First Project problem with G7

I seem to be having problems getting my G7 to work. Changes I’ve made have been a 240v to 12v secondary transformer in PSU as a 220 to 15v doesn’t seem to be available in Australia. The 10K resistor between the two 470u/200v electrolytic has been replaced with a 30K to give;
Unloaded
Pins  1-2  182.9v
Pins  6-7    6.1v
Pins  1-5    91.9, 0.0,182.9v
With mic attached;
Pins  1-2  125.0v
Pins  6-7  5.9v
Pins  1-5  63.0,0.0, 126.6v
Some other voltages are;
Between 470k resisters    66.0v
Pin1-capsule, Front 0v, Center 14.6v, Back 0.6v.
The valve is warm but not glowing.
Big thanks to Jakob and all else involved. I’ve already started another two projects but have to finish this first.
Any help would be appreciated.
http://members.iinet.net.au/~tinasime/Grille.jpg
http://members.iinet.net.au/~tinasime/IMG_3180.jpg
http://members.iinet.net.au/~tinasime/valve%20trans.jpg
http://members.iinet.net.au/~tinasime/set.jpg
http://members.iinet.net.au/~tinasime/psu2b.jpg
http://members.iinet.net.au/~tinasime/mic-stuffed.jpg
http://members.iinet.net.au/~tinasime/capsule%20Z%20area.jpg

sethyrish said:

Pin1-capsule, Front 0v, Center 14.6v, Back 0.6v.

Click to expand...

That seems to be a problem, center of the capsule should be at about 80V. If I were you I'd check all conections once again and perhaps redo those solder joints on the right side of mic-stuffed.jpg picture.

5.9V for heaters is less than 5% from 6.3V so that should be ok.

Also it would be helpful to provide some voltage readings of the pins of tube if capsule polarisation isn't an issue.

hope you get it working soon, great lookig build btw! did you make a capsule mount?

Thanks michal_k,
I'll re-do those solder joints and check all connections.
Yes I made the capsule mount. I drilled an old knife board with a 34mm forstner bit. Then shaped it with a Dremmel. I think Gus mentioned it once. It worked a treat!

hellooo...

I finaly finished my G7, but it 's not working, and I think I have the same problem.....
Only I heard in my speakers was a loud hummmm.

I check all the connections, and read the META, but unfortunately the reference voltages on the Omnipressor's website are not available... so, I test mine are there's my results:

Obs.: I aquire this results with the mic plugged in, so... load.

ALL DC Volts!!!!

at the PSU: Heater F+ to F- -> 6V
                 Pin 2 -> 110V
                 Pin 5 (pattern): Cardioid -> 52,8V
                                       Figure 8 -> 110V
                                       omni     -> 0V

at the MIC: Heater F+ to F- -> 6V
                Cathode (K) -> 0,77V ------> Is it too bad?????
                Anode (A)   -> 60,6V
                Pin 2 -> 110V
                Pin 5 (pattern): Cardioid -> 52,8V
                                      Figure 8 -> 110V
                                      omni     -> 0V

So far, the cable is good, as you can see above, an the voltages are what they should be... more or less  :-\

at the CAPSULE:   
                                      Cardioid  |  Figure 8  | Omni
                          Front         0V      |      0V     |    0V            ------> here is the problem? should be around 3,27V at any pattern? isn't it?
                          Back         1,64V  |     3,27V  |    0V
                          Center     0,054V  |     0,06V  |   0,06V

I'm using Peluso's CEK-12 and connected the capsule this way: choose on of the casule's fronts (red wire) and connected to the "Front" pad... connected the 2 blue center wires to the "center" pad position and the remaining red wire to the "back" pad on the mic's PCB.
The Tube is glowing and getting warm, not too much, but it glows a little.
45V across each 470k resistor, and the Lundahl LL1538 are connected nicely with this resistance with all the mic turned off:

1 + 2 -> 25,4Ohms
3 + 4 -> 25,4Ohms
5 + 6 -> 803Ohms

Pin2 and 3 are truly conncted, so I have 0Ohm between Lundahl's pin 1 and 4.

Any Idea? I checked out for any solder bridges or cut traces, I made the audio test with the mic body fitted, to avoid any humms... pin 1 from 7 pin's XLR are connected to IEC Starground at the PSU, and the pin 1 in the mic are connected to the mic Body...
Maybe I'll try to swap the 10k resistor at the PSU Board to elevate the amount of voltage to the circuit, hey Sethrish, 30K is ok?

I'll try to put some photos too...

Thanks a lot for the patience! I will be very greatful if someone could give me some working voltages for this project!

Cheers,

Eddie  ;D
                         

Wow... 80V at the front side of the Capsule... mine is 0V in any position... damn  :-\
Hey Seth... same problem ahn??? your mike looks gourgeous!!! nice work on that!

Eddie  :-[

You can't trust measurements made at the capsule because of the very high resistances involved. If polarisation voltages are correct at xlr5, it is right.

It dosen't make sense to measure psu voltages without mic connected.

