AKG C28ish with 5840 tube

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geebeeVIE

Member
Joined
Jan 23, 2015
Messages
22
Hi All,

i am extremely happy with my two matachung C12s, and I want to build now something different.
I'd love to build a preamp for my AKG CK-1/61/etc capsules.
As it would be easier if it all would fit into a standard mic body, I'd love to use it on a C480 size (21mm diameter) body.
I think this should be possible by using a 5840 tube.

I have sourced the oiginal schematics of the C28 (AKGs old SD tube design), see attachement. it is basically a C12 circuit without the pattern switching.

The 5840 is of course different to the 6072, but it could run on the same Heater (6,3V) and B+ (120V), and it should be fine with the T14/1.

I'd love to use Chungers C12 for that, but if I am not mistaken, the 5840 needs a lot more current, so R1 and R2 in the PS need to be changed to lower values. Could you help me figure out what would be a good starting point?

What other changes  (in the overall circuit) should be made?

Thanks for your help.

Gee
 

Attachments

  • c28.jpg
    c28.jpg
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I did the same, with a (Sylvania) 5840W tube, housed in an Apex 460 body.
I used 1 G. for R8+R9 and R5+R7.
Cathode resistor is 2.7 K in my case.
I had to increase the value of C3, because with 25 (22) uF I lost a lot of low frequencies.
The microphone has a very low self-noise and sounds good.
At first I was surprised by the measured THD (mainly second harmonic), but that is what probably makes the 'tube sound'...
 
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