AKG solidtube

Discussion in 'Electronic Repair Modifications DIY' started by gus, Jul 6, 2003.

  1. gus

    gus Guest

    Anyone have a schematic of the solid tube? or know the transformer ratio?

    I can trace it but I thought I would ask.

    I bought one on closeout today. Stock out of the box did not sound that great. I pulled the foam inside the grill and replaced the 12ax7 with a GE 6072A, that helped alot. The stock output level makes me think the transformer might be about 10:1 or 4:1 if they use a CF. If I change any other parts I will take pictures.

    12ax7 in a microphone does not make sense to me. I am also suprised in how cheap the power supply case looks and feels as well as the plastic frame the insides are mounted to. The list price is about $1,300.00.
  2. scenaria

    scenaria Guest

    I would like to hear about the results you get with the changes to this mic.

    I have one in my closet that was used ONCE and since then has sat there worthless..
  3. Jbuntz

    Jbuntz Guest

    Forgive me for being stupid, but why is a 12ax7 bad in a mic? Lack of flat response?
  4. gyraf

    gyraf Guest


    Are you sure that there's a transformer in there at all? I've never had one apart, but judging from it's sound the output transformer has been "rationalized" away. My guess is a cheap solid-state balanced output stage.

    Adding a (decent) transformer could do wonders here. (If I'm right that there's no transformer in it as is)

    Jakob E.
  5. buttercup

    buttercup Guest

    It does have a transformer in it and it is the same one used in the C12VR.
    I scabbed a circuit diagram from AKG service a while ago who didn't seem to have the same problem
    that many people have with keeping things under wraps.
    Or perhaps I put on a voice and pretended to be another service engineer - I can't quite remember but I did pick up some handy persuasion skills from somewhere.
    I actually have one of these tx's and it's a twin bobbin thing with 2x input windings and 2x output
    windings. The blurb on the C12VR sheet talks about the large core and less distortion in the bass end of things but it is obviously cheap production line stuff like the capsules.
    Now I think of it, perhaps if it sounds rubbish I might not use it.
    Bloody fifty quid too!!!!!!
  6. gyraf

    gyraf Guest

    Is there any chance we could see that schematic?

    I know a couple of people that would be happy to know if there was upgrade possibilities in this one..

    Jakob E.
  7. buttercup

    buttercup Guest

    It would be a pleasure to get the schem scanned-
    in for those who are interested.
    Er......as soon as I can find it!
    I made 2 copies.
    One is at home and the other may be here at work
    but if not I shall whizz it over to you tomorrow.
  8. gyraf

    gyraf Guest

    Thanks Buttercup,

    I really appreciate that..

    Jakob E.
  9. buttercup

    buttercup Guest

    I have just emailed the thing to Jakob who definitely will have more space than I do to host
    things at the moment as I have an almost free but jampacked tiny website.
    Interesting reading though.

  10. gus

    gus Guest


    I have a 3.3 megpic sony with a good lens. I can take picture if you want. I have had it apart the transformer is built like stated above it Has more core mass than the china built microphones I have been in it looks like it might be about the same size as the transformer you use in the G7. I did not see any film caps just electros. Some of the electros are nichicon.

    Pulling the foam inside and changing the tube to a GE6072a helped alot. A few cap upgrades or even a circuit change using a ef86(if the transformer is >= 5:1)

    I don't have a web site:however I will take pictures of he microphone in the different sets of the mod.

    12ax7 have to much voltage gain IMO for a microphone circuit not "musical". It looks like this microphone uses a CF to the transformer I think this is because the Rp is to high in the 12ax7. To my ears med mu low Rp tube sound good in microphones also the construction grid leakage current etc seem to affect the "sound"
  11. gyraf

    gyraf Guest

    Sorry to disappoint you. This one is basically not upgradable to pro standards - the capsule used is an electret type!


    .. I could'nt understand why they would sink the rather high HT voltage all the vay down to 9V for polarisation voltage.. Yes, there is a 9V1 zener there, fed from HT. But in the components list (not included) all became clearer, it says:

    "Part no. 2512Z003 - electret capsule"

    So yes, upgrading the tube to 6072 - or maybe 12AT7/ECC81 - as well as changing the electrolytics into polyester will help. But I fear that it will never get close to even a cheap chinese "real" condenser tube microphone.

