Hello

I bought a radio from Ebay last week and need a schematic to help me do a cap job on it.
I'll send some images so they may get attached to this post later.
The radio looks like its from a radiogram, has trimax 315-0-315 power , trimax 10k2 and 8 ohms output and a trimax choke.

I've identified the glassware as x61m...6sk7-gt...kt-61 x 2...6j5...ebf35 plus the rec tube which I havent pulled.

There are six knobs across the front, it looks like a seven band shortwave.

On the glass panel on the front in the bottom right hand corner in small letters is printed 'EFCO U S L 46 AWA'
Any help would be appreciated
Thanks

Can you link to photos on ebay, or post some of your own?

Any numbers stamped on the back?

Something sounds odd EFCO made dials. AWA is possibly a dial they used? I cannot see AWA using the Philps valve even during the war. HMV 661 used EBF2G with a circuit mod during the war when the tubes ran out: That was superseded by EBF35 / EBF32; KT 61(Tetrode) is also not a common OP valve.

Marc

Photos uploaded.

I can't find anything with that particular valve line-up in the usual places. Maybe they are not the original/correct valves.

Hello

thanks for the help

The efb35 may not be an efb 35.
Most of the red paint had baked off one of the top anode octal based valves, all that was left was a '35' about a third the way up the glass, and a two lined code that was in very good readable condition.

K51
A5G

There is no etching on the glass ( I removed most of the paint that was left in order to check)
In addition, ' 5L' is printed in white next to ' Miniwatt'.

From the two lined code I determined from the 'k5' on the first line that the valve is an ebf35.
All the other valves are clearly marked.
These transformers look like mid to late sixties, I thought the earlier trimax had a more contoured end-cover.

There is kind of a printed tag on the back with what looks like "ARTS&P" printed over what looks like some kind of licence agreement, and there is a number that looks like a serial number right there on that tag, that number is 17033.

There was a kt61 parked there with the other missing.
Other observations...It doesn't look like anything has been added, no vacant holes except for two around the speaker socket on the back panel, and the internals look like the ones in an AWA at21 xmitter I have here from ww2, lots of waxy dark brown components and large resistors.

I think I'm going to need a schematic..from memory those sausage shaped resistors end up drifting real bad.

The string thing is going to be a challenge, there is string still around the corner pulleys and large dial, however I also have a section of string loose with a small spring attached to it.

Any further help is appreciated and needed, I want to hear the bee gees on this thing ASAP.

There is kind of a printed tag on the back with what looks like "ARTS&P"

What colour is that label? It may give a clue to the set's age.

The tag is light blue/light green...more blue, and the ARTS&P is printed in a darker blue.

The grey tube is likely the pentagrid and the set clearly has an RF stage ahead of it. Definitely looks like some hefty PP tubes like 6L6 in the OP. Big remaining tube looks like a filament rectifier like 5Y3 or heftier.

The potentiometer count looks like that of the HMV chassis on the bench where it has a volume, pot & two for Hi & low tone. That would account for three pots & the tuning shaft and a band switch and the other possibly switching the PU.

Interesting that it has plastic wire yet retains the crappy worn out caps & some "different era" resistors one looking overheated.

That says "worked on" and should ring alarm bells. Powering not recommended.

Can't remember seeing a grey ECH35 , perhaps ECH3G,

The size of the OP & the extra tone says it may not be intended for a radio, but perhaps PA

Marc

Let's presume for the moment that the valves are the correct ones.

- 6SK7GT, remote cutoff pentode, used as RF amp, bottom left of photo.
- X61M, frequency changer with metallic grey sprayed-on shield, centre left with red top-cap wire.
- EBF35, IF amp with 2 diodes, centre bottom with blue top-cap wire.
- 6J5, metal valve, triode with 2 diodes. It only has a gain of 14, cannot yet see the point of having it.
- unknown valve at bottom right? It's in the position where the phase inverter would go.
- 2 empty sockets which form the output pair. If the KT61 is hard to get, you could try a pair of 6V6GT which are much more common.
- unknown large filament-type rectifier, at centre

This adds up to 8 valves.

Like the others have said, I'd be very carefully checking that the wiring matches the valves. All those capacitors should be replaced, and the resistors checked.

Those transformers are massively oversized for the job, but appear to be original. Left hand one is the power transformer, right-hand one is the speaker transformer, centre one I guess is a choke.

Hello

Thanks for your help.

