Groove Tubes GT55 circuit mods

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
I'll be also thankfull for the doc via email.
I can't use my old account on micbuilders group.
 
Potato Cakes said:
No caps soldered directly to the XLR pins, FYI.

Thanks!

Paul

Did you checked that?

nielsk said:
Sorry to hijack this, but does anyone have a schematic for the GT66 mic they can post to the tech docs?

Thanks!

Some time ago i also searched it, didn't found anywhere.

 
It would be great to see ricardo doc  - anyone have it and could share?

After it wecan start fun with other mods ;)
 
Great ;)

Am courious ricardo mod in output part of the circuit, then we can talk about output cap and other things :)
In the meantime i will try to find most effective connection of deemphasis network components.
Did you tested new connection of the capsule?
 
I haven't pulled R12 and 13 as you suggested yet. The capsule is connected with the diaphragm connected to the right 1Gohm resistor with the FET attached and the backplate to the left one. Both silver mica and ceramic capacitors by the 1Gohm resistors have been removed. I'll pull R12 and 13 and put jumpers in and let you know.

Thanks!

Paul
 
No, don't remove R12 and R13!!!

I had in mind only to remove it when there are also caps on xlr socket!
I will try to trace circuit (or even part of it) in a free time so for now don't  change anything at the output.

In fact i was wrong about silver mica cap becuase it looks (on the pics from recordinghacks) connected different than it is in reality.
Anyway if you want to use deemphasis, you don't have to put it back.

Am rather courious did you tested new capsule connection, especially if you have unmoded microphone to compare :)
 
I was curious about the silver mica caps as well. On both of mine one leg is connected to the 1Gohm resistor on the left and the other is attached to the C2 solder pad right above it.

Thanks!

Paul
 
Yes it's connected to ground for filtering but am courious what's the value?
Micas are usual low ranga capacity and polarisation filtering caps usual starting from 10nF.
WIth deemphasis topology isn't used.
 
oh sh..t - so i totally didn't saw it proper on the pic :D

I didn't recieved anything still - did you send it on go2 or g02?
should be go2 ;)

If there's schematic, we are in home :)
 
Thanks!

Are there any notes from ricardo?
I sent you email by gdiy, please check did you received it, if so try resend to it ;)

I see few things to improve, tomorrow i will also try some simulations.
 
Here's a deemphasis  and two  caps (FET output and PNP output) mod.
In a free time i will look for different things and try to verify current mod.
I couldn't use original fet model (is it really 3D37F? - Paul please check that;) ).
In deemphasis network for now capacitor 2.2nF is the only part which you should test with different values.


 

Attachments

  • GT55 deemphasis mod.png
    GT55 deemphasis mod.png
    160.4 KB · Views: 112
The writing on the FET is hard to read, but it does appear to say 3DJ7F.

I'll have to order some of those caps as I don't have all those values. Fortunately, I need those same ones for another project.

Thanks!

Paul
 
Ok, fet symbol is correct now :)
If you will find square shaped Evox RIfa MMK 1uF/100V (or 63V) these are great for the outputs.
330nF also could be Rifa, but these are hard to find, so Wima MKS or ROE MKT will be good for it.
For 2.2nF you could look for Wima FKP or MKP, small sized high voltge ratings.
I will try later verify few things and look for other improvements as also i look hot to simple add deemphasis circuit on the PCB.
Do you have oscilloscope and generator?
 
Back
Top