Official G7 microphone help thread

[ln76d]

In case like this:
- cheapest donor body with decent headbasket and proper diameter
- two flat bars (there's many options to mount PCB)
- order new longer pipe (steel is pretty good - much better than most alu in chinese mikes)
- professionl powder painting of pipe

= Good looking microphone

 

[knight84]

Thanks for the reply and advice. Another option, how about Alctron HST-11a ? 46mm- 240 mm. Is that enough for G7?
Possibly If that doesnt fit, not much option left for donor body.

 

[hitchhiker]

That should work, the G7 pcb  measures 120 mm by 42 mm. Keep in mind you need room inside for the wires that go to the mic's cable connector and some clearance for screw heads connecting the basket to the inside rails etc.

 

[museic]

I'm finally getting back to this project and looking for a little help. I built 2 g7's mics about 2 years ago. Had an issue with the power supply that I never solved.

My transformers are putting out 12.8v and 140vac before being connected to the psu board. So I believe they are hooked up correctly. As soon as I connect the 2 9v psu points to the 12.8v my 140v drops to 28v.

Is it possible the 2 transformers aren't large enough?  I'm using amveco 62013-p2s02 and 62021-p2s02. 62013 is 2x15 and 107ma. 62021 is 2x9 and 278ma.

I'm not measuring any voltage at the pinout to the mic. The first transformer is also getting quite hoøt.


Any advise on what to look for?

 

[museic]

All is now good! I had the rectifiers rotated wrong. Make sure to pay attention to the plus orientation when soldering 

Now I have some ground hum to deal with. I have a feeling I didn't ground the mic body yet.

 

[museic]

After grounding the mic body and the tube I have two perfectly clear and amazing sounding g7's! Thanks to gyraf for a really amazing project. And to all the other posts for inspiration on this build.

I am getting a hum from one of the power supplies, but I should get that sorted out.

 

[Alexandru marian]

My first G7 is working, and passing nice sound from the first try   Amazed by how much "gain" it has.  Thank you Jakob!
Beezneez K7, Sowter 9995 (1:5), Amplimo 1n1729 power trafo (125V/6.3V))    To get a  reasonable capsule voltage for first tests I changed the dropping resistor from 10k to 37k which gives 132V under load.  Anode voltage after the 100k is 68-70V.  In cardiod I don't have any problem, only in 8 if I speak right into it the back capsule will intermittently cut off so I will have to lower and refine the voltages in the dividers. 

Having two problems:

1. hum.  I think I made the grounding well. It is built p2p, all components use the 0V ground which goes to pin1.  Heater pair is twisted from valve socket to xlr 6/7, where I connected together: pin 1, pin 7, mic body/chassis, one transformer input and transformer casing. 

https://soundcloud.com/user757801519/hum

2.  heater voltage is kinda low.  6.1 mic disconnected, 5.6 loaded.  I get noticeably lower DC than theoretical AC x1.4  after the diodes, even if the trafo is rated 1.5 A      Is it possible to "slightly" damage the diodes when soldering?

Thanks for any suggestions!

 

[gyraf]

Hum. Every part of housing + screen must be grounded. Try wrapping it all in alufoil and see if hum goes away. If so, work on grounding/shielding - if not, work on hum in PSU voltage(s)

heater at 5.6V loaded is probably fine (does it work?) - but make sure that it's regulated, so you don't get hum in that way

Jakob E.

 

[Alexandru marian]

Thanks Jakob!

Alu wrap doesn't change it.  It is not strong enough to ruin a recording but it is well hearable too (I have a soundcloud link above) 
If I unsettle casing or grille connection, the hum explodes. 

A few mentions: 

-the PSU is not in a metal box yet, that part is late in mail. So everything (trafo, parts, connectors) spread open on a table, but the mic is well away.
-HV caps are 220uF, same as I have in my G9.  I tried bumping the second cap to 500uf with no effect. Didn't try to add to the first one.
-I do not have Earth connection in my house, if that matters (other than protection)   

An interesting thing is that I have the same exact hum, although at a lower intensity, coming from my G9 when no mic is attached.  As soon as I plug mics in it (regular SDCs) the noise vanishes and it goes dead silent. If I plug the new G7 I don't get the same effect, G9 hum continues. 

----------

heater:  yes it seems to work, at least it has about 2-3db more gain than a DPA 4011 or Oktava 012... Sounds good too, even if a bit tinny at times, I suspect it is because the grille is not yet finished (just one rather open mesh layer) and the tube is maybe not rigid enough (1mm alu)

The PSU is just like the schematic with the addition of a on-on switch (changing from 1.8 to 1.2k) for PF86 operation.  That one gets a truer voltage.  I'm puzzled by the low output after the bridge, the trafo is good quality. A bigger one same series works perfectly in my G9.    I think I will put a 1.9k instead of 1.8, that should fix it. 

