G7 Capsule Recommendations Please!

[ln76d]

Hi In76d,

I got around to testing some mods. The headbasket test proved to be very subtle. So I don't think that's a problem.
The output cap got switched out to 1uf (I know you recommended 0.5uf, but 1uf was all I had at hand) and it definitely reduced the low end. Then I removed the 220r resistor and replaced with a jumper. Then installed a 4k7 trimpot and tested various bias settings.
Everything was going along well until I modified the section near the capsule. The two 1 gig resistors swapped for 100Meg, the removal of the 1n cap between the one of the 100M resistors and the center of the capsule. The swap of 1n, 10n capacitors and all the rest as shown in red on the mod scheme. Now I get very low gain and bad sound. I've checked and double checked and I can't see any error I made. Not sure what to do now!

By the way, here are some A-B test files if you care to listen.

Unmodded G7:  https://app.box.com/s/9zf237xy2h3ibpz3jp132aw2l32e9sho
1uf output cap:  https://app.box.com/s/s2ypnihcav8wll8prdrf7w4rra53uq9q
220r jumpered:  https://app.box.com/s/riq2gjex996mj3hi9ak0e3421ij72w1y
2.7k bias :  https://app.box.com/s/t59bd3itg1gp00vbcb84yamv8hrymd5f
2k bias:  https://app.box.com/s/felx41e8rkp77f63x3n2i082ya5ok0ou
4.7k bias:  https://app.box.com/s/cjcp29lq732di57wxk1glv65n8y8rq39
700r bias: https://app.box.com/s/jnlrafbuqgdhi6c5b71fgdv62ag92f5t
input mods: https://app.box.com/s/okbg1zy3711ux5dzikxcqux53h1n4h4n


And just for reference, this is the other unmodded G7 from a recording session today. It's on the ukelele and also the vocal. No EQ on the vocal, just compression and reverb :  https://app.box.com/s/wl2aso1a5kmc3rxrxxodlslpqkavcjfn

First question Did you adjusted microphone preamp gain after each step?
Second question - related last step - input topology - are you sure, that you made proper connection - didn't you left grounded front diaphragm? You can expect lower sensitivity, but 2-3dB is max. What is that hum and noise? Is it high microphone preamp gain or faulty connection?

 

[Freddy G]

First question Did you adjusted microphone preamp gain after each step?
Second question - related last step - input topology - are you sure, that you made proper connection - didn't you left grounded front diaphragm? You can expect lower sensitivity, but 2-3dB is max. What is that hum and noise? Is it high microphone preamp gain or faulty connection?

First answer:  Mic preamp gain was not adjusted at all for any of the steps.

Not sure what you mean by "didn't you left grounded front diaphragm" If you are talking about the blue line in the scheme I am posting here....yes i removed that connection as per you scheme.

That hum and noise must be a fault in the circuit...the mic pre had not changed.

 

[ln76d]

First answer:  Mic preamp gain was not adjusted at all for any of the steps.

Not sure what you mean by "didn't you left grounded front diaphragm" If you are talking about the blue line in the scheme I am posting here....yes i removed that connection as per you scheme.

That hum and noise must be a fault in the circuit...the mic pre had not changed.

Yes, the blue line. Are you sure, that you didn't messed any other connection in the input, yes, because it sounds like fault.
Or...  Didn't diaphragm stucked to backplate? Wasn't there higher noise which go down after few seconds?
Did you measured anything during setting up bias potentiometer? Cathode voltage, anode voltage?
If you want to try again something, then for my schematic connection, disconnect 1nF between diaphragms (from the front diaphragm)  and set omni position in PSU. This should at least show is it front diaphragm and backplate connection made properly as also you will be able to judge non multipattern cardioid connection.

 

[Freddy G]

Just disconnected the 1n cap from the front capsule. Now the gain is back up (working in cardiod and omni) but still lots of crackling and popping

 

[ln76d]

Just disconnected the 1n cap from the front capsule. Now the gain is back up (working in cardiod and omni) but still lots of crackling and popping

So you did something wrong, with 1nF it can't work as omni or figure eight, it is just cardioid not dependent on switch.
Check again connections. Also this connection should give you slightly higher senstivity than original G7 topology.

 

[Freddy G]

Thanks for your help.

I will trace everything again. Maybe even draw up my new component and connections layout on a PC board for visual confirmation.

 

[ln76d]

Or just make the pic of the board connection?!? At the first attempt to change input topology, if you replaced 10nF with 1nF direct on the board and connected front diaphragm to 1st grid of the tube, then you have grounded input - just for clarification
Also if you could make potentiometer connection on the board photo, then would be great (of course if pot is still on the board).

 

[Freddy G]

i will post more tomorrow...thanks!

 

[Freddy G]

Or just make the pic of the board connection?!? At the first attempt to change input topology, if you replaced 10nF with 1nF direct on the board and connected front diaphragm to 1st grid of the tube, then you have grounded input - just for clarification
Also if you could make potentiometer connection on the board photo, then would be great (of course if pot is still on the board).

