VF14 question

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Mworthin

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I am not a tube guy, and I didn’t want to clutter up the Russian tube thread with these comments, but a few things are striking to me about the U47 mic with the VF14 that could be tested, to understand the magic of this combination. I am not a tube guy, so forgive me if these are frankly stupid.

Looking at the Telefunken U47M (mic) and U47N (power supply) schematic at Dan Alexander dated 9-1-55 (https://danalexanderaudio.com/wp-content/uploads/2018/01/Schematicscans002-1080x716-1080x675.jpg):

(I am sure this is in our archives, but this is the one I saw)

1. We all know the heater and plate come off of the same 105 V supply, which is dropped by resistors to 34 V at the plate and 36 V at the heater. There are no electrolytics in the mic body, only in the power supply, connected by a cable . It seems to me that the performance of the mic would be prone to B+ fluctuations, which would affect both heater and plate simultaneously.

Would strong signals be prone to an interaction between heater and plate, not unlike sag in a guitar amp? If so, could it act as a form of mild pleasant compression on strong signals, accounting for some of the “magic?”.

2. The voltage at the plate seems low to me compared to other tubes. What sonic signature does a “starved plate” impart on a microphone?

I was just thinking if it wasn’t the microphonics of the VF14 tube, could it be something else. The fact that there is only one capacitor (the output cap) in mic body schematic jumped out at me.

I will never own a vintage U47 with a VF14, so I can’t test these ideas, which may be just wrong. I do know if you overengineer a 1950s Fender guitar amp like a 5F1 with more modern transformers or more robust power supply filtering, you lose some of the charm.

Forgive me if these ideas are frankly stupid.
 
The U47 supply is a series supply acting like a shunt power supply.
This also a reason you always want it to have a load on it to keep the B+ voltage under control (not too high a voltage)
.55mA plate current. How is the tube starved?
The U47 is more fixed bias about 39.5mA that goes around the tube and sums at the "top" of the 29 ohm.
The heater, resistor, plate current and the 29 ohm loads the power supply.
.55mA is used by the tube so you have about .55mA vs a more constant around 39.5mA
SO
the voltage across the 29ohm is mostly a constant voltage because the current is mostly constant.
 
1. We all know the heater and plate come off of the same 105 V supply, which is dropped by resistors to 34 V at the plate and 36 V at the heater. There are no electrolytics in the mic body, only in the power supply, connected by a cable . It seems to me that the performance of the mic would be prone to B+ fluctuations, which would affect both heater and plate simultaneously.
That's the case with most tube equipment using non-regulated power. However, the variation is very small, and has been considered not important when the U47 was designed. Later the PSU were regulated, though, mainly because, within definite noise specs, it dispensed with expensive inductors.
2. The voltage at the plate seems low to me compared to other tubes. What sonic signature does a “starved plate” impart on a microphone?
I can't dissert about the sonic signature, but a low plate voltage is beneficial to noise performance.
 
I was just thinking if it wasn’t the microphonics of the VF14 tube, could it be something else. The fact that there is only one capacitor (the output cap) in mic body schematic jumped out at me.
There are three capacitors in the U47 body.

VF14 are microphonic but I wonder if it plays a major part. A lot of factors influence the sound of U47, some more than others. Today the most important is the condition, first and foremost of the capsule and tube. Many U47 are in bad shape and can't be used as a reference. Most important for the sound is the capsule (M7 in my book), the resonant body, the transformer and the tube. Carbon film resistors, MP capacitors, passive PSU and I guess microphonics contribute as well but are less important.

Personally I wouldn't mind having a VF14 without microphonics in my U47. I recently serviced a U67 that sounded quite bad and strange. My acoustic guitar sounded like a twelve string out of tune. Turned out the tube was very microphonic. I changed the tube and the microphone instantly sounded great.

I do know if you overengineer a 1950s Fender guitar amp like a 5F1 with more modern transformers or more robust power supply filtering, you lose some of the charm.
Yes, but a tube microphone is not a power amp so building a more robust PSU won't make as much difference. If you change the electronics in the microphone to modern parts and especially if you install a new K47 capsule the microphone will loose a bit of its classic sound.
 
Not really starved plate, but because of this u47 can't really take high spl without distorting.
This and perhaps the resonant body (and microphonics?) are the reasons that U47 is not a very good all round microphone. It sounds best on soft voices and various instruments. Then you get the large sound (almost like there's something going on an octave down) with very natural prescence, that feels like a finished mix. Some sources especially screaming tenors can make the microphone sound sibilant, thin and not very pleasant.
 
I have access to measuring seven U47 with VF14, two of them VF14M. I would like to get a better picture of the amount of microphonics and how much it varies. Which is the most reproductive method for measuring it? I guess tapping with a pencil won't do.
 
I think i remember reading about factory testing microphonic characteristics of some russian tubes by releasing a certain weight from a certain height and simply recording the sound. So maybe something like that, rubber weight hitting the Neumanns at certain velocity, recording and listening. This kind of test i'd do with capsule engaged and listening for differences in decay.
 
What about positioning a speaker so that it’s addressing the microphone under test, and playing back the same loud signal at the same level after calibrating mic distance carefully?
Correct. Since it is a comparative test, it just requires a repeatable stimulus and set-up. Swept sinewaves are a good choice although not strictly necessary. Any recorded signal that has significant content in the audible range is adequate. Pink noise is another possibility. However swept sines may offer more discrimination in pinpointing resonances.
Of course the capsule should be disconnected and replaced with a similar fixed non-microphonic capacitor. Alternatively, on an externally-biased capsule, the bias voltage could be disabled.
 
Alternatively, on an externally-biased capsule, the bias voltage could be disabled.
Even with no polarization there is still some very faint but noticeable signal picked by the capsule, so i'd avoid this.

Also there should be a test performed with capsule engaged, like the one i suggested with a weight hitting the body/grille, which will showcase the simple microphonic properties of the whole assembly. Some mics suspended on a thin thread ring like a church bell. You need capsule engaged for this.
 
Even with no polarization there is still some very faint but noticeable signal picked by the capsule, so i'd avoid this.
Correct. Grid current puts some bias on teh capsule.
Also there should be a test performed with capsule engaged, like the one i suggested with a weight hitting the body/grille, which will showcase the simple microphonic properties of the whole assembly. Some mics suspended on a thin thread ring like a church bell. You need capsule engaged for this.
Also correct. My suggestion was to test the tube microphonics (and probably some capacitors).
 
I'm finished with the testing. I made a new thread

Microphonics in VF14 and U47

Also there should be a test performed with capsule engaged, like the one i suggested with a weight hitting the body/grille, which will showcase the simple microphonic properties of the whole assembly. Some mics suspended on a thin thread ring like a church bell. You need capsule engaged for this.
I made an attempt to do this test but I couldn't reproduce it. The amplitude and frequency differed to much between separate hits even though I had a rubber ball on a string and released it from the same height.
 

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