Ok,....We've got the T-Bone
SCT800 tube mic of Jankees on the work bench. JK made some vocal test samples
with the T-bone and Rode NT-2. The T-Bone was more "warm" and friendly
where the Rode was more direct and, in my opinion, does have some, too, sharp
edges to it.
So, if possible we need to find the golden middle way between those two.
After opening up the T-Bone we recognize the Chinese style,....Very neatly build, solid construction just very good value for money!
Also the power supply is a neatly build up one, transformer is big enough and,
at that point, I did not for see any problems.
I hooked up everything to the mic and fired it up. After warming up a few
minutes (take at least 60 minutes it you will use it for recordings!) I measure
first the filament voltage, 6.5 Vdc,...Perfect! I measure the high voltage at
the entrance,...200Vdc, perfect! Than I measure the anode voltage, that was less
than 100 volts by only 0.5 milli amps,....And that's really in my opinion way to
low. It sure will give you the warm "tube sound" but don't be
mislead,....That's just a truck load of harmonic distortion! So we've to deal
with that.
So, now the solder iron is set and ready,.....:-)
Closer look to the power supply revealed it was filtered with a number of RC
networks and on the end stabilized with zeners to supply a nice 200 volts DC.
So, first we scavenge the power supply. I hate 3 RC filters behind each
other,...This one was 3 times 22u and 15K in series. Yeah, quite but also very
slow and possible noisy too!
I decide to implant my own standard pcb fitted FET stabilized power supply. It
fitted without any problems into the original power supply case. I set the first
two caps in parallel and give this to my FET power supply. 290 vdc on the input
and 200 vdc out. Perfect! The third 22u cap was placed after the (slow start)
FET supply. So, this job was done.
Now the mic,...I removed the ecc81's anode 100K resistor and replaced this for a
FET circuitry. Kind of mu stage solution. I decide to bias the tube at 100 volts
Va and I want to yank 2mA through the anode,...(Take notice that the ECC81 is a
low noise NOS mullard tube, it is NOT the standard tube (something vague from
China) that comes with this microphone!)
I lower the cathode resistor to 390ohm giving 0.78 volts at around 100 volts
anode voltage giving me the desired 2 milli amps current I wanted.
0.78 volt of negative grid voltage looks little of keep in mind that there is
only a few milli volts of swing out of the mic capsule so,...No issue here! 2mA
looks pretty high for a ECC81 but don't worry, it's way inside the limits! It
lowers harmonic distortion dramatically compared to the 0.5 mA in the original
design. It will produce a more open and clean sound without the excessive warm /
brown feeling sound.
Second practical finding was that the decouple cap for the condenser element was
a styroflex cap,....one of the worst choices to use in your audio path! So, that
was replace by a silvered mica capacitor, the only one you should want to see
there. (Maybe a paper oil type it turns out to be to clinical but first choice
is silvered mica)
Third practical finding was the el cheapo cathode decouple cap,....Yank it out
and replaced for an Elna Cerafine type. This will also contribute to a more
clean, direct and open sound.
One really weak point no left is the output transformer. This is a real el
cheapo one. It might be worth the money to insert there a better one.
If there are enough interested folks I even might consider to make a new PCB
using the ECC81 as an LTP and that way give a balanced output with the advantage
NOT to use the transformer. Future will tell,...:-)
So, after a few hours of soldering and constructing I plugged in the new
kit,.....Aaaarrrghhhh,....LOADS of HUMMMMM on the oscilloscope, ....After a few
minutes I realized I have to put the mic his (her?) jacket on,....:-) After I
did that it looks real cool,....a few millivolts (Peak peak) of noise on my
noisy work bench and no detectable hum,...looks promising!
So curious as I am I decide to bring it straight to Jankees so he can try it
out. I also want to hear myself what the base noise would be on a pair of
headphones,....Well,....VERY quit is was!!
Jankees would make some comparison recordings, I'll wait for that but I'm pretty
sure it will turn out pretty good!
--
Met vriendelijke groet / With kind regards,
Rob Spigt
Machinator sound products
Hoofddorp the Netherlands
Phone fixed: +31 (0)23 5654330
Phone Mobile: +31 (0)628 594198
E-Mail: i.info@machinator.nl
WWW: www.machinator.nl
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The natural sound of pure power.