Mystery amplifier

[Radio Wrangler]

One thought in the back of my mind.... What if those output transformers have only 15 Ohm secondaries?

That could be awkward with modern speakers.

Speaker development over the years has made significant improvements in quality, but usually by sacrificing efficiency. They've been taking a free ride on cheaper power from solid state amplifiers. The dominance of 8 Ohm speakers is to suit solid state amplifiers.

Tracing the output circuitry should help, and transformer turns ratios can be measured.

David

[toprepairman]

Looking at the pictures of the underside again it would appear that the transformer secondaries have a number of taps so should accommodate all speaker impedances.
I would recommend the OP gets in touch with Grim Joseph of this parish who I believe is not too far from him and and he could easily guide you through the amp.

[knobtwiddler]

Good advice re: GJ. He will have a bench HT PSU with current limiting, as well as dummy loads, which will make for the safest way to revive the amp.

[Trigon.]

As trobbins pointed out earlier, the two secondaries are wired differently. It appears that each transformer has two secondary windings, wired in series on one channel and in parallel on the other...



The jumper wires are neatly formed so it doesn't look like it was just for a quick test. I wonder why it should end up that way??

It seems unlikely that it's to correct a manufacturing error in one of the transformers, so one explanation might be that at the end of its previous life it was being used with two different speakers with different impedances ?

Although given the attention to detail in construction that might seem out of character?

Cheers

[Kokotoni Will]

Originally Posted by Radio Wrangler

So, one important step to take with your amplifier is to trace out its circuit and post it here. The known published designs will get recognised quickly, and any variations on their themes will be spotted. Only then will you really know what you've got. David

I'll have a go. Should be fun!

Originally Posted by Welsh Anorak

Might it be an idea to pop to the auction house and have a chat.

Good idea. I was planning to go today but weather forbade.

Originally Posted by toprepairman

I would recommend the OP gets in touch with Grim Joseph of this parish who I believe is not too far from him and and he could easily guide you through the amp.

He's a little out of the way, but his website gives a promising impression. I'll certainly bear him in mind.

[Kokotoni Will]

I've done some resistance checks between the amplifier output transformer pins, and there seems to be a short between pins 1, 2, 3, 4, H, J, K, and L (numbered as per photo). Am I right in thinking this does not bode well?

Here are the other results I got; all just rapidly increasing or decreasing numbers - no fixed results. Is it worth checking the remaining pins?

1-2, 3, 4: short
1-5: 100k, increasing (200k setting), or 500k & decreasing (2M setting)
1-6: 29k & increasing (200k setting), or 68k & decreasing (2M setting)
1-7: 15k & increasing (200k), or 30k (2M)
1-8: 1.3M & decreasing
1-9: decreased to 75k, then started increasing
1-10: 3M and decreasing
1-A: 2M and decreasing
1-B: 1.5M and dec.
1-C: 1.2M & dec.
1-D: 1.5M & dec.
1-E: 1M & dec.
1-F: 1.2M & dec.
1-G, H, J, K: short
1-6, 1-7, 1-8, 1-9, 1-A, 1-B, 1-C, 1-D, 1-E, 1-F, 1-G: open
1-H, 1-J, 1-K, 1-L: short
2-3, 2-4: short
2-5: dec. to 63k, then inc.
2-6: 2.5M & dec.
2-7: 1.4M & dec.
2-8: 1M & dec.
2-9: 15k & inc.
3-4: short

[Station X]

Are you sure it's a short and not low resistance?

[Silicon]

My guess is that 5 and A are linked together because they are the HT connection.
The Orange wires may go to the screen grids (G2) and 7 & 8 go to the Anodes.

Please measure the resistance from 5 to 6 and then 5 to 7.

Then measure 5 to 9 and 5 to 8.

Then measure B to C and B to D.
Then measure B to F and B to E.

[Kokotoni Will]

Originally Posted by Station X

Are you sure it's a short and not low resistance?

Yep.

Originally Posted by Silicon

Please measure the resistance from 5 to 6 and then 5 to 7.

Then measure 5 to 9 and 5 to 8.

Then measure B to C and B to D.
Then measure B to F and B to E.

