musix2000

I`m building an ELA M250e clone with a CT12, JAN GE 6072a mic. grade and a 13114 transformer by cinemag. As in the original schematics I use a 1k8Ohms (1k82) cathode  resistor, instead of the no more produced 8M resistor I use one with 7M96(7)Ohms  For tests and some experiments I have the capsule of a Rhode K2 in it. Because Telefunken have a 2k7Ohms (2k67) resistor in their C12 and ELA M251e I`ve also tried this: with 1k82Ohms 1.065V and with 2k67Ohms 1.37V are coming from the cathode (B+ is 120,6V). It seems: higher resistor values > higher cathode bias voltage. I was told that the cathode bias should be 1.1V, otherwise, with more voltage, the tube could be damaged.  But I think the guys at Telefunken know what they do. They also have changed from the 8M resistor to a 10M (what I still haven`t tried, maybe this brings the cathode bias down to 1.1V??).  Whether in their C12  resistors were changed or not I didn`t find out, instead of fixed bias they also use a 2k67Ohms resistor.
I still haven`t tried the microphone under studio conditions, I`ve just finished it with the K2 capsule and it`s in my workshop for the next days.
Is it ok to have a higher or lower cathode bias? What is the influence to the sound?
Does anybody know about this?
« Last Edit: June 16, 2018, 08:13:04 PM by musix2000 »


I think you are being to picky about values. 8meg vs 10meg wont play huge role here.  Some use 1 Gig here. There are a lot of threads about what those components do and how to pick them.

You shouldn't look for voltages but for the sound. Those 1k8 and 2k7 are biasing resistors, look for adjusting bias topics. IMHO those two values won't make a world of difference if any. And those voltages aren't hurting the tube. Try both and use one you prefer. Ideally you should chose the value that makes least amount of THD and that could be neither of those.

RuudNL

My experience is that the value of the cathode resistor is not very critical.
Usually I aim for a value of about 1/2 of the supply voltage of the tube stage on the anode.
Tubes aren't exactly 1% tolerance devices!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Matador

The cathode resistance sets the idling point of the tube.  Given most small signal triodes in these microphones have only about 2V of usable grid swing when run at 120V supply voltage, then a 1V bias point is a reasonable point for most tubes.  So in short, the stock value of 1k8 works well across many tubes.

If you want to be super pedantic, you would do a signal swing analysis on a scope, and bias the tube for minimum distortion (much like you would with a JFET).  But tubes aren't exactly meant to be perfectly clean so I wouldn't worry about it.

You won't damage the tube by adjusting cathode bias in any case.

musix2000

Thank you all for your answers, they make the thing much clearer for me!
I have a scope, would an adjustable resistor to find the lowest THD be a choice? Does lowest THD also mean best working point of the tube?
« Last Edit: June 17, 2018, 09:49:39 PM by musix2000 »

Last time I made a tube mic and adjusted it to lowest possible THD it cancelled 100% in null test with same capsule in the same body but with FET circuit. In other words the mic sounded exactly the same as FET mic when used in normal conditions.

Like Matador mentioned I don't think  tube mics are about lowest possible THD. 

« Last Edit: June 18, 2018, 09:36:26 PM by kingkorg »

musix2000

Hi Kingkorg....
This would mean that the cathode resistor is the component that makes the tube sounding like a tube or not, is this right? In this case an adjustable resistor would be a good choice to create the tube sound to it`s best. Values could be changed as long as the optimal sound is reached.

Yes, you can put a VR there and adjust at will, or several resistor values and a switch.

RuudNL

When I first started building tube microphones, I thought that there was something seriously wrong with the design!
My solid state microphones usually had distortion figures of 0.02% at high output levels, but for a tube microphone 0.8% seemed 'normal'. I analyzed the distortion and came to the conclusion that it was mainly second harmonic, probably the kind of distortion that people like about a tube microphone. Even top class tube microphones produce this kind of second harmonic distortion!

By the way: adjusting the cathode resistor for minimal distortion on an oscilloscope isn't very exact.
Before you start seeing distortion (on a sine wave) on an oscilloscope, the THD is already in the order of 3%.
Personally I use a distortion analyzer to find the bias point for minimal distortion.
There is a solution for every problem!

http://www.vansteenisaudio.nl

Matador

At low cathode resistance (meaning that as the cathode voltage approaches ground), ground-referenced input signals will tend to distort as the grid is driven more positive and the tube becomes progressively non-linear.  The same is true for operating a higer cathode resistances (meaning the cathode voltage is increasing), as the tube current will pinch-off as the plate voltage swings upwards and intersects with the 120V power rail.

So the first case is that the porridge is too hot, and the second is that the porridge is too cold, so we want to be right in the middle. :)   



So here is a typical tube with a typical load line as you would see in a microphone application.  The cathode bias sets where on the red line the tube idles, so if you are far to the left of the line (low bias voltage) you have positive grid operation, and far right on the line is where the curves become 'bendy' and the tube current cuts off.  Somewhere right in the middle is where the -1V and -2V grid lines intersect, which tells you that you need to run a cathode voltage between +1V and +2V.

This chart is for a 12AT7, but you can calculate the exact resistance from any tube type in a similar fashion.  If you want 1V cathode voltage, follow the -1.0V grid voltage line down until it intersects with the red line, then follow it over to the left and see what plate current this happens.  On this chart, it's about 0.7mA, so 1V at 0.7mA means 1/0.7 = 1.428k ohm.


Killer post!

maybe it's punctilious, but why not choosing the tube (and its biasing voltage) depending to the max output voltage of the transducer (just before the capsule starts to really distort, maybe around 1%...), given by the manufacturer (if he measured it...) at its optimal voltage polarisation (usually around 60v)?
So , we could have the lowest distortion and the optimal bias point at the same time...
« Last Edit: June 22, 2018, 04:13:39 AM by granger.frederic »

Matador

That's pretty much exactly what is done.

If we assume a typical 20 mV/Pa sensitivity rating (@94 dbSPL) for an LDC, then we need about 30dB of gain to bring this up to line level at that average sound level.  94dB is fairly loud already, meaning that we can get to standard line level under stock conditions, and the 22mV input swing is well within the operating point of a tube under most conditions.  Also remember that we have an output transformer that reduces the gain by 20dB, so the net gain of the system (as seen by the output jack) looks like 10dB, or an average signal level of 100mV under pure stock conditions.

If we switch to a super loud source (say 130 dBSPL), then the capsule might put out near line level by itself.  If 1V RMS, then the tube biased at -1V will still be 'fine', the plate will swing 30V peak-to-peak, and the output signal will be 3-4V at the output jack.


 

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