ST12 - 6AK5 - SELA T12 rip-off - SIMPLEST TUBE MICROPHONE IN THE WORLD, FULL PRO

[ricardo]

There's no Grid connection in your schematic.

Yes there is

Diaphragm to grid - and that's the funny story
It's really work

If it's convenient, can you please measure the DC voltages on Plate, p4 and Cathode, p7

Thanks

 

[ln76d]

Can we wait with that?
I would like to measure whole circuit after i build PSU for it.
I did test with S16 PSU where i had to use dropping resistors.
After tests i restored PSU for S16 use and truly i don't want to solder it again.
Two weeks and i should have complete microphone with PSU, so i'll be able to make measurements on finished build.
I promise that i will not forget, i always measure microphone when it's done

 

[Matador]

I think you misunderstood Ricardo - it's not that there's no grid connection, it's that there's no DC reference for the grid.  This is going to make the tube highly prone to oscillation through the grid->cathode capacitance, possibly up in the MHz range.

It's a simple addition:  just a 1G from grid to ground:  nothing else has to change.  It will also guarantee that the diaphragm connection won't float to strange places and reduce the polarization voltage seen by the capsule.

 

[ln76d]

I know what ricardo had in mind
I know what for is high resistance at the grid
This is a part of controversy in  this circuit and that's why i wanted to build it - out of curiosity.
If microphone work very well, even with not suitable PSU - why change that?  What for?
Due to theory?
What's the difference between sela circuit and Royer circuit in case of grid connection?
For the many years Royer mod was made by many people - does anyone complain about oscilations or any inconviniences due to circuit design?
Doesn't the same Royer circuit isn't in his Mojave mike?

 

[Matador]

Of course, you can do whatever you want.

Without a DC reference, you're relying on the tube to somehow establish an operating point based on what the grid and capsule float to "in the air".  Bias point can drift based on the orientation of the earth, how the mike is positioned in the room.  If it oscillates and aliases down to the audio spectrum the squealing will be ... unbearable.   Or it will radiate MHz energy out into the room and play havoc with everything with an antenna.

 

[ln76d]

We will see how this microphone will work
There's always a time to add the grid resistor and as i mentioned in first post -  if there will be need for improvements i will update schematic.
First what i need - good PSU and check 10:1 transformer.
Then i will make more sirious tests.
What do you think about Royer schematic?

 

[ricardo]

Without a DC reference, you're relying on the tube to somehow establish an operating point based on what the grid and capsule float to "in the air".  Bias point can drift based on the orientation of the earth, how the mike is positioned in the room.

Actually, there are a couple of mechanisms that allow certain FET mike designs to get away without '1G' gate resistors.

Guru Scott Wurcer, myself and AN Other from the Yahoo MicBuilders group have done some investigations, the results of which may or may not be in Guru Wurcer's "Linear Audio" series of articles.  (As a beach bum, I don't get to buy & read these excellent articles  )

Which mechanism is effective determines how the FET is 'biased' ... which is why I asked for Anode & Cathode voltages from ln76d.

An important effect with FETs is the gate-source junction is a diode, which rectifies RFI picked up even in the headbasket of a mike.  This changes when you connect a capsule which kills most RFI

This could be happening in these tube circuits but grid-cathode diode threshold voltage is MUCH greater than the 0.5V of a silicon FET gate-source.

The anode & cathode voltages will give me a clue.

Getting rid of the '1G' gives better noise.  The big problem we found with our FET experiments was that the extended LF response at the gate makes the mike VERY susceptible to 'pops' and gives VERY poor, > 1 sec, overload recovery from these.

It would be good if ln76d, or anyone with these mikes w/o '1G' grid resistors on a Tube mike, could test for 'pop overload recovery'.

 

[ln76d]

There's no "pop" problem - at least was not yesterday
When am making voice tests am a real barbarian!
I tested it also without front mesh - and if we add the facts that circuit wasn't shielded also and PSU not suitable for it - microphone was clean! There wasn't noise problem at all

I will make many test and some measurements of the final build - but Gentelmans, what's most important, we have interesting topic and some kind of debate!
This what i like - not all the:
"follow instructions...", "for R7 use 100k..", "check the picture 6", "wire capsule according schematic...", "black wire to ground...", "use your brain..." etc.

What do you think about Zener regulation of plate/polarisation voltage?
From the begining i decided to make it "passive" (why the hell people call it "passive"?) due to zener noise, but maybe i'm wrong?

 

[ricardo]

There's no "pop" problem - at least was not yesterday
When am making voice tests am a real barbarian!

Thanks for this ln76d.

If it is convenient, can you repeat the test (voltage & pop) with 10u across R1, your 510 cathode resistor too.

I don't think I'll ever build another tube device but I'm finding this stuff really fascinating.  It's rare to have someone do careful tests like you do. 

What do you think about Zener regulation of plate/polarisation voltage?
From the begining i decided to make it "passive" (why the hell people call it "passive"?) due to zener noise, but maybe i'm wrong?

If the exact voltage isn't important, "passive" is quieter than a Zener and you can usually get rid of at least one capacitor too.