5-6: 130 ohms
5-7: 290
5-8: 227
5-9: 92
B-C: 86
B-D: 216
B-E: 277
B-F: 121

[Techman]

I agree that it won't be a short but just a very low resistance, which would be expected on an output transformer secondary. I actually think that unless anyone has done something really stupid with this amplifier in the very recent past then there will be absolutely nothing wrong with ANY of the transformers. It's been very neatly constructed, probably the best I've ever seen and anyone messing about with it would have likely disturbed some of those very neat connections and there's absolutely no sign of that.

The one thing that we don't know is how well it originally worked. I say this because it's obviously homemade, but to a very high standard. There's some odd things about it that perhaps point to problems when first tested in that very strangely the power supply chassis has been spray painted black on the inside - not a good idea! There's a lot of fixtures that have been made without consideration for efficient grounding and you can see evidence of where there's been a problem with the transformer mounting bolts having to be removed and the paint scratched off the mounting points for a couple of them. Valve hats on the output valves is also a bit of an odd one to me.

I agree that the missing control unit is almost certainly a passive unit and this is likely to have been mounted into a built in cabinet of some sort, probably a large hi-fi corner unit next to a fire place in a living room somewhere. I'm not sure whether the mains comes from the Bulgin input and goes straight to the transformer. You'd expect there to be some sort of 'user' control panel mains switch somewhere, although these often get bundled with the wiring up to a control panel, this isn't a good idea as regards to hum pickup, so may have not been done this way in this instance. I suspect that this amplifier setup was built and installed in a hi-fi cabinet built into a house living room somewhere, and the person who built it died and house clearance people came in and removed what was easily accessible and left behind what wasn't, ie, the bolted in control unit and all the interconnecting wiring loom with its plugs and sockets that would have still been connected to it. Those two units, the amplifier and its power supply would have been easy to remove from the cabinet, as it's obvious that they weren't fixed and were just free standing - the brass threaded inserts on the bottom of the amplifier chassis would have been for fixing a base cover plate, if one were ever fitted to it...well that's my opinion and there's a lot of guess work going on here, but I suspect that I'm not far off the truth!

You definitely need to go back to the auction house ASAP and see if you can find out where it came from. I suspect that the missing parts won't be there, but still in the house that got cleared out, so you need to be quick!

[Kokotoni Will]

Originally Posted by Silicon

The Orange wires may go to the screen grids (G2) and 7 & 8 go to the Anodes.

Continuity tests have shown you're right. The orange wires go to the screen grids via 10k resistors.

Originally Posted by Techman

I agree that it won't be a short but just a very low resistance, which would be expected on an output transformer secondary. I actually think that unless anyone has done something really stupid with this amplifier in the very recent past then there will be absolutely nothing wrong with ANY of the transformers. It's been very neatly constructed, probably the best I've ever seen and anyone messing about with it would have likely disturbed some of those very neat connections and there's absolutely no sign of that.

The one thing that we don't know is how well it originally worked. I say this because it's obviously homemade, but to a very high standard. There's some odd things about it that perhaps point to problems when first tested, in that vary strangely the power supply chassis has been spray painted black on the inside - not a good idea! There's a lot of fixtures that have been made without consideration for efficient grounding and you can see evidence of where there's been a problem with the transformer mounting bolts having to be removed and the paint scratched off the mounting points for a couple of them. Valve hats on the output valves is also a bit of an odd one to me.

I agree that the missing control unit is almost certainly a passive unit and this is likely to have been mounted into a built in cabinet of some sort, probably a large hi-fi corner unit next to a fire place in a living room somewhere. I'm not sure whether the mains comes from the Bulgin input and goes straight to the transformer. You'd expect there to be some sort of 'user' control panel mains switch somewhere, although these often get bundled with the wiring up to a control panel, this isn't a good idea as regards to hum pickup, so may have not been done this way in this instance. I suspect that this amplifier setup was built and installed in a hi-fi cabinet built into a house living room somewhere, and the person who built it died and house clearance people came in and removed what was easily accessible and left behind what wasn't, ie, the bolted in control unit and all the interconnecting wiring loom with its plugs and sockets that would have still been connected to it. Those two units, the amplifier and its power supply would have been easy to remove from the cabinet, as it's obvious that they weren't fixed and were just free standing - the brass threaded inserts on the bottom of the amplifier chassis would have been for fixing a base cover plate, if one were ever fitted to it...well that's my opinion and there's a lot of guess work going on here, but I suspect that I'm not far off the truth!

