Cheap 251 Using Russian 6S6B-V valve and other leftovers. Noisy results!

[pinchemotherloaf]

I was bored so I decided to go though my leftovers to see what could be made. Came up with, guess what, another 251 type build done in a few hours to see if these Russian 6S6B-V tubes I bought made any sound. Yes they are a bit noisy, but I think my bias is off and the fact I used carbon grid/cap bias resistors did not help either. The other problem was the cheap capsule I put on it has a small tent so It really needs some more work to be functional. Any Bias advise for the 6S6B-V tube would be appreciated! See both my schematic and the original 251. I omitted the 100pF cap at the output. Is this really needed?

Blank Body

Board Installed

Voltage divider and capsule bias resistor placement

Grid dip bias and plate current resistor placements. Other lead is the cathode bias resistor and output cap underneath.

Cathode bias resistor placement

Cathode bias and output caps placed.

Tube placement

Capsule and output transformer placements. 7 pin XLR placed with ground and power leads soldered in place.

Capsule bias capacitor placed as well.

Finished other side.

Familiar Apex body

My circuit kludge. I don't like the wires to cross unconnected like the old schematic!

 

[RuudNL]

I have used many 6S6B-V tubes. They all had a very low self noise!
For a microphone company I have tested hundreds of 6S6B-V tubes, and the self noise was always very consistent and low. Deviation in self noise from tube to tube was usually in the order of plus or minus one dB!
So, I have a feeling that if your microphone is very noisy, it may have a different cause.

 

[pinchemotherloaf]

Yes, I suspect my Cathode bias is way off and I am using carbon bias resistors and the grid dip and capsule. Also, I have a defective capsule with a tent so that alone could be a noise issue!

 

[Murdock]

It Looks like you have the grid and capsule Connection on the board. These are high impedance sections and should be isolated with teflon insulated Pins or the connections float in the air.

 

[pinchemotherloaf]

It Looks like you have the grid and capsule Connection on the board. These are high impedance sections and should be isolated with teflon insulated Pins or the connections float in the air.

Thanks, That makes sense!

 

[Papa Tango Charly]

Hello beautiful can you tell me what 3U transformer you are using?

 

[pinchemotherloaf]

Hello beautiful can you tell me what 3U transformer you are using?

I'm using the 3U GTZ-12 and can be found here:
https://www.ebay.com/itm/111540701945

 

[Papa Tango Charly]

Merci

 

[Papa Tango Charly]

I was bored so I decided to go though my leftovers to see what could be made. Came up with, guess what, another 251 type build done in a few hours to see if these Russian 6S6B-V tubes I bought made any sound. Yes they are a bit noisy, but I think my bias is off and the fact I used carbon grid/cap bias resistors did not help either. The other problem was the cheap capsule I put on it has a small tent so It really needs some more work to be functional. Any Bias advise for the 6S6B-V tube would be appreciated! See both my schematic and the original 251. I omitted the 100pF cap at the output. Is this really needed?
View attachment 95450
Blank Body
View attachment 95452
Board Installed
View attachment 95453
Voltage divider and capsule bias resistor placement
View attachment 95455
Grid dip bias and plate current resistor placements. Other lead is the cathode bias resistor and output cap underneath.
View attachment 95456
Cathode bias resistor placement
View attachment 95457
Cathode bias and output caps placed.
View attachment 95458
Tube placement
View attachment 95459
Capsule and output transformer placements. 7 pin XLR placed with ground and power leads soldered in place.
View attachment 95461
Capsule bias capacitor placed as well.
View attachment 95462
Finished other side.
View attachment 95463
Familiar Apex body
View attachment 95465
My circuit kludge. I don't like the wires to cross unconnected like the old schematic!
View attachment 95466

 

[OneRoomStudio]

It Looks like you have the grid and capsule Connection on the board. These are high impedance sections and should be isolated with teflon insulated Pins or the connections float in the air.

I think this is probably the biggest culprit.

Also, I think you might be better off with a 200K plate resistor and a cathode resistor closer to 3K. The AC701 version of the circuit (the schematic I posted in the other similar thread) uses 200K plate and 3K6 cathode. 6s6b probably wants to see near the same.

