Measuring tube mic voltage

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johnnybregar

Active member
Joined
Nov 22, 2013
Messages
25
Hello,
I am a studio owner, and I’m having some issues with a tube mic. Can someone point me to a resource (or describe to me) how to measure the heater voltage, the control grid voltage, and the plate voltage? I need to see if any of those are out of spec because tubes seem to be dying early with this mic. They start to crackle, then become unusable.

I’m good with a multimeter, but I just don’t know which pins are which, and if I measure this on the mic while it is powered up, or if I just measure from the output of the power supply.

Thanks in advance for any help you can offer.

Johnny
 
There is no "universal" answer to your question. If you can volunteer some models of the mics you're having trouble with, perhaps the schematics (and connector pinouts, if necessary) can be found or determined, otherwise... No can do.
 
There is no "universal" answer to your question. If you can volunteer some models of the mics you're having trouble with, perhaps the schematics (and connector pinouts, if necessary) can be found or determined, otherwise... No can do.
I'm trying to measure the voltage to the EF800 tube in a Stam SA47 mic. Thank you!
 
Thank you - I was hoping it might be easy. I think I'll bring this to an audio repair shop, this is way beyond my skill level.
 
Is the tube underheated (run at under 6.3VDC) Some tubes don't run well being underheated and the lower the voltage is the faster issues happen
Is the heater at 5.8VDC or lower VDC?
Can you post an inside picture it should not be hard to find a fil voltage voltage node to probe
 
Found this link look at the pictures does the inside look like the one picture?
https://www.amazona.de/test-neumann-u47-peluso-2247-le-stam-audio-sa-47/If so it should be easy to measure the heater voltage

On another note look at the power supply pictures.

Is the microphone based off this
http://www.tab-funkenwerk.org/artic...-microphone-schematic-archive/u47-schematics/is the heater at 5.05VDC?
In my testing some tubes will have issues at lower than around 5.7VDC 5.8VDC
If you increased the voltage you will need to redo the biasing resistor(R2) in the heater circuit
 
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Thanks - the heater voltage measured at 4.75 volts. It has been blowing through tubes.

I just increased it to 6.1, because I read that's where an EF800 wants to be. But perhaps my biasing resister now. needs to be changed.

Does anyone know what that pin 2 should read for voltage? Pin 2 seems to be the only one hooked directly to a resistor on the back of the board.

I'm definitely in unknown territory here...
 
Pin 2 should be zero volt and connected with a high value resistor. From 60M to 1G ohm. For my experience pin7 8 9 around 40-50v. Pin 4 or pin 5 around 5-6v. Pin 1 and pin 3 around 1v. Hope that can help you.
 
I will guess it is based of the link below, does the microphone circuit match the link?
http://www.tab-funkenwerk.org/artic...-microphone-schematic-archive/u47-schematics/Use ohms law
from the schematic link 5.05VDC/(29 ohms + 100 ohms) = what current is needed for the bias voltage
That current x 29 ohms = the bias voltage
now you have 6.1VDC so you can increase the 100 ohm resistor value for the same voltage at the cathode
 
So I just learned some important information from the manufacturer.

The control grid (pin2) is shunted to ground via a high value resistor, which puts it very near ground potential by design. The amplifier is in auto-bias configuration, so the fixed positive cathode voltage sets the bias point.

This voltage is derived from the heater voltage, which should be set to 5.05 volts. This way the voltage divider on the cathode plus filament DC resistance form a voltage divider that puts 1.1V on the cathode, that is the historically correct value for the U47 bias voltage.
 
Did you read my posts? I showed how to calculate the bias voltage

Good luck with 5.05VDC at the heater I think you will still have the same tube issue, I did give some hints if you do.

Did you buy this microphone new if so how was it so far out of adjustment?

Also I hope the power supply heater supply is very clean.

The u47 is fixed bias(the current in the 29 ohm has more bias control than a 29 ohm cathode resistor) and a higher value grid to ground resistor is what you often find in a microphone.
 
Did you read my posts? I showed how to calculate the bias voltage

Good luck with 5.05VDC at the heater I think you will still have the same tube issue, I did give some hints if you do.

Did you buy this microphone new if so how was it so far out of adjustment?

Also I hope the power supply heater supply is very clean.

The u47 is fixed bias(the current in the 29 ohm has more bias control than a 29 ohm cathode resistor) and a higher value grid to ground resistor is what you often find in a microphone.
Yes, I read your posts.

I'm now caught between what Joshua Stam is telling me (who makes the SA47 mic), and what people on this board are telling me, and I don't have the technical expertise to argue either side.

I purchased the mic new, but it needed some work after I bought it, and I never checked the voltage until my tube started crackling.

To be clear, this is not a Neumann U47 - it is a Stam Audio clone.

