modify g7 mic for 60v polarisation voltage?

Jonkan · Dec 14, 2007

I am wondering how i could modify my g7 mic psu so that i could get around 60v polarisation voltage instead of the usual 80 volts?

I read from Dale that makes the m7 clones that m7 capsules are usually polarized around 60v and i have one of his m7 clones that i want to put in my g7. Ive had some problems with the membrane getting sucked into the backplate in another g7 mic with the same type of capsule, so i was hoping that lowering the polarisation voltage some would remedy this.

Am i right thinking this? And how could i accomplish this change?
/J

ioaudio · Dec 14, 2007

the two 470k resistors form a voltage divider.

-max

Gus · Dec 14, 2007

Look around the meta you should find things to give you an answer

Salossi · Dec 15, 2007

Build it without any modifications, and you will get around 60 volts polarisation... :wink:

In other words: in my circuit, plate- and capsule-voltages are very low (perhaps depending on the kind of trannnys used). They ARE about 2x 60 V.
I have to change at least one of the transformers to get the voltages higher.

So my advise is: first build it the usual way, then measure the psu-voltages (with the mic connected, but WITHOUT capsule mounted, if it can only handle about 60 volts; use two caps instead as a substitute for the capsule; around 68p will be fine).

Then you can see, if the voltages are dangerously high.
But anyway: Changing the voltage divider will not work (then you´ll have the center electrode at 60V, the front at 0V but the back electrode still changing from 0 - 160 volts. This means 100V over the rear system in fig 8!! And changing the psu voltage will of course affect the plate voltage too. But my g7 runs accepteable with low plate voltage...

What kind of capsule do you like to use??

Sascha

Jonkan · Dec 15, 2007

thanks. Ive read basically everything in the meta, but i wasnt 100% sure anyway.

I have a dale m7 clone capsule. A friend had problems with the same type of capsule getting sucked into the backplate in his g7 mic.

Salossi, do you mean that i should connect two 68pf caps from the center connection to the front and back connections respectively to be able to accurately measure voltage in the mic?

Will the tube be ok with 120v plate voltage?
/J

Salossi · Dec 15, 2007

Hi Jonkan,

Yes, two 68pf´s, one from the backplate to the front, the other from b.plate to the rear.
If you just want to measure the voltages, I think you don´t need this capsule substitution. It makes sense, if you´d like to run the circuit without using a capsule, perhaps for also checking the output etc.

But I would not recommend to use a capsule with 80 volts, if the diaphraghm is getting sucked to the backplate. There might be danger of destroying the capsule if you´re getting a dielectric breakdown...

Perhaps you can change the 2x 470k divider AND change the divider in the psu (3 instead of 2 resistors), so that you´ll get 0v - 60v - 120v - 160v instead 0v - 80v - 160v. But then you´ll need a 8-pin connection between mic and psu for separate connections of the 160 V (plate) and 120V (capsule) voltages. If you´ve got such connectors and a cable with enough leads you can do so.
Greets, Sascha

Jonkan · Dec 15, 2007

ok, thanks. Thats what i figured too.

I have the standard 7 pin xlr and 7pin cable+shield, so that could be hard.

Unless it is possible to connect the heater ground and the other grounds in the psu, and just use pin1 in the cable as a carrier for all grounds up to the microphone (that would free up pin 7)

Would that create problems? Im not sure i understand why you keep the grounds separate anyway (especially since you can connect heater ground to the other ground at either the mic or psu end), but there must be a good reason for it im sure.

But if the tube would be happy with around 120v or so plate voltage, then i could just build the psu to stock values and hope that my capsule will be happy with that.
/J

Gus · Dec 16, 2007

Use ohms law

You have two voltage dividers, one in the microphone the two 470Ks and one in the power supply the two 100k pattern control

Here is one way
built stock
Measure the B+ under load microphone plugged into the power supply
Note If your B+ is 120VDC you don't have to change the circuit

http://www.gyraf.dk/gy_pd/g7/gic_s.gif

Change the top voltage divider resistor value to give 60VDC to the backplate inside the microphone. The two 470K that set the backplate voltage part of the circuit

You know you want 60VDC so start with 60VDC divided by 470K this gives the series current. B+ - 60VDC give the "top" resistor voltage drop and you know the current so calculate the "top" resistor value

Next add a resistor between the B+ in the power supply and the top 100K of the pattern control switch so you have 120VDC at the top of the 100K move the top connection of the pattern switch to the node of the top 100K and the new resistor. You are looking for a series current of 60VDC / 100K =.6ma. You have 200K already so the total value will be power supply VDC / .6ma. So if you have a 160VDC power supply under load, 160VDC/.6ma=267K total, add a 68K

now go back and check the voltages adjust the values, because if your power supply is weak the B+ will move up a little because of less loading via the voltage dividers

that said I use another way

Jonkan · Dec 16, 2007

Thanks gus, that seems fairly straightforward!

I will try the psu stock and see what kind of values i get. im still waiting for the 1g resistors, so i can check the voltages with the mic plugged in. Next week i hope they will arrive. Everything else is ready though, so my fingers are itching to finish this project (or atleast do some more experimenting)

If i understood you right: If I get voltages around 120v when loaded i can just leave everything as is in the psu and be happy with that?

