[Gus]

+6 will work the cathode to grid voltage difference is what counts.

  The 2nd schematic would need to be redone for the first tube bias.

Look at the first M49 at this link
http://members.tripod.com/~coreyeng/neumann.htm

[lolo-m]

hi synthi,
Do you mean that the original transfo out is 10k impedance? This would explain the better match with the vt-737 because of it's input tansfo... I'm not sure about the low quality of the transfo because the sound is really interesting like this. Do we have to get real hifi in a studio? This is THE QUESTION !!! Sometimes vintage gear is more interesting than new one because of its defaults that make it better on some applications... There's a few values I changed in the G7 power supply... I changed the second transfo to a 12:220V and the 10 K resistor between the two filter caps to 12k or 15k (sorry I don't remember) to get 130V output. I didn't mod the mic itself so I didn't have to get the -6V. I think that you can get it using another transfo 220:9 and a LM317 (be carefull, the wiring of the pins is different ). Don't forget to put a heatsink to the LM337 or LM317 because the lamp is very hungry with current...

[synthi]

Thanks lolo,

I mean the mic 10K bandwith, with the stock electronics the frequency response is up to 10Khz only!
The stock trafo would be ok for voices but will saturate easily with loud sources.

Best Regards,

Synthi

[lolo-m]

I will check if the bandwidth is really 10K with a spectrum analyser during the week end. I'm a little bit surprised by what you're telling me because I was really pleased with some tests I made with my mic... But it's possible... What transformer should be more suitable to increase the bandwidth?

[synthi]

check the original specs:



I`m not sure if is the trafo what put the 10K limit. BUT if you like it as is, then don`t change it!!  :thumb:

I don`t know the ratio for the original trafo, I`ll use haufe trafo for the "neue" version I think that a standard mic trafo 1:5 to 1:11 will work there... GUS?? :?:

Synthi.

[lolo-m]

You seam to be right. The difference between mine and the one in the ads is the caps. The one on the ads has got a RFT caps and mine is a M7... I don't know the specs of the RFT caps but it can be the reason why... I never heard about RFT to be a great caps maker... I'll keep you informed.

[lewilson]

I did this mod to a 7151.  You must have negitive heating  or else you are unable to bias the first  tube correctly. You are borrowing some negitve voltage from the heater to create -1 or so volts to the first tube grid.

The stock output transformer falls off at the top end  once you get over 10 k and also falls off at the bottom You can change the output transformer or play with response of the amp a bit to make it better. I got mine within plus / minus  1.5 db from about  from around  30cps to 50k with stock transformer.

It was a fun project and I hope to do more of them soon

[synthi]

Hey Mark! Nice to see you here!  :wink:

Synthi.

[lewilson]

I think we have spoken over  e mail before?

[synthi]

Yes, ...er past week???  :cool:

Best Regards,

Synthi.

[mig27]

Heya bottle lovers,

My technician just finished refurbishing my 7151 bottle.

He's been hunting high and low for a matching connector / stand mount and finally gave up. Instead, a 3/8 mount was installed next to a 5 PIN XLR connector leading to the now external PSU.

Still, my tech wasn't happy with the performance: Good s/n ratio, but questionable frequency response.

Now the good thing is that he used to work as chief technician for the east german radio network and there must be a good amount of pride involved. He simply could not accept a less than stellar performing RFT product leaving his home lab.

So he installed a second output transformer, taken from nothing less than a Neumann U67. I'll be able to change from the original RFT xformer to U67 by the flick of the switch. And he didn't even have to drill new holes, using the place where you formerly had the hum balance pot. How neat is that?

The parcel with the 7151 should arrive tomorrow afternoon. I'll keep you updated on my findings. The attached pics were taken by my tech, to sweeten my waiting time. I'm convinced that this is going to be a love affair...

Cheers.


Michael

[synthi]

Wow! looks great

Synthi

[Tillmann]

Hey Michael,

U67 transformer!  :shock:  That's pretty damn cool.

I'm interested in having mine worked on here in Berlin.  Who did the work?


Cheers,

T

[drpat]

...

[drpat]

...

[ubxf]

Hello,
i got a couple of EF12 spez tubes
http://groupdiy.twin-x.com/displayimage.php?album=lastup&cat=0&pos=0
and i just realized they might not have the same pin out as the EF12
Do you know if they can be used in the 7151
Thanks
francois

[3phase]

Hey Michael,

U67 transformer!  :shock:  That's pretty damn cool.

I'm interested in having mine worked on here in Berlin.  Who did the work?


Cheers,

T

The best expert for all this old mikes is Andreas Grosser.
Half of the work he has to do theese days is to remod the mods of US vintage specalists. Simply because there is quite a portion of knowledge how to handle valves correctly lost.
He knew the original designers of that stuff when they was still alive.
And he is working in this field for 20 years or more by now.
I think i dont tell lies when i say there is no living person with deeper knowledege about german valve mikes on the planet wright now.
And it will probably stay like this because the source of this knowledge is
about passing or allready passed away. He however is a specialist in fet mics aswell..
All big german mic manufactors have consulted him by now.

And the best thing..he lives in Berlin ...

Btw:
He also designed the most noisefree valve mic pre with over 127db as a demo projekt to proove to some solid state fanatics that this can be done...

sorry to be so enthusiastic about it..but he is one of my personal heros ;-) The guy that allways know an answer or sombody that knows something about it.

[3phase]

While having the AC supply in the mike body sounds like a dumb idea, this was not a cheap mike and I'm sure it didn't hum (much) when working right.

This mikes was known for humming...
To have the power supply inside was indeed a dumb idea...But at the time this mike was developed the parameters for the developers had changed and became east standards...
Sonic performance wasnt allways at No1 of the priority list anymore.
Many good technicans anyway used the oportunity to go west before the wall was build. There is some good east german stuff..but also a lot of underdeveloped or badly made equipment... As older as better defenetly applys more to DDR produktions than to anything else in the world.
The kommunist buissenes model dont worked well for highend gear.

But a  bottle with a M7 capsule defently can become a very good mike when the wright changes are done.

[ioaudio]

hi francois,

here´s your picture again, quote my post to see how i did.




i found the data for ef12s here (bookmark that page):
http://tdsl.duncanamps.com/tubesearch.php

pinout is the same as ef12 and ef12k. the k version should be less microfonic, k stands for "klingelarm", meaning it makes a softer ping when you touch it.
so just dont hit your mic while recording  :grin:
if i´m not mistaken the ef12spez tube is the long-life version of the ef12.
it should work in your mic without any further changes of the circuit.

[3phase]

After the wall opened we all was going for cmv 563´s and didnt touched the rft bottles...
i refused to get them for free because of the hum...
when i see the actual going rates i could kick my *** :-)

defenetly a mike to be moded..but the space inside after getting rid of the internal powersupply allows to turn this mike in everything you want it to be..

The M7 is the same as on the pre war models ( cmv3 )

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