CCDA mod for Fame SKEA002/W

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CCDA mod for Fame SKEA002/W

Postby Pavezer » Thu Jul 19, 2018 3:32 pm

Hello!

I've had a cheapo chinese tube mic lying around for a long time, never been excited at all with it, and I've been thinking that this would be a good candidate for CCDA type mod with AK47 capsule and BV8 trafo. Microphone is Fame SKEA002/W. Mic is supposed to be same as Apex 450, but I can't confirm that. Anyway, I have not been able to find schematic anywhere so I traced out the circuit. Mic does have a pad and a hpf rolloff switches but i will dump them and these are not included in the traced schem, which is attached here (hopefully no mistakes...):

https://www.dropbox.com/s/8y74896iylgvhwz/Fame_orig.png?dl=0

I've been reading around about CCDA circuit and mods should look like this (red indicates mod):

https://www.dropbox.com/s/wwhgyzjva9w5mqk/Fame_mod.png?dl=0

Can anyone confirm this is ok? Anything important that I am missing?
Couple of questions:

- Original PSU gives 200V b+. This should be dropped to 124V with two 62v zeners at the PSU, right? At the moment there's two 100V zeners which I located already.

- If dropping the b+ to 124V, should R5 & R7 (51M) be changed to something else?

- If dropping the b+ voltage to 124V, would the PSU multipattern switch work as supposed, without modding?

- Heater voltage (pin 2) at PSU, without load, no cable etc, is 6,7V. Is this ok and some voltage dropping is excpected when there is load? I have some GE JAN 6027A tubes and I dont want to run them hot.

- In the original circuit, there is a heater voltage 10uF filter cap, C11. Is this needed or can I dump it (marked blue in mod schem)?

- What is prefearable value of C8 (cathode output cap) within CCDA circuit with K47/BV8? 1uF or 2,2uF? I know this affects the bass response, but in general, what would be overall better choice for this application?

- 1st stage cathode resistor from pin 3 is in original circuit is 3k, should this be changed?

- 1st stage grid resistor R1 is 200M, ay thoughts on this?

Thanks a lot in advance!
Pavezer
 
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Joined: Tue Jul 17, 2018 11:57 pm

Re: CCDA mod for Fame SKEA002/W

Postby Dave Thomas » Mon Jul 30, 2018 9:46 am

Hello, thanks for the inquiry. Yes, if that is the schematic for the SKEA002 then you modifications would work.

R1 can be a 1G (1000meg). They are using a 200M which will start to roll out the low end at about 40-50hz this is good for vocals. With the 1G the response is flat down to 20hz.

R2 the cathode resistor should be 2.2K to optimize the circuit for a 6072a. Yes, change the 100v zeners to two 62v.

C8 can be a 2.2ufd for a flat response. A 1 ufd capacitor with the BV8 will give you a more traditional U47 sound. You will get more even order harmonic in the low end. The 2.2ufd will yield a more linear LF response. This difference is subtle.

Cheers, Dave
aamicrophones.com
Dave Thomas
 
Posts: 171
Joined: Mon Dec 30, 2013 2:46 pm


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