/J

Jonkan · Dec 25, 2007

Ok, i got the psu up and running after some strange problems.

I get about 172v unloaded for Ht voltage.

Loaded i get 156, and exactly 6.3v heater (should i lower this a bit, or should it be exactly 6.3?).

Is that about right? It seems that many people get alot less voltage after loading the psu with the (capsule less) microphone.

Im also worried that this polarisation voltage 156/2=78V might be a bit too much for my capsule...Can i hurt my capsule if i connect it and see if it works at this voltage or not?

/J

Jonkan · Dec 25, 2007

Ok, calculator in hand. Trying to remember what they told me in school so many years ago...lol

I have about 152Vdc (fluctuating a bit) when loaded so thats my +B voltage. I had 156 earlier, but maybe because everyone in my building is using their tvs, ovens, etc the voltage could be down a bit?? Will this have much effect on my mic?

Is this calculation correct?

Voltage divider in microphone:

60v/470000 ohm= 0.0001276595744680851 ampere

152Vdc -60vdc= 92Vdc

0.0001276595744680851 a x 92Vdc= 720666.6666666667 ohm.

So i need to change the top voltage divider resistor to about 721k ohm.

Psu divider:

60Vdc / 100000 ohm= 0.0006 a
152Vdc/ 0.0006 a= 253333.33333333334 ohm = 253.33k ohm total

253.33k-200k= 53.3k

So that means i need to put a 53.3k resistor on top of the 2x100k voltage divider, right?

Sorry if this is extremely basic, but i havent really used ohms law much since i learnt it 15 years ago or so...

I understand almost everything, but this:
Why is it you calculate the current in the voltage dividers over just one of the resistors, and not both of them? Theres probably an easy answer, but i d really appreciate if someone could explain it to me. God i feel stupid sometimes.

/J

/J

Whizard · Dec 25, 2007

Have a look at your thread in the Swedish forum... :roll:

plumsolly · Mar 10, 2009

I know this is an old thread, but how did this end up working out, Jonkan?
Thanks, Ben

Jonkan · Mar 10, 2009

It worked out just fine. I redid the voltage dividers according to the calculations i did (after much help getting them correct), and now the voltages match better from capsule side to side, and i dont get the sucking of membrane to backplate that i got when using stock values.

Im sure the max voltage varies from capsule to capsule though, as there seems to some people who have had no problems with higher polarisation voltages on the dale m7 capsule.

/J

mamiti · Oct 25, 2010

Really old topic but I happened to think about the same thing.

What about if I'd just make the 10k resistor at the power supply bigger to lower the maximum 160V? 33k will get the voltage down to 135V but I don't know whether there is a reason to modify dividers rather than this one resistor?

Then, I could make 100k resistor smaller in the mic just before Anode to get that voltage higher. Am I going to get into trouble somehow by doing this?

Cheers,

MT

gyraf · Oct 26, 2010

Modifying the 10K in powersupply will bring down entire mic supply - including that feeding the tube.

You will (probably) want to keep the ca. 160V for the tube, and then take the polarisation voltage for the capsule down a bit by adjusting one of the two 470K dividers inside mic.

Possibly also make a voltage divider that takes the external polarisation voltage down to 0-120V in stead of 0-160V (to ensure even sensitivity front/back in Omni mode).

Jakob E.

marktokach · Dec 12, 2013

Gus said:
Use ohms law

You have two voltage dividers, one in the microphone the two 470Ks and one in the power supply the two 100k pattern control

Here is one way
built stock
Measure the B+ under load microphone plugged into the power supply
Note If your B+ is 120VDC you don't have to change the circuit

http://www.gyraf.dk/gy_pd/g7/gic_s.gif

Change the top voltage divider resistor value to give 60VDC to the backplate inside the microphone. The two 470K that set the backplate voltage part of the circuit

You know you want 60VDC so start with 60VDC divided by 470K this gives the series current. B+ - 60VDC give the "top" resistor voltage drop and you know the current so calculate the "top" resistor value

Next add a resistor between the B+ in the power supply and the top 100K of the pattern control switch so you have 120VDC at the top of the 100K move the top connection of the pattern switch to the node of the top 100K and the new resistor. You are looking for a series current of 60VDC / 100K =.6ma. You have 200K already so the total value will be power supply VDC / .6ma. So if you have a 160VDC power supply under load, 160VDC/.6ma=267K total, add a 68K

now go back and check the voltages adjust the values, because if your power supply is weak the B+ will move up a little because of less loading via the voltage dividers

that said I use another way

I would love to hear about the other way you use if it's ok with you Gus.

marktokach · Dec 12, 2013

What is Gus referring to as the "top" resistor in the mic or power supply? The one farthest away from the start of the circuits? Does changing only these two "top" resistors in the mic and psu complete the process? Or is there more to be done with pattern switch resistor values to get the mic functioning at 60 volts?

I guess this attached file of mod for 60 volt is another way to do it? I see here he has added resitors to get the patterns right.

I have searched, I find this confusing, I' close to getting this thing working! With the mic plugged in with no capsule I'm getting 87.3 volts on the right side of the left 470k resistor in the mike. I need a little more help. Thanks

modify g7 mic for 60v polarisation voltage?

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