Here is a pic of the actual board;

 

[Freddy G]

and also a drawing:

 

[ln76d]

Theoretically looks proper but i will look more carefully later, currently need go out.
Naming - your "front plate" should be front diaphragm, "center" should be backplate and your "backplate" should be back diaphragm. Is it that connection?
Where is connected bare wire (or is it insulated?)  marked with red arrow on the pic?
Doesn't piece of wire marked green don't touch the ground trace?
I would also check solder joints, because don't looks so healthy.

 

[hop.sing]

Hi
I think that board might be too dirty somewhere. That could explain the symptoms you experience. Lack of volume and noise.  Did you clean the area around the 100m resistors and he area  close to the capsule thoroughly with alcohol? That is often the remedy for those problems, even when on visual inspection things look ok .
Tobias

 

[Freddy G]

Theoretically looks proper but i will look more carefully later, currently need go out.
Naming - your "front plate" should be front diaphragm, "center" should be backplate and your "backplate" should be back diaphragm. Is it that connection?
Where is connected bare wire (or is it insulated?)  marked with red arrow on the pic?
Doesn't piece of wire marked green don't touch the ground trace?
I would also check solder joints, because don't looks so healthy.

Thanks for clarifying the naming convention. Yes that all corresponds.
The bare wire (is actually insulated) marked with the red arrow connects to the backplate.
Yes the soldering is on the sloppy side, I have cleaned it up.  Also I cleaned everything just now with alcohol but still making lots of noise. So here are A-B files between the unmodded mic and the modded mic. As you can hear, the tonailty of the modded mic is very, very different and I also hod to boost gain on the mic pre by 12db to get in the same range.

unmodded:  https://app.box.com/s/40cymzkqzrkua77hg9e5ef6ak2e1vb22

modded:  https://app.box.com/s/vfgrem7pq7vi223mryhjy486v2r0za8g

 

[ln76d]

Thanks for clarifying the naming convention. Yes that all corresponds.
The bare wire (is actually insulated) marked with the red arrow connects to the backplate.
Yes the soldering is on the sloppy side, I have cleaned it up.  Also I cleaned everything just now with alcohol but still making lots of noise. So here are A-B files between the unmodded mic and the modded mic. As you can hear, the tonailty of the modded mic is very, very different and I also hod to boost gain on the mic pre by 12db to get in the same range.

unmodded:  https://app.box.com/s/40cymzkqzrkua77hg9e5ef6ak2e1vb22

modded:  https://app.box.com/s/vfgrem7pq7vi223mryhjy486v2r0za8g

Still something seems to be wrong. 12dB lower is definately too much also if everything would be ok, there shouldn't be any noise.
What was the bias setup (cathode resistance) in this comparison?
I would take multimeter and check with diode checker (beeper) isn't there any continuity between these points and ground as also check 1nF capacitor, isn't it shorted.
I will listen it tomorrow, now is to late for this

 

[granger.frederic]

it looks like the 100Mohms resistor is damaged , at the right side of the 0.001uF capacitor.
the varnish is damaged due to a probable overheating during the soldering process...
i'd replace it first...
regards

 

[ln76d]

Yes definately it can be 100M resistor mentioned by Fred. I think i never before seen resistor with black stripe after gold (tolerance) stripe or maybe i just didn't paid attention. Anyway it would be good to measure these resistors.

 

[Moby]

thanks for the reply guys!

I have a pair of Dale's m7 capsules. Got them in 2007 or 8. The transformer is a Lundahl.

No, it's not that I'm unhappy or anything is bad....just thought maybe I could do something to make me really dig these mics again.  I've been using a couple of U87s from the early 70s for vocals and I like the quality they have in the midrange....totally different than the G7. Very thick and forward. Perhaps that tube change might be an inexpensive experiment to start with.

My opinion about Lundahl transformers in this kind of circuit is not so positive. Generally, those transformers tend to be "flat" and "lifeless".

 

[Freddy G]

it looks like the 100Mohms resistor is damaged , at the right side of the 0.001uF capacitor.
the varnish is damaged due to a probable overheating during the soldering process...
i'd replace it first...
regards

I just replaced that resistor. It was however still in spec. I measured the other resistors as well...all good.

 

[Freddy G]

My opinion about Lundahl transformers in this kind of circuit is not so positive. Generally, those transformers tend to be "flat" and "lifeless".

OH? Very interesting! I have found that exact same issue with some Lundahl transformers for mic pre input stage. I built a two channel mic pre that had a Lundahl on one side and a Cinemag on the other. Everyone preferred the Cinemag! Yes, the Lundahl sounded "blah"
What's your preference for a transformer for this mic?

 

[Moby]

OH? Very interesting! I have found that exact same issue with some Lundahl transformers for mic pre input stage. I built a two channel mic pre that had a Lundahl on one side and a Cinemag on the other. Everyone preferred the Cinemag! Yes, the Lundahl sounded "blah"
What's your preference for a transformer for this mic?

Again, I will not call it "blah", just without character  Some people prefer that kind of sound and it's OK .  I must be honest, I have never built G7. But it looks like any quality microphone input, or maybe better mic output  transformer (Ratio between 6:1 to 10:1) will do fine in this circuit .