Firing up an old valve amp of unknown provenance

[Simon Barden]

A friend of mine has unearthed a Sound City 50 Plus valve amp that has been in his loft for at least 10 years. It's a non-master volume amp with normal and bright channels. I'm interested in it, but he's scared of powering it up as he can never remember it being used. It was his partner's son's amp (he moved out 10 years ago), and he knows that the son never had a cab for it, so he may never have used it.

He's had a look inside and there are no signs of leaky or damaged caps. He's vacuumed it out but there wasn't a lot of dust in there.

I've told him to take the valves out first, check the fuses are OK, then fire it up with no valves in and make sure no fuses pop or smoke appears. Then stick the pre-amp valves in and run it again. If still OK, put the power valves in. (All done with the volume off and a cab connected).

I know some people recommend using a variac to increase the voltage slowly, but there is none handy.

Is my start-up suggestion OK? Anything else to consider.

[Marcel]

What you've suggested Simon is pretty much what I would do if I didn't have access to a variac.

An alternative is to wire a mains voltage 100W incandescent bulb in series with the amplifiers mains, which if there is any short in the amp will cause the lamp to light up brightly, and will only glow dimly and reduce the voltage slightly to the amp if the amp is okay. The best way of doing this is to make a jig with a lamp holder and a suitable power outlet both securely and safely mounted to a small wooden board. Some may choose to include a switch to bypass the lamp to allow full voltage checks without having to re-plug things.

Something to consider... Short of going over the entire amp with all the appropriate meters my usual practice when powering up old devices like this is to leave the amp on for a few minutes during the test... Caps are often a little slow to react to being put back to work.... Applying full voltage for 5 to 15 minutes once all the tubes are back in while maintaining a comfortable distance will inspire confidence that all is okay. Often it may take 5 or 10 minutes for a bad cap to build enough internal pressure inside before horribly and loudly detonating in one of those few moments while you are otherwise distracted..

[Simon Barden]

Thanks Marcel.

100W incandescent bulbs are rare as hen's teeth now in the UK. They were banned several years ago, so everything (apart from speciality bulbs) are now low-energy bulbs, which are small fluorescent or LED types.

But yes, I understand the purpose/use. We often used to spec 'fuse failed' neon indicators on safety-related control circuits for HVAC control panels and MCCs.

[Marcel]

Yeah, the impacts of some political and executive decisions are hidden, and sometimes only add inconvenience and cost to a once very cheap processes.

I have seen one 'jig' that used 110W worth of halogen down lamps (2 x 55W) with two step down transformers in parallel, and another jig that used a whole bunch of 15W incandescent oven lamps. What is needed is a purely (or mostly) resistive mains rated passive load capable of at least 2 or 3 times the wattage of the device under test and which does not care if it gets less than mains voltage applied to it. Nil SCR or dimmer control at all, but some minor inductance or small capacitance can be permitted. Due to the way they work I seriously doubt if twenty 5W LED style fittings in parallel would be entirely suitable for our field of audio purposes.

[Simon Barden]

I think not. Luckily one can still get bigger wattage incandescent bulbs of the more specialised type. I think we've probably got some screw fitting standard incandescent bulb lurking somewhere that were accidentally bought instead of bayonet fitting ones.

[dave.king1]

What about heat lamps for medical purposes or the bathroom or for that matter what about bar heaters or space heaters if they still exist

[DrNomis_44]

You could also make sure you have some kind of a dummy load handy that you can plug into the speaker outs on the back of the amp, you don't want to end up frying the output transformer which could be expensive to replace, make sure the dummy load is the correct impedance and is rated for about the same power that the amp can put out.

First thing I would do is give the underside of the chassis a good look over just so that there's nothing obviously wrong, next thing I would do is use a multimeter to check that there are no shorts to ground from any of the mains power wiring going to the power transformer primary, also check to see that there are no shorts to ground on the secondary side of the power transformer too, depending on how old the amp is I'd be wary of the electrolytic caps if they look original, I think the useful lifespan of some electrolytic caps made back in the 60's or so used to be something like 20 odd years for some brands, nowadays the electrolytic caps we have available tend to be more reliable.

Having had first-hand experience of an electrolytic cap exploding, I can say that it's not a pleasant thing, they also tend to give off a quite pungent smell when they do go bang.

Those Sound City amps are supposed to be pretty good and worth quite a bit if they're in working condition.

[Simon Barden]

Not worth anything like as much as a Marshall of similar vintage. This was an early 70's model. Obviously they'll be more common in the UK than elsewhere, but a very good condition version of that amp would go for around £350 max (around Aus$560). But even not working and sold for parts, they sell for around £220 (around Aus$350), so there's only a small margin for economic repair.

[Simon Barden]

The main problem is that my mate lives about an hours drive away (without snow getting in the way) and he's not keen to fire it up. I've made an offer to take it as-is, so I'll see what he says. I don't need it, but another amp is always nice, especially as I don't have an EL34 amp at all, either 6V6 or 6L6. I'd then need to make a suitable cab, probably 2x12", which would then be another project for me when the weather turns warmer. It would look a bit silly sitting on top of my 1x12" Marshall cab.

[DrNomis_44]

If it still works properly, then I reckon it's worth hanging onto, all you'd need is a speaker cab with the correct impedance and enough power handling, hope it all works out.


I'm definitely hanging onto my Fender Super Twin amp, I've got a great amp with EL34 tubes in it, and now I've also got a potentially great amp with 6L6 tubes in it.

I'm just wondering if those Sound City amps are based on the Marshall JTM45, which was based on the Fender Tweed Bassman, from looking at the front panel layout, it looks similar to a JTM45.