T.Bone SCT800 modifications - C12 and other type circuits.

[ELM]

Thanks for the reply!
Im sure you´re right about the impact of the capsule.
I have no deeper understanding of tube circuits but I think i have an idea of what i want the circuit to do (more than amplification  )
I think what i look for is subtile compression and subtile slowing down of transients. I have a feeling (but I know nothing about) that larger output transformers have a role there. Do you have any idea if your mods makes the sound cleaner/tighter or Warmer/softer?
Do you have any recommendations about transformers?
It wonder if i could get a Ami T49 in there if i get violent with my dremel...

 

[ln76d]

Probably you wouldn't hear much difference between AMI and chinese transformer. Both are clean linear.
Some will argue with core size etc and low end response.
I would leave transformer alone especially if you want to put it in original circuit with original capsule.
You want to test transformer, buy cheap tiny neutrik NT10/3 and see do you hear difference.
You will fit it everywhere.
I really like small beyerdynamic transformers in tube microphones or bigger funkwerk leipzig (these are hard to find with higher ratio).
If you want to keep original capsule i can draw new schematic for you, but you will be tester of it, because i didn't tested it by myself.  It's about tuning few parts value for best performance. Do you have skills to solder it?
It would be using U87 style deemphasis to damp HF of k67.

My overall idea of modding tube microphones is making it warmer, more neutral with deeper low end than you will find in most of chinese and many even better "modern" tube mikes.
Anyway it's hard to make with k67 without deemphasis circuit.
Usual people are satisfied from my work. Mostly am using proven parts, usual so-called "vintage" in these parts of the circuit where it needs.

 

[ELM]

I will probably buy Thiersch m7 so no filtering needed. Thanks for your advice about transformers, that may save me som cash  .
There is a lot going on in my garage at the moment, I'm building both tube preamps and EMT 140 clones at the same time. Your mods seems very interesting, I might try them when the time is right. Ive never heard of a U47/Elam251 mix mic!

 

[ELM]

A small update so far:

I´ve modded one mic and compared it briefly with the other one that is 100% original.
The modded mic:
Rebuilt circuit to "Maliq mod" ciruit. + changed all electrolyte caps for quality brands, same value. Tube changed for a TAD high grade selected 6072. Original capsule, original transformer.
PSU rebuilt as described. I also changed all electrolyte caps for quality brand, highest value I could fit.

How did it change the sound?
The differens is subtile but detectable, even blindly.
The modded mic has deeper lows, easy to hear when playing with the proximity effect.
The modded mic has more detail and clarity, easy to hear on soft sounds like whisperings.
The only downside is that the modded mic has a grain more noise.
Any Ideas if the noise can be reduced?  It´s hard to believe that a tested TAD is noisier than a Chinese noname tube?


Very fun and interesting project! Many thanks to the designer! I wonder how the mic sounds with a nice M7 capsule?

 

[Gus]

Does the 6072a have green printing? 

 

[ELM]

My TAD is printed in silver grey. Maybe it will benefit from burn in? It had only been on for a hour or so.

 

[ln76d]

My friend Maliq also have problem with noise, we currently discuss  that via email

For original capsule (which is truly good capsule, even if hundreds people say opposite) you could try to use 100pF-270pF capacitor from plate to ground. Styroflex or wima FKP/MKP works good at this position. High voltage rated (wima is small sized).
It should smooth HF response. Blindly i would go for 150pF/200pF. This could also prevent RF.
Did you cleaned boards after soldering? Especially high impedance section is important.
There's also probability that original high impedance resistors 200M/51M are noisy.
I didn't had that problem, but some members noticed that in other builds.
Could you measure voltage on B+ section and heater?

 

[ln76d]

BTW.

sorry for some mistakes on schematic. C1 and output capacitor should be non electrolytic for better performance.
Do you have any other tubes like 12at7 or 12ax7?
If so, could you swap it for for test? Is there still noisy?
If you would like to see what you can squeeze from original capsule, then try mentiond cap from plate to ground as also you could change R3 for 1.3M resistor (optional you can use 1.5M).
After resistor change, you need focus on low end response, if it doesn't change then is good
It should improve signal/noise ratio so the output will be a little higher.

