Oktava MKL2500 schematic

[zebra50]

Spent an hour last night with an Oktava tube mic. This is what I got.....

http://www.omnipressor.com/other/othermicpix/MKL2500schem.jpg

Looks odd to me - did I screw up? Certainly different from amny of the tube mic schematics kicking around. The transformer has to take some DC.

The tube heater supply is at 4.5 V which is serious underheating :shock:
. Should be 6.3V for normal heating. You can adjust the heater at the PSU.

The PSU provides 135V unloaded, 120V with the mic connected.

Stew

[EDIT: Schematic updated]

 

[sismofyt]

I think you screwed up :green: Underheating have some effect on sonics and specs. It's being talked about at Klaus' forum.

Why don't you email Taylor at the Soundroom? He may be willing to give you a schematic :thumb:

 

[Gus]

Looks interesting. What size is the transformer. A number of things could be part of the design. If the transformer is large and has a good amount of Henrys it could a kind of choke load. If I understand choke loading it "fixes" the tube to a a part of its curves.

It kind of reminds me of the mc012 schematic CF with a load/ SF with an active load.

Looking at that you need to think DC bias and AC operation. The caps "drop out" at DC but at AC the cathode is "more connected" to the transformer.

Is the transformer gapped for DC?

Need to look for russian tube specs maybe the tube has a lower voltage heater. Marik would be the person to find the tube specs.

I thought the 2500 had a solid state output section?

 

[BradAvenson]

http://www.mif.pg.gda.pl/homepages/frank/sheets/112/6/6Z1P.pdf

Nope, looks like its underheated. It could be unintentional.

 

[zebra50]

Hi Gus,

I checked this pretty carefully as I thought it looked strange and it wasn't what I was expecting to see. I think it is mostly correct but maybe I missed a connection somewhere.

There's no solid state section in the mic. I didn't check the PSU yet but the pins from the transformer are in continuity with the 3 pin XLR +/- so I doubt there is anything funny going on there.

I think a 'choke load' is right.
The transformer is not large, but a little bit bigger than that in the 219/319s.

I have looked the data sheet (it is at Frank's tube data site) and it should be 6.3V so it is very underheated.

 

[sismofyt]

Yeah, but the VF14 in a U47 is way underheated too, like 36V instead of the 55/60 in the spec sheet. That's done 100% on purpose.

Could also just be an Oktava fuckup I seem to remember this mic was developed in the US as a response to all the other cheap tube mics. IT's not really a russian design. Don't quote me on that though..

I'd MUCH rather get my hands on the other tube mics Taylor is selling. They look very tasty!

 

[microx]

No polarizing voltage on the capsule, is it electret. Apart from that looks OK. The DC in the tx would be small.
Steve

 

[zebra50]

No polarizing voltage on the capsule, is it electret.

Hi Steve
Look again, R1 polarises the capsule at about 64V. It's a 'proper' capsule similar to those in the other LD oktavas.
Stewart

 

[Gus]

The math does not work out right for the resistor values. I get .15ma in the 6.5K bias setting resistor (65v-64v)/6.5K

In a series circuit the current should be the same. -1 (65-64) volt bias makes sense so I believe the 62K is a 6200 ohm 64/6200 = .10 ma not the same one or both of the resistors could be off from the marked value.

Things I would try replacing both of the electros with good ones pansonic nichicon elna etc. I would raise the heater supply above ground and filter it better, also I would try the heater at 5.4V

Its a choke loaded CF with about 64V on the capsule

 

[microx]

Yeah I see it now. It's not as daft as it looks.Drawing it from the cathode uses the valve as a second rectifier/stabilizer.
I would lay money that the guy who designed it is old school from way back, it's typical "old" thinking, I like it.
The "underheating" can go down lowe than you would think, just play with the voltage and watch the noise to find the optimum.
Steve

 

[Gus]

Another thing can you test the transformers ratio? I would guess it is 2:1 to maybe a 4:1. In the two 219s of mine the transformers are marked for ratio in pencil 1.6:1.

