This was just to sweet a deal to pass. A rather odd old Swedish made tube amp,or more to the point complete PA really.
In stock form quoted at a mere 20 watts,but..rest assured...we´re gonna surpass that with a friggin landslide this time out,too.
Not exactly your mainstream tube setup i´d say. Hoooooowever...it gets worse. It gets worse as this old thing carries an output transformer that simply isn´t to mix with modern day n era speakers. Sorry to say,but that´s the short end of it. No matter tho as OT´s i´ve got laying around to spare..
The 4654K tube in turn...can be made to deliver alright. Very alike the EL-34 really..
Mains enter the "hood" of this thing right. And see said hood needs to be "inserted" correctly as them pins there in the pic is how current reaches the amp. Talk about trying to F doing a handstand in a canoe going down rapid waters...
Right. As large as the powertransformer is..just as small is the OT. Wierd.
Innards. Now,as this thing hadn´t seen service since stonehenge i didn´t even try.. So out the ESR meter and what not came.
The Farad caps...the intro year for the 4654K´s...it all adds up,and i´d wager to say that this thing is either real late -40s or real early -50s.
Uhu. Them brown thingys indeed are speaker enclosures. As it turns out tho the speakers are of 4 Ohms a pop...which relates real bad with the minimum 137 Ohm out output transformer. The answer to that is right there...there´s conversion transformers for each speaker.
Amp is cathode bias stock right. What´s more it carries a setup that is...different. For those of you out there that are in the know,that there is a choke alright and back in the old days it was cheaper to produce a choke for ripple control then to add,very expensive,electrolytic capacitors.
So.
B+ out of the tube rectifier,that side of the choke,brings a ripple of approx 170VAC or so. The other side of the choke,which carries a smoothing cap of course-sees like 5VAC. B+ came all in at approx 410VDC.
So. What i did was to add a smoothing cap before the choke too and...DAAAMN!!!
B+ voltage soared! Inxs of 600VDC for a brief moment until i flicked the mains again!In short...we´re back to the old ways. A pulldown resistor to stabilize voltage before the powertubes start to digest what they need. So..installed a 10k/25w such and indeed.. B+ was now around the 500V mark. WAY more manageable.
As such i got to try the amp out and...nah. Didn´t sound all that hot TBH.
Now...please have in mind,bench work. ALL capacitors leaked you won´t believe!!! ALL of them,sans..the e-lytes
. The old Swedish made Rifa e-lytes were still good to go. Friggin amazing.Anyhow.
Tried to amp out again and...better. Way better.
OT tho needed to be replaced and i did so with an old Dynacord one for twin EL-34´s. They take about the same primary impedance as the 4654´s...
Said and done and HELLO! NOW we were getting some serious power from this unit!
For the win in short.
Drilled...tapped...and what not,and then cleaned the chassis out entirely. Paint...in the pic regular primer...
Right...
As cathode bias is out i fabbed a small board to cater to negative bias needs.
Here in turn the socket for the tube rectifier. Added "saver" diodes...and a pair of 150k resistors to feed the bias board..
...to be continued
