Funkwerk Leipzig MVT 4050 T tube mic preamp

[adamasnan]

Hello everyone, I just took a chance on a couple of old DDR Funkwerk Leipzig MVT 4050 T tube mic amps from the 1950s. This 'T' variant is for dynamic mics, with an IP impedance of 200 ohm, whereas the standard 4050 and 'M' variants are for crystal mics with an IP impedance of 1M Ohm. I know nothing about tube amps really, but took a chance in the hope they can give me an extra colour in amongst all my transistor based mic amps.

Has anyone come across these before? Does anyone have any guesses/opinions as to how they might sound by looking at the schematic (attached, non 'T' variant)? Sorry to ask, but perhaps some can determine such things from having prior experience with a similar circuit..

Should I not get my hopes up too high? I found an old listing for one saying that Martin Eich had done a full service on it, I wonder if anyone knows who that might be referring to?

I have sent pictures to Andreas Stenzel here in Germany and he said they can be good preamps, but I am gathering any extra info I can before I have restoration underway.

I have not received them yet, but will take comprehensive photos once received, if that helps?

Thanks for your insights!

Adam

 

[adamasnan]

Can anyone please help? It's tube!!

 

[shabtek]

unclear what you need help with.
never seen these before but they can probably be made to amplify a signal cleanly, assuming parts are functional.

 

[adamasnan]

Hi, thanks - yes, this is already some insight. I think I ma asking mostly what those people with greater experience think of this circuit, and yes - whether these can amplify a signal cleanly. Actually, more specifically, is there anything to suggest that there might be a good low end response?

I have also more information attached below:

Not sure how to equate "200/400 fach" (200/400 fold) as a db level.. but that is VERY high.

Thanks again!

 

[rock soderstrom]

Hi, thanks - yes, this is already some insight. I think I ma asking mostly what those people with greater experience think of this circuit, and yes - whether these can amplify a signal cleanly. Actually, more specifically, is there anything to suggest that there might be a good low end response?

I have also more information attached below:

Not sure how to equate "200/400 fach" (200/400 fold) as a db level.. but that is VERY high.

Thanks again!

Hi Adam,

I don´t know this mic pre but I think this could be a really cool sounding tool in your studio.
You have asked for extra colour and this amp will deliever.

Unlike the West German counterparts with Ef12K tubes, this amp has no negative feedback, which is not necessarily worse in your case.

Several options are available. I think basically you should first get the amp back to the start and see if you like what you hear.

Then you can implement some useful mods.
I would upgrade the power supply with more capacity, I would throw out all the electrolytic capacitors and replace them with tasty f&t caps.

With the coupling and cathode capacitors you can adjust the bass behavior. Maybe make them switchable as well.

I would give the amp an additional DI input. This is done very quickly and offers many more uses.

Cool would also be an adjustable gain and/ or as needed a switchable input PAD.

You can also keep the core unit functionally original and only fix the necessary things. Your decision, I would probably favor the mods to make the part fully suitable for modern studio use.

I think this project can sound very good with a little love.

What did you pay for these two mic pres?

PS: hunt down some NOS Telefunken EF12k for V1 position and compare them to the RFTs for better SNR.

 

[rock soderstrom]

but will take comprehensive photos once received

That would be nice. I am always interested in some good gut shots from old gear.

 

[adamasnan]

Hi Adam,

I don´t know this mic pre but I think this could be a really cool sounding tool in your studio.
You have asked for extra colour and this amp will deliever.

Unlike the West German counterparts with Ef12K tubes, this amp has no negative feedback, which is not necessarily worse in your case.

Several options are available. I think basically you should first get the amp back to the start and see if you like what you hear.

Then you can implement some useful mods.
I would upgrade the power supply with more capacity, I would throw out all the electrolytic capacitors and replace them with tasty f&t caps.

With the coupling and cathode capacitors you can adjust the bass behavior. Maybe make them switchable as well.

I would give the amp an additional DI input. This is done very quickly and offers many more uses.

Cool would also be an adjustable gain and/ or as needed a switchable input PAD.

You can also keep the core unit functionally original and only fix the necessary things. Your decision, I would probably favor the mods to make the part fully suitable for modern studio use.

I think the project can sound very good with a little love.

What did you pay for these two mic pres?

PS: hunt down some NOS Telefunken EF12k for V1 position and compare them to the RFTs for better SNR.

Wow great suggestions, thanks so much!

I will send them to a tech to undertake these things, but yes all of these mods sound good! I think the absolute first essentials would be an IP attenuator and an OP attenuator (passive balanced fader). But yes the rest should be done too.

I would love to keep them also looking like this, so hopefully some of the revisions can be implemented into the housing as is (with some extra panels to conceal the sides.

When you mention DI, do you mean to bypass the IP transformer? I am not entirely certain if the T version has a slight difference on the IP of the schematic after the transformer, I would assume so?

I got the pair for €220. I was too curious.

I'll be on the look out for west german EF12K's but also will get some extra RFT EF12K's too.

Thanks again!

 

[rock soderstrom]

Not sure how to equate "200/400 fach" (200/400 fold) as a db level.. but that is VERY high.

46dB/52dB gain, this is not "Very high" quite normal for a two stage mic pre without NFB.
I am not sure if you will need an OP attenuator.

You will find gain/dB calculator here:
http://www.sengpielaudio.com/Rechner-db.htm

When you mention DI, do you mean to bypass the IP transformer? I am not entirely certain if the T version has a slight difference on the IP of the schematic after the transformer, I would assume so?

Yes, DI for instruments bypasses the input transformer (and its zobel network and resistive termination) for a high impedance input. Easy job, your tech will know how do that.

