NWDR U17a tube diode bridge compressor on Eurocard

[DaveP]

I translated the operating Instructions, attached:-

It looks like the germanium diodes in the 50's were not very reliable and that they needed to be changed. I guess this is the reason that the later U23 and U73 went over to vari-mu designs.
best
DaveP

 

[DaveP]

Looking at the schematic in detail, it is in fact a slightly modified V72 circuit, using ECC81's instead of EF804's

These are the common points:-
~100k of DC NFB.
Rk1 is 1.25k
50k of inter-cathode DC NFB
V72 Choke loaded output tube.
2uF output Cap.
V72 type OPT.
The date of the V72 drawing is August 1952, just 2 years before the U17a was drawn.

This means that choke loading would be absolutely necessary to provide enough drive for the large amount of DC lost through the 100k NFB resistor.
Having said that, it would be possible to insert a 2uF cap after the 100k NFB resistor, as was done in the later V77 (1959).
best
DaveP

 

[dirty1_1garry]

Hi!
interesting topic, interesting compressor!
I have redrawn schematic for greater clarity and understanding.
Abbreviation "abgl" has been replaced with a question mark after value. I think it's about adjustment.

 

[rock soderstrom]

Thanks for all the information! Interesting device, no question.

I think the NWDR U17a has a good chance of winning the title of “audio device with the most chokes & transformer per channel” worldwide. My ancestors were not stingy when it came to using iron in their gear.

 

[dirty1_1garry]

a good chance of winning the title of “audio device with the most chokes & transformer per channel” worldwide. My ancestors were not stingy when it came to using iron in their gear.

U17 is a good contender, but the U23s are definitely up a notch.
Three audio transformers, one side chain transformer, two plate chokes, one power choke. IRON!

 

[ruffrecords]

Hi!
interesting topic, interesting compressor!
I have redrawn schematic for greater clarity and understanding.
Abbreviation "abgl" has been replaced with a question mark after value. I think it's about adjustment.

The polarity of C29 is wrong.

Cheers

Ian

 

[ruffrecords]

U17 is a good contender, but the U23s are definitely up a notch.
Three audio transformers, one side chain transformer, two plate chokes, one power choke. IRON!

There are no plate chokes, just a pair of capacitor coupled output transformers (no gap required).

But yes, a lot of iron

Cheers

Ian

 

[dirty1_1garry]

The polarity of C29 is wrong.

Cheers

Ian

Right, fixed

 

[dirty1_1garry]

There are no plate chokes, just a pair of capacitor coupled output transformers (no gap required).

But yes, a lot of iron

Cheers

Ian

Yes, I'm wrong how to name them. Nevertheless, U23 is rich for iron.
L1 - looks like is a part of LPF
L2 - Ro4 supressor choke
L3 - some sort of a Ro5 cathode choke
L4 - power choke

Not the point, I probably should study the thread about the U23 more closely)

 

[MidnightArrakis]

Yes, I'm wrong how to name them. Nevertheless, U23 is rich for iron.
L1 - looks like is a part of LPF
L2 - Ro4 supressor choke
L3 - some sort of a Ro5 cathode choke
L4 - power choke

[U23 is rich for iron] -- And.....it also produces lots of lovable vintage "rusty" sound!!!

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