Philips-NEFA 2890 - a 1953/54 three-to-one mixer/console

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Conviction

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Hi all!

I recently acquired this gray monster: a three-channel tube mixer/console made by Philips-NEFA for the Swedish Radio around 1955-56 1953-54. It has some features that I don't see much use for, like the commando (talkback) amplifier. Not sure how the designer intended one to use it (see the schematic/block diagram). There’s also a pre-listening function for incoming line level signal. Typical early broadcast feature.

The speaker amplifier (8 & 500 ohms) could prove useful for re-amping etc.

When I bought it I was certain it would be full of Jørgen Schou input transformers, but for some reason the designer(s) chose Unitran E 10E (JS was otherwise the go-to make). The Unitran’s are wired for 50 ohms pri, but has 200- and 500-ohms taps as well. The rest of the iron is Philips’ own creations.

Tube lineup for the preamp section is two EF40’s per channel. The output make use of EL91 in push-pull. Should be able to output quite a high level. Haven’t measured anything yet. Listening amp consists of a EF40 and a EL42.

I’ve slowly started to go through it, replacing dried out electrolytics etc. At some point I think I’ll (have to) change the input wiring, since the isolation is made of rubber and it literally falls apart if you bend it even the slightest.

Also: one of the 100k plate resistors on channel 1, a wire-wound with “bifilar” printed on it, is cracked. Should I replace it with a modern metal film? Or am I missing something here? Wire-wound=stability? What was the intention?


Well, I’ll update this thread once it’s up to par. It’s a nice piece of equipment for sure. Can’t say I find much use for a 3-to-1 mixer though. As is, I kinda see it as a huge and rather impractical (unjustified, space) one-channel preamp.

Some photos (yes, the schematic is glued to the bottom, need to take a high res photo and Photoshop it a little):

dDSguxnl.jpg


ixQFogrl.jpg


DFh8dh3l.jpg


wWu2OV7l.jpg


TqD2NO1l.jpg


SMDHteil.jpg
 

emrr

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Interesting that it's Weston and Cannon P.  Dig the pullout attenuator access panel.  Is that top rear panel spare tubes? 
 

Tubetec

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Reportage mixer suggests that was a portable rig , its still quite a lump to carry around Id imagine .

Maybe the 100k resistor is a non inductive wire wound ,  I do know that sometimes special winding schemes are used for this purpose.
 

Conviction

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EmRR said:
Interesting that it's Weston and Cannon P.  Dig the pullout attenuator access panel.  Is that top rear panel spare tubes?

Indeed it's a row of spare tubes! Almost a full set.

Yeah, apparently those big Cannons were standard here in the mid 50's. Seen them on almost every ex-Swedish Radio (ribbon) microphone or preamp I've come across from that period. Westons too.

Any thoughts about the plate resistor? Like tubetec I was thinking in terms of inductive/non-inductive. But a bifilar winding method really makes them l e s s inductive... But compared to what? A carbon of that era?

I guess a modern 1% metal film would be suitable?
 

Conviction

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Tubetec said:
Reportage mixer suggests that was a portable rig , its still quite a lump to carry around Id imagine .

Maybe the 100k resistor is a non inductive wire wound ,  I do know that sometimes special winding schemes are used for this purpose.

I think you're right! So. Back then it must have been intentional. But compared to a modern metal film - would it matter? I don't see anything special there around the EF40's.

Yes, portable  ;D 22 kilos. Then add the power supply, which is another 20 kilos. I think it also had a section for plate and filament batteries (and a vibrator transformer) for remote work. Only seen it in a picture.
 

Conviction

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ruffrecords said:
Very nice indeed. Looks like all B7G tubes.

Cheers

ian

B7G and B8A (rimlock)  :) Have to read up on EF40's. I would guess they're very similar to their successor, the EF86.
 

emrr

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Wirewound was the quietest option.  All the early pro RCA is all wirewound.  If it measures fine then it should be good. 
 

ruffrecords

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Conviction said:
B7G and B8A (rimlock)  :) Have to read up on EF40's. I would guess they're very similar to their successor, the EF86.