I am thinking if the ratio is high enought >5:1 you could use a 100k plate load cathode biased with bypass cap. One reason I am thinking this is I don't like the Oktava 219/319 capsule it is "dull" with clean electronics. think ela M251 circuit at 120V with the ac701 schematic make the grid to ground 1 gig use the russian tube. Play with the bias point buy trying different cathode Rs keep the bias at -1 or lower (-1.2 -1.4) etc. This is something I have been wanting to do with a switch on some tube microphones.

The bias point affect the harmonics. That capsule if it is the same as in the 219/319 it needs all the help it can get!

I am not even sure what design it is based from there seems to be to much open area in the backplate.

 

[zebra50]

I'll check the values again tonight, but I have 'Blue-red-orange-gold' written down which I make 62K? (Put me right if I'm wrong!). I only wrote down the voltages to 2 sig figs - I'll try to get more accurate numbers.

Caps look pretty crappy. I have some audio grade caps coming - I'll let you know the difference.

Heater is on a regulator with a trim pot to adjust, which is nice.

stewart

 

[Gus]

Stewart

Looking again 64V/62K is just a little over 1ma.
1ma makes more sense than .1ma.
most tube microphone circuit fall between .4 and 1ma
1ma and 1V would make the bias R 1K?
Color code does give 62K.

 

[zebra50]

Gus, I agree with your maths. I didn't make a note of the code for the bias resistor so I'll double check that & the xformer tonight.

 

[Marik]

Stewart,

Thanks for posting the schema. It is funny--it's topology looks exactly like one half of my PP CF output amplifier on 6550...

I talked to Elisabeth Papapetrou of Sound-Room (it was about a week before she passed away) and asked her about this mic. She told me that they don't carry it (at least at that time) because of chipo PSU, which limits it's performance. Please let us know what's going on with that.

If you don't mind spending some money, I would suggest replacing electro caps with Black Gates.

6Ж1П Western equialents are EF95 and 6AK5, FWIW.

 

[adrianh]

Yuck cathode follower.
But an interesting one.
Stew thanks for the schematic.

Under heating will raise the input impedance
of the Pentode (wired as triode) and reduce
grid current.
Got to try this in my 319.

 

[Marik]

[quote author="adrianh"]Yuck cathode follower.
But an interesting one.
Stew thanks for the schematic.

Under heating will raise the input impedance
of the Pentode (wired as triode) and reduce
grid current.
Got to try this in my 319.[/quote]

Hey Adrian,

You are just a CF hater :evil: :grin:
Yeah, I know, but transformer coupled CF sounds different. I've discussed it with Kevin a few times, and he seems to agree with that.

 

[zebra50]

Cathode followers can be fun! I've built a couple of CF mics and have enjoyed them.

Right then...

Gus was bang on with his calculations for the cathode/bias resistor: it is 1k (Brown blk orange gold). I must have been tired last night.

Voltages tonight were
126V on plate (6V higher than last night!)*
67.85V at cathode
66.63V at top of the transformer
66.36V at bottom of transformer.
(*I think out local supply is fluctuating a lot at the moment - something to do with the football european cup matches!).
So about .3V DC dropped over the xformer

PSU has LM317 for heater and adjustable B+.

Someone has sanded the top of the xformer to remove the ratio (this has been written in pencil on the other oktavas I have seen). Possibly the UK importer being paranoid. The PSU has the UK importer's name written all over it (I won't give them advertising space here!).


And now for the pictures....

 

[adrianh]

OK Marik you sold me. But I am not changing my Avatar.
I am going to try it.
Any of you tried it or have a guess on the sonics let me know.
I have 1 319 and 5 219 mics.
Relax gus I got them cheap.
I do need them to sound good.
Hope I can use the 219 kind of like a Sennheiser MD421.
Yea I know a long stretch but I can try.

I had discussed with Gus about using the 6AK5
so wow they did it. They are dirt cheap, small
and unless it sounds real bad I am going with it.
With the DC through the primary this may be an issue.
I was going to use amphorous core Lundahls.

 

[Gus]

The last picture is a good shot of the strange capsule design.