Groove Tubes GT55 circuit mods

[Potato Cakes]

Here are some more voltages. This is for the modded schematic.

 

[Potato Cakes]

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]

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]

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

 

[ricardo]

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.

 

[ln76d]

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]

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)

 

[Potato Cakes]

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]

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]

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

 

[kingkorg]

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]

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

 

[kingkorg]

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.

 

[Potato Cakes]

I've been on a bit of a tear finishing projects and got the pair of GT55s out after kind of forgetting about them for two years and put them on the list of projects to complete. I implemented the last schematic update from kingkorg and now I FINALLY have these guys up and sounding really nice. For the one with the K87 style capsule, I used 220pF as is and changed the 12nF to 22nF. The other guy I would up swapping out the original capsule for a K47 (I didn't have another K87) and also kept the inner mesh in the head basket. For this one I removed the 220pF but used the same 22nF as before. It was slightly darker but still clear. However this is the one which I was reporting sounding like a constant ocean sound was in the background, which is why I swapped the capsule in the first place. The microphone itself sounds great and very full bodied (like a U47/FET47) but it just has the constant noise in the background. I suppose it could be the FET or the PNP transistor. I've had noisy zener diodes before when used in a compressor, but that sound was more like a high noise floor/hiss/impedance mismatch than what I am hearing. I'll look to see if I have any compatible FETs or transistors. I think the only ones that I have that are PNP are BC560's, but the HFE on those are much higher than the C9012. I have no idea what would be a good substitute for the 3DJ7F that are currently in there.

Thanks!

Paul

 

[Potato Cakes]

The closest thing I had to a C9012 was a BC327, but for some reason barely any signal was able to pass. I put the old C9012 back in and it returned to the state it was in earlier with the ocean noise. The specs look similar but there much be a spec I am not looking at to determine compatibility.

Thanks!

Paul

After having a struggle with my dyslexia, I realized that the pinouts for the two were backwards. I even drew it out beforehand to avoid making a mistake and I still got it wrong. The good news is that replacing the C9012 with a BC327-40 fixed the ocean noise. Also, it really brought the mic to life in an unexpected way. The K87 version now sounded dull and lifeless in comparison (better transient response?) so I put a BC327 in that one as well. Man, these sound really fantastic now. I was hoping that they would just function properly and perform decently but these really exceed my expectations from when I started out to revive these mics. Many thanks to kingkorg and ln76d for your knowledge and help with modifying this circuit.

On a side note I think I prefer the K87 capsule in this circuit, but only by a small margin. The K87 has a little more openness to the top end (depending on deemphasis tastes) like you would expect when compared to the K47 capsule. These are also on the lower middle of the range of capsule quality, so that should be taken into condersation. Regardless, I'm very glad I decided to revisit this project and was able to finish them without buying more parts.

Thanks!

Paul