GT Microphones design (LA parts sale)

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ChuckD

Well-known member
Joined
Jun 3, 2004
Messages
578
Location
Irvine California USA
OK I just wanted to start a thread on the GT mic parts etc I picked up at the LA sale this weekend.

Firstly, Aspen was selling of all the boards and parts so I think the design is fair game now.

The MD1 is a Cathode Follower design into a Cinemag CM-9336 transformer. The PCB around the tube uses the 7,8,9 side of the tube as the first (AKG C12-C24) type gain stage then like the Chinese models it uses the 1,2,3 side of the tube as a Cathode follower. I will try to draw a schematic tonight.

Here is the Transformer info straight from Cinemag:



We have not made data sheets for either of these transformers. Sorry, but we manufacture so many that it is impractical to compose one for each.

The CM-9911 is a 5:1 microphone output transformer. Aspen's price of $20 was a give-away compared to what Alesis paid for it. Be sure that the turns ratio is what your circuit is suited for.

The CM-9336 was a 4:1 cathode follower microphone output transformer. It was inexpensive, but did a decent job. It is not in current production.

Aspen owes us lunch for all of the inquiries we are getting about these transformers. lol :)

Best regards,
David Geren


I spoke with them on the phone as well and they do not recomend the CM 9336 as it is a single bobbin and relatively cheap in design for a high end build.


I also mentioned that Aspen was recommending that the CM9911 could be used as the CM2480 as a great replacement in the Chinese mics. This should be taken with a grain of salt as the 9911 is a 5:1 ratio while the 2480 is a 10:1. So with the 9911 a PAD would be required most likely. This is very design dependent but the APEX460 etc. mod should use the 10:1. I think I will try one just to see.

I know many other picked up this stuff too so post some ideas!

-ChuckD
 
[quote author="ChuckD"]I also mentioned that Aspen was recommending that the CM9911 could be used as the CM2480 as a great replacement in the Chinese mics. This should be taken with a grain of salt as the 9911 is a 5:1 ratio while the 2480 is a 10:1. So with the 9911 a PAD would be required most likely. This is very design dependent but the APEX460 etc. mod should use the 10:1. I think I will try one just to see.[/quote]

Probably best to try it using the original cathode follower or using a different valve configured for lower output impedance then.

The MD1 was supposedly a very nice mic, but I believe the capsule was by Josephson which would at least partly explain that.
 
OK here's my 1st attempt at the MD1 :

Groove_tubes_source_v1.gif


Some things look a little strange ... comments?


-Chuck
 
The 2 gig from capsule to capacitor looks wrong - might go from the capsule backplate to something around 60-80V. (often derived from B+ via a voltage divider)

220 uF in parallel with the otput tranny looks wrong - might be bypassing the cathode resistor of the first tube stage.

EDIT: What is the 10n doing? It looks to be in parallel with the first tube stage? EQ? or maybe an error?
 
On my MD-1 schematic, it's a 12AT7, an electret capsule, and no variable patterns:

http://www.twin-x.com/groupdiy/albums/userpics/GT_MD-1_MIC.GIF

Jakob E.
 
[quote author="zebra50"]
220 uF in parallel with the otput tranny looks wrong - might be bypassing the cathode resistor of the first tube stage.[/quote]

Flipped round in Jakob's schematic, it's decoupling. I'm guessing the random 100n there might be bypassing this cap if it were put in parallel.

EDIT: What is the 10n doing? It looks to be in parallel with the first tube stage? EQ? or maybe an error?

I'd guess it's be there for HT filtering, but it's not in the right place.

Note, there seems to be four versions of this mic, MD1, MD1A, MD1B and MD1B FET:

groovemd1binternals.l.jpg
 
I would guess the 220uf is the output coupling cap and the .1uf is like Jakob's linked schematic being a B+ bypass

It might be the MD2 microphone circuit that had the variable capsule voltage control IIRC.

The 220uf is rated at 25volts and R5 10meg, R6 1meg so 1/11 of the B+ is at the grid of the CF section looks like a design to get a "TUBE SOUND" with the CF closer to ground and bending the waveform more than a "proper" operating point.