You definitely need to go back to the auction house ASAP and see if you can find out where it came from. I suspect that the missing parts won't be there, but still in the house that got cleared out, so you need to be quick!

I agree. I'll have to go in next Tuesday. I bought them on 20th August, so have left it a bit late, but fingers crossed. There were a lot of other vintage radios and boxes of old testing equipment sold that day, but nothing that I can remember bore any resemblance to a control panel.

[Techman]

Originally Posted by Kokotoni Will

I bought them on 20th August, so have left it a bit late, but fingers crossed. There were a lot of other vintage radios and boxes of old testing equipment sold that day, but nothing that I can remember bore any resemblance to a control panel.

Ouch! That is a bit of a while ago, but fingers crossed for you and I really do wish you the very best of luck!

If the reception give you the cold shoulder, don't be afraid to go out the back if you can and have a word with some of the other staff such as the porters etc. They may be able to tell you who brought the units in if you approach them the right way.

[See_Mos]

Silicon is spot on with the primary side of the transformer connections.

Techman may be incorrect saying the missing control unit is almost certainly a passive. I suspect that the regulated HT voltage would be for an active pre amplifier or possibly a tuner?

The unit looks suspiciously like a stereo version of the ultra linear Mullard 5-10 but the transformers do look a bit on the big size for a ten watt amp.

From the first pictures the Jones connector on the back appears to have only four pins so simply 6.3V heaters, HT negative and HT positive. The colours of the wires make it obvious which is what.

Instead of speakers ten ohm ten watt resistors would make a safe load.

I am not suggesting that the unit should be powered up without taking the usual precautions though.

[Techman]

I originally thought that it might have been a 'powered' preamplifier, but then changed my mind, however, you could well be correct.

I always tack in watty wire wound resistors under the chassis onto the speaker connections in place of actual speakers as a 'load' when I'm working on an amplifier like this, as suggested in the above post, it saves a lot of 'noise'!

That amplifier will likely just need several of the original paper type coupling capacitors replacing and then it's likely to just work, that is once the interconnections are sorted out.

[Silicon]

Although the resistance is not directly related to the number of turns, the screen grid tap on the output transformer primary is about 40-45% of the anode winding size.

For a given number of turns, the windings on the inside of the core need less wire than the windings on the outside of the core and therefore have a lower resistance.

[Kokotoni Will]

After a bit of cleaning up, the transformers appear to be stamped "P.P.L.7".

[Kokotoni Will]

Originally Posted by Techman

Ouch! That is a bit of a while ago, but fingers crossed for you and I really do wish you the very best of luck!

If the reception give you the cold shoulder, don't be afraid to go out the back if you can and have a word with some of the other staff such as the porters etc. They may be able to tell you who brought the units in if you approach them the right way.

Thanks! I'll probably need it!

[Techman]

Originally Posted by knobtwiddler

Good advice re: GJ. He will have a bench HT PSU with current limiting, as well as dummy loads, which will make for the safest way to revive the amp.

I would curse if someone brought something like that to me with that multipin interconnecting cable and plugs missing! It would have to be completely hard wired and then how are you going to handle it when the two units are all hardwired together? It'll be a right hand full!

That multipin Jones type plug might be available from somewhere. Someone might even have one in their junk box.

I've noticed that there's a period Leak control unit that could well be matched to that amplifier in the 'for sale' section of this forum at this very moment if it hasn't already been sold. It's got 'octal' connections that would have to be matched up.

Edit: third item in the link below:-
https://www.vintage-radio.net/forum/showthread.php?t=222694

Edit again: that control unit is probably mono, so no good for this amplifier. It's also 'collection only'.

[Kokotoni Will]

Originally Posted by Techman

That multipin Jones type plug might be available from somewhere.

https://www.ebay.co.uk/itm/326471868107

This looks like it would work provided that that one thick round pin could be removed. There's a few on eBay, but they all have that one thick pin. Do you think it's worth buying?

[daveobuchanan]

In line with my A.W.A guess, I wonder if it was a monitoring amplifier for some of their pro gear, thus well made and rare, especially in this country.