Transition period transistor preamp

[Conviction]

Hey all,

I found this little cutie yesterday. At first I thought it was a tube preamp, fooled by the looks. This seems to be some kind of a prototype build and I'm quite sure it's from the former NEFA factory here in Sweden, where Philips built a lot of broadcast equipment back in 50's and 60's (remember the Philips-NEFA thread? There's lots of similarities). I don't think this one ever was in production or even considered. Hard to tell. It was, however, tested 5th of may 1969, so I guess someone used it now and then.

The unit was designed so that it could run on a 6.3V battery (5x1.25 battery pack) or mains. Unfortunately the mains circuit seems faulty, but I guess even a wall wart could do the job (if I'm not able to fix it).

The transformers are all Jörgen Schou, which is nice. It even sports a Daven attentuator (see pics).
The transistors are OC603's, OC604's and OD603's.

I got very little knowledge in early transistor technique and am a little curious about the performance of these (germanium) transistors.
Do you think I should put it to use or put it on, well... display?


Here's some pics (without it's brown tolex cab):

... The actual preamp sits inside an internal shielded housing (here without the filter caps, gotta change those asap):

Best,
Olle

 

[PRR]

> Do you think I should put it to use or

I don't see why not. A late-1950s transistor preamp was objectively as good as most tube preamps.

 

[sahib]

Why not put it to use? You can never know. It may be the right tool for a particular job. I once had  an early 70s transistor Watkins guitar amp. It was generally awful. But in one occasion it had the right sound.

 

[CJ]

those black epoxy transistors usually sound good,

but you need to rip out that Daven and put in an Alpha pot, then send the Daven over here, 

 

[mjrippe]

Probably sounds awful, like that early germanium Neve stuff that nobody wants ;-) 

Seriously though, replace the rectifiers if they are selenium types typical of that era.

 

[emrr]

Looks cool.  Mainly depends if it is the type of germ transistors that tend to leak humidity and get noisy, or types that are better sealed.  To date I've only heard a rare few early germ amps that are as quiet as a same era tube amp.  Might have competed then, but not after 60 years of humidity leakage.  I have rarely rebuilt germ amps with much lower noise.  Sometimes, but not usually.  Modern or NOS germ is a crapshoot. 

 

[Conviction]

Thanks for the input guys. 

I changed the filter caps and tried it out yesterday, but this one leaves me with a lot of questions. First, when on 6.25VDC, the microphone input is highly distorted. Down on approximately 3,5VDC, almost half of the voltage, it works just fine. But then, naturally, the output stage is limited + the oscillator is down at 392Hz when it should be 440.

They all, of course, share the same B+ node.

It's rather hard without a schematic...

__

Edit: turning the VU light on, thus draining some current, smoothens things out. But this still seems like a faulty circuit to me.
Sounds fine tho!

 

[Conviction]

but you need to rip out that Daven and put in an Alpha pot, then send the Daven over here, 

Sure thing. You can have the ugly Weston VU-meter too. I'm going to replace it with a buffered LED circuit in some funny pattern!
And lot's of colors. Who doesn't like colors?

The input transformers seems kinda overkill too. I think I'll replace them with balanced opamp inputs...

;D

 

[Baltazar]

It's funny how that VU lamp is interacting with Philips-Nefa gear..
Like i told you about my 2891A preamp, when the lamp is on, the heater goes down to 3V (4,5v recommended) and all the noise goes away...

If that thing just sounds as good as it looks....

Good luck!

 

[Conviction]

Yeah, it's weird indeed!

Assuming B+ should be around 6.25VDC (total of the battery packs) the microphone stage gets way overvolted and doesn't pass anything except distorted signal.
Around 3.5VDC, as I wrote earlier, the stage passes audio fine and actually sounds very good. Thing is the VU meter bulb then is very faint and I'm guessing that wasn't intented in the original design.

The mains rectifier circuit is faulty, there's something weird with the transformer, and this makes me unable to do just more than guessing cause I'm unable to rule things out.

Either I try to drop the voltage to the microphone stage, or simply run it on 3.5VDC. But i'm thinking this will probably limit the output stage.
Any ideas?