AKG Solidtube

[rock soderstrom]

In a long ago article, Dave Royer used the Primo EM21 electret in a tube cathode follower mic that applied many 10s of volts of grid leakage to the capsule; no issues.

The schem is still the Jensen website: https://www.jensen-transformers.com/wp-content/uploads/2014/08/as076.pdf

Do you still have this article on your computer? I've been looking for it for a while.

 

[k brown]

I assume you mean "The Country Boy's Capacitor Microphone" article - no, it came up on another thread recently, and I searched my files and never found it.

My best recollection was that it was nearly identical to this one, but used the Primo EM-21 capsule, mounted on a handmade body.

 

[Tubetec]

Inspirational to me , Daves original tape op articles

 

[rock soderstrom]

I assume you mean "The Country Boy's Capacitor Microphone" article - no, it came up on another thread recently, and I searched my files and never found it

Yes, exactly this article I've been looking for for a while. I have also started the thread in question, so far without success.

 

[Tubetec]

Tape -op magazine Rock ,
https://tapeop.com/tutorials/by/david-royer/thats one of the articles and theres others
From humble beginnings to the toast of the industry ,Dave's the man .
We could offer him honorary lifetime membership here .
Maybe Pucho knows Dave .

 

[rock soderstrom]

Tape -op magazine Rock ,
https://tapeop.com/tutorials/by/david-royer/thats one of the articles and theres others
From humble beginnings to the toast of the industry ,Dave's the man .
We could offer him honorary lifetime membership here .
Maybe Pucho knows Dave .

Thanks, but I know all the Tape Op articles from Dave Royer. That's why I would have liked to read the original Glassaudio "The Country Boy's Capacitor Microphone" article. I guess the content won't be much different than the Tape Op article, I'm just curious. Probably this one is still copyright protected and can't be published even by the author.

 

[k brown]

Really a great guy - his profile is probably too high for him to do something like this now, but around 2000, he actually sent me (without my even asking) scans of a couple of schems for 6AK5-based tube mic preamps, since I'd inquired about their use as a cathode follower in mics.

6AK5s have very similar characteristics to the 5840 submini he uses a lot. I used his 6AK5 circuit to convert an Oktava 319 to tube operation. They can be quite microphonic, however.

 

[Tubetec]

I have an MKL 2500 I bought years ago , its good little mic , not overly sibilant in the top end ,same capsule as the 319 .

 

[cyrano]

MKL?

I've never heard about MKL. Can you enlighten me please?

 

[Tubetec]

https://www.soundonsound.com/reviews/oktava-mkl2500MKL is just the code Oktava use for their range of tube mics .

 

[cyrano]

Thx, Tubetec. I had searched a bit and came across the Oktava, but Goofle also suggested MXL

 

[Groutfinger]

Hi, looks like my circuit board is knackered on my old solid tube. Is there a way to get a replacement? Or a whole new power unit?

Regards, Mac

 

[mikeyB]

I need to fix this!

Tried a Nixie driver for HT but switch noise bleeds through the HT supply!

I just looked at a 12VA EI tx and it is too big to fit in the case. So I guess it must have been a 6VA model I bought years ago, it almost fits the mounting holes.

Heater requirements -
So with rectification multiply V x 1.414 and I x 0.707? So with a 12V winding I'd end up with 12x1.414 = 17V Subtract 1.2V for bridge drop = 15.8V and further 2.5V for regulation 13.26V. This should be ok for 12.6V. Current required is 150mA /0.707 = 212mA. Therefore 0.212Aac x 12Vac = 2.54VA so a 3VA winding should be good?

HT requirements - from schematic R10 drops 45V across 22k so that draws 2mA.
The schematic also 10mA draw through the 2 transistor stages in parallel T2 and T52.
I guess this is to ensure the Zener string (248V) works correctly?
270Vac specified. 10mA/0.707 = 14mA ac therefore VA rating for HT is 270x14mA = 3.78VA.

So I'm guessing it MUST have been a frame rated more than 6VA??

But the 6VA frame does seem the right size when comparing how it mounted (from memory)!!

I was thinking of trying to hack a 6VA 12 + 12 tx by removing a 12V winding and replacing with 270V winding.

Might be worth trying on test bench with back to back as done in gyrafs G9 but with dual 12V 6VA txs??

Any thoughts from the gurus would be much appreciated.
Thanks in advance.

 

[mikeyB]

Does the Mic need a stable HT or could I just bin off the zener string and CRCRC the output from back to back winding to the correct value? IE - Bridge CRCRC - 205V?
Also what is the purpose of the T3 and zener cct - is it a soft start for the HT combined with a failsafe if the heater fails??
Would some knowledgable person explain the operation of this to me/all interested, please?
Thanks again!!

 

[mikeyB]

Ok - so my client is desperate to get his Solidtube back to work!
I've made some progress with the little Nixie psu boards (for the HT) but still not happy with the switching noise. (It's low, but noticable in the micpre is cranked up. Need more time to play with screening and additional filtering.

