Finished AKG C12 clone pics

[gyraf]

Bezen: You are referring to somewhere-near maximum specs of a tube - there's no design requirement that you go to it's limit.

Getter material is flashed at factory, and will catch minute traces of gases over time, even when stored in it's cardboard box - tube operation does not affect this at all.

Lower heater voltage is probably more of a problem - but most tubes are designed for +/-20% tolerance for heater voltage (as it should be able to run on batteries). Some cathode materials are more sensitive to massive underheating than others - I have VF14's in u47's running on 50'es year on 50% heater voltage, but experimenting with EF40's in this range has shown as little as 20-30hours of operation before problems arise.

Dmp: take a look at 12AY7-based amplifiers, note grid resistance range.

Jakob E.

 

[bezen4uk]

Hi Jakob,
Thank you for note.
Just checked again - getter needs heating to adsorb gases.
I had tubes that have pop noise after 20 working hours. After heat treatment most tubes become good.
All in all, if the tube is already noisy, it worth to try this method.

 

[gyraf]

Getter is activated from outside by induction heating during production, not by using the tube's own heat

 

[bezen4uk]

Looks like we both are right (from Wiki):

While the tube envelope is being evacuated, the internal parts except the getter are heated by RF induction heating to help free any remaining gases from the metal parts. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating...

The getter continues to absorb small amounts of gas that may leak into the tube during its working life.

 

[david-p]

Now the only thing separating C24 and C12 would be the fixed tube bias of the C12 coming from the PSU.

Would there be another way of achieving the fixed bias without supplying it from the PSU?

There is simple way : add resistor in series with heater network like it was in U47 circuit (don't forget about cathode pin). Resistor should be R=Ubias/Ih, so about 3-5 Ohm.

Interesting. I wonder how it will affect the sound... I remember reading that at least mr.Heyne is very fond of fixed bias.

Has anybody tried that U47-type of bias method in a C12 type of circuit?

btw, later when AKG designed the ELA M251 for Telefunken, they used similar tube bias than in C24. Not the U47 type. Wonder why, if the fixed bias would have been somehow better...

Actually, if you look  carefully at the C12 circuit, it is not materially different from the C24.  The 1k resistor (R8) could have been put in the mic (directly in series with the cathode), but then possibly there would not have been room in the mic for the 2µF (C6) in parallel with it.  Either way, the tube would see the same biassing situation.

David

 

[david-p]

They might meet the spec. They might struggle to pass selection for noise and microphony.

Are low noise and microphonics not part of the spec?  Are they just a question of luck?

Low noise and microphonics, when used in a very high grid resistance circuit like mics, is unfortunately not part of the specs a new-production tube aims to meet. Remember, we use these waaaay out of specified range, and hope for a tube design/build that'll be forgiving when pushed like this.

Jakob E.

Thanks.  I will buy a batch of EH 6072s and check them out.

David

 

[telefunk]

Now the only thing separating C24 and C12 would be the fixed tube bias of the C12 coming from the PSU.

Would there be another way of achieving the fixed bias without supplying it from the PSU?

There is simple way : add resistor in series with heater network like it was in U47 circuit (don't forget about cathode pin). Resistor should be R=Ubias/Ih, so about 3-5 Ohm.

Interesting. I wonder how it will affect the sound... I remember reading that at least mr.Heyne is very fond of fixed bias.

Has anybody tried that U47-type of bias method in a C12 type of circuit?

btw, later when AKG designed the ELA M251 for Telefunken, they used similar tube bias than in C24. Not the U47 type. Wonder why, if the fixed bias would have been somehow better...

Actually, if you look  carefully at the C12 circuit, it is not materially different from the C24.  The 1k resistor (R8) could have been put in the mic (directly in series with the cathode), but then possibly there would not have been room in the mic for the 2µF (C6) in parallel with it.  Either way, the tube would see the same biassing situation.

David

Hmm, i don't have the expertise to argue with you but my understanding is that in C12 the cathode is connected directly to ground and the fixed bias voltage feeds directly the grid bias resistor making it quite different bias scheme than in C24.

 

[e.oelberg]

Thanks.  I will buy a batch of EH 6072s and check them out.

David

I did that with no success at all, every old tube i tested sounded better.

 

[telefunk]

Thanks.  I will buy a batch of EH 6072s and check them out.

David

I did that with no success at all, every old tube i tested sounded better.

Better, in what way?

 

[david-p]

Thanks.  I will buy a batch of EH 6072s and check them out.

David

I did that with no success at all, every old tube i tested sounded better.

Well, I ordered a dozen already.  Wish me luck!

David

Better, in what way?

 

[david-p]

Hmm, i don't have the expertise to argue with you but my understanding is that in C12 the cathode is connected directly to ground and the fixed bias voltage feeds directly the grid bias resistor making it quite different bias scheme than in C24.

