Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #60 on: September 24, 2016, 06:46:39 PM »
Here are some more voltages. This is for the modded schematic.


Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #61 on: September 24, 2016, 06:56:08 PM »
And this if for the stock schematic.

On thing of note, on the modded circuit, the voltage for D kept creeping up once it got past 8VDC. It was very slow, but it would keep increasing. I stopping watching it after 8.7.

I'm going to set this one aside for a couple of days and order some 1G resistors (the ones on the circuit are very flimsy and feel like they are falling apart) and start working on the 1176 Rev A kits that I have sitting around. I've been staring at this one for too long and I'm just doing more damage to it at this point.

Thanks!

Paul

Gene Pink

Re: Groove Tubes GT55 circuit mods
« Reply #62 on: September 24, 2016, 11:43:29 PM »
One thing to try, on your "modded" schematic, under the capsule is a grounded 6.8K in series with a 0.33uF cap to the capsule.

Measure the voltage across that 6.8K resistor, if it is anything other than zero, replace that cap.

The voltage drops on those 1M resistors means about 60uA DC current is flowing, and the only things downstream are supposed to be capacitors. Something is leaky, that current is going somewhere.

Gene

Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #63 on: September 25, 2016, 03:24:27 AM »
One thing to try, on your "modded" schematic, under the capsule is a grounded 6.8K in series with a 0.33uF cap to the capsule.

Measure the voltage across that 6.8K resistor, if it is anything other than zero, replace that cap.

The voltage drops on those 1M resistors means about 60uA DC current is flowing, and the only things downstream are supposed to be capacitors. Something is leaky, that current is going somewhere.

Gene


Sounds good. I'll get it out after load in tomorrow and measure it.

Thanks!

Paul

Re: Groove Tubes GT55 circuit mods
« Reply #64 on: September 25, 2016, 06:12:50 AM »
Haven't been following this closely but I think you have been a little hasty in your mods.

You haven't told us what is wrong with the sound of the stock mike that you want to change.
  • Removing the HF disc will change the sound.
  • So will removing parts of the mesh.
You should have changed these one step at a time and checked against the stock mike to see if it is moving in the required direction.

Was the output low before you modded it?

If not, its likely the mod that is responsible.

A quick glance at your modded circuit suggests the added 2.2nF and top 330nF will reduce gain by more than 50dB.

Also not sure what the bottom 6k8 is doing but it certainly introduces noise.

Is your DMM by any chance 1M input resistance on the range you are using to measure your 17.6V

Similarly for your 1.76V on the capsule

The meter needs to be MUCH greater resistance than the impedance/resistance of the circuits it is measuring .. more than 2M in your case so you need at least 20M.

It looks as though the capsule can be isolated from ground which allows some simplification and even better noise.  But this may not be what is bugging you.

« Last Edit: September 25, 2016, 06:18:05 AM by ricardo »

ln76d

Re: Groove Tubes GT55 circuit mods
« Reply #65 on: September 25, 2016, 07:27:01 AM »

A quick glance at your modded circuit suggests the added 2.2nF and top 330nF will reduce gain by more than 50dB.

Also not sure what the bottom 6k8 is doing but it certainly introduces noise.


It's deemphasis network only.
2x330nF, 2.2nF, 1M, 6.8k

Gus

Re: Groove Tubes GT55 circuit mods
« Reply #66 on: September 25, 2016, 09:49:56 AM »
here is a post to maybe get some ideas from. It is one adjustment you can try with the N channel to PNP circuits
However the Sterling microphones I have measured, do have good operation points at the transistors
http://groupdiy.com/index.php?topic=47569.0

Now to fix your microphone test the unknown capsule in the good microphone
if it is still working
Undo the network you added and return the circuit to stock and get it working again
Then adjust the circuit

As I posted you can have low output with a broken transformer winding (there is capacitance in the windings)
« Last Edit: September 27, 2016, 04:51:09 AM by Gus »

Re: Groove Tubes GT55 circuit mods
« Reply #67 on: May 07, 2017, 04:00:06 PM »
Hi guys, sorry for waking this post up from the dead, but i just wanted to confirm that this mod works like a charm. However the mic i modded is GT md1b, which is basically the same thing.