I would make a first guess of 120VDC B+ like the linked schematic why make different power supply designs?

I don't think the first stage would have a cathode resistor bypass because you have voltage gain and then the CF and a 4:1 transformer you might not want the extra gain.

The input looks odd IIRC the capsule in the MD2 was a D.J. designed and built one.

10nf looks like it works with the first stage plate output resistance as a low pass
 
I need to look over this again. I looks like I have the transformer part upside down. But Jakob's schematic is missing some resistors a caps... strange I think

I Think I have the MD1a , Jakob's schematic would most likely be the MD1. It's address is Sylmar which is close to San Fernado but a different city I wonder why?

I believe the MD2 line was done in china by Alctran or 767. Not sure really. But I didn't see any extra PCBs at the shop for the FET version. Which makes me think that all the parts were left overs from before they moved production to China.

I will redraw later tonight. I try to scan the pcbs and upload those.

-Chuck
 
I was posting about the early MD2.

I have not found the GT brochures but I seem to remember a black body with a capsule voltage control mounted at the bottom by the DB9 with a D.J. designed and built capsule.

Jakob's linked schematic is a plate out to CF and the other one with the additional resistors and caps looks like it is redesigned to force the CF section bias operating points closer to ground causing more distortion as the level goes up.

How many versions of the GT MDs are there?

found this
http://www.soundonsound.com/sos/1995_articles/oct95/groovetubes.html
 
Good Article.

I see that it would seem that the MD series was all made here in LA and not shipped overseas as I thought?

I have a MD3 internal circuit that I got at the sale as well. I am going to try and draw that after I finish the MD1 circuit so we can see the difference.

I'll take some photos tonight.

Hey Jakob do you have any more MD schematics?

-Chuck
 
Ok here's attempt #2
The PCBs are marked
4-27-93 Rev2
and
3-15-93

Groove_tubes_source_v1_big.gif


Photos of the PCBs

pcb1_bot.jpg

pcb1_top.jpg

pcb2_bot.jpg

pcb2_top.jpg


Comments?

I don't think I will stick with this design. Probably just use the socket as it mounts nice to the posts. Maybe put the 1/2 - C24 circuit I drew inside and switch the transformer. The Pipe is nice though!

-Chuck
 
Ah ha ! Thanks Jakob!

It look like the parts I have are to MD2 from the schematics. But was converted into the MD3 hence the second 2G resistor and cap. I guess we are looking at the genesis of the MD3. I almost had it correct except the part connected to the capsule. I think I saw the 3rd PCB in the boxes but didn't think it went with these two.

I was drawing out my second bunch of parts last night and it looks like it is marked MDA-3. But the PCBs are dated 1997 and the layout is different. Also the parts are much nicer! Caps are all Mallory and Wimas. I may put a capsule and tube on this one to see how it sounds. But I think the transformer would need an upgrade. 


-Chuck
 
B+ of 120VDC is not low for a tube microphone.  Look at the voltages in older classics. 

Chuck

    I would try the stock design.  The transformer might work to your advantage with some capsules.
 
Chuck
I came across some Cinemag CM9711 transformers which are said to be similar
to the 9911 but a different turns ratio. Cinemag has all the info on these according
to Aspen. the CM9711 were for 12AX7 and CM9911's for 12AT7. The ratios you mentioned
previous on the CM9911 are correct.
 
Did anyone pick up any extra power supplies for md1's or md1a's etc? I have an original MD1 but have no power supply. If anyone needs me to take any pictures of the internals let me know!
 
Call Aspen directly they will sell you one

http://www.aspenandassoc.com/contact.html
 
maxwall said:
Call Aspen directly they will sell you one

http://www.aspenandassoc.com/contact.html

I did and they called me back and said that they didn't have any of the original power supplies left. I have found the power supplies for the newer GT tube mics, but they have different connectors and different amount of pins so I'm not sure if I could mod one of them to work or not. These MD1 uses a 9 pin D connector where as the newer GT tube mics use I believe a 7 pin round connector.
 

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