Vox AC50 - a question for the amp gurus

NPP

Evening all, 

please bear with me for a rambling question. Short version of it: I have a 60s AC50 which I want to revive. 

Longer version: when I wanted my first proper amp in the mid-80s just about the only thing in the local music store I could afford was an AC50 which was sold to me as a bass amp but actually had a really nice (cleanish) Hendrix sound when hit with a boost. So I bought it and played it for a few years; it was one of those by the way, no. 6796 http://www.voxshowroom.com/uk/amp/ac50bb2.html - the most common, least desirable solid-state rectified version. I later decided that playing in a covers band I needed a more versatile rig but had the good sense to hang on to the Vox. Since the early 90s, it has lived (slept?) under a dustproof cover in a dry bedroom at my parents' house, but I have now decided to take it back and revive it. It is now sitting in my study looking clean and ready to go. 

This brings me to the questions:

- can I just switch it on and expect it to be ok, or should I get it checked over first to avoid harm to the amplifier or myself? It was working flawlessly when I put it away.

- are there any checks I can perform myself - never having worked on amps myself, I hasten to add?

- I don't have a cabinet (never had an original Vox cabinet, used a home-built 4x12 with it which I eventually sold). What would be the best speaker to go with it? And could I get away with a 1x12 cab? 

The amp is in Germany so I'm afraid there's no use recommending UK techs who might take a look. 

ICBM

NPP said:

- can I just switch it on and expect it to be ok, or should I get it checked over first to avoid harm to the amplifier or myself? It was working flawlessly when I put it away.

Unwise. Electrolytic caps can degrade completely in 20 years of non-use if they weren't new then, and even if they were fine when it was put away. If one fails on power-up, several bad and potentially expensive, and in some cases originality-damaging, things can happen. Powering it up slowly with a Variac can at best save the caps, and at worst let you know there's a problem before something goes bad. It's also a good idea to warm the transformer through thoroughly over a period of several hours as you bring the voltage up, since they can absorb moisture even if you think they've been stored somewhere dry - the worst case scenario is that you could blow the power transformer, which is probably about a £200 job to replace, and will devalue the amp by a fair amount even if the work is done well.

OK, this is not *very* likely, and from what you've described it will *probably* be fine if you just turn it on, but it's an expensive and avoidable disaster if it isn't.

NPP said:

- are there any checks I can perform myself - never having worked on amps myself, I hasten to add?

Not really. You need a Variac, test meters and a good knowledge and experience of what to look for if it isn't working right.

NPP said:

What would be the best speaker to go with it? And could I get away with a 1x12 cab?

Any 8 or 16-ohm cabinet with a power rating of more than about 75W will be fine. The *best* would be something like a 2x12" with Celestion Gold speakers, but that isn't going to be a cheap solution! You'll get pretty close for much less money though.

A 1x12" will work fine too, as long as the speaker is rated for enough power. Something like the new Celestion G12H-75 'Creamback' might have the right sort of sound, although I haven't tried one yet.

jpfamps

As the man says, I would advise caution when energizing this amp.

Co-incidentally I had a VOX AC30 in for repair that had been powered up after several years of neglect with a shorted main filter cap, which can caused the rectifier to expire.

You have been warned.

NPP

cheers guys, I have been warned and won't try anything rash. Not having a cabinet means I can't just give it a go anyway ...

I'll have a look around locally for a tube amp specialist. Reußenzehn are not far from me I think (i.e. from my German place where the amp is, mostly I'm in Sheffield though) but I will first try to find someone closer to home. 

57Deluxe

...it has a fuse right? well then - someone has to 'turn it on' sometime...! Just as long as is not come in from a low low temp environment or has damp inside....But Before turning on you could probably remove and reseat valves to ensure no oxide build-up.

ICBM

57Deluxe said:

...it has a fuse right? well then - someone has to 'turn it on' sometime...! Just as long as is not come in from a low low temp environment or has damp inside....But Before turning on you could probably remove and reseat valves to ensure no oxide build-up.

No, this is NOT the right thing to do. The fuse is a last-ditch emergency measure which is mostly there to protect the *upstream* wiring once the amp has actually gone faulty - although it can in many cases (but not all) limit the damage in the amp as well. It will never prevent a component from failing since it cannot blow *until* there is a fault.

You can easily do serious damage to the amp by just turning it on, *before* the fuse blows - even if it then does. And that's even if the fuse value is correct… often it isn't, and this needs to be checked as well.

Removing and reseating the valves will do *nothing* to prevent a filter cap failure or a transformer arc, although you should have done that anyway to check for problems in the sockets (surprisingly, spiders and spider webs are electrically conductive…) and to clean the valves - you don't want the smell of the dirt on them burning off to hide that there's something else burning.

By powering the amp up on a Variac, you are not just "turning it on" - you're slowly increasing the voltage from zero. This allows you to monitor the current draw and internal voltages at a much lower point - I start at 24V, which allows a very simple quick check of all the secondary voltages which should read a tenth of the proper values. If there's a fault, the current draw will also be much too high even at this point. After that, you slowly increase the voltage - for an amp which has been stored in a cold or damp place, I literally do it over a couple of hours, adding 24V every quarter hour or so and going back and monitoring what's going on every time.

If I had a hundred pounds or more for every time someone has brought me a badly fried amp that they "just turned on to see if it was still working OK"… actually, I do .

impmann

+1 for ICBM there - you need a Variac to ensure you don't blow something much more expensive elsewhere on the amp. A friend brought up an old Selmer for me a while back that had been found in a cellar at a soon-to-be-knocked-down Working Man's Club. He got to just under 100v when he noticed something wasn't right and powered it down - there was a fault in one of the caps that would/could have done big damage elsewhere had it been just powered up.

As it was, a complete set of filter caps (and a new rectifier valve, as it had a tiny crack in it) saw it running well on its original transformers and Mullard valves. Sadly, I didn't keep it!!