Shoud I replace the EF12 or keep them in this micpre ?

Hi,

Quite a long time ago I baught a mic preamp (for kristall mics, but I think I wanna try to adapt it so it can handle dynamic microphones) and I just used help from a friend to repair it. It's a very old deseign from the 50ies, based around 2 EF12 tubes.

I was wondering if, to get it to better specs and try to squeeze its best juice, I should replace the EF12 with any other pentodes (such as a EF804s, for example).

Here's the shematic :


Thanks a lot,

Sam.

No, I wouldn´t replace the tube type. If they are weak, get another pair of EF12. Juts try a good qualitry input transformer at the input and you should be able to use dynamic and modern condenser mics OK, and actually results could be quite impressive. In old tube preamps I would replace the electrolytic caps also...

Wow Check out that power supply!
That's the first half wave circuit I have ever seen.
That's a real relic.
If your broke, take that output x-former and slap it on the input, with the 600 ohm side hooked to an xlr jack.
Then just use the cap and ground as your output signal.
But inspect the leads on the transformer first. If it looks like thay are going to turn into powder when you touch them, you best leave everything alone.

Yes to the filter cap replacement.

Thanks a lot for those answer !

So, I might try to find NOS EF12 !
For the caps to be changed, yes ... I think it's a must to be done ... But I really don't know if I'm gonna be able to find such good looking caps :love'em:

If I swap the input tranny and replace it with the OP tranny wired opposedly, I did'nt understant where I should get the exits from ?

Something that's wierd though, near the input, just after (if I remember well) the first EF12, if I remember, there is a component that really looks like an input tranny ... wierd that I can't source it on the shem (without the pre near me, I left it at a friends so he could help me working at this) ...

We should have some pictures made to help this going !

Anyway, that's a real beauty !!

Take your output directly off the 0.25 output cap. Make sure it isn't leaky, you don't want any B+ on the next piece of gear in line.

Thanks !

For the input tranny, I'll have to take pictures ... That looks like someone had that same idea back in the 60ies and did his little improovment by its own (there is something inside that looks like an input tranny and it's not on the shems ... so ... )

Do you have any ideas of what chemical caps works best with old tube gear ? Elna's caps maybe ?

Those values are so low that you might consider using non lytic caps. Especiall on the 1 and 2 uf.

You might want to replace the single diode with a bridge, I don't think the voltage would go up to a dangerous level as the current draw is probably pretty low? I don't have a spec sheet on the EF-12.

But then, if you want to keep the original sound, any old lytic cap out of a Mouser or Digi-Key catalog will work.

Is the output transformer an open frame model or is it in a can? The most important job of the input transformer is to cut down on hum. If un-shielded, you might want to rig up a DIY can out of something.

I'm not sure that I understand the difference between the two, but if yes, it's open frame !

And yes, input tranny to keep hum out of it ... The one that seems to be an input tranny is in a can ...

For the caps, I could swap the electrolytics for other ?? I didn't knew that ... I'll see, but as I'm making a huge part order, I migh try and benefit from the low prices to get a good set of caps to try inside this mic pre !! :grin:

For the bridge, that's an excelent idea !

Whowho, I can't wait to clean this oldie back to hear it !

Anyway, thanks for your help guys !

If anybody's got something else to say about this, I'd appreciate

Peace,
Sam.

Ho, and does anybody have technical information about the EF12 ? a datasheet maybe ? serched a long time on google, but could'nt get any revealing results ...

http://tdsl.duncanamps.com/

http://frank.pocnet.net/

specifically:

http://tdsl.duncanamps.com/pdf/vm079.pdf

http://frank.pocnet.net/sheets/076/e/EF12.pdf

http://frank.pocnet.net/sheets/030/e/EF12.pdf

How's your Deustch?

http://frank.pocnet.net/sheets/082/e/EF12.pdf

We need a Tube Data Sheet Meta.

Anybody?

You could try changing those 4uF caps at the filters for some film types... With all this resistance going on on the filters, the secondary of the transformer might be pretty high voltage, so try 4uF/630v or 2x 2.2uF/630v in parallel. It should make a big diference. Go polypropylene if possible.

Wow, you guys are both wonderfull !!!!

Thanks so much ! (and those tube links are saved to my fav !! )

So, changing the filter caps to poly Hi voltage ones, swaping the chemical caps for new ones ...
See what to do at the input ...

Nice programm !

Thanks guys !

Peace,
Sam, happy DIYer !

Ho ho ... as expected, I'll have trouble to get those 4uF in very high voltage in europe ...

I found these on eBay, they might fit with the retro look of the preamp, but don't know if they'll fit inside the box :grin:

http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&category=4662&item=7524553663&rd=1&ssPageName=WDVW

Yes, might be a good try, but I would still try some 2.2uF polyprop in parallel instead. What gets my attention is that this circuit has no high value caps. You could also try some film caps at the cathode bypass caps. If you can´t get a 22uF low voltage film cap, try two 10uF in parallel. Should be enought. I´m sure you can find 22u / 63v or 100v around. look for wima MKP red caps. Very good sounding!

This is a good candidate for a no electrolytics tube mic pre.

Make sure bigger film caps will fit in the box!

For the 22µF, I found a few things :

First, a polyprop 22µF 250V (hard to find lower values ... and REALLY expensive)
http://www.selectronic.fr/upload/produit/pagecatalogue/5-21.pdf

Then a few others ... Do you think it might work with these ?

http://www.selectronic.fr/upload/produit/pagecatalogue/5-33.pdf

http://www.selectronic.fr/upload/produit/pagecatalogue/5-34.pdf

http://www.radiospares.fr/
part number 215-5758 ...

If you have any idea of where to find these values in europe ... I'll be happy if you hare it with me !

Thanks,

Sam.

Also found these, but seems to be quite expensive !
http://www.scr.fr/produits/condensateurs/pdf/acoustique/scr_condensateur_acoustique_FEPmet.pdf
And
http://www.scr.fr/produits/condensateurs/pdf/acoustique/scr_condensateur_acoustique_MKPaxiaux.pdf

There' a shop selling these in Paris ...

Hi,

to your first question: I guess yuor question aims at the V72? Well, you should know that the V72 was the sucessor of the V41. It was slightly more noisy than the V41 but smaller. It also didn't have variable Gain. And the V41 was bulit around 3 EF12! So I guess your amp is a kind of east-german bottleg or something of the V41. Try to get the V41-schematics and compare them.

regards

Chris

It was slightly more noisy than the V41

Click to expand...

Wow, I knew the rest, but for this point ... I thought it was the opposite :shock:

So I'll definitly give those EF12 a deep try !

Thanks a lot for the answer !!

I found this info in a german book from the 50's where this is mentioned. It says that the V72 is a bit more noisy but that this wouldn't matter because the noise of the condensor-microphones to connect is still higher. The book is reissued:

Otto Diciol

regards

Chris

Thanks for the info, but my german might not be good enough ... Usualy, my girlfriend translate german for me (she was born and lived 17 years on the border, so ... ) but I'm not shure she'll be happy to "read me thoses kinds of stories" before going to bed at night :lol: