Groove Tubes GT55 circuit mods

[Potato Cakes]

No caps soldered directly to the XLR pins, FYI.

Thanks!

Paul

 

[ln76d]

I'll be also thankfull for the doc via email.
I can't use my old account on micbuilders group.

 

[nielsk]

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

Thanks!

 

[ln76d]

No caps soldered directly to the XLR pins, FYI.

Thanks!

Paul

Did you checked that?

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.

 

[ln76d]

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

After it wecan start fun with other mods

 

[Potato Cakes]

I'll forward it to you...

Definitely no caps soldered directly to the XLR pins.

Thanks,

Paul

 

[ln76d]

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?

 

[Potato Cakes]

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

 

[ln76d]

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

 

[Potato Cakes]

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

 

[ln76d]

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.

 

[Potato Cakes]

One of them is 15J and the other is 12J. The schematic I sent to you shows it as 20pF and connects to one end of the Pad switch.

 

[ln76d]

oh sh..t - so i totally didn't saw it proper on the pic

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

 

[Potato Cakes]

go2

I copied and pasted what you sent me.

The schematic is in a .doc file.

I'll resend.

Thanks!

Paul

 

[Potato Cakes]

I lifted the schematic in case the email didn't go through.

 

[ln76d]

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.

 

[Potato Cakes]

There are some notes. Should be in the email I sent you. Let me know if you don't get it by the morning.

Thanks!

Paul

 

[ln76d]

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.

 

[Potato Cakes]

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

 

[ln76d]

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?