My last commercial mike design used "passive" to replace a Zener with good effect.

Depending on all sorts of things (cleanliness of PSU, how long is the mike cable etc), you might want a 10u across your 60V too.  It should go to your 'star earth' in the mike itself.

 

[ricardo]

This is a part of controversy in  this circuit and that's why i wanted to build it - out of curiosity.
If microphone work very well, even with not suitable PSU - why change that?  What for?
Due to theory?
What's the difference between sela circuit and Royer circuit in case of grid connection?
For the many years Royer mod was made by many people - does anyone complain about oscilations or any inconviniences due to circuit design?
Doesn't the same Royer circuit isn't in his Mojave mike?

http://tapeop.com/tutorials/34/david-royer/
Here David Royer says

"In my tube mod to the ADK A-51 SC mics, I am relying on the leakage resistance among the elements of the tube to form Rg and I am using grid leak bias (or contact-potential bias) rather than self- bias. This circuit has been proven to work very satisfactorilly, at least with single-diaphragm mic capsules. (I have found that the circuit needs to be modified to be satisfactory for a double diaphragm multi-pattern capsule, but this issue will have to wait for another time.)"

I've not heard the words "grid leak bias" for a very LONG time. 

"grid leak bias" is somewhat unreliable and the reason why it is a problem with multi-pattern is that the "leak" is not to a fixed voltage.

I'm not sure Bernhard Weingartner would be happy to use this.  He designed C12 for AKG and was a good friend of Clem Beaumont, my mike mentor.

There's several schematics for the "Royer mod" including both Cathode Follower & Plate Loaded (like the SELA).

Also a number of kits for the "Royer mod".  Some of these kits re-introduce the 1G resistor.

 

[Matador]

Actually, there are a couple of mechanisms that allow certain FET mike designs to get away without '1G' gate resistors.

I suppose one could rely on grid leakage current to form a phantom grid resistance.  Seems like it would vary drastically from tube to tube.

 

[ricardo]

I suppose one could rely on grid leakage current to form a phantom grid resistance.  Seems like it would vary drastically from tube to tube.

Also the "better" tubes like ef86, 408A & 6072 preferred by the old gurus like Bernhard Weingartner of AKG, would have less grid leakage and so this would be even more unreliable.

Anyone tried this biasing system with these tubes .. preferably NOS Mullard / Philips / Telefunken?

I'm not knocking the idea.  If there are commercial mikes using this system, then it must 'work'.

Just trying to understand how it works. 

I know it can work with FETs but I'm not sure it is the same mechanisms in a tube mike.

Voltage readings of such a mike would be a clue.

 

[Matador]

I'm not knocking the idea.  If there are commercial mikes using this system, then it must 'work'.

Just trying to understand how it works. 

I know it can work with FETs but I'm not sure it is the same mechanisms in a tube mike.

Voltage readings of such a mike would be a clue.

Agreed.  At least with a FET, you have  a (hopefully) consistent doping, and the characteristics of the body/substrate diodes could be measured and would likely be fairly consistent.

I'll be curious to see the other readings as well!

 

[ln76d]

Thank you Guys for really interesting discussion!
If i'll be able (due to time) i'll try it with E80F in my S216, but can't promise
All the things with ST12 i'll be checking when it will be done.

 

[micaddict]

Thank you Guys for really interesting discussion!

+1

 

[granger.frederic]

i always find that this kind of schematics are not very stable due to the "floating" grid biasing method , and very dependent to the tube's electrical characteristics...
i don't like the lack of B+ filtering also

 

[ricardo]

i always find that this kind of schematics are not very stable due to the "floating" grid biasing method , and very dependent to the tube's electrical characteristics...

Which tubes did you try with this?
What did you find?

 

[granger.frederic]

true for me and for virtually  all kinds of tubes ...
it's my conception, but i'm not trying to force anyone...

this schematic counts on the inter electrodes specs to avoid instability/oscillation, wich may vary from one tube to another, and also with tube usage
i also note that this schematic is not very popular in all the audio tube devices found in the world, especially in microphones

but it works , i've tried it a long ago with 5840 , it just not my cup of tea...

too many dummy variables...

 

[ricardo]

true for me and for virtually  all kinds of tubes ...
it's my conception, but i'm not trying to force anyone...

this schematic counts on the inter electrodes specs to avoid instability/oscillation, wich may vary from one tube to another, and also with tube usage
i also note that this schematic is not very popular in all the audio tube devices found in the world, especially in microphones

but it works , i've tried it a long ago with 5840 , it just not my cup of tea...

I think the theoretical pros & cons are quite clear.

Just want to hear from people with practical evidence.

Was hoping you'd say something like, "tried one with 5840 and it was OK but didn't work with EF86".

Looks like 5840 works  for Royer and Fred.  Anyone have had experience of tubes where it DOESN'T work?

 

[granger.frederic]

quite clear but not for you
you can make it work with virtually ALL tubes as long as its inter-electrodes specs allow operation
you  will have more chances with higher inter-electrodes capacitance , leakage, etc  ...