The unlisted unknown valve is a 6sn7-gt
I like these trimax, they dont get hot or smell and the o/t's are so linear with a nice flat freq response, at least to my ears.
The o/t states its rated at 42db..15 watts or thereabouts..but i'd say thats its clean rating, it could probably safely handle at least 35?

I sprayed the thing down underneath with some carb cleaner spray, quickly air blew it out, then did a little soak job on the switchery with metho, then blew that out too.

I did a quick cap job on 4 of the the filters, used some 22uf500 axials
I did not however change out the two preamp/phase inv filters as I didn't have any laying around, I figured the voltage is low there so it wouldn't be too fatal if they popped.
I snipped out that large dark brown slimy cap, very dodgily connected..maybe someone was trying to remove some hum?
I then popped in a couple of 6l6gc...installed a 2a slow blow fuse ( they don't have fuses??) and turned it on watching for cherry on the output tubes.
No cherry and then after twiddling some knobs I caught an evangelist peddling his bullshit somewhere on the broadcast band no doubt.
Flick the knob to P.U and that works too, Ill hook a phono up to that later.
Lovely big sound, loud and lush, a couple of those knobs just seem to turn up the gain, I'd call them lush controls, they make it sound really warm you just can't get that sound on modern day amps.
The audio disappears then fades back in again after you wiggle the on/off knob a bit.I don't think it;s the DPDT switch, I think its a coupling cap close by there.
I want to do a diagram and make a board for this thing, this point to point is just so hard to work with

Are parasitics an issue with these ..or should I just struggle through the thing and change out the coupling caps ( and those two filter caps) and check and replace resistors leaving the point to point as it is?

6SN7GT (2 triodes), sounds correct, one triode is a preamp, other half is the phase inverter.

Using 2x 6L6GC sounds fine.

No fuses in vintage stuff, if something shorted out it could start smoking or burning. So putting in a fuse is a good idea.

I find point to point is actually easier unless it is a very complex gadget. Just replace the parts leaving the new ones in the same place as the old ones were. The main problem is changing the parts on the 6SN7GT as they are under the power switch. You might need to remove it to get in there. Working around the bandswitch could be a pain too. The rest looked straightforward.

The main thing is that it works already.

Thanks for your advice.
i really like the sound of this radio and I will do as you advise.
ive ordered a few more filter caps.
i will remove the switch...note where everything goes...pull the p.i tube socket..put a nice new one there...replace all the carbon comps with new ones and try to replace just the leaky coupling caps.
ive observed the solder is 'clumpy'..i reckon new solder on all the joints is required for reliability.
Also i'll measure cathode voltage and divide into resistor and then use some good old ten watt wire wound resistors for the output tubes.

Are these coupling caps re-usable..they seem to be covered in honey-like goopy stuff

Throw ALL the old caps out, not just the ones that show leakage on your meter. The gooey stuff is wax. Even if they measure good, they WILL leak when hot, and even a 30megohm leak can upset valve operating conditions by allowing high voltages to get to the grid circuits. (Yes, valves are that sensitive).

The only caps I'd leave in are the brown lozenge or square-shaped ones (I see one of each in your photo). They can go faulty too, but when they do it is obvious, such as loud crackling noises which is not affected by the volume control.

I did post on this seems to have gone awry? I have sent Brad a pic of a circuit consistent with what I am thinking.

The set possibly needs some reverse engineering. With a 315VAC secondary, that could produce 275VDC, depending on it being the correct rectifier & 5Y3 is looking wrong: KT61 will handle that voltage and more if triode wired.

The filter caps will give a clue if their working volts are around 500-600VDC.

Regarding the 6J5, (http://frank.pocnet.net/sheets/093/6/6J5.pdf) that gain may be wilful, considering the next tube is a twin Triode. It may well be that all the the 6J5 is doing is feeding the 6SN7. 6SN7 (http://frank.pocnet.net/sheets/155/6/6SN7GT.pdf) was a common driver for paraphase Push Pull and its triodes are quite likely individually providing the drive to the grids of the OP tubes, so a massive gain at the second detector would not be desirable.

If there is a Cathode resistor (pin 8) 250V plate expect 90 Ohms and 100 Ohms on the screen to prevent parasitic oscillation. 6L6 may have same. The cathode of KT61 is rated at 6Watts (http://frank.pocnet.net/sheets/084/k/KT61_Radiotron.pdf)

How a 6SN7 fits into a circuit