GT Microphones design (LA parts sale)

[Marik]

Look at the way Jakob does the biasing for G7:

http://www.gyraf.dk/gy_pd/g7/gic_s.gif

Leave in divider on 2 470K resistors (you can use your 1M) and also use 1G and 100n capacitor.
If you don't need multipatter connect the back side (now connected to 1000Pf) to ground for omni, or short with the other side for cardioid pattern.
For grid resistor I'd use something like 150M-200M.

Best, M

 

[3nity]

like this?

 

[rodabod]

Mark, I'm guessing they accepted they were using a CF for the output stage (along with cheaper transformer). So, I wonder if the first stage there (set for high gain) is deliberately to add a touch of distortion. They could have easily gone for a typical 100K as the gain is unnecessary in my opinion. Thing is though, a lot of people have said the original mic was quite nice. Maybe it was partly the capsule.

Here is a crap edit of your drawing which should work:

There are tonnes of other things you could try though. Off the top of my head, you could try a single-stage cathode follower like say a C37 / Royer Country Boy, etc. or maybe try doing a plate-out with some lower gain, lower Rp tube like an ECC82 depending on how well your transformer performs. Maybe think about what sort of sound you'd like from this thing, and if you'd like high level handling.

 

[3nity]

Thanks Rod.
I'll start by doing as you schem.
We'll see if i like it or not!
Thanks so much.

 

[MM1100]

3nity - I will buy the electret from you. I have a GT DT-1 that I would like to mod into a microphone. Yes/No - let me know.

 

[kdramsey]

Chuck,

I have been looking this schematic over lately and I noticed that the area around the LM317 doesn't match up with the way it is on the circuit board layout you also posted........Is the circuit board an actual scan of the PSM1?  I am currently building a few power supplies for some GT MP1's I got and I was looking for an original layout to work from.  Do you have any info on the transformer in your unit like make and model?  Or do you know the AC voltage outputs for both of the secondaries?  By my calculations I am thinking they should be 140Vac and 30Vac.......But it would be nice to have some confirmation.

Thanks
-Kevin

So I was comparing the schematic posted by Jakob above for the PSM1 and my broken PSM1 that I got for cheap at the Big sale last fall.
They are not the same at all.

I decided to scan in the PCB into Eagle CAD and reverse engineer it for everyone to use. Here is what I have:

Are there any corrections that someone wants to point out? I did this rather quickly late last night.
If anyone wants the BRD eagle file just throw me an email and I send it to you.

I was thinking I might change this PCB to match Max's MK7 PSU? Is that version better then this one for noise?
Other comments?

-ChuckD

 

[ChuckD]

Yes the PCB is an actual scan of the board I have.
I never ended up using this design though.


Chuck

 

[kdramsey]

Do you happen to know what value the D25 zener should be?  Or do you remember the part # by chance?  I am going to make a couple of these shortly but I plan to modify it a little......I will use a 7812 for the 12v circuit.  I will also use a transformer with 140v and 24v secondary taps.  I figure the D25 zener should be 120v and resistor R25 is there to control the current dropped across the zener.

Thanks,
Kevin

Yes the PCB is an actual scan of the board I have.
I never ended up using this design though.


Chuck

 

[moomilk]

Couple of words regarding the capsules:
model one \ MD1 \ MD1a (can't say if it is true for later MD1B) used Japan made "Primo" medium diaphragm capsule,
MD 2 \ MD2A and MD3(a) capsules were designed by David Josephson

 

[GabrieleP]

A couple MORE words:

capsules for MD2 and MD3, both "not-A" and "A" versions, were designed AND built by Josephson.
Upon recent purchase of an MD3 (serial in the low 2k), I was able to get in touch with Mr. Josephson himself, which kindly explained that.
Let me add that the design of the MD3 capsule is obviously inspired to the element mounted in the SONY C-37 and also that it is still adopted by Josephson in their C715.
Here follows an excerpt from the .pdf available at the company web site:
"The C715 features the new single-sided capsule with a 5 micron evaporated gold diaphragm. The backplate has an adjustable vent, reached through an opening in the back of the grille, that allows the mic to be adjusted from cardioid to omni."

This is exactly the way the MD3 works.

I am also the owner of a pair of MD1 (one of them is actually labeled MODEL 1 but not with the first font adopted, rather the later ITALIC one, serials are 14xx and 17xx) and in response to the very first post of this thread (looong time ago!) the transformer in both is labeled CM9121. The CM9336 is actually present in the MD3. There's pictures posted here by MikoKensington showing an MD1 with 91xx stamped on xfrmr.

I thought it worth a contribution with some additional info, just in case these tools become the new vintage...

Gabe

 

[kato]

I thought it worth a contribution with some additional info, just in case these tools become the new vintage...

It's always OK to bump old threads here when you're contributing potentially useful information; we're in it for the long haul. So thanks for adding.

 

[ppa]

the input tube polarization is good for very low noise but the 10k resistors should have an electrolytic and a polyester cap in paralllel to reduce to zero the noise that this 10kohm resistor makes there. 

 

[SteveB]

I recently acquired an early model Groove Tubes Model 2 microphone, serial number 2032. From research I understand this model and the model 3 utilized Josephson capsules. Upon receiving the microphone, the capsule looks rather unique, and not quite what I expected. It looks like the electret capsules I have seen pictured in the model 1. Attached are photos

I am curious if this is indeed a Josephson capsule or do I have a transition model 2 or something. Anyone familiar with these?

 

[hodad]

It looks like the electret capsules I have seen pictured in the model 1.

Yeah, it does. It looks pretty much identical to a photo of a model 1 capsule that I have in my files. It's definitely not the same electronically as a Model 1 (Model 1 does not have VR1, which is the obvious bit that's different), but I have no idea what capsule was typically in the Model 2, or if it changed over time, or anything like that.

 

[SteveB]

Ok- I think the capsule mystery is solved - from David Josephson himself. So nice that he took the time to reply.

As I recall, the capsule in the Model 2 is the same as the Model 1
except GT exchanged the electret diaphragm (silver color) for a gold
mylar non-electret diaphragm that was made by us. The Model 3 capsule
was made entirely by us.