Choping 'n' Racking RFT MV4056

[Dr_J]

Hi,

i had an old GDR tube preamp (1956) laying around. Some time ago i build a JLM GoBetween Box for it and it was working so far.
A few days ago i decided to put this little thing into a 19" rack.
A had along discussion with that machine, should I chop you? You've survived nearly 60 years! And now i'm gonna kill you!? 
It took me some beer and nearly a hour, but in the end i sad "Fu** you Honi, the GDR was sh*t, i chop this machine!"

But unfortunately i was disturbed by chopping the preamp and it had only white wires in it... grrr.

So now i can't get it back working in his Neue Heimat ...   

I feel that i can't see the wood of all the forest, it's something that i'm missing or don't understand.... 

The audio signal is going to the EF86 @pin#9, ok! But I can't trace it coming out of the tube. @pin#3 i have an signal but modulated on a big hum @pin#6/8/1 nothing...??

Any hints where to look at??

 

[Dr_J]

Another picture before copping...

 

[Dr_J]

I only have a very bad picture of the schematic so i did a redraw as far as i could.

 

[Dr_J]

And here we have the patient laying on the bench: dead. 

 

[Dr_J]

Ok, here we go!
I just missed the negative rail coming from the rectifying pcb.  :

Now this thing is working again  ;D  ;D

I'm just wondering about the low anode voltage @EF86 (35v, @EC92 108V)?

I would like to add an output transformer (1:1 600:600 would be straight!?).

What's the best way to find out which ratio will fit?

ROCK-ON!

 

[Dr_J]

Here is the finished unit. 

 

[Dr_J]

...and the inside!

 

[sodderboy]

That looks great, and even better that you sussed it yourself.  You will want some vent holes above and below those bottles.
Mike

 

[Dr_J]

Yes Sodderboy, it took some time 

And yes, now I did some vent holes but only above the tubes, they don't really get warm.

Back to my output transformer plan.
So, I did some measurement and calculation (Ri=RL(U1/U2-1) of the output impedance, it's some what around 13.5k.
That means it will fit the "impedance class"10k.
What do you guys think, is a 10k:600k worse a try or better use a 10k:10k? (Going direct into the DAW).

Any hints on that?

ROCK-ON!

 

[valco]

Hi Dr_J

I´ve been working on a similar racking project. Its been sitting for a while cause I ran out of steam. I recently revived it and luckily found this thread.
I have got it together with a Beyerdynamic 1:5 input transformer and inserted a volume pot after C1, but the signal out is way too low with a dynamic mic. Do you have the same problem into the daw? I thought that i might get away without using any output transformer, but maybe i need one after all. What specs on the output transformer did you end up using?

 

[Dr_J]

Hi Valco,

yes, I noticed the same problem after finishing my racking, the output level is quit low.
I didn't had the time to check on that.
As a D-Box it works quit well, if i remember correctly.

The output transformer was just a plan, didn't test it yet.


ROCK-ON!