M7 in C800-g Circuit

I have

• brand new blue line Thiersch M7
  
• 2 x C800-G Power Supply and Mics
  
• 1 MA-55 board which is the mic mother board.
  
• A load of russian and sony 6au6a's
  
• few different vintage and new transformers at 6-1 ratio.

I'm either going to just pop the m7 into one of the sony's and see what happens or going to do a simple build using the spare ma-55 board. Maybe add a 100pf / 270pf shunt cap for those high highs the circuit produces.

Just want to make sure the polarization voltages are right. I think the Sony is supposed to be 45v at k67 capsule. Not sure if this is actually the case though.

Anyone can chime in on best voltages for the m7 and whether i would have to add a different divider?

Cheers.

Jesse

I'm really enjoying this back and forth.. really. 



Such an easy mod. the Thiersch mount fit perfectly even to the exact same screw thread size.

As predicted I lost like 3-6db of gain. Ive got another c800g untouched so I can go back and forth. What I gained was beautiful sibilance, smooth tops and more forward mids. I was originally going to add a smoothing cap 100pf / 270pf but it really doesn't need it. If anything its maybe a little TOO on the dark side now. Amazing, it just goes to show how shit the k67 capsule design really is inside this circuit.

For now I want to see if its possible to get that 47v at capsule up to the 63v like in the u47. Would it be as simple as just lowering the value of R101?

After that I'd like to replace all the lyrics with film types.



I have a spare MA-55 that I can mod and drop in as to keep the mic returnable to stock.

Why did you lose 3-6dB? Is the M7 just less sensitive than the K67?

I believe its because it needs higher polarization voltage and is 75pf instead of 50pf

Curious why you didn't build a complete mic in a donor body with the M7, instead of modifying the C800? Is there something about the C800 that you like, that is special, in the circuit or body design?  I'm curious because I've wanted to experiment with a 6au6 mic circuit but haven't gotten around to it yet.

JessJackson said:

I believe its because it needs higher polarization voltage and is 75pf instead of 50pf

Click to expand...

I wonder if the K67 in the C800G is actually designed for 45V, or if it's like a normal K67.

Melodeath00 said:

JessJackson said:

I believe its because it needs higher polarization voltage and is 75pf instead of 50pf

Click to expand...

I wonder if the K67 in the C800G is actually designed for 45V, or if it's like a normal K67.

Click to expand...

Thats a very good question but i believe its Sony's best attempt at an identical copy of the K67

JessJackson said:

For now I want to see if its possible to get that 47v at capsule up to the 63v like in the u47. Would it be as simple as just lowering the value of R101?

Click to expand...

Yes.

After that I'd like to replace all the lyrics with film types.

Click to expand...

No point unless your films are hand carved from solid BS or Unobtainium by virgins  8)

Anyone know what the Peltier element does?

is the Peletier supposed to be a heat dissipation device ?

JessJackson said:

... Amazing, it just goes to show how sh*t the k67 capsule design really is inside this circuit. ...

Click to expand...

Amazing how subjective ears (taste) are! I've dealt with just about all types of mics in my entire life and
the C800G is one of the most unique & beautiful sounding mics I've sang through. Although depending on
the vox, like all mics, it handles sibilance very well & the balance of hi's & lo's and proximity very smooth.

It shows they spent years coming up with the relationship of the Sony 67 capsule & the 6AU6 tube which
is great. Don't really think the Peltier heat sink does anything.

And on the other side their legendary 37 capsule can be found in all the other models (including the C800) &
is different & unique in it's own way.

I THINK they use the Peltier element and heat pipes to reduce any outgassing from the glass walls.

I have built a microphone using 6AU6s at the voltages and currents in the C800G with no tube cooling. The higher Plate current has its Plus.

Here is a hint if you change the capsule voltage by changing a resistor(s) look close at the circuit and think about the currents in the different paths.

i was told that the reason for the peltier is that the tube's noise figure can be improved ever-so-slightly this way: I've heard 1 or 2 dB mentioned.

An article in Japanese that may (or may not) explain this: http://www.gyraf.dk/schematics/schematics.html - scroll down to "A Japanese article about the Sony C800 Microphone:"

Anyone that can translate?

Jakob E.

Peltier Element is a mini refrigerator for your tube. https://www.youtube.com/watch?v=Ipt8xqKbCSw

Threw in a 150k and the voltage is now 57v

Replaced R103 with a carbon 150m

Replaced C103 with a film 1uf

Replaced C104 with a film 0.47uf

Sounding much nicer now. fuller mids, smoother top end but transients still a little sharp and brittle. Either the remaining elecrolytics or the transformer or the plate voltages.

very interesting experiment for me

----

Two questions

in u47 c101 is non electrolytic 0.01uf but with no resistor in parallel, what serves the purpose of R102?

what serves the purpose of super caps c105 / 106 and r106 / 107 across the heater supply?

JessJackson said:

Peltier Element is a mini refrigerator for your tube. https://www.youtube.com/watch?v=Ipt8xqKbCSw

Threw in a 150k and the voltage is now 57v

Replaced R103 with a carbon 150m

Replaced C103 with a film 1uf

Replaced C104 with a film 0.47uf

Sounding much nicer now. fuller mids, smoother top end but transients still a little sharp and brittle. Either the remaining elecrolytics or the transformer or the plate voltages.

very interesting experiment for me

----

Two questions

in u47 c101 is non electrolytic 0.01uf but with no resistor in parallel, what serves the purpose of R102?

what serves the purpose of super caps c105 / 106 and r106 / 107 across the heater supply?

Click to expand...

Don't quite get the component swaps. Wasn't C104 film already? Reducing the value only reduces the low end and moves the resonant bump higher (if there's a bump, depends on the trafo inductance). Making C103 smaller should only make the mic noisier, it's a filter cap isn't it? R103 swap also possibly makes the mic noisier (carbon) and reduces the very low end.

Regarding the first question: R102 is the other resistor of the polarization voltage divider. Because there's no large "polarization resistor" like in most mics, C101 has to be large for full low end.

The second question puzzles me also.