 

[shot]

I assembled my first G7 yesterday and powered it up. No luck with first powering. Still having some problems.

My PSU measures 187vdc without mic connected. I'm using 9vac 7VA and 15vac 3VA transformers. Seems okay.

But when I connect my mic (without capsule fitted) I get only 38vdc both on HT and Pattern rail!
Can that be from inadequate mains transformers?

I went to check if my wiring is wrong but it seems correct.
No solder bridges, as far as I inspected. But I'll check again, I feel I need to check (I had to travel today but I'll check that first thing when I get back home in few days)
All the components seem ok. Electrolytic cap is orientated in right direction (checked against schematic).
I've used beyerdinamic 1:7 transformer and I checked if the pinout is correctly soldered - it is.

Any other ideas where to look for an error?

 

[Alexandru marian]

Shot, check above posts, someone had a similar problem (very low DC) and turned out the diodes were poorly orientated. 


Jakob:  some good news.  I put in a PF86 (for this one the PSU gives a full 4.5V) and the hum is reduced.  Not 100%, I can still clearly hear it on high gain but for real word recording it is unobtrusive.  I'm not 100% but I think the mic is even hotter (louder) than before, if I record stereo with a DPA 4011 I need to set two gain steps lower on the G9.  The low preamp gain needed helps reduce the hum.  Speaking of DPA 4011, I think the G7 holds its own against very well. The G7 has a rather similar mids/high quality, as in being precise and not too bright,  but the bass is meatier, without losing focus either. 

Update:  hum comparison at full preamp  gain between PF86 and EF86.  The difference is bigger than I initially thought!  PF is for the first 3 secs. 

https://soundcloud.com/user757801519/humm

I also tried another EF which makes even more noise.  This one I got for free from the seller of the other, he said "current is too low to ask for money" but insisted the other measured extremely well on his test machine.  Oh well. 

 

[shot]

Shot, check above posts, someone had a similar problem (very low DC) and turned out the diodes were poorly orientated. 

I don't think it's problem with bridge rectifier. But I'll check that also just to be sure.
My PSU when unloaded is performing fine. Transformers don't heat up.
It's just when I connect the mic the dc voltage drops more than 150vdc
Also, I get correct heater voltage on the tube - that one doesn't drop!



Luka

 

[Matt Nolan]

Maybe you have a short or a mistake in the mic?

Try the PSU with a dummy load instead and see how much the B+ drops.

 

[ln76d]

I had also unidentified problem in my S216 project mike (i need to correct schematic with new values).
Microphone worked really fine but i had strange too high current on plate, it cused higher drop.
When Murdock build that circuit and sent me meaurements i started digging inside mike, improved some connections and the current consumption now is proper. But still don't know what was the cuse.

 

[shot]

Maybe you have a short or a mistake in the mic?

Try the PSU with a dummy load instead and see how much the B+ drops.

That's what I was thinking also. I need to inspect every trace for possible short or break...

What would be the dummy load in this case? Would 5W 150r resistor be enough? That's what I have at hand now.

 

[Matt Nolan]

That's what I was thinking also. I need to inspect every trace for possible short or break...

What would be the dummy load in this case? Would 5W 150r resistor be enough? That's what I have at hand now.

No. You want something that will draw approximately the same current as the mic would. Someone on here will know. It's going to be very small numbers of mA, so something like 60k or more, 1W. The point is for it to be the same as the mic so that you can isolate whether your issue is in your PSU or in the mic (or cable).

 

[Alexandru marian]

Regarding my 6.3V trafo, I just learned each diode eats half V, so this explains why I was reading so low after the bridge.  I think the regulator doesn't have enough delta between Vin and requested Vout to work well.  So I need a different regulator, new trafo (excluded) or limit myself to PF86. Or schottky diodes maybe?

 

[gyraf]

Go with PF86.

In my experience, these work significantly better (on average) than the EF86. Maybe different cathode material because of the series-heating-mode?

And they're much cheaper because the hifi-people can't use them as plug-in-replacements.

Jakob E.

 

[Alexandru marian]

I've seen Telefunken EF806S going from 80 to 150e and been tempted - really silly. 

What is your experience, soundwise, with the plate voltage?  With the resistors I have I went from 120 to 175V  (always keeping the capsule in the 130-120 / 65-60V range)  but for the moment I am not sure which sounds better.  I see the U67 has 210V,  75 at the tube. 

 

[Murdock]

Hey alexandru, which resistors exactly did you change to get these voltages?