AKG Solidtube

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mikeyB

Well-known member
Joined
Apr 12, 2005
Messages
570
Location
Manchester UK
Hi, I seem to be in Mic repair season!!
Got an AKG Solidtube on the bench with guess what?.... a dead transformer!!
This is the 5th I've got to repair.
A replacement TX is around €90!!!! so looking for an alternative method. (Long gone the days when I could contact "The" Mic guy and get a TX for £25 ...... retired long ago).
Was looking at the thread re the schematic and the "mad" heater design.... 23Vac to drop to 12.6VDC.

I was thinking along the lines of a 15V or 18V SMPS feeding the heater reg and also a step up nixie type driver to get the HT required.
Has anyone experimented with this idea? - would save me re-inventing the wheel!!

Thanks in advance.....
 
Yeah Khron,
Mains TX is dead, new one €90!! So looking at an off shelf 15/18V for heater and a nixie step up circuit for HT (205V). Would only cost around €10!!
 
Transformer failed, they get too hot, varnish melts and windings short, ka-boom! fuse blow instantly.
Varistors nuked as well, so assuming primary, will measure when I get on it.
BTW - it is a 23Vac winding.
 
I have an AKG SolidTube mic.
I grabbed it off of Reverb being sold as "for parts". The seller didn't know if it worked.
It didn't come with the cable or a shock-mount. I had a cable made (as it is a 6 pin cable), hooked it up > turned it on > and nothing.
Fuse was blown. Replaced the Fuse and blew it again.
Now I see there are known issues with the Transformers and Varistors in these.
Anyone know someone who can get parts and replace them?

I'm a bit of a novice with a soldering iron. I've built guitar pedals and made cables but I haven't done much else.
 
AKG underestimated the load and undersized the transformer (I think solidtube and the c12vr uses the same power transformer) btw AKG made a replacement transformer but it is not available anymore as they are closed. I think a simple toroid can be made as substitution.
 
While the AKG solidtube is a real tube mic its not an LDC ,
the prepolarised capsule in it probably costs no more than 50 cents .

A broken Solidtube if you get it at the right price is still a great basis for a project , the circuit isnt all that different to several nice tube mics which have first stage anode loaded and cathode follower/transformer output .

The Nixie board I showed in the 'Battery Tube mic supply' post with both HT/LT rails seem like a good option for repairing a solidtube without the need for a special transformer. , It only supplies 6.3v to the heater so you would need to parrallel the tube filaments and possibly elevate the supply a bit so you remain within the limits of the heater/cathode volts of both sections . Some extra RF filtering of the rails might also be a good thing . And dont forget to allow some extra HT voltage to drop across your usual RC filter chains in the HT .
 
Tubetec said:
While the AKG solidtube is a real tube mic its not an LDC ,
the prepolarised capsule in it probably costs no more than 50 cents .

A broken Solidtube if you get it at the right price is still a great basis for a project , the circuit isnt all that different to several nice tube mics which have first stage anode loaded and cathode follower/transformer output .

The Nixie board I showed in the 'Battery Tube mic supply' post with both HT/LT rails seem like a good option for repairing a solidtube without the need for a special transformer. , It only supplies 6.3v to the heater so you would need to parrallel the tube filaments and possibly elevate the supply a bit so you remain within the limits of the heater/cathode volts of both sections . Some extra RF filtering of the rails might also be a good thing . And dont forget to allow some extra HT voltage to drop across your usual RC filter chains in the HT .
Click to expand...
Absurd statement.
LDC means Large Diameter Condenser; the capsule is well within the Large category, and it's a condenser. The fact that it's a pre-polarised condenser does not disqualify it at all. Also, the cost of a capsule has nothing to do with whether it's an LDC or not. More anti-electret snobbery; many fine mics are electrets - ever heard of DPA? A couple of AT's most highly-regarded mics have electret capsules.
 
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Solid tube schematic courtesy of Jakob .
I stand corrected , it does qualify as an LDC ,
the polarising voltage across the capsule in the solid tube is held by the zener at 9.1volts
Any references I found to the solidtube capsule point towards it being very similar to the modern 414 or C12 cv
more here
https://www.akg.com/on/demandware.s...efault/dw201dd21b/pdfs/solidtube_cutsheet.pdfgold sputtered mylar foil .
 

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It's a Large Diaphragm Condenser whether it has 'volts across it' or not.

Nobody ever said the (25mm electret) AT3035 wasn't an LDC.
 

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Agreed an LDC could be either electret or externally polarised , I havent found literature from AKG specifically saying one way or another if the capsule is electret either though .
 
Same capsule was used in the C3000B, where it receives a bit of 'leakage' from the circuitry, but I've used the capsule in DIY mics that supply no voltage to the capsule, and it works fine.
 
Did you ever try applying the usual 60 volts bias to one of these capsules ?
snap crackle and pop ,most likely .
 
Tubetec said:
Did you ever try applying the usual 60 volts bias to one of these capsules ?
snap crackle and pop ,most likely .
Click to expand...
I have, no issues. The applied voltage is in parallel to the pre polarized voltage.

You can even remove the film from the backplate by sanding it off, and turn the capsule into a regular, non electret one. Sometimes you can just turn the backplate around and use the side that's not pre polarized.

Take care if you disassemble Akg capsule. It uses screws internally to tune the response. You need a measurement setup to put it together properly.
 
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Cheers Kingkorg and R.Brown
I was thinking ,
could we have Groupdiy tshirts made up ,
GDIY
'Highly concentrated extract of the last 100 years in audio design'
 

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