DIY Tube Mic project

[rock soderstrom]

I think your voltage divider resistor will get quite warm for a short time at the moment of switching on, when the tube is cold....if you don't take any measures against it.

 

[Emmathom]

Yes filaments are relatively "current hungry" when cold !
Let's figure double current = (350mA^2)*6,8Ω = 0,83w ! > let's put a 6,8Ω@3w here...

 

[thor.zmt]

What's the best solution ? a divider with 7,5v + 6,8Ω or a cathode R 240Ω ?
If divider is this schem. correct ?

Leave that out for now. It needs extremely clean heater power or it's noisy.

 

[thor.zmt]

Just an another example, this is how Neumann did it in the U67. Kathode is grounded, heater voltage negative.
View attachment 130044

More critically, the negative heater supplies a negative grid bias of -1.6...1.8V via R9, R8 and C7 to filter the bias voltage. The negative grid bias is directly applied to the capsule connection.

Current incidentally should be around 0.9mA.

Thor

 

[thor.zmt]

I think your voltage divider resistor will get quite warm for a short time at the moment of switching on, when the tube is cold....if you don't take any measures against it.

I would probably feed the heater from CCS, not a normal regulator.

Thor

 

[Emmathom]

Hi Thor, I just received the 6AK5 socket so I will begin building the mic.
Just to be sure : the E-352 must be connected anode to B+ (90v) and cathode to 6AK5 anode, right ?
For 6AK5 cathode R I will use 2x470Ω 1/4w in // so 235Ω, ok ?
BR

 

[thor.zmt]

Just to be sure : the E-352 must be connected anode to B+ (90v) and cathode to 6AK5 anode, right ?

Ring on glass towards 6AK5 for simplicity.

For 6AK5 cathode R I will use 2x470Ω 1/4w in // so 235Ω, ok ?

Sure.

Thor

 

[Emmathom]

I rebuilt the PSU because there were some undos, some things I wasn't satisfied with.
Now this version is clean, improved and 100% efficient.
Several hours of burning test with 6,3v under 210mA (175mA target) and 90v under 3,8mA (3,5mA target) = stable voltages
Let's build the mic now...

 

[Emmathom]

Leave that out for now. It needs extremely clean heater power or it's noisy.

Another solution is to use 2 diodes (1N4148) in serie to get 2x0,6v... ?

 

[Emmathom]

Here are the 6AK5 connexions :
Pin 1 = signal from capsule (diaphragm)
Pin 2 = R cathode to ground
Pin 3 & 4 = heater (6,3v@175mA)
Pin 5 = to anode (90v@3,5mA)
Pin 6 = connect to anode > output
Pin 7 = internal connexion with cathode (no further connexion)
Is it correct ?

 

[rock soderstrom]

Pin 5 = to anode (90v@3,5mA)
Pin 6 = connect to anode > output

If you keep your way of labeling consistent, then it would be:

pin 5 = to cathode of E-352 and output cap (or in the original schematic: to anode resistor and output cap
pin 6 = connect to anode/ Pin5 of the tube (makes the 6AK5w to a pseudo triode)

You sometimes refer to the anode when you mean the operating voltage, which is somewhat misleading, I would call it B+

 

[MicUlli]

Sorry guys, I really don't understand the hype around tube mic designs. A lot of additional electronic components, high current consumption and expensive transformers with one outcome: Heat, noise and distortion

 

[rock soderstrom]

Heat, noise and distortion

You forgot one thing in your list Ulli: sound!

This is of course highly subjective, but I have often made direct comparisons. Tube microphones come much closer to my idea of sound than any other technology. Whether that justifies all the extra effort is something everyone has to decide for themselves.

From a purely technical point of view I can understand you, but I don't build measurement microphones, I build microphones that make their recordings (at best) larger than life. I'm not interested in reality, I don't record movie sound or classical music. I see microphones as part of the design of the recording, technical values don't really interest me that much, as long as you can make professional recordings with them.

Somehow your post reminds me of a post by a dog lover in a cat thread.

 

[Emmathom]

First I want to test the voltages in the mic before connecting a capsule : do I must connect the grid to ground thru a R ? (1GΩ ?) or thru a 56pF (or so) capacitor ? or can I leave the grid not connected ?

 

[MicUlli]

You forgot one thing in your list Ulli: sound!

This is of course highly subjective, but I have often made direct comparisons. Tube microphones come much closer to my idea of sound than any other technology. Whether that justifies all the extra effort is something everyone has to decide for themselves.

From a purely technical point of view I can understand you, but I don't build measurement microphones, I build microphones that make their recordings (at best) larger than life. I'm not interested in reality, I don't record movie sound or classical music. I see microphones as part of the design of the recording, technical values don't really interest me that much, as long as you can make professional recordings with them.

Somehow your post reminds me of a post by a dog lover in a cat thread.

Yes Thorsten,
I want my post to be understood as an opener for arguments

 

[Emmathom]

You forgot one thing in your list Ulli: sound!

This is of course highly subjective, but I have often made direct comparisons. Tube microphones come much closer to my idea of sound than any other technology. Whether that justifies all the extra effort is something everyone has to decide for themselves.

From a purely technical point of view I can understand you, but I don't build measurement microphones, I build microphones that make their recordings (at best) larger than life. I'm not interested in reality, I don't record movie sound or classical music. I see microphones as part of the design of the recording, technical values don't really interest me that much, as long as you can make professional recordings with them.

Somehow your post reminds me of a post by a dog lover in a cat thread.

And tubes are fun ! and easy to build & bias

 

[Emmathom]

I found an unployed capsule (mono diaphragm) so I was able do to some voltages tests with it (@thor.zmt)
Signal from diaphragm / Polar. to backplate

B+ = 90v
Polarisation caps. = 58,28v
Anode volt. = 56,25v (thru E-352)
Cathode volt. = 830mV (thru 235Ω to ground)

I haven't put 1GΩ from grid to ground... capsule is connected straight to grid

No sound test yet since I have to wire the NTE10/3 and I wait for @thor.zmt comments on these voltages...

 

[rock soderstrom]

3.53mA current, as predicted. Voltages look good. Soundcheck, lets go!

 

[Emmathom]

Some picts.
I installed "hoops" to properly maintain the tube support circuit.
I use a 5 x 20AWG shielded multi-conductor cable (it is rather thin and flexible)
The 1uF MKP is what I got in stock...

Not much more to say with this ultra-simple circuit using only a few components...
Glad to get good voltages (I think the PSU is well done, well filtered > anyway it holds both tensions without a wrinkle)

 

[rock soderstrom]

I installed "hoops" to properly maintain the tube support circuit.

What is a “hoops”?