Yes, you are right there are some B8A types in there. I have not seen any of them since the 60s when I used to strip down old TV sets for parts. I checked out the EF40 and according to the Valve Museum it was later developed into the EF86:

http://www.r-type.org/exhib/aad0126.htm

Cheers

Ian
 

PRR

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> the commando (talkback) amplifier. Not sure how the designer intended one to use it

The three reporters could be out of sight, even blocks away. The guy at the console could feed the Kom.-Mik through an amp to the reporter's microphone. Dynamic mikes make OK small speakers. Reporter is told to keep mike near ear. When he hears a tinny "You're on in 3-2-1(clik)" he starts talking.

There would logically be another switch so the reporter mike goes to the console/director, to be sure he was alive/awake and to last-check any details; I don't see that.
 

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nashkato

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Hi !
Your philips mixer looks gorgeous and your schematics should be  very useful in bringing my Siemens mixer  back to life .
( which isn´t in such neat condition as your philips , but seems to have a similar design , same tubes etc  )
https://groupdiy.com/index.php?topic=73860.0
Thanks a lot for sharing .

If i don´t ask too much : you  don´t happen to have the powersupply schematics , too ?
 

Winston OBoogie

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Very nice piece.  Thanks for posting the pics. 
The first batch of V72 amps (pre the standard Siemens and Halske and Telefunken units) used EF40 tubes before the EF804S was utilised, and the EF40 was itself a replacement for the earlier EF12 used in units such as the V41.   
Most of the European broadcast and studio valve equipment of this period was built using wire wound resistors and, although it's a bit of a misnomer, you can still buy 'non-inductve, wire-wound' resistors from places that sell audiophile components if you needed or wanted to replace that 100K.  'Mills'  is one brand I can think of.   

 

ruffrecords

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Wire wound resistors were used in those days because they were much lower noise than the regular carbon ones that were the only other type available then. Carbon resistors produce a lot of excess noise which is proportional to the current flowing in them. This is why wirewound types are often used as the anode load of the first stage of preamps.

Cheers

Ian
 

Conviction

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ruffrecords said:
Wire wound resistors were used in those days because they were much lower noise than the regular carbon ones that were the only other type available then. Carbon resistors produce a lot of excess noise which is proportional to the current flowing in them. This is why wirewound types are often used as the anode load of the first stage of preamps.

Cheers

Ian

Thanks for the explanation Ian! I know carbons are notoriously noisy in these positions.

One of the 100k (Vitrohm, bad rep) plate resistors in preamp 1 is actually not only cracked, but broken. So it needs to be replaced. I don't know if I should be true to the design/era and find a good wirewound or simply replace it with a modern metal film.
Are there a n y audible advantages compared to a modern metal film resistor? Perhaps wirewound "compete" with metal foil? Is this in the domain of audiophoolery?
 

ruffrecords

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Conviction said:
Thanks for the explanation Ian! I know carbons are notoriously noisy in these positions.

One of the 100k (Vitrohm, bad rep) plate resistors in preamp 1 is actually not only cracked, but broken. So it needs to be replaced. I don't know if I should be true to the design/era and find a good wirewound or simply replace it with a modern metal film.
Are there a n y audible advantages compared to a modern metal film resistor? Perhaps wirewound "compete" with metal foil? Is this in the domain of audiophoolery?

The designers used WW only because it was the only choice available to them. I am sure if they were designing it today they would use a good quality metal film. Higher wattage types tend to have lower excess noise so use a 2W or 5W type.

Cheers

Ian
 

ombudsman

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I've got some nos EF40 with no particular purpose, a swap could be arranged. Very cool piece.
 

emrr

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EmRR said:
Wirewound was the quietest option.  All the early pro RCA is all wirewound.

RCA stopped using WW by WWII.  I can't think of it in any other post-war American pieces besides some obscure Cinema Engineering and Western Electric stuff.  You'll see it as precision resistors in cathodes for exact unchanging values related to metering taps, not about noise there. 

The usual American approach was to used 4x (or more) required wattage, which also reduces noise in carbon.  Normal to see all 2W carbon resistors in lots of American preamps. 
 

mjrippe

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I'll have to remember that higher wattage bit.  I've got a bunch of old preamps and two drawers full of Motorola 2 watt carbons.
 

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