Groove Tubes GT55 circuit mods

[ln76d]

I couldn't find datasheet for FET but i found information that it's similar to 2SK30A.
With 2SK30A model simulation showed completely different results...
Anyway i think you should try first what i attached above.
If this wouldn't be proper then there's option to change a little input configuration and FET.

 

[Potato Cakes]

Just ordered all the parts and had them rushed so I can work on this before I leave out on tour again. The 1uF caps I have are WIMA and Kemet. The 2.2nF I have are ceramic.

I do have an oscilloscope with a generator.

Thanks!

Paul

 

[ln76d]

I hope ceramic is NP0/C0G type, otherwise it wouldn't be good option
Good that you have scope and genertor!
If there shouldn't be great result, then there's option to improve microphone circuit with use of both tools

 

[ln76d]

I need a good pic of the backside FET PCB (with visible traces at the top of PCB).
Then i'll be able to mark connection of additional parts.
Could paste links to the parts which you bought?

 

[Potato Cakes]

It should be the second picture I posted on the first page.

Here are the links for what i bought:

http://www.mouser.com/search/ProductDetail.aspx?r=598-CD5EC200JO3F

http://www.mouser.com/search/ProductDetail.aspx?r=80-R82EC1220DQ50J

http://www.mouser.com/search/ProductDetail.aspx?r=80-R82EC4100Z370K

I also have a couple of WIMA 1uF caps here already just in case.

http://www.mouser.com/search/ProductDetail.aspx?r=80-R82EC3330Z370K

I already have the other things you listed on the schematic.

Thanks!

Paul

 

[ln76d]

It should be the second picture I posted on the first page.

I don't see uper connections, since wires covers it and it's blury - that's why i asked for another one

 

[Potato Cakes]

Lets try these. For some reason the pics are blown up really big when enlarging them. You may have to download them.

 

[Potato Cakes]

And the other side

 

[ln76d]

Now i see everything

 

[ln76d]

Of course you need decide what better is put on the front and what on the back
20pF as it was in original.

 

[Potato Cakes]

When the parts come in I'll get right on this.

Thanks!

Paul

 

[Potato Cakes]

I got the parts, but I'm going to have to wait a couple of days as I'm about to head back out on tour. I'll find time on a day off and report back.

Thanks!

Paul

 

[opacheco]

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

Dear Porato Cakes,

I made this schematic by hand years ago following all the cooper tracks and wires!....but the serious problem with this mic is the capsule I believe !!...

Opacheco.

 

[opacheco]

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.

This is a 3DJ7F.....I am sure because I draw this diagram by hand.
Opacheco

 

[Potato Cakes]

I found some free time to work on this after load in, but I left some of the parts I needed at home! I'm going order some more parts and send them to one of the hotels in the states am at when I have a day off. Just have to finish up a couple more Canadian dates.

Thanks!

Paul

 

[Potato Cakes]

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 am using Kemet film capacitors for the 330nF and 2.2nF and WIMA for the 1n. I think they'll be okay. Maybe later I'll experiment with different caps.

Thanks!

Paul

 

[Potato Cakes]

So I made all the changes, but it still has very low output, but no buzz. I feel that there may be an issue with the capsule. When I took off the high frequency disc, there was a small brass spacer that was being used between the disc and the capsule, which I reused when I put on a standard connection to the capsule. I don't know if I should have left it off.

The schematic that you sent back to me had two connections to the backplane, which I was a little confused. I attached that same schematic with some voltages in relation to the capsule connections. I checked these with the stock GT55 that I have for comparison and it they are the same. I connected the diaphragm to the section that is +40V as in the stock version and the backplane to the 330nF cap before the 6k8 resistor to ground. At least I'm pretty sure about this. I'll double check this when I get back in my work box during load in tomorrow.

The good news is that there is no buzz or hum or any crackling noise, just very low output. Either the capsule is somehow bad, or there is some other component that is not letting signal pass efficiently. However, when I tap on the body of the mic the handling noise is about the same as the stock GT55.

The pad switch also does not do anything at this point.

Full disclosure, I did fail in testing this mic before doing the mods, and it has been sitting around being unused for several years. What I may do is if don't succeed in getting this up and running, I may start doing the mods the other guy minus anything capsule related. I really don't use these mics anymore, so I'm treating this as a fun experiment/educational project while I'm out touring. If it does turn out that the capsule is somehow bad, the question will be what type of capsule (87, 47, etc) could I drop into the modded circuit without having to change a whole bunch of components?

Thanks so much for your time. I know you are probably very busy with other projects and I greatly appreciate your help with this.

Thanks!

Paul

EDIT: Revised voltages Below

 

[Gus]

I would have done this a different way
First for the full range I would also disconnect C7 in the full range setting
The emitter to transformer 1uf is a good selection

Look for the Phantom C schematic

Check the transformer windings at the PCB, a broken winding can cause a low output.
If the voltages at the drain and emitter look good you could try moving the capsule from the other microphone into the microphone that is not working or you could move the capsule to the good microphone

 

[Potato Cakes]

All rightly. I got to my work box and have been able to double check some of my work. The 40vdc label is incorrect. That should be 0.4vdc on the stock (working) mic and 10vdc on the modded one. I may try to get some 1G resistors and swap one of those back where the two 1M resistors were placed. I'm going to look over my work and compare it your modded schematic and make sure it's not something silly that I am doing.

Thanks!

Paul

 

[Potato Cakes]

Also, after correcting some errors, their is no capsule output anymore. But it still transmits signal when I tap on the body.

Thanks!

Paul