Brauner Valvet x VLF very low freaquency noise (rumble)

Quiet environment - I have noise VLF rumble from this mic.
Plan the schematics with some voltage measurments.
Fault the valve or the design?
Any ideas?
Thanks in advance

How clean (or dirty) is the capsule? Flux residue in the hi-Z area?

Does your microphone look like one of the ones in the following link?

https://www.gearslutz.com/board/geekslutz-forum/831269-brauner-change-capsule-across-whole-range.html

If you want to disconnect the capsule front wire to substitute a 50pf to 100pf cap to ground(threads are at this forum about testing for capsule noise) it looks like you will need to remove the two screws that hold on the capsule holder to reach the wire that goes to the front of the capsule.

Thanks for the schematic.

If so I looks like the tube is soldered to an 8 pin T0-5 carrier for an older round  8 pin ICs

If you want to check the tube by substitution you might need to solder on an adapter

Some links. I have not worked on a Valvet so I am guessing something like the following might be what was used.

Also if yours is built like the pictures and you solder on an adapter you want a longer length of leads between the tube and adpater. This is to minimize the heat and mechanical stress at the tube lead vacuum seals.

https://www.digikey.com/product-detail/en/te-connectivity-amp/8058-1G32/1-1437508-6-ND/2259044&?gclid=EAIaIQobChMIkt-ww5vu4wIVBhgMCh3ENgUuEAQYASABEgJmz_D_BwE

https://www.digikey.com/products/en?FV=ffec2bfa

https://www.digikey.com/product-detail/en/te-connectivity-amp-connectors/4-1437508-0/A113639-ND/1153459

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Customer+Drawing%7F1437508-5%7FA1%7Fpdf%7FEnglish%7FENG_CD_1437508-5_A1.pdf%7F4-1437508-0

Do you have LED lights nearby? Or RF source? Try moving the mic around, to another room.

That headgrille seams very open to me, could be the cause. And obviously check grounding and if the grille is grounded well.

capsule front

capsule back

valvet x tube socket

Might be this
https://www.digikey.com/products/en/connectors-interconnects/sockets-for-ics-transistors/409?k=&pkeyword=&sv=0&pv571=268&sf=0&FV=ffe00199&quantity=&ColumnSort=0&page=1&pageSize=25

https://www.digikey.com/product-detail/en/mill-max-manufacturing-corp/917-47-210-41-005000/ED11274-ND/7364149

Use a soldering iron touch the bottom of the pin heat it and it should push out to convert the 10 to an eight.
I would guess the solder leads will plug right into the socket. 

Capsule looks fairly clean only looks like dust. Do not use a can of air to clean the surface the cans have stuff in them that might hurt the capsule. I use a squeeze bottle to blow off the dust, if needed I use a very soft and clean optics type brush to help remove dust if the bottle does not remove it. The clear area is what you need to examine.

I would guess it is the tube before the capsule from the pictures.

Do you feel confident to work on this microphone unscrewing things and taking things apart?

Another thread about the VM1
https://groupdiy.com/index.php?topic=63305.20

correction schematic: R6 is 150K and R4 is 270K.

Any ideas why the C3 is 470uf?(Cathode ).And why this cap is parallel with C2 (1nf)?

The 470uf can "hold"  the cathode voltage a little more stable than a lower value. This is a "taste" adjustment when you select the value. Think tube guitar amps with cathode or fixed bias output stages.

I find the film cap bypasses to be more of an "believed" improvement with new electrolytic caps(not years old ones).

I am looking for one of the data books I have that has an example of a bypass cap causing issues instead of helping. I forget if it was in an old AD or LT data book.

Circuit looks like a standard pentode wired as a triode plate out. Do you know what type laminations are in the transformer? Amorphous, Mu etc.

tom3 said:

correction schematic: R6 is 150K and R4 is 270K.

Any ideas why the C3 is 470uf?(Cathode ).And why this cap is parallel with C2 (1nf)?

Click to expand...

The 470 uF capacitor is a cathode bypass.  Depending on the value of the cathode resistor in parallel, there is a time constant associated with this.  Typically, an electrolytic capacitor (470 uF) will NOT bypass any stray RF signals to ground.  The 1 nF should,  and standard practice is to parallel them.

A link to thread I will not post in
https://groupdiy.com/index.php?topic=72839.0

rmburrow
I understand what you posted about RF however IMO RF can be addressed in different ways.

For the group
I do not see any RF filtering at the connector.

Bypass ceramics on a PCB with digital signals or "sharp" edges are there because of the trace length LRC. A bypass cap right next to the electrolytic in a slow audio circuit does not make sense to me.

Gus said:

The 470uf can "hold"  the cathode voltage a little more stable than a lower value. This is a "taste" adjustment when you select the value. Think tube guitar amps with cathode or fixed bias output stages.

I find the film cap bypasses to be more of an "believed" improvement with new electrolytic caps(not years old ones).

I am looking for one of the data books I have that has an example of a bypass cap causing issues instead of helping. I forget if it was in an old AD or LT data book.

Circuit looks like a standard pentode wired as a triode plate out. Do you know what type laminations are in the transformer? Amorphous, Mu etc.

Click to expand...

The transformer is custom from lundahl. LL4112.

pages 26,27,28.
and calculator:
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/