Aim for 110-160V HT vith mic connected, and all is well

Remember to have full shielding around the mic electronics connected to ground, otherwize you'll only get hum.

all figures seems right to me.

Jakob E

Thanks for the replies . I found a broken solder connection on a capacitor. It seems to be working now. Voltages on Pin 5 of XLR are
63.7,
0,
127.4
with mic connected so I guess all is fine. I just have to sort out some hum now. The grille is stainless steel instead of brass. Not magnetic enough to hold a hard drive magnet. Is this ok? I’ve searched the Meta’s and will add some grounding links in the mic as suggested. Thank once again for for a great project.

simon

Hum in G7 is nearly always from missing or too little shielding - or because the housing/mesh is not properly grounded.

I see no problem in using stainless mesh, other than the obvious that you need to attach a ground wire to it without soldering.

Jakob E.

Well... I tried and tried and tried... maybe my mic's body and grille are not that good... i can hear my voice below the hummm when I speak to it... so, I think my mic is not propertly shielded around the capsule, I used brass for the body and Steel for the outside grille, and Stainless steel for the inner one... tried to attach the grille with the 7 pin XLR's pin 1 with a wire... the hummm in continuous... is there someone who can tell me where can I buy some brass grille? or someone who can sell me a Grille... here in Portugal is awful... we have only wine and corks here... damn I'm in a wrong country to dig into DIYs... hehehehe

Cheers,

Eddie  ;D

I found this info. Quote from: "gyraf"
"Yes, the heater 0V, the chassis ground, the HT 0V, and the audio ground has to be connected. Preferably in the microphone". I found the audio ground connection in particular reduced the hum in my mic.

"Yes, the heater 0V, the chassis ground, the HT 0V, and the audio ground has to be connected. Preferably in the microphone". I found the audio ground connection in particular reduced the hum in my mic.

Click to expand...

What's you're saying is that we must ot connect pin 1 and 7 on the mic PCB?

Cheers!

Eddie  ???

Yes and pin 4 audio- to pin 1 as well. This had a greater effect at for me. Getting rid of hum altogether. Mind you I'm a complete newbie myself. It could still be your grille. Search for G7 Hum for some other pointers.

Hello,

Connecting the pin 4 ("Cold" of the XLR) to the pin 1 (0V7GND) you are unbalancing your circuit, did you notice that?
Well, I'll try to connect the Pin 4 (HT -) to the Pin 1 as you said and see what happens!

Thankssss

Eddie 

No. It seems quite to me. Maybe I'll disconnect and try pin 4 to Psu chassis/earth or pin 1 to PSU chassis/earth. Would there be any problems with this? Thanks for pointing that out. 

Hi!!! mine is finally working and dead quiet!!!
I just connected the pin 1 from the psu to the starground (where I've connected my IEC ground too), connected in the mic PCB, the XLR's pin 1 to the grille and the XLR's pin 1 with pin 7 (HT -)!

Actually if you see in detail, there's a unfinished track near the end of the Mic PCB... that actually should connect pin 1 and pin 7 on the mic PCB! I left the pin 4 untouched because is the cold of the balancing XLR, and I'm still thinking if you connect the pin 1 (GND) to the pin 4 (XLR -) what you have is: "Hot (XLR +)" and "Cold (XLR -) + Ground", that unbalances your signal in a "hard" way.
Somebody please correct me if I'm wrong with that...
If you still have some noise with your mic, try to disconnect your pin 4 from the pin 1, and "wire" the grille, hehehe... in other way, connect the grille to the xlr's pin 1 with a wire

Cheers

Eddie  ;D

Thanks for that information Eddie. I'll do the same. Probably the reason mine is quite is the mic cable is only 1m long.
Simon

Just a note - when describing G7 circuit, be very clear about what XLR you refer to by pin numbers (XLR3, XLR7). Had some misunderstandings that way.. :-\

If you have hum in the G7, check:

- that your mic housing and grille fully encloses the electronics (it is extremely sensitive to external electrostatic noise)
- that heater ground and HT ground are connected - either in the mic or in the supply box.
- that this ground is also well connected to the mic housing and grille inside the mic
- that your powersupply voltages are healthy and approximately right - Heater 6.2V and HT 110-160V DC
- that ground is connected to chassis in your supply box
- that your output XLR3 is wired correctly (1-gnd 2-out+ 3-out-)

These are common errors in approximate order of probability

Jakob E.

Thanks Gyraf!
Mine is working very well right know.It has a great sound indeed! just a little to much of the highs with the Peluso's CEK-12, but very warm sound an "in your face".
Just a quick question: Is the PSU/Mic protected against any phantom power that can be send it to it by mistake? Must verify on the schematic... if not what do I need to do? some diodes on the XLR + and XLR -? hummm...
Cheers,

Eddie  ;D

My mic‘s working great now too. I’m now waiting for custom toroid for a G9 to go with it! :

Is the PSU/Mic protected against any phantom power that can be send it to it by mistake?

Click to expand...

Yes, completely protected. This is the beauty of transformer-balancing

Jakob E.