Telefunken ECC8100 for microphone project?

[nmark72]

Hello everyone on this great forum,
I am currently working on a conceptual project of a quality tube studio condenser microphone, which would not be a simple copy of a well-known one such as the U47 and U67. I am considering the possibility of using NOS Telefunken ECC8100. Does anyone have experience with that tube?

 

[RuudNL]

What is the advantage of using an ECC8100 compared to other tubes?

 

[nmark72]

What is the advantage of using an ECC8100 compared to other tubes?

As for me, the biggest advantage is that I have two ECC8100 Telefunken NOS . What I could find in the specifications, is that they are very low noise and very resistant to microphonics. Their gain is significantly lower than that of the ECC83. That's why I ask the question, does anyone have experience with them? From what I know, they have their advantages and disadvantages, but I believe they can be used well in audio projects.

 

[Delta Sigma]

Common cathode? Do you know triode plate resistance?

I'm not experienced enough to say whether a tube is suitable or not, but I'd worry less about gain and more about: having the right output transformer (and load resistor) for the anode, capacitances, and grid current.

 

[micolas]

ECC8100 Telefunken

 

[nmark72]

ECC8100 Telefunken

I already found that original spec, but thanks for the help anyway micolas

 

[nmark72]

Common cathode? Do you know triode plate resistance?

I'm not experienced enough to say whether a tube is suitable or not, but I'd worry less about gain and more about: having the right output transformer (and load resistor) for the anode, capacitances, and grid current.

Well, my idea is to do a complete project, and that includes making my own transformer. I found pemalloy cores on ebay...

 

[micolas]

Well, my idea is to do a complete project, and that includes making my own transformer. I found pemalloy cores on ebay...

Bravo! That's DIY!

 

[hop.sing]

I guess you are on your own with this tube, it might be working well in a microphone circuit, or not...
The tube is optimised for cascode circuits in HF applications but it might as well work in a microphone.
Since you have to heat up both halfs of the tube anyway you could use both triodes in parallel to reduce the output impedance.
I think the Lucas CS1 did that.
But the increased capacitances could be detrimental to the sound. You have to try.
Choose a typical microphone tube operating point around, or with less, than 1mA Plate dissipation per triode and than measure and listen.
If I would tackle such a project, I would first decide for a capsule, would wind something like a 10:1 transformer and than decide for a circuit topology and a fitting tube. If you search a bit you can find recommendations for russian miniature tubes that have low noise, low microphonics, sound good and cost something like 2 Euro a piece. And those have proven to work well.
So if the ecc8100 has too much noise or microphonics, you can still change the circuit for some other tube. Experimentation is the fun of DIY.

 

[nmark72]

I guess you are on your own with this tube, it might be working well in a microphone circuit, or not...
The tube is optimised for cascode circuits in HF applications but it might as well work in a microphone.
Since you have to heat up both halfs of the tube anyway you could use both triodes in parallel to reduce the output impedance.
I think the Lucas CS1 did that.
But the increased capacitances could be detrimental to the sound. You have to try.
Choose a typical microphone tube operating point around, or with less, than 1mA Plate dissipation per triode and than measure and listen.
If I would tackle such a project, I would first decide for a capsule, would wind something like a 10:1 transformer and than decide for a circuit topology and a fitting tube. If you search a bit you can find recommendations for russian miniature tubes that have low noise, low microphonics, sound good and cost something like 2 Euro a piece. And those have proven to work well.
So if the ecc8100 has too much noise or microphonics, you can still change the circuit for some other tube. Experimentation is the fun of DIY.

Thanks hop.sing for the helpful suggestions. Connecting both triodes in parallel is an option that I will definitely experiment with, it seems interesting to me because of the noise reduction. Also, Miller's capacitance is drastically reduced by the tube design, to only 0.4pF, so that shouldn't be a problem. The input capacitance is relatively large, 5.5pF, but given that a typical mic capsule capacitance is between 30pF and 65pF (I haven't decided on a specific capsule yet), I don't think that should be a problem either. I don't believe that increasing the input capacity would significantly affect the frequency response, but measurements will show . However, my first option for the experiment is still the common cathode configuration. It seems to me that this is the optimal configuration for this tube, considering that the output impedance is not too important to me, because I will design the output transformer according to the resulting output impedance of the circuit. I will start without the bypass capacitor, so based on the measurements I will experiment with maybe adding a bypass capacitor of smaller capacities.
For now, my goal is to experiment with these tubes I have, because I'm very interested in what kind of sound they will make.