Lastly, why 500M (!) resistors from the capsule? The original uses 8M. 30M is plenty. That may be contributing a bit of noise too.

 

[Delta Sigma]

I personally wouldn't do such a big output cap as the 6072 schematic above. 1uF as the hand drawn one shows should be sufficient.

 

[Papa Tango Charly]

Thank You Guys Fresh Copies of the schematic

 

[RuudNL]

Lastly, why 500M (!) resistors from the capsule? The original uses 8M. 30M is plenty. That may be contributing a bit of noise too.

500 M might be a bit high, but the higher the value, the lower the noise.
8 M is a very low value and may act as a high-pass filter in combination with the capsule capacity.
(Even 30 M and 80 pF will create a high-pass filter with a corner frequency of 66 Hz, with 8 M it will be as high as 248 Hz!)
Noise from the bias resistor (also 500 M) will be filtered by the 4800 pF capacitor.

 

[Tubetec]

Ive attached a sub mini tube to a small piece of vero board , then used strands of headphone wire to make the connections , the grid wire doesnt attach to the board , instead its scaffolded up towards the capsule and floats like suggested by Murdoch.
The sub assembly is mounted to the mic frame with thin bungee cord type bands , providing a short circuit to any vibrations reaching the tube itself .

Small M3 PCB antivibration mounts for quad copters and drones are readily available now , these are a great way of building in extra isolation for tube or other components subject to microphony ,
Sony put quite an elaborate iso-mount in the C-800G but the 6AU6 is another bucket of bolts altogether from a submini .

Subminis are typically good with regards microphony , but maybe its worth the extra attention to detail with regards mounting to reduce the chances of vibrations at very high SPL's .

 

[OneRoomStudio]

500 M might be a bit high, but the higher the value, the lower the noise.
8 M is a very low value and may act as a high-pass filter in combination with the capsule capacity.
(Even 30 M and 80 pF will create a high-pass filter with a corner frequency of 66 Hz, with 8 M it will be as high as 248 Hz!)
Noise from the bias resistor (also 500 M) will be filtered by the 4800 pF capacitor.

You're right about the lower Johnson-Nyquist noise with the higher value. I was thinking potential noise as a result of the higher impedance with his PCB connections (rather than off-board connections).

As for the high-pass, you're right about the math of course, but I often wonder about the reality of it. Many people love their original 251's, and the ones I have heard (export versions with 30M resistors) never sounded thin to me. I wonder why? CK12's are often around 80pf...so you would think a ~70Hz roll off would be audible.

 

[Delta Sigma]

500 M might be a bit high, but the higher the value, the lower the noise.
8 M is a very low value and may act as a high-pass filter in combination with the capsule capacity.
(Even 30 M and 80 pF will create a high-pass filter with a corner frequency of 66 Hz, with 8 M it will be as high as 248 Hz!)
Noise from the bias resistor (also 500 M) will be filtered by the 4800 pF capacitor.

What was the target customer for the non-export version? Broadcast? I could see the severe high pass being useful in some broadcast applications in that era.

Isn't the risk with a high grid resistor, grid current?

 

[kingkorg]

This is elam FR from recording hacks. Wondering what grid resistor is in this one, and if the owner of this particular one claims it's the best mic ever.

http://recordinghacks.com/images/graphs/_gen/small/0/0858.png

 

[rock soderstrom]

This is elam FR from recording hacks. Wondering what grid resistor is in this one, and if the owner of this particular one claims it's the best mic ever.

That looks like a good vocal microphone. There's no need for a downstream HP filter, the infrasound and the overemphasis caused by close miking are already gently filtered out. I think this is also the reason why even mics with even less LF can sound very good and correct on vocals (especially in the mix later).

 

[kingkorg]

That looks like a good vocal microphone. There's no need for a downstream HP filter, the infrasound and the overemphasis caused by close miking are already gently filtered out. I think this is also the reason why even mics with even less LF can sound very good and correct on vocals (especially in the mix later).
View attachment 118963

Agree 100%, but having the price in mind i'd rather have it switchable.

 

[rock soderstrom]

Agree 100%, but having the price in mind i'd rather have it switchable.

+1 You can definitely see it that way.