Greetings all, I recently came into one of these dangerous little guys and decided to make it into a functioning piece again. I've added an iso transformer (no bridge...yet) and an earth'd cord. When I received the amp, the output transformer was completely disconnected (loose in the bottom of the cabinet) and I wasn't able to see exactly from where it had been removed/come off. I've been hard-pressed to find a schematic, so I'm wondering if any of you smart folks might be able to offer some advice on reconnecting it.
Cheers!
Welcome billtrevaskis.
I'm glad you recognise that these are seriously dangerous as original.
In a small output transformer the primary side typically has stranded wires and the secondary typically is single core enamel-coated copper wire. The primary will have a resistance of 10's to 100's of ohms, while the secondary will only be a few ohms as most and may look like a short.
If it's a single-ended amp (one output valve) then one end of the primary will go to the output valve anode and the other end will go to the B+ supply, the secondary naturally going to the speaker.
If it has been removed (as opposed to being torn lose by the amp being dropped, hit by a truck or whatever) then it may be faulty.
You can use your ohm meter to check for continuity of the primary and secondary, and that they aren't shorted together, but you will need a little test rig to check for a shorted turn, the other most common fault.
(http://www.ozvalveamps.org/repairs/xform_test.gif)
http://www.ozvalveamps.org/repairs/trannytest.htm (http://www.ozvalveamps.org/repairs/trannytest.htm)
Roly, this is great info. Many thanks. Ohm test seems to yield normal results (190ohm-primary, continuity≈secondar). When the amplifier was given to me, the transformer looked like it had shaken loose from years of sitting around and then being moved a few times.
I've now connected the one side of the primary to the plate of the output valve (50L6GT) and the other side of the primary to the cathode of the rectifier(?) (35Z5). Secondary has been connected to the speaker and now there's quite a buzz coming from the speaker. It's uncontrollable via the 1meg volume pot (which when ohm tested looks to be working fine). Any thoughts?
Just a thought this morning: could the hum be a bad filter cap? Funny enough, the original cap which is still installed says "good for one year" on it.
What pre-amp tube do you have? Take a look at this and see if anything looks similar to your set-up.
A reasonable level of hum tends to suggest that the tranny doesn't have a shorted turn.
Quote from: billtrevaskiscould the hum be a bad filter cap? Funny enough, the original cap which is still installed says "good for one year" on it.
Yes, it could. Given that "one year" possibly ran out about 40-50 years ago (and there isn't a lot more in the amp anyway) it would be a fair bet.
The simple test is to tack a suitable cap across the filter cap(s). If this cures the hum then properly fit the cap(s).
Any other caps that are waxed paper type (typically a brown cylinder with a waxy coating) should be replaced as well as they will just be resistors by now. And talking of resistors, in an amp of this vintage I'd check all the resistors are still within 10% of marked value and replace those that aren't.
Okay, I've got a few caps coming via mail (I live on an island, so going to the local component store is not really in the cards for me).
I'll keep you all updated on the progress. Thanks for your help!