If the tube is supposed to be getting 6.1 volts, someone more knowledgeable than I am needs to take it up with Josh and company, because I can't make heads or tails of what I'm supposed to do in this situation.
 
I will guess it is based of the link below, does the microphone circuit match the link?
http://www.tab-funkenwerk.org/artic...-microphone-schematic-archive/u47-schematics/Use ohms law
from the schematic link 5.05VDC/(29 ohms + 100 ohms) = what current is needed for the bias voltage
That current x 29 ohms = the bias voltage
now you have 6.1VDC so you can increase the 100 ohm resistor value for the same voltage at the cathode
I don't have the skill to be able to tell you if my mic matches either of the mics in that link.
 
Does the inside of the microphone look like to one in the link with the reviews?

I would guess the microphone might be a direct copy of the AMI schematic.

Try it at 5.05VDC and if it goes noisy increase the heater voltage

I have built a few microphone with that biasing and have found that the lowest heater voltage I could run a tube was 5.4VDC other times I needed to increase the voltage to about 5.7VDC. This does not mean you can't run some tubes at 5.05VDC heater. I have not used an EF800 maybe it can be run at this voltage.

I then changed the resistor bias divider by increasing the "top" resistor value and/or decrease the 29 ohm for the desired bias voltage.

The lowest voltage you can under heat a tube seems to be related to the metal alloy of the cathode along with the oxide coating and the manufacturing process. It seems to be a try it and find out what happens.

Your tubes are most likely OK I would install them in a circuit and run them with the heater at 6.3VDC for a few hours and then try them again.
 
Thanks.

Yes, my mic is a Stam SA-47. So it is the same as the Stam in that review.

My heater WAS 4.7. The EF800 that was in it started crackling after about 4 months. Then I started trying to figure out what was up, and I have gotten confused. Some think I need to run it at 6.1. Some think 6.3. Some think I probably need an EF860 in it. And Josh Stam, the designer, thinks I should run it at the factory spec of 5.05. If I bring it up, I wouldn’t know what resistor to use.

I brought it up to 6.1 AFTER putting in a new tube. It sounded fine. Maybe a little more harmonics. Then I spoke with Stam and he said that’s too high because of the pin 2 setup, and it should be 5.05. At 5.05 the mic is quiet and seems to sound great.

You are saying that if I run my crackly tube at 6.3 for a few hours, it will restore it?
 
Thanks.

Yes, my mic is a Stam SA-47. So it is the same as the Stam in that review.

My heater WAS 4.7. The EF800 that was in it started crackling after about 4 months. Then I started trying to figure out what was up, and I have gotten confused. Some think I need to run it at 6.1. Some think 6.3. Some think I probably need an EF860 in it. And Josh Stam, the designer, thinks I should run it at the factory spec of 5.05. If I bring it up, I wouldn’t know what resistor to use.

I brought it up to 6.1 AFTER putting in a new tube. It sounded fine. Maybe a little more harmonics. Then I spoke with Stam and he said that’s too high because of the pin 2 setup, and it should be 5.05. At 5.05 the mic is quiet and seems to sound great.

You are saying that if I run my crackly tube at 6.3 for a few hours, it will restore it?

Data of 1.1 V, if known, applied to the cathode bias for VF14. However, each tube behaves differently. Moreover, VF14 operated at 55V/50 mA, but was strongly underheated to around 34V/40 mA (in U47). Each EF tube behaves differently as well. You can't rely on a schematic with 1.1 V. What voltage did you measure in the Stam clone (cathode bias)?

The cathode bias sets the operating point, which is also related to the anode voltage. Schematics are valid for VF14; for individual tubes or nuvistors (in U47 13CW4 or 8393), the operating point, anode voltage settings, and other components necessarily change.

Everything also changes during so-called underheating of the tube. Especially modern EF-type tubes do not tolerate underheating very well. VF14 and generally V-type tubes (at 55–60V) used to have a double, helical filament, operated differently, had a different cathode coating, etc., etc.


P.S.
I'll add another experience with a nuvistor, I believe it was 8393, the successor to 13CW4 (13,5V / 60 mA), which the late Mr. Neumann recommended as a replacement for the VF14 tube when it became scarce because Telefunken stopped producing it. Normally, the U47 is powered at 105V / 40mA; for the nuvistor version, Neumann released a document, increasing the voltage to 110V / 50 mA. If I had the nuvistor underheated, it produced strange sounds; it heats up again and works, but these new tubes, including the glass EF ones, are not prepared for excessive underheating. They can behave strangely, produce peculiar sounds, and can be invasively destroyed by improper handling (coating on the internal parts).

First, I recommend measuring the heater voltage of the tube, then try another tube that I know is correct.
 
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