ELA M251 Inspired Microphone - Build Thread

[chunger]

Sorry for the recently broken photo links. Some forum migration may have broken them, and now, the forum is not allowing me to re-insert more than 30 photos per post. I am uploading documentation to my blog and will link there.

 

[chunger]

Build guide has been uploaded here with properly linked full photo set. Sorry I was not able to fix this thread in place.

ELA M251 Build Guide

 

[lightningbefore]

Thank you @chunger, was just going to ask!

 

[SubSpec]

Sooo, I've just dropped a CT12 into my JRR NOS 251, which is essentially this build with a 6021 (on the Matador PCB). It's always sounded decent but had a noise floor high enough that I don't reach for it often. It could just be a noisy tube (NOS GE 5-star), but rather than fool with swapping for other 6201's I'm leaning towards reverting it back to the 6072a configuration.

I need to check the components used in the JRR mic, but as I understand it the typical arrangement for switching from 6072a to 6201 would be dropping R17 to 47k. So assuming that's how it's configured, anything else I should take into account before swapping in a 6072a?

 

[Accelerator]

@SubSpec​

Changing the Anode Resistor (R17) 100KΩ to a 47KΩ anode resistor for an ECC81 - 12AT7 - 6201 tube works pretty well in a C-12 microphone which is a fixed-adjustable biasing microphone (so that with the 47KΩ anode resistor you can properly biasing it at -1.02V), but it doesn’t work as good for the Cathode Biasing ELA-M 251E microphone…

But let’s see the 47K case with the ECC81 - 12AT7 - 6201 tube:

120 DC Volts Vb / 47.000 Ohms = 0.0025531914…A aka 2.55mA so we have the 2.55mA as a marking point and we can draw our load line…

As we have the 120 DC Volts as Vb and we like to take 60 DC Volts as Quiescent Operating Point for Center biasing, we will see in the load line that at the 60 DC Volts the Grid Bias Voltage (V) is -1.02 Volts and the Iq(mA) is 1.28mA…

-1.02 Volts / 1.28mA = 796,875 Ohms, aka the nearest value is 800 Ohms for the Cathode Resistor (R1)…

The 800 Ohms is very low value for Cathode Resistor (R1) to performs well with the 22μF Cathode Bypass Capacitor (C1) a properly LC/HP Filter for a microphone, so in this case you have to increase the value of the Cathode Bypass Capacitor (C1) to 47μF or higher…

If you like to use the ECC81 - 12AT7 - 6201 tube in an ELA-M 251E microphone it is more preferable to keep the 100KΩ Anode Resistor (R17).

So let’s see this case, the 100K case with the ECC81 - 12AT7 - 6201 tube:

120 DC Volts Vb / 100.000 Ohms = 0.00120A aka 1.20mA so we have the 1.20mA as a marking point and we can draw our load line…

As we have the 120 DC Volts as Vb and we like to take 60 DC Volts as Quiescent Operating Point for Center biasing, we will see in the load line that at the 60 DC Volts the Grid Bias Voltage (V) is -1.58 Volts and the Iq(mA) is 0.60mA…

-1.58 Volts / 0.60mA = 2.630,333333 Ohms aka the nearest value is 2.630 Ohms for the Cathode Resistor (R1)…

So you can use the Vishay Dale CMF55 2.61k Ohm, 1% as Cathode Resistor (R1) and with this value it will work pretty well in combination with the 22μF Cathode Bypass Capacitor (C1) to performs a properly LC/HP Filter for a microphone, so in this case you don’t have to increase the value of the Cathode Bypass Capacitor (C1) to 47μF or higher…

Keep in your mind that back in time the AKG AG used the 100KΩ Anode Resistor – 2.7KΩ Cathode Resistor for the legendary C-24 microphone, but today we can have more precision in today’s value choices…