the required alternate resistor values to utilize the 12AT7 family tubes."

I'm hoping to learn which resistors and values are considered alternates, compared to the values intended for a 6072 build.

Thanks.

 

[Matador]

It's R17:  it's 100K for 6072A tubes and 47K for 12AT7.

I used to recommend 33K for R1 and R2 with a 12AT7, but several builders have said it's trim-able with the stock 68K values.

 

[chunger]

the current parts kits contain 68.1K resistors (intended to center the adjustment range for 6072A tubes)

. . .and 33K resistors (intended to center the adjustment range for 12AT7 option)

We used to use 91K resistors in the kit but it seemed the majority of builds were on the "top" end of the adjustment range and a few customers reported needing to lower the value to obtain 120V B+ particularly on the few power transformers that were spec'd at 175V instead of the standard 200V on the secondaries.  The change was also instituted after the "meter hack" that put an onboard LED volt meter into the case which drew a bit of current from the B+ when inserted.

If the 68K also trims out 12AT7 tubes, so much the better!

 

[trans4funks1]

Thank you for the information.

R1 and R2 are part of the power supply and R17 is a plate resistor.

Matador, can you elaborate on how and why the lower value plate resistor is a good choice for the 12AT7 tube substitute?

I am asking primarily because you seem so effective at explaining how  components interact in a circuit and I think I will enjoy learning the thought process with which you seem so at ease with.

Thank you.

 

[pedroplanet]

Has anyone used/tested the Telefunken TK51D?


http://store.t-funk.com/p/tk51d-capsule?pp=24

 

[saxmonster]

This seems like an awesome option,  what is telefunken using in the  newer elam 251's.