Look carefully at the C12 circuit, particularly at the 1k resistor, R8. It is true that the cathode is connected directly to ground, but the grid resistor, R15 is not: it is connected to R11 and thence, via R10 to the negative end of R8.  The other end of R8 is connected to ground.  There is current passing through R8 (actually the return from the HT or B+ line to thebottom end of the lower winding of the transfomer).  This makes the left hand end of R8 negative with respect to ground.  Thus the grid is also negative with respect to the cathode, as is normally achieved by means of a resistor in series with the cathode. (See the C24 circuit.)  The method of achieving the bias is not unlike that in the U47 circuit which also relies a on a voltage across a resistor (R3) fixed by a larger current also flowing through it -- in that case the heater current.

The connection between R10 and R11 may be obscured if you are looking at the version of the C12 circuit on which someone has kindly drawn a fat vertical line with arrow heads on it.  This one is cleaner: http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

David

 

[pasarski]

Look carefully at the C12 circuit, particularly at the 1k resistor, R8. It is true that the cathode is connected directly to ground, but the grid resistor, R15 is not: it is connected to R11 and thence, via R10 to the negative end of R8.  The other end of R8 is connected to ground.  There is current passing through R8 (actually the return from the HT or B+ line to thebottom end of the lower winding of the transfomer).  This makes the left hand end of R8 negative with respect to ground.  Thus the grid is also negative with respect to the cathode, as is normally achieved by means of a resistor in series with the cathode. (See the C24 circuit.)  The method of achieving the bias is not unlike that in the U47 circuit which also relies a on a voltage across a resistor (R3) fixed by a larger current also flowing through it -- in that case the heater current.

The connection between R10 and R11 may be obscured if you are looking at the version of the C12 circuit on which someone has kindly drawn a fat vertical line with arrow heads on it.  This one is cleaner: http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

David

Doesn't all that make it fixed bias (bias doesn't change with the signal as in cathode biased circuit)?

 

[david-p]

Look carefully at the C12 circuit, particularly at the 1k resistor, R8. It is true that the cathode is connected directly to ground, but the grid resistor, R15 is not: it is connected to R11 and thence, via R10 to the negative end of R8.  The other end of R8 is connected to ground.  There is current passing through R8 (actually the return from the HT or B+ line to thebottom end of the lower winding of the transfomer).  This makes the left hand end of R8 negative with respect to ground.  Thus the grid is also negative with respect to the cathode, as is normally achieved by means of a resistor in series with the cathode. (See the C24 circuit.)  The method of achieving the bias is not unlike that in the U47 circuit which also relies a on a voltage across a resistor (R3) fixed by a larger current also flowing through it -- in that case the heater current.

The connection between R10 and R11 may be obscured if you are looking at the version of the C12 circuit on which someone has kindly drawn a fat vertical line with arrow heads on it.  This one is cleaner: http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

David

Doesn't all that make it fixed bias (bias doesn't change with the signal as in cathode biased circuit)?

The method just places the resistor that would normally be connected between cathode and ground to the position of R8.  See http://www.hans-egebo.dk/Tutorial/biasing.htm, where it is described as "Power supply backoff".

David

 

[0dbfs]

That's pretty cool!

Any ideas why we don't use a dedicated negative supply? How would this affect the sound of the amp?

Why does self-bias and fixed-bias "sound different" anyway?

Thx,
jonathan

 

[MagnetoSound]

Doesn't all that make it fixed bias (bias doesn't change with the signal as in cathode biased circuit)?

Yes, it is fixed bias (in case you are still in any doubt).

 

[saxmonster]

Does sound quality differ from the c12 and 251 because of where the patten controls are?  251 has switches on the capsule assembly but the c12 is done by the power supply.  If I am modding my apex 460 into a 251 do i need to be worried about this?


What is grid coupling?  Is it different on the two mics?

Thanks
-Scott

 

[david-p]

Does sound quality differ from the c12 and 251 because of where the patten controls are?  251 has switches on the capsule assembly but the c12 is done by the power supply.  If I am modding my apex 460 into a 251 do i need to be worried about this?

I cant see that this should make any difference at all.

David

 

[pasarski]

Does sound quality differ from the c12 and 251 because of where the patten controls are?  251 has switches on the capsule assembly but the c12 is done by the power supply.  If I am modding my apex 460 into a 251 do i need to be worried about this?

I cant see that this should make any difference at all.

David

Actually 251 type pattern switch improves S/N ratio about 3 db's (IIRC) in cardioid mode because the rear membrane gets disconnected from the circuit. Also it enables the elimination of capsule to grid coupling capacitor in cardioid and omni mode. But you also need a capsule with separate backplates for this. I'd love to see someone build this kind of pattern switch.

 

[david-p]

Actually 251 type pattern switch improves S/N ratio about 3 db's (IIRC) in cardioid mode because the rear membrane gets disconnected from the circuit.

I dont see this: reactive sources do not produce noise.  Or are you referring to the pick up of ambient noise?

David

 

[MagnetoSound]

The two membranes form a potential divider, or 'mutual pad', as the capacitance of each is effectively loading the other (regardless of polarisation).

There is therefore a corresponding loss of signal at the grid (or gate), which leads to a lower S/N ratio after the amplifier, compared to that when only one membrane is connected.