Questions about the "big" resistors in a tube mic

Hi there,

I've been trying to understand this for weeks... :roll:

The Gyraf G7 Page

CAPSULE BIAS RESISTOR

1.1) Is the polarization voltage of the capsule determined only by the two 470k ?
1.2) As long as these two resistors have the same value, the polarization voltage will remain 80 volts, right ?
1.3) Why choosing 470k ? Do these resistors act as a filter in combination with other components ?

2) As I understand it, there is no DC current across this resistor, because there is no path to ground after this resistor. If there is no DC current, there is no voltage drop of "the 80 Volts". So the 80 volts are only a potential reference brought at the capsule and the 1GigaOhm has no effect on this voltage. Is that true ?

3) About AC current : as I understand the Jakob's explanation, this resistor "kills" the leaking currents(=discharges) from the capsule ? What would happen if the value of this resistor is decreased ? It would reduce the voltages variations applied to the grid ? And the microphone would have less output ?
I'm asking because for example the U47 has a 100 MOhm.

4) I've read it's not possible to make a voltage measurement with a "traditional" multimeter because of the high impedance of that part of the circuit. Is it still true if the measurement is done "before" the 1G ?


"The pattern" RESISTORS : 33M and 10k

1) What is the role of the 10k ? Does it make an low pass filter with the 100nF ? Is there AC current in this part of the circuit ?
2) What's the role of the 33MOhm ?
3) There is no DC voltage drop across this resistor, right ?
4) I can't measure this polarity voltage with a traditional multimeter, right ?

I'm sorry, it's a lot of questions... :wink:

Thank you,

eD)))

EDIT : I've changed the title of this topic as these questions apply to all tube microphones and not only the G7.

I'll offer what I know, but it would be better if others chipped-in as there are people here who's forte is condenser mics.

Capsule Bias & Grid Resistor:

1.1 Correct. The polarisation of the backplate. There are also the two diaphragms to think about.

1.2 Correct. And assuming the HT voltage is still 160V.

1.3 Well, you want two equal resistors to split the voltage, and you don't want to waste current, so a high value resistor is a good choice. The 100n caps filters any residual AC to earth.

2. Correct.

3. I wouldn't use the word "kills" but I see what you mean. We are talking about the 1G grid to ground resistor, right? The current from the capsule develops a voltage across the resistor. I believe using a smaller value will give you less output, but it also affects the frequency response as you have a filter developed by this resistor, the previous cap, and the miller capacitance of the valve.

4. Typical DMMs have an input impedance of 10M or so. You could measure the polarising voltage at the node between the two 470K resistors and ground.

Pattern Resistors:

1. Correct. There is residual noise which is filtered.

2. Well, I don't know enough here to comment on this resistor.

3. Correct

4. Correct

[quote author="rodabod"]I'll offer what I know, but it would be better if others chipped-in as there are people here who's forte is condenser mics.
[/quote]

Thank you so much for your answer ! I've been trying to understand this for weeks ! :?

[quote author="rodabod"]
Capsule Bias & Grid Resistor:

1.3 Well, you want two equal resistors to split the voltage, and you don't want to waste current, so a high value resistor is a good choice. The 100n caps filters any residual AC to earth.
[/quote]

When you say you don't want to waste current, you mean in this part of circuit ? => "keep the current to the plate" ?

In the U47 for example there are two differents resistors one 2M (which drops 42 volts) and one 3M(which drop 63 volts) for getting 63 volts at the backplate.



Another example, for the Apex 460, does it mean that the voltages polarizations will be :

133/2=66.5 Volts if B+=133 Volts ?
154/2=77 Volts if B+=154 Volts ?
182/2=91 Volts if B+=182 Volts ? 91 volts at the capsule ? :shock:

(133, 154, 182 volts are 3 usual B+ stock values for the Apex 1050/460)



[quote author="rodabod"]
3. I wouldn't use the word "kills" but I see what you mean. We are talking about the 1G grid to ground resistor, right? The current from the capsule develops a voltage across the resistor. I believe using a smaller value will give you less output, but it also affects the frequency response as you have a filter developed by this resistor, the previous cap, and the miller capacitance of the valve.
[/quote]

I was talking about the 1G between the junction of the two 470k and the backplate, I think you're talking about the grid resistor. Besides, your answer is interesting...

Thank you,

eD

[quote author="vertiges"]
When you say you don't want to waste current, you mean in this part of circuit ? => "keep the current to the plate" ?[/quote]

It's just a simple voltage divider there to get a good capsule voltage. I think we could replace it with two 1R reistors, but we'd end up driving about 100A through them.

In the U47 for example there are two differents resistors one 2M (which drops 42 volts) and one 3M(which drop 63 volts) for getting 63 volts at the backplate.

Click to expand...

Yep, seems reasonable.

Another example, for the Apex 460, does it mean that the voltages polarizations will be :

133/2=66.5 Volts if B+=133 Volts ?
154/2=77 Volts if B+=154 Volts ?
182/2=91 Volts if B+=182 Volts ? 91 volts at the capsule ? :shock:

(133, 154, 182 volts are 3 usual B+ stock values for the Apex 1050/460)

Click to expand...

Yep, and it's probably too high for some capsules as you are correctly suggesting.

I was talking about the 1G between the junction of the two 470k and the backplate, I think you're talking about the grid resistor. Besides, your answer is interesting...

Click to expand...

Well, the current from the capsule will still drop across that resistor too. In which case the two 1G resistor should look like they are in parallel since the 1n cap looks more like a short to audio frequencies.