I used 1uF cap, as GT capsule is not so bright, not like original k67 anyway. I measured the circuit response with 1uf and 2.2, and they look like this:

1uF:

Re: Groove Tubes GT55 circuit mods
« Reply #68 on: May 07, 2017, 04:00:45 PM »
And 2.2 uF:

Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #69 on: February 24, 2019, 01:37:26 PM »
So I blinked and two years went by and I still had finished this mod. I thought it was a dead capsule, so I put on a K87 version that I had in a box and it still wasn't working. I poked around some more and found that I had not properly connected the backplate to where it was supposed to go. I made that correction and it was working great. Mostly great. The mod in this thread was for the stock capsule and with this one the swapped cap values made it a little too dark. With some application of EQ it sounds nice, but I would like to figure out where to make adjustment for the top end attenuation. From the modded schematic, I'm guessing it's either C5 and or C4, but that is a guess and a poor one I think. I'd like to not mess with the capsules anymore and leave the K87 one in there and possibly learn something new about FET style mics.

And I need to sort out why the high pass and pad aren't working.

Thanks!

Paul


Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #70 on: February 24, 2019, 02:16:24 PM »
Found the issue with the pad. I wound up having to remove the 20pF as it is shown in the modded schematic and connect it between the G connection point on the FET and the pad switch. The roll off still doesn't be doing anything but I don't find an overwhelming amount of low end that needs to be rolled off or maybe something with the capsule and this mod already attenuates the low end so high passing the same frequency doesn't do anything audibly. I have another GT55 that I was waiting to do this mod after I got the other one working that I will use to make comparisons when I complete making the changes.

Maybe Iater today I'll get to it.

Thanks!

Paul

Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #71 on: February 25, 2019, 02:58:07 AM »
I did the mod to the other one and it does seem a bit darker than I expect like I experienced with the other one with the K87 capsule installed, but it does reflect what kingkorg posted with the frequency plot. Maybe I'm expecting to hear a bit of a presence boost that we've grown accustomed to hearing, but it still sounds much more pleasant than before and with a touch of EQ it sounds great. And I'm glad the unnatural sounding high frequency hype is gone.

I noticed the stock capsule with the mod sounds wind is rushing by and the K87 capsule does not. I failed to take note if this was something that was occurring before I modded this guy, so it could be the capsule. Maybe I just need to get another inexpensive K87 or K67 style version.

Also, the HPF on the mic does work. In my great wisdom and thoroughness of testing procedures, I had the 100Hz HPF engaged on the little mixer I am using. Classic me...

A far belated thanks to In76d for this mod. It really transforms the sound quality and usefulness of this mic to a much higher level.

Thanks!

Paul

Re: Groove Tubes GT55 circuit mods
« Reply #72 on: August 12, 2019, 11:34:45 AM »
I just wanted to do an update with latest schematic in case anyone stumbles upon this thread. I changed values for the capacitors so that low end is also tuned perfectly. Added 560 ohm resistor to create calibration input for biasing and deemphasis measurement.

220pF can go up to 470pF for brighter capsules. 12nF can go higher for more bass response.

However, with updated values i made the mic indistinguishable from u87 i had for testing. It actually sounded closer to it than another u87 i loaned.

Potato Cakes

Re: Groove Tubes GT55 circuit mods
« Reply #73 on: August 12, 2019, 11:56:43 AM »
Thanks for sharing this! I'm guessing this is with the same stock capsule, correct?

I'm hoping to update my pair of GT55's soon. I used the original schematic you posted and I liked the "improvement" I heard from the stock components.

Thanks!

Paul
« Last Edit: August 12, 2019, 12:02:24 PM by Potato Cakes »

Re: Groove Tubes GT55 circuit mods
« Reply #74 on: August 12, 2019, 12:23:50 PM »
It is not exactly the same capsule, but is of the same family, and measures almost the same. One db+-

That is why 220 or 470 pF (or even 12nF) can be tweaked to taste, as i found myself a real u87 can vary that much in response. Even Neumann used diferent values here.

Original 2.2nf was way too much, made it sound like a ribbon. 330nF was too much as well, didn't attenuate the bass enough.