It's a 1939 Rickenbacher (note pre-war spelling here, although there's no markings on it.), made in the US and designed for 110V 60Hz power. Norman had just finished restoring the lap-steel guitar that it came with, as they were sold as a kit.
The amplifier is a very simple design, featuring a 5Z4 full wave rectifier, a 6N7 double triode, one triode as an input amplifier, the other as a phase-splitter, and a pair of 6L6's in push-pull output.The power supply deserves some mention. The speaker does not have a permanent magnet, so there's an extra set of windings in the back of the speaker, which forms an electromagnet. This winding is fed from the power supply, after a small smoothing inductor and capacitor. It's in series with the actual amplifier HT rail, so the current supplied to the amp, passes through, and energises, the electromagnet. Now, all things being equal, this is a stupid idea... but it's put to a clever use. There's some rippe on the HT supply, as the smoothing caps are quite small in value. Here's the clever part... the amplifier is busy humming away, but the field coil is so connected as to effectively cancel out the hum! So not only does it improve the filtering (as it's an inductor itself) but also cancels out a lot of hum. Once working correctly, this works fantastically!
A note about those resistors... they're dog bone resistors... and were about 20-25% accuracy when new. They don't have the normal striped colour code. There's an article on Radio Museum about them here.
All the resistors check out within 20%, and I'm surprised!
The internal connecting wire is cloth-covered and in good condition. The mains lead is rubber, and is also in reasonable nick, but will have to go anyway.. read on!
After a recap, and a check of the integrity of the transformer insulation with a 500V Megger test, I gently power the amp up via the variac to 110VAC. Nothing, not a jot. There's heaters and HT, but no audio. There's no volume control (or indeed any controls of any kind!) but there is nothing coming from the speaker.
Checking around shows there's HT on the anodes, so the output transformer primary isn't open circuit, and the field-coil in the speaker is OK. A check on the secondary shows that's not open circuit. Damn, it's the voice coil....
The chances of getting another speaker are about a million to one, so I CAREFULLY disassembled it, lifting off the field coil, and gaining access to the voice coil. Sadly, I negated to take pictures of this process. I could see the break where the coil connects to the back of the cone. Carefully, I tinned the wire, and soldered it back to the remains of the wire on the cone. I check the resistance of the coil with the meter. 18 Ohms... Bingo! I painted the joint with a blob of lacquer to give it some insulation and mechanical support, and left it to dry before reassembling.
Powering up again, there was some very slight "blow" through the speaker. I plugged in a guitar and gave it a strum. Horrible distortion. It was clear that the voice coil was fouling on the field coil inside the speaker. I slacked off the 4 slotted screws holding the coil assembly onto the speaker basket, and tapped it slightly to one side. A couple of goes later, and it's working well. The level of white noise (or "blow" as it's called in guitar amps) is minimal. There's no hum whatsoever! That anti-phase field coil arrangement is working well!
Electrical safety, or lack thereof...
This amplifier has a metal cabinet, and metal chassis.... and a 2-core mains lead. It's transformer isolated, so I'm ok with that, but that metalwork needs to be connected to a safety electrical earth. I remove the rubber 2-core lead and fit a modern 3-core PVC lead. The owner has a 240-110V auto-transformer to supply power, so that mains is still "live" and not isolated. I fit a 3-pin american plug to the end, and, for peace of mind, perform another insulation test. It's all good. Sadly, although not unexpectedly, as the chassis carries the signal and HT grounds, adding a safety earth has introduced a small amount of hum pick-up. It's not significant though, and better to be safe and humming, than dead and not. As this amp was designed for 60Hz operation, there is some concern that the mains transformer could saturate when being run at 50Hz, and get hot, resulting in failure. I ran the amp for a few hours, and the transformer was barely warm. I pronounce it fit, clean up the years of dust, and Norman is delighted!
Arty valve shot (still getting the hang of the new camera!)
Norman has kindly supplied some pictures of the guitar.
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