 

[Gus]

If you look at https://frank.pocnet.net/sheets/093/6/6072.pdf note the grid to ground resistor is .1meg(100K)
Do you know what grid to ground resistor was used to test for noise?
Do you only have one 6072? Do you have a 12ay7

Have you tried the original tube after the changes to the circuit? if so is the noise lower with the original tube? Not clear if you swapped tubes after the changes.

What is the filament voltage at the tube? You can try to reduce grid noise with a lower heater voltage, however some tubes don't underheat well.  Underheating a tube is often a try it to test it.

 

[ELM]

Thanks for all input! I will hopefully have some time for experimenting this weekend. Clean the pcb is on top of the list.
I'l be baaack...

 

[ELM]

I´ve been experimenting a lot and found that the noise is caused by the modified PSU. When I used the modded mic with the unmodded PSU and the unmodded mic with the modded PSU the unmoved mic had even more noise.

I´ve even done some measurements on the modded mic connected to the modded PSU:
Heater: 6,68V
B+: 118V
Plate, at tube socket: 72,7V

The unmoved PSU, No mic connected:
Heater: 6,7V
B+: 197V

The interesting thing is that the modded mic, even with the unmoved PSU still is a nicer sounding mic. It has a richer sound.
What is the point with lovering the PSU voltage? Is it only to lower the capsule polarisation voltage or is it even to lovering plate voltage?

 

[lampas]

You want to test transformer, buy cheap tiny neutrik NT10/3 and see do you hear difference.

NT10/3 is 1:10/3 ratio transformer. But, is this also mean 1:7 ?

 

[ln76d]

Shoot me, i don't remember...
I think i mesured it for 10:1, 3:1, and 6.x:1, but with the last one truly am not sure.
What for you need 7:1?

 

[lampas]

Shoot me, i don't remember...
I think i mesured it for 10:1, 3:1, and 6.x:1, but with the last one truly am not sure.
What for you need 7:1?

Just asking as on specs they put just 10:1 and 3:1 but not 7:1...

 

[ELM]

Any ideas why the mic is noisier with the lower voltage psu? The heater voltage is the same so it cant be that.
Could it be capsule voltage that is to low? There is no way to actually messure capsule voltage, right?

 

[ELM]

I should ad that the noise is wideband, even noise, sounds like a waterfall. No hum or pops.

 

[ln76d]

Sorry ELM, i missed your post with measuraments.
Great that you found source of problem.
I didn't modified 300V PSU, rather 200V etc.
Noise came probably from dropping resistors and zener diodes.
Definately 120V is more suitable for microphone circuits, maybe someone will explain it better.
Lower voltage is for tube operating point, polarisation voltage can be change without any problem.
In Maliq mod, polarisation voltage, was set for chinese k47 - ca. 60V which works good with that capsule.

Hmm... Maybe just simple voltage divider will work better for dropping voltage.

If you want to use original capsule, then check my post with resistor value.  Original capsule should work fine up to 75V (you need test it for low end response).  If you will set higher polarisation voltage, then remember that you will need lower voltage for most of other capsules.

 

[ln76d]

BTW.
Try one thing - remove R4 in PSU.

 

[ELM]

BTW.
Try one thing - remove R4 in PSU.

R4 removed, didn't change noise level.

I´m thinking there must be something about the lower voltage levels that isn´t optimal, tube or capsule voltage. As i wrote earlier, the modded psu made the original mic a lot noisier to. It would be wonderful if someone explained more about the art of tube biasing (or linked to some interesting reading)! Six month ago i had no clue about how a tube worked so i have a lot to learn...
Thanks for the advice about getting the original capsule to sound god. In my case my plan is to change for a high quality M7 when I hopefully get the circuit right.

 

[ln76d]

Point is optimal. Polarisation voltage too.
There's need to better drop voltage.

Ok Guys!
What do you think about simple voltage divider after rectifier?
Then it will be 150V. So two 1k resistor and three 47uF should be enough.