Official G7 microphone help thread

[lightningbefore]

I'm having a tough time finding transformers (I'm in the US) can anyone help point me find them?
(110V Model)
1 pcs. Transformer 110:9V, ~5VA
1 pcs. Transformer 220:15V, ~3VA

 

[rock soderstrom]

I'm having a tough time finding transformers (I'm in the US) can anyone help point me find them?
(110V Model)
1 pcs. Transformer 110:9V, ~5VA
1 pcs. Transformer 220:15V, ~3VA

Wouldn't it make more sense to look for a suitable transformer right away, instead of the (2 transformer) back to back solution?

 

[lightningbefore]

I'm not sure - I hope so! But what would a suitable transformer be? I'm just following the components list on Gyraf's site (and I don't really know what I'm doing).

 

[rock soderstrom]

I'm not sure - I hope so! But what would a suitable transformer be? I'm just following the components list on Gyraf's site (and I don't really know what I'm doing).

I don't want to question the proven design of Jacobs G7 but the back to back solution (which definitely works) has some disadvantages and was probably due to the supply situation for such transformers at the time.

A dedicated tube transformer brings some advantages, the schematic says that you need 135VAC and 9VAC at least on the secondary side. The 135V rail must be able to supply one(!) EF86 with one or two mA, the DC heater voltage according to the data sheet for the EF86 must be able to supply 200mA.

You need such a transformer or similar (means with larger values), which you then make suitable for your purposes.



The schematic for the G7 microphone specifies well filtered 160V operating voltage and 80V (in cardoid position) for the polarization voltage and regulated 6.3V DC for the heater. Where they come from doesn't really matter, as long as the values and the quality are right.

Especially for beginners one should always point out that working with mains voltage and high tube voltages is dangerous for the environment (fire hazard) and life (electric shock).

Professional assistance and qualified testing and final acceptance by an experienced, certified technician are mandatory.

Under that circumstances, I also find the idea of converting an existing power supply to your needs not unattractive. This will save you some headaches and probably cannot be topped in terms of price.

Some examples...

https://www.thomann.de/de/the_tbone...AQQViZz6YOkOyGvB3LpSbefpoXuOplN4aAuTJEALw_wcB
https://www.thomann.de/de/the_tbone_psu_sct_2000.htm
Just my 2€ cents, stay safe and have fun. Good luck!

 

[lightningbefore]

Thank you! Similar dummy question, how would I mod the t.bone PSU to fit the G7? Literally just take out the PCB in that and put in my G7 PCB? Or is it simpler or more intricate than that? I'm still very confused about trying to get any kind of transformer for the G7 PSU vs. doing something like you suggest.

 

[rock soderstrom]

Thank you! Similar dummy question, how would I mod the t.bone PSU to fit the G7? Literally just take out the PCB in that and put in my G7 PCB? Or is it simpler or more intricate than that? I'm still very confused about trying to get any kind of transformer for the G7 PSU vs. doing something like you suggest.

I just have power supply from a t.bone SCT700 on my workbench. This should be pretty similar and would be directly usable for the G7 with a small difference. B+ of 150V instead of 160V, would be ok or quickly adapt it by a swap of the Z diodes and one resistor.

Schematic:


If you want to go original G7 you can just swap the PCB as well, if it fit into the PSU.

Ahh, yes the Z diodes are drawn in wrong direction

 

[jonathan9473]

hello guys,so i have built the g7 microphone but it lacks of high frequency, i am not so sure what is happening, maybe because of the tube? but everything was fine, 6.3v heater, 1.3v cathode and 65v on plate.

I have tried to change the tube from wing c to rft ef86, it helps a little bit.

Then I try to put the feedback in the circuit, but oddly, once I put the feedback in the circuit, the high frequency sounds natural. Isn't the feedback feature will lower the high frequency? any possible problem? I have search many threads about this, but I can't find any information.


capsule:beesneez ck12 (65v)
tube:rft ef86
transformer:lundahl 1578xl

thankyou

 

[LinusGrane]

Hello, I have almost finished my G7 microphone.

I get sound out of the mic but it seems like only the back of the capsule works properly.
I only get a low and muddy sound out of the front, so if i use the figure 8 or omni the backside of the mic sounds good but if i try cardioid and use the front it sounds low and “muddy”.