    That is a sad design - regarding the fact that AKG have access to the decent CK12 capsule at manufacturing cost. And CK12 is machine-made, so cost won't be that high.

    Looks to me like AKG here has has made a prosumer microphone, that they can be sure won't ever compete with their "real" ones. Harman International Inc. has struck again...
    Jakob E.

    EDIT: What would happen if we just cut the zener and the associated 16V electrolytic, and inserted a 1nF polypropylene cap between the capsule and the tube grid, and a second 1G resistor from grid to gnd.? Would the electrect capsule collapse, exposed to the high polarisation voltage, or would it be possible to use an electrect as a real-condenser? I mean, there is a conducting membrane, and there is a backplate.. all we need for a real-condenser.. Or would it just collapse and burn? Anyone?
  12. gus

    gus Guest


    I believe the electret is applied to the backplate. I read in an AES book electret films do not make good diaphrams. Inside my microphone I don't have an LED that I can see, unless it is in the tube socket, the tube glows red thur the red plastic AKG window?? My PCB is marked 2800z1201 akg soprint9.brd.

    I wonder why 9.1 volts is being added? or subtracted? to the charge of the electret. The 9.1v goes to the white wire to the backplate the diaphram goes to ground via the green wire.

    Jakob the capsule does look like the one used in the c300b IIRC.

    Is there any specs on the transformer?

    What was AKG thinking building this????? It looks like a toy. I think I am going to ask if I can return the microphone.

    Is this a copy of the Groove tubes MD1 electret microphone?
  13. MPlancke

    MPlancke Member

    Mar 27, 2001
    I've had a Solidtube here at the studio since it was released way back when. I pulled the internal foam pop filter and put a Telefunken 12AT7 smooth plate in there. Made a big difference, it has been my front of kick microphone for as long as I can remember and works great in that application.

    I paid around $700 for mine brand new and it is definitely targeted at the project studio market. I suspose that there are worse things I could stick in front of a kick drum. ;)

  14. MPlancke

    MPlancke Member

    Mar 27, 2001
    Yeah, but how does it sound?

    Ultimately that should be the deciding factor shouldn't it?

  15. Shalimar

    Shalimar Guest

    I think it utter blows. No clarity, freq (or freak) response is hills'n'valleys. No detail, murky. Does have a 'sound' of course. Just never had the need for that. I wouldn't bother modding it even. Not even if I got one for free.

    Sorry, that's a pos, imo.

    It's clearly been designed to get home reccers to buy a tube mic. I think it was the first affordable tubemic, btw. This garbage mic have never been designed with a great sound in mind. If they had, they'd yanked out the tube and done a proper capsule.

    Too lame.
  16. gus

    gus Guest


    It does not sound that great to me I hope it is the electronics and not the capsule. If it was about $200.OO it would be OK I payed about $600 for the solidtube I bought it because I did not have an microphone with a CK12 type capsule. I think it was in the store from 1998.
  17. MPlancke

    MPlancke Member

    Mar 27, 2001
    Keep an eye out for some 414EB Silver bodies, they are multi-patterned, have a real CK12 capsule in them, unless they have been replaced and can be had for pretty low dough. I've got a pair here at the studio and I use them all the time. AKG stopped making good sounding CK12 capsules when they switched from a brass capsule, used in the sought after CK12's to the teflon CK12-like capsule used in their VR, The Tube and in the current 414TL derivatives.

  18. hermitsonic

    hermitsonic Active Member

    Jan 1, 2008
    Gus, I have a few questions regarding the solidtube mods you mentioned. You said you pulled out the foam from the inside of the basket. I opened mine up last week and could not figure out how to separate the basket from the lower part of the mic and didn't want to force it and damage anything. Step by step how did you get the foam out. I have a telefunken tube in mine but still think the mic sounds less then stellar, it has an edgy and unappealing mid range, seems compressed in certain frequencies and generally just doesn't sound all that great. Mine is in absolute like new condition as I have only use it maybe a half dozen times over the years. I have had it at least 15 years, bought it new and wish I would have bought something else instead. Hope you see this replay and respond. Thanks
Similar Threads
  1. grant

Share This Page