I got the pair for €220. I was too curious.

The price is very good for a pair.

I'll be on the look out for west german EF12K's but also will get some extra RFT EF12K's too.

The Telefunken Ef12k are considered superior to the RFTs. For the first stage in mic pres a good investment from my point of view. The GDR had quality problems at the time due to material shortages. I once made a (completely non-scientific) comparison in this position, the Telefunkens were noticeably quieter.

 

[adamasnan]

Just received! One has had a quick clean.. they are VERY dusty on the top, but actually pretty good condition on the underside..

Curiously came with a pair of 600ohm Siemens transformers, that were installed in the preamp > speaker amp enclosure..

 

[rock soderstrom]

Just received! One has had a quick clean.. they are VERY dusty on the top, but actually pretty good condition on the underside..

Very nice! Thanks for the photos. It would be cool to have an overview photo from below that has better resolution and sharpness. Then we can also read the labels of the components.

Overall your amps seem to be in pretty original condition. The Siemens transformers are an add-on (directly from the BRD "Klassenfeind") to make the amps compatible with 600 ohm lines. I found the original ebay kleinanzeigen anounce. Apparently your amps were in use as PA preamps.

As far as I can see, the input transformer is in the (mu metal?) box on top of the chassis, the output transformer can be seen from below.

Post a better overview photo and I (and maybe a few other interested user) can trace the setup and help you to understand these beautiful amps a bit.

 

[Tubetec]

You have three 4 uf electrolytic caps in the HT supply , these are best replaced , however if you change the value of the first filter cap to a higher value the HT voltage will rise appreciably , so Id use a 4uf metalised foil 250 volt motor run cap(theres a good choice of mounting styles ,box ,can etc ) as first filter then add a couple of large electrolytics after the dropper resistors . You could also consider putting a modern silicon diode in place of whats there now or even replacing with a bridge , although voltage dropping resistors might need adjustment in that case.
Once you have attended to the psu and checked for any other loose wires or bad solder connections Id chance powering them up , make sure you add the mains earth connection bolted down to chassis or at least soldered to a ground tag adjacent to the power transformer .

Check the HT and make sure the coupling caps arent leaky .

The units you show have balanced input via step up transformer , 200 ohm , which is a bit lower than expected these days , it should still work well enough with dynamic mics .
output transformer gives the option of two ratio's 200/800 ohm ,

Its the output transformer on top of the chassis in the mumetal box , the input is below deck

 

[rock soderstrom]

Its the output transformer on top of the chassis in the mumetal box , the input is below deck

Are you sure? It's possible, but isn't it a little unusual to run the input transformer without a shield and to protect the output transformer with a Mu metal box instead?

I need to take a closer look, I can't really see it in the photos at the moment.

 

[rock soderstrom]

I was right, the transformer in the "mu metal" cage is the input transformer, which makes more sense. It looks good, quite big, I'm curious to see what it can do. I just bought an MVT 4050T in original condition, I will test it properly and post it here.

 

[Cqwet Dbdfte]

Are you going to restore it to museum condition to look original, or upgrade the parts to make it perform as good as possible?
Old parts have issues, insulation qualities were weak, like wiring, paper caps, electrolytic caps, etc.
The power transformer looks to have some "flux band".
I just went thru a 1950's HP200ABR, tone generator, exorcising every single cap, all tubes were OK except one.
Ahhh that funky smell!

 

[rock soderstrom]

Are you going to restore it to museum condition to look original, or upgrade the parts to make it perform as good as possible?

That's a good question. At the moment I'm inspecting the unit, it's in very good condition visually. I think the amp was rarely used, it came in the original box.

Old parts have issues, insulation qualities were weak, like wiring, paper caps, electrolytic caps, etc.

That's right, the electrolytic capacitors here are leaking and definitely need to be replaced.

I just went thru a 1950's HP200ABR, tone generator, exorcising every single cap, all tubes were OK except one.

Photos? Project info online?

Ahhh that funky smell!

OMG, when I opened the unit I was met with a biting odour that still lingers. I'm not sensitive but this is really extreme...

I have a good idea what I could do with the amp, I'll document it in a separate thread, but first I want to get the preamp to play in its original form. Stay tuned.

 

[rock soderstrom]

A little guessing game, what kind of component is this? I was really surprised...

 

[Cqwet Dbdfte]

Looks like a crapacitor...

 

[lukas]

...input transformer looks like UI30 lamination used.

 

[Cqwet Dbdfte]

Measure the mystery part. Buy an impedance meter.
https://www.ebay.com/p/20032261934

 

[rock soderstrom]

Looks like a crapacitor...

I wholeheartedly agree. The material of the shell is the same as that of the neighbouring PSU electrolytic capacitors.

Measure the mystery part. Buy an impedance meter.

Of course I did this (with two measuring devices), the result was as follows. To be sure, I removed the component, then the labelling was also legible, unfortunately it does not match the result tenfold.

Okay, the capacitor is shot like the others or just mislabelled, what's your problem Rocky?

My problem is that the capacitor is in the exact place of the rectifier diode, otherwise there are no diodes in the whole unit. I have traced the whole circuit and drawn this preliminary schematic. It differs in a few places from the MVT4050M model, for example one RC filter in the PSU is missing or my amp has a bit NFB.


Could it be that this "capacitor" is a combination type, with a parallel rectifier diode that is dead? That would explain a lot, I'm about to cut this thing open...

What do you think?

Edit: The capacitor is polarised, quite unusual for a 0.01uF type. It also has this marking on the anode side:

No max. voltage markings...