Who in (Germany?) made these? Valve rack units, small power amps and a mystery box

[Disco Volante]

Yeah, I had a Fisher amp eons ago that would eat EL84's. Swapped it for a Quad 33/303 that came along asking for help. We've been friends ever since.

 

[Disco Volante]

That old Fisher would probably be hot on Fleabay these days.

 

[Disco Volante]

I traced the schematic of the original modules, EL95 and E80CF.
The NFB part is a bit confusing, it seems to be mixed with the grounding of the first pentode. Though I can't entirely rule out a mistake on my part ;-)
Also, I've never seen a diode in the cathode of a PP stage before?

 

[abbey road d enfer]

I traced the schematic of the original modules, EL95 and E80CF.
The NFB part is a bit confusing, it seems to be mixed with the grounding of the first pentode.

ATM I don't see any global NFB.

Though I can't entirely rule out a mistake on my part ;-)

Indeed. I suggest you check the 100uF cap and 1k resistor. They probably should go the pentode's cathode.

Also, I've never seen a diode in the cathode of a PP stage before?

Combining fixed and cathode bias...
Fixed bias is not necessarily via a negative voltage on the grids. It can also be a positive voltage source of some sort in the cathode.

 

[Tubetec]

I have a couple of hifi amp modules with a similar phase invertor setup and EL84's , I ended up putting in a blues deville transformer in one of them , sounds great for guitar . It has what I'd describe as a growl/swirl when over driven , Im not sure the technical reason for this .

 

[ruffrecords]

I suspect there may be a couple of errors in your tracing the schematic. It does not make sense as it is. In particular the 100uF on the left does not make sense. Neither does the semiconductor diode attached to pin 8 - it is reverse biased. And the resistor to the right of that diode, is it 100K or 100R?

Cheers

Ian

 

[abbey road d enfer]

I suspect there may be a couple of errors in your tracing the schematic. It does not make sense as it is. In particular the 100uF on the left does not make sense.

Correct.

Neither does the semiconductor diode attached to pin 8 - it is reverse biased.

Probably a zener, in order to develop enough voltage to ensure partial fixed bias. In that case, orientation is correct.

And the resistor to the right of that diode, is it 100K or 100R?

Works only if it's 100R.

 

[Disco Volante]

Ok, found at least one error. Got a pin wrong on the first tube socket, hehe..
So there doesn't seem to be any global NFB in this thing?! Unless of course it was part of the EQ circuitry and placed outside the module...
Except for the apparent zenerbased bias (yes, it's a 100R resistor) it's a pretty straight-forward small PP output stage.
One thing puzzles me: the 4R resistor in the ground path of the output transformer and the first pentode...
And what is that RC doing over the OPT primary? for stability?
redrawn schemo:

 

[abbey road d enfer]

So there doesn't seem to be any global NFB in this thing?!

Correct.

One thing puzzles me: the 4R resistor in the ground path of the output transformer and the first pentode...

Test point for output stage quiescent current.

And what is that RC doing over the OPT primary? for stability?

Zobel correction on the primary.

 

[ruffrecords]

How big physically is the zener? I am not sure what the 1308 or 1B08 designation on your schematic means. From the EL95 datasheet it looks like there could be around 40mA flowing through it so its dissipation could be quite high.

|Cheers

Ian

 

[CJ]

Those are called snap in lams.

Was the coil scatter wound? Using nice layers with insulation will improve reliability.

 

[abbey road d enfer]

From the EL95 datasheet it looks like there could be around 40mA flowing through it so its dissipation could be quite high.

Assuming 40mA idle, so about 4V in the 100r, then a 8-10V zener, the dissipation in it would be less than 1/2 W. Looks feasible.

 

[My3gger]

I have a set of transformers with the same purple foil on the outside, they came from a Siemens tube radio.

The laminations are called M, in contrast to EI. They were (rarely still are) mostly used by German companies as far as I am aware. They have the advantage of less leakage, tighter air gaps, and more winding space per core cross section compared to the standard EI cores. When the laminations were still widely produced they also could be bought with a specified air gap for DC applications. Disadvantage is cost because the two strips that are stamped out are waste, and they can only be stacked by hand I think. EI laminations are "wasteless", 2x E + 2x I form a rectangle without any scrap. They were also developed into a couple different forms to further reduce magnetic losses.


What tubes do you intend to use, at what impedance and which secondaries? What is the edge length of the lamination and how thick is the stack? Hifi/studio or guitar?

Friend has stock of this lamination, he told me they were most often used in Germany for DC and AC coupling, chokes, etc. They came from Germany or Austria for high quality stuff as we always had good choice of EI lams.

 

[Analog_Fan]

Since I'm not planning on ever owning or making any 200 ohm speakers, I might as well learn how to rewind the transformers. Also, I'd like some UL taps in the primary and some impedance options on the secondary. This was intended as a low-power amp for testing some high-efficiency speakers...

Anyway, started taking them apart. Interesting lams, they look like EI, but they're only split inside, and wedged together with a piece of Pertinax.

Would it hurt the magnetic properties if I clean off the rust with a stainless wire brush? Don't want to glass-blast them because of the possible warpage...

Thanks

View attachment 98506

You could use Electrolysis, vinegar, soda to clean them.

 

[Disco Volante]

@ruffrecords: Zener is about 8mm long, with a small metal protusion, probably 10mm in total. 6mm Ø. Very clearly marked 1308 and a solitary L on the rear side.
close-up:


@CJ: Coil was scatter wound. I am aiming for a more controlled winding with proper interleaving and insulation. I've got one unwound and mounted in the lathe for some fun as soon as I get my old rev counter sorted.

@Analog_Fan: I have good experiences with chemical cleaning. Thanks for the tip;-)

 

[ruffrecords]

Those Zener dimensions are very similar to the modern 53 series 5W Zeners which would be conservatively rated in this application. Interesting way of self biasing a PP output stage without the need for a hefty electrolytic capacitor.

Cheers

Ian

 

[Disco Volante]

Some progress on the winder.
This is my 60's Mazak lathe as a makeshift transformer winding machine. Footpedal connected over the test-button allows me to feed with two hands. It takes 4 revs to stop the gearbox when I release;-) The rev counter was built by a very young me somewhere around 1988, originally used with a big power drill to make crossover coils. Found it and jumpered some bad traces;-)
The sponge makes a little extra tension happen, laying it down nice tidy and tight...
Two layers of Kapton between S and P, one between layers.

 

[Disco Volante]

All wound up. Volker was right, it all fit with even a mm to spare. Inspired by CJ's post Fender Blues DeVille OPT I went with the Partridge-style interleaving, so 5 primaries, 4 secondaries. Secondaries are bifi-wound with 0.5mm, and I decided to make a full layer of each, so 4x48 turns which should give me a 16 ohm tap for the feedback.
It is quite a bunch of wires flying around, but I have my notes from the winding session and most of the primary leads are marked...