I don't want to say too much more because as I said before, condenser mics are not my forte, and others here will be able to explain component choices better (eg. Jakob).

By the way, regarding the voltage dividers used here, Zebra50 pointed out that they are not exact bacuse of the various 10Ks added for filtering (from what I remember) so the balance between capsule sides is a little off. You can find the thread here somewhere.

[quote author="rodabod"]
By the way, regarding the voltage dividers used here, Zebra50 pointed out that they are not exact bacuse of the various 10Ks added for filtering (from what I remember) so the balance between capsule sides is a little off. You can find the thread here somewhere.[/quote]

You're talking about the G7 here ?

Thank you,

eD

[quote author="vertiges"]You're talking about the G7 here ?[/quote]

Yes.

[quote author="rodabod"]
By the way, regarding the voltage dividers used here, Zebra50 pointed out that they are not exact bacuse of the various 10Ks added for filtering (from what I remember) so the balance between capsule sides is a little off. You can find the thread here somewhere.[/quote]

Those 10k wouldn't have any influence on the DC voltage... :roll:
They are just part of filter for the AC, aren't they ?

That's interesting, I'm gonna try to find his post.

Thank you,

eD

[quote author="rodabod"]

Another example, for the Apex 460, does it mean that the voltages polarizations will be :

133/2=66.5 Volts if B+=133 Volts ?
154/2=77 Volts if B+=154 Volts ?
182/2=91 Volts if B+=182 Volts ? 91 volts at the capsule ? :shock:

(133, 154, 182 volts are 3 usual B+ stock values for the Apex 1050/460)

Click to expand...

Yep, and it's probably too high for some capsules as you are correctly suggesting.
[/quote]

That means differents versions (=different B+ values) of the 460 with the same circuit, same compents, same capsules must sound completely different ! The ones with the higher voltage will sound more thick/aggressive in the mids, harsh, bright... right ?

eD

[quote author="vertiges"]
That means differents versions (=different B+ values) of the 460 with the same circuit, same compents, same capsules must sound completely different ! The ones with the higher voltage will sound more thick/aggressive in the mids, harsh, bright... right ?[/quote]

The higher voltage versions will eb more sensitive and will possibly sound different depending on the capsules used. I agree though that in theory there may be less bass on the higher voltage versions since the diaphragms will be tensioned higher as they are pulled towards the backplate. I'm not sure if this effect is negligible or not though; you'd really need to test.

Those 10k wouldn't have any influence on the DC voltage...
They are just part of filter for the AC, aren't they ?

Click to expand...

At least in one case, the 10K adds to the top section of the 470K/470K divider which gives you 480K/470K instead. There is more to it than that though.

[quote author="rodabod"]At least in one case, the 10K adds to the top section of the 470K/470K divider which gives you 480K/470K instead. There is more to it than that though.[/quote]

You're right, you're right, you're right ! The 10k is in series with the 470k !


eD

The role of the bias resistor is still not totally clear for me.

I know that this resistor in series with the grid resistor, the 100nF and the capsule capcitance acts as a low cut filter.

It's the part when this resistor "receives" the discharges of the capsule I don't understand very well. What would be the consequences if its value is reduced for example ? There would be less voltage "applied" to the grid ? And the tube will have less gain ? :roll:

Thank you,

eD

The role of the bias resistor is still not totally clear for me.

Click to expand...

You mean the 1 GOhm resistor, right (didn't read the entire thread)?

The capsule capacity and this resistor form a high-pass filter; as the capsule capacity is some pF only you need a very high value resistor in order to preserve a good low-frequency bandwidth.

And the tube will have less gain?

Click to expand...

The sensitivity of the capsule would decrease. That's how the polar pattern change is done--one side gets higher bias and the other lower.

Samuel

[quote author="Samuel Groner"]You mean the 1 GOhm resistor, right (didn't read the entire thread)?[/quote]

Yes.

[quote author="Samuel Groner"]
The sensitivity of the capsule would decrease. That's how the polar pattern change is done--one side gets higher bias and the other lower.
Samuel[/quote]

I don't understand that... because the capsule bias resistor doesn't influence the polarization voltage. I understand how the polar pattern chang is done though.

You mean the capsule bias resistor has an influence on the sensitivy of the capsule ? Sensitivity is that equal with the capsule output voltage ?

Thank you,

eD

My bad--no no, the value has no influence on the applied bias voltage. Its value is just about the low-frequency cut-off.

Samuel

[quote author="vertiges"][quote author="Samuel Groner"]You mean the 1 GOhm resistor, right (didn't read the entire thread)?[/quote]

Yes. [/quote]

Well, there are two; the capsule backplate polarising resistor, and the grid leak resistor.

Does the capsule not see the parallel combination of these seeing as both have a path to earth?

By the way, do we just model the capsule as a voltage generator with internal capacitance? Is that fair enough?

[quote author="rodabod"]
Does the capsule not see the parallel combination of these seeing as both have a path to earth?
[/quote]

yes, as explained in this thread by PRR.

[quote author="rodabod"]
By the way, do we just model the capsule as a voltage generator with internal capacitance? Is that fair enough?
[/quote]

It's a good question... :thumb:

eD

Do we just model the capsule as a voltage generator with internal capacitance? Is that fair enough?

Click to expand...

It is fair enough, I think.

Samuel

Does the bias capsule resistor influences the "output" of the capsule ?

eD

Err, which resistor are you now talking about..? Still the 1 GOhm?

Samuel

[quote author="Samuel Groner"]Err, which resistor are you now talking about..? Still the 1 GOhm?

Samuel[/quote]

Yes, the 1G just "before" the capsule, not the grid leak resistor one.