When i first tested the mic i had 174.7v att pin 2, and now i have lowered the voltage to 128.5v by installing a 40k resistor in series with the 10k resistor in the psu.

 

[gyraf]

Capsule collapsing? Phase? (listening to your own voice in headphones)

 

[kingkorg]

Hello, I have almost finished my G7 microphone.

I get sound out of the mic but it seems like only the back of the capsule works properly.
I only get a low and muddy sound out of the front, so if i use the figure 8 or omni the backside of the mic sounds good but if i try cardioid and use the front it sounds low and “muddy”.

When i first tested the mic i had 174.7v att pin 2, and now i have lowered the voltage to 128.5v by installing a 40k resistor in series with the 10k resistor in the psu.View attachment 104290

Also you shouldn't have 51v at the voltage divider, might be something wrong somewhere. You should be getting 60v. Otherwise pattern will be off. Some of the edge terminated capsules i've tested (RK12 type) can't withstand 60v. I use them at about 50v. But then you have to adjust the pattern control to be 100/50/0V.

Not sure what you get by installing that 40k resistor, try lowering the b+ by using two 60v zeners in the psu. If we are talking about regular chinese psu.

 

[neowalla]

I see the PCBs are back in stock on the Analogue Classics website. For anyone who has built from these, what body did you use? The largest I can find is the U47 Chinese bodies out there and the max PCB dimensions for those are 130mm x 45mm. The PCB dimensions on Analogue Classics says 6x2.5" which amounts to 152.4mm x 63.5mm

 

[johnchantler]

G7 here with PF86. I have some cicada-esque high frequency oscillations that kick in after a minute or two. Heater voltage is also dropping over this time and lands around 3,4v when the oscillations start.

swapped in another tube. same thing.

something to do with the regulator for H+ ? no heatsink on LM317. it's being fed by a 10V/20VA transformer.

otherwise passes a strong clear signal. noise floor as expected.

any ideas on what to check welcome!

 

[rock soderstrom]

I guess your LM317 is getting too hot and shuts down --> inbuild thermal overload protection.

Think about it, the heater needs 4,5V and 300mA.

Your 10V AC from the transformer will be around 14V DC after the rectifier, minus the drop voltage of diodes, there will be 12.5V DC at the input of your LM317.

The regulator will transfer 8V at 0,3A into heat.

8V x 0,3A = 2.4W

Thats a lot, especially without heatsink.

In reality it is possible even more because your transformer is not fully loaded.

 

[johnchantler]

Thanks. I can heatsink the lm317 with the psu chassis and see if that solves it!

 

[rock soderstrom]

Thanks. I can heatsink the lm317 with the psu chassis and see if that solves it!

Good idea, but even with that it is possible still too hot. You could share the heat on an additional resistor in front of or after the regulator. Something like 10 Ohm/3Watt type will take 3V or 0,9W load from the regulator.

Try without the resistor first. Good luck!

 

[johnchantler]

This seemed to have done the trick! No more cicada oscillations so far.

Regulator isn’t doing the full drop to 4,5. Ended up with 2 x 6,8ohm resistors in parallel on the output which gives a reading of 4,54v at the microphone which is hopefully close enough.

Thanks!

 

[rock soderstrom]

This seemed to have done the trick! No more cicada oscillations so far.

Regulator isn’t doing the full drop to 4,5. Ended up with 2 x 6,8ohm resistors in parallel on the output which gives a reading of 4,54v at the microphone which is hopefully close enough.

Thanks!

Cool 4.54V is right on target!

 

[chrispbass]

Anyone recommend a cheap test capsule that’ll withstand the voltages?

eBay, Ali Ex etc

Thank you

 

[LevinGuitar]

Check the filtering caps, in a similar psu I found a problematic one

G7 here with PF86. I have some cicada-esque high frequency oscillations that kick in after a minute or two. Heater voltage is also dropping over this time and lands around 3,4v when the oscillations start.

swapped in another tube. same thing.

something to do with the regulator for H+ ? no heatsink on LM317. it's being fed by a 10V/20VA transformer.

otherwise passes a strong clear signal. noise floor as expected.

any ideas on what to check welcome!

 

[chrispbass]

I’ve got the majority of parts to build my first G7 now, but a quick question if someone can help…

Reg, the 1G resistors, do we need high tolerance values, i.e 1%, or will 5 or 10% be ok?