But for now!......

The only replacement I could find a while back was from a European (Belguim) company. It was an original AKG part priced at €90 + shipping + taxes, bringing the total in at well over £100. This deterred me, hence the road to an alternative solution.
I've since found a UK replacement along with a pair of varistors which always go ("yes they do!") for the price of £62.62 inc VAT and shipping, this is much more bearable ( I hope!).

Here's what I found when I received the transformer:


Dead AKG


Replacement EMS Tx


EMS underbelly (mmmm!)


EMS smaller casing!!



Replacement Varistor (pair provided)


Difference in transformers:
First thing/s I noticed was a difference in size and weight - The EMS was less in both at about 6mm shorter in height

and


Weight approx. AKG 350g compared to EMS 280g

The appearance of the bottom of the EMS appears to have the old casing - certainly the same brown colour? Is it a refurbished AKG transformer??? I will be asking EMS!!

The AKG seems to measure ok for DC winding resistance - probably the varnish/insulation fails when powered up.

AKG - each half primary measures 71R and 81R - full primary measures 140R.

secondaries: Heater winding 7R, HT winding 1.05K (1K05)

No shorts between any windings.



EMS - each half primary measures 174R and 326R - full primary measures 500R.

secondaries: Heater winding 12.2R, HT winding 1.822K (1K822)

No shorts between any windings.

This (to me) indicates the use of thinner/lighter windings for the EMS replacement, probably resulting in a smaller volume, hence the smaller casing???

Please comment!!

I'll be contacting EMS to ask if it is a refurb product - I was told it was new, and made to the exact same specs as the AKG transformer. If the AKG Tx is renowned to fail for being under speck then, this "lighter" specked EMS is more likely to fail than the AKG!!

I'm just wondering if I should go ahead with this replacement or return the tx and go back down the alternative route!

Any comments and advice greatly appreciated. Once again, thanks in advance!!

 

[rock soderstrom]

The AKG seems to measure ok for DC winding resistance - probably the varnish/insulation fails when powered up.

Any comments and advice greatly appreciated. Once again, thanks in advance!!

Are you sure the original transformer is dead? Have you measured it outside the circuit? What voltages does it have on the 2 secondary windings?

I've made some progress with the little Nixie psu boards (for the HT) but still not happy with the switching noise. (It's low, but noticable in the micpre is cranked up. Need more time to play with screening and additional filtering.

I would rather go a conventional route. My Nixie PSU experiments for sensitive amplifier stages were not particularly successful. At the end of the day, the microphone doesn't care how its clean and stable voltages are created. But the emphasis is clearly on clean, even a classic, unregulated CRCRC will be good enough.

 

[mikeyB]

Are you sure the original transformer is dead? Have you measured it outside the circuit? What voltages does it have on the 2 secondary windings?


I would rather go a conventional route. My Nixie PSU experiments for sensitive amplifier stages were not particularly successful. At the end of the day, the microphone doesn't care how its clean and stable voltages are created. But the emphasis is clearly on clean, even a classic, unregulated CRCRC will be good enough.

Yeah, it's popping fuses! I think the winding insulation is failing when voltage is applied, I'll try bringing it up slowly on the variac monitoring the current with a keen eye and one hand on the power switch!

 

[terry setter]

I fixed one of these about a year ago. The culprit was the varistors. They had gone bad. Doing some research, it seems they get old and start shorting out. Often that takes the transformer with them (which is probably why your transformer was sent with two new varistors). But in the case I worked on, just pulling the varistors out - FOREVER - stopped the fuse blowing and the transformer was still alive. The varistors, I'm told, are there to ensure against over-voltage on cold startups. Not too many of the great mics in history have varistors in them, so I put it back together without the varistors and it's worked fine ever since. I hope you get off as luckily. By the way, the transformer in the SolidTube runs REALLY hot, which contributes to the failure of various parts in the supply.

 

[mikeyB]

I fixed one of these about a year ago. The culprit was the varistors. They had gone bad. Doing some research, it seems they get old and start shorting out. Often that takes the transformer with them (which is probably why your transformer was sent with two new varistors). But in the case I worked on, just pulling the varistors out - FOREVER - stopped the fuse blowing and the transformer was still alive. The varistors, I'm told, are there to ensure against over-voltage on cold startups. Not too many of the great mics in history have varistors in them, so I put it back together without the varistors and it's worked fine ever since. I hope you get off as luckily. By the way, the transformer in the SolidTube runs REALLY hot, which contributes to the failure of various parts in the supply.

Thanks for the reply Terry, yeah I have to get this back in use so I probably will go without the varistors, I have lots of Varistor related repairs at the moment, including my own Mackie 1640i mixer supply!!

 

[mikeyB]

Oh - and by the way, I used to get a replacement EI clamp frame Tx from a UK mic guru who has long retired. It only cost £30 and I had about 10 units I repaired, Wish I'd bought a couple as spares. He told me to do away with the varistors also!!