Gefell mics heater voltage and transformer question

[David_H]

Hello community,

I have a question regarding the 5,8V heater voltage in Gefell mics that perhaps can be helpful to other people as well.
The situation is the following - with a simple mod, the Chinese PSUs for Tube Mics that are cheap and easily accessible can be made to put out ca. 120V polarisation voltage, which is ideal for the Gefells. However, the heater voltage is 6,3V. The EC92 will accept this and combination does work - but here's my question:

Will this damage any components in the long run? I don't quite see how it could, since there is so little current flow thorugh the tube, but I just wanted to clarify this for me and any other potential "cheap & dirty" kind of users. I know that powering the mic this way alters the impedance the capsule sees and possibly the noise characteristics, but somewhere I also read that this results in a transformer mismatch - can the OT be damaged this way?
Because otherwise, this really is a perfectly acceptable solution for everyone that's not running a commercial studio in my opinion!

Best regards and thanks for the help,
David

 

[Gus]

Some power supplies use a 7806 with a 1n400_ in series with the ground leg. This sets the voltage to about 6.6VDC in the power supply.
I am guessing the "design" works with the voltage drop from the cable heater wires resistance to reduce the voltage

I would adjust the heater circuit for the 5.8VDC measured at the tube in the microphone

 

[David_H]

Hey! Thanks for the reply! Yes, that does seem to be the case with the supply I am working with - would changing this diode get me closer to 5,8V? The 6,3V were measured in the Mic by the way.
Regarding the orginal question - other than noise characteristics and loading of the capsule - is there actually any harm done in just using the mic with 6,3V heater voltage? I am refering to this schematic/mic by the way

 

[emrr]

You can also consider this approach. Those are available at various adjustable voltages.
https://groupdiy.com/threads/tube-mic-pre-re-purposing-upgrade-filament-regulator.77039/#post-986392

 

[David_H]

Thanks for the suggestion, but that seems way over my head in terms of knowledge/skills. Are the 5,8V really that critical to begin with? I mean that is within 10% of 6,3V, wouldn't the variations in the mains power lines back then been more of an offset on any given day? I guess what I am asking is - are there any real world drawbacks to just use the mic with 6,3V heater voltage apart from maybe tube life?

 

[mjrippe]

EC92 is designed for 6.3v. That's what the "E" stands for in European tube numbers. The "C" is for triode. Stop worrying and make records!

 

[David_H]

Thanks for the suggestion, will do ! I was just trying to play it safe and preferably not fry my vintage m582 or orignal M7 capsule . Anyway, I guess it should be ok. File under "for future reference for other Gefell users".
Thanks for the help guys!

 

[Gus]

What I have done with supplies like that is to take out the diode and ground the ground pin of the 7806. That should get you in the 5.7VDC to 5.9VD range with the voltage drop in some cables

Or better you can add a series resistor between the output of the regulator and the filter cap after it, you will need to cut a trace and bridge the cut with the resistor
This does two things.
One it helps current limit the the starting current to the heater when you power up.
Two it helps filter the regulator noise some being a RC

Think ohms law if you have 6.3VDC in the microphone you will want to drop .5VDC
.5VDC/150mA(or what ever the filament current is)= the resistor value in ohms about 3.3ohms (Three10ohm resistors in parallel should work)
then use DC power math for the resistor wattage
Filiments can be somewhat nonlinear like a light bulb so you might need to adjust the resistor value a little

 

[David_H]

That is an excellent suggestion and seems very doable for me ! Just to double check, so these 10Ohm resistors in parallel would come before c10 in the attached schematic?

 

[David_H]

Just a little follow up in case anyone stumbles across this thread in the future. I did not go ahead and modify my Chinese PSU, but I got hold of a N61 power supply for a reasonable price. This is the original PSU for this mic and either the 5,8V or the different design or both do make a difference! The mic is quite a bit less noisy now - the hiss is a lot less prominent now. Interstingly, it is not measurebly more silent in any significant way ( the noise is pretty much the same on a dbfs scale), but the hiss has shifted into a lower register where it is barely noticeable. It is a VERY clear difference in perceived noise level that could be picked out in a blind test 10/10 times.