Groove Tubes GT55 circuit mods

[Potato Cakes]

Hello, Friends!

I have a pair of old Groove Tubes GT55 mics that were pretty good when I bought them, but being compared to some really nice mics that I'm building now, they don't seem to hold up in comparison. Even the cheap Chinese mics that I've modded seem to be more preferred. Aside from changing out the capsule, I was wondering if anyone has had any success altering the circuit to make noticeable sonic improvements. I saw an old thread on here from quite a ways back, but there were no conclusions posted.

Thanks!

Paul

 

[pucho812]

those have the high frequency disc  in use with the capsule IIRC... I never liked that

 

[Potato Cakes]

I had also seen somewhere in that older post that removing the inner grill seemed to help.

 

[ln76d]

Judging only by pics and some informations about it
http://cdn.recordinghacks.com/images//mic_extras/groovetubes/GT55-pcb.jpg
Measure polarisation voltage at the second pin (from the left) on the four-pin connector.
Remove resonator disk and replace it with standard capsule connector.
Remove capacitors between two 1G ohm resistors.
Connect diaphragm wire to the 1Gohm resistor on the right (with Fet gate connected).
Connect backplate to the 1Gohm resistor on the left.
Swap transformer wires.
Find output capacitor.
There's few other options of improvement but at least i need to know what the value of polrisation voltage.

 

[Potato Cakes]

Thanks for the info!

It'll probably be first half of next week before I can dig into this thing. I'll send you the polarization voltage as soon as I can.

Thanks!

Paul

 

[ln76d]

Don't worry about the time, when you will be ready, just back to the topic.
Good pics of the tracks sides on both PCBs would be great

 

[Gus]

What don't you like about the GT55?
That will give ideas for what could be adjusted

I have a few sterling microphones that surprised me with how good they sounded. I bought them just for the body and I left them alone for now thinking about what to adjust.  The bias of the n channel and pnp was good. 

What is the capsule charge voltage?
Look at the China n channel, pnp circuit that should help trace this circuit

 

[ln76d]

Could you be more specific about "China n channel, pnp circuit"?

 

[Potato Cakes]

I am getting +38.3 VDC for both of my GT55s. Phantom coming down the XLR is +47.8VDC.

Thanks!

Paul

 

[Potato Cakes]

And for pictures. This is the front side of PCB which to measure polarization voltage.

 

[Potato Cakes]

The back

 

[Potato Cakes]

Front of the other guy

 

[Potato Cakes]

And it's back

 

[ln76d]

Measure it again at empty pin where is R2 marking and check voltage before and after D1.
To have sure

I will look at the pics tomorrow with fresh look, now going sleep.
You can make deemphasis circuit to damp high frequencies

For quick - it looks like output capacitor is at the bottom check what's the value.

If you would like to use deemphasis network there will be also need good pics of the switches board.
Overall it's adding of three (or even two) resistors and three capacitors. 
Then you will don't have to change the capsule.

Check also are the two capacitors (ceramic probably) soldered direct on XLR pins - you will have to take off metal cover for transformer. If so, you can remove both as also R12 nd R13 (put jumpers instead).

Now am going sleep

 

[Potato Cakes]

Here's a pic of what the capsule looked like when I took off the HF disk. It seems to have the tiniest tear, but this is completely covered up when I put a new diagram connector on it. I reused the brass washer as before.

I did all the initial changes in the earlier post to one of the GT55's, and it has noticeably lower output than stock, and when I bring up the gain there is a 60Hz hum that can be heard. But this is probably because I haven't finished doing the other steps that you just posted. I'll give those a go and see what happens.

Thanks!

Paul

 

[Potato Cakes]

The cap at the bottom center is 10uF 50V.

The Voltage at the R2 pad is 38.3. The side of D1 facing the transformer is 20.6 and the side towards the capsule is 20.4.

Keeping the same capsule would be great. I'll working on getting you the other pictures.

Thanks!

Paul

 

[ricardo]

GT55.doc in my Yahoo MicBuilders Files has some recommendations for circuit mods.

Mainly to reduce noise & increase headroom

You have to join

 

[Potato Cakes]

Pictures of the switch.

top

 

[Potato Cakes]

Bottom

 

[Potato Cakes]

Ricardo,

Thanks for the info. I'm going to keep down the path I've started and keeping the other GT55 I have stock as the control for this experiment. Maybe when I get this done I can do your mod and compare. Would you mind emailing it to me?

Thanks!

Paul