I bought an old Vox Valvetronix VT50 for 50 bucks about 8 months ago. It worked perfectly until last week. Then I was playing one morning, took a break and switched it off. Came back an hour later and nothing happened when I pressed the power button. Completely dead!! There were no signs before that of anything strange - always powered up straight away and there were no strange noises etc.
I’ve checked the fuse and it looks fine.
Does anyone have any idea what the problem could be?
This amp was v cheap. But here in Switzerland it will cost me about 100 bucks just for someone to look at it and give me a repair estimate. Then it’s about 100 bucks an hour labor. A new amp of similar quality wouldn’t cost much (250 bucks) but I hate throwing things away and it might be a simple fix.
It has a nice 12” 8 ohm speaker in it and the cabinet is quite nice so the another possibility would be to convert it into a speaker cabinet and buy a small head. Orange Micro Dark looks nice. However, I really do like some of the modelling amp tones I get out of this combo, esp the AC 15& 30 amp models and some of the models simulating some classic fender amps.
Could anyone advise me about what to look at to identify the problem with this amp. i don’t want to just throw it away but i don’t know anything about electronics.
Yes I’m fully aware of that - even cheaper on Anibis.ch. But thats not really the point. My suspicion is that 99.99% of the parts in this amp are perfectly good and it’s a shame to just throw something out that could be repaired, and still used, rather than being dumped in our “dechetterie” (rubbish tip).
I would replace fuse and pull the power cord and reseat that. Theni would open the back an carefully wiggle some components around a little and see if there is bad soldering joint. That’s what I would do on my own.
I would think you are correct that most of it is fine. If components like the valve or speaker were the problem it would still power on.
I bought a small behringer tube/SS that didn’t power on for $20. Re-soldered the power switch leads and good to go.
So think logically and use a meter if you are comfortable. First pull the fuse and test for continuity (they don’t always look obviously burnt), then open it and look to see if anything looks obviously burnt, inspect the connections of the power cord, then check the switch for continuity when on.
Beyond that you will need to know more about electronics and testing. It is probably in the cord connections, switch or power supply.
I think I’ll follow your advice. Yes the fuse looks ok but I thinks it’s worth putting in a new one just in case. I also think it’s likely to be the switch or something in the socket where the power lead plugs in. I’m sure it’s not the tube.
have tested the lead itself with another amp and it’s fine.
Luckily I recently bought an Orange crush 35RT to use at my holiday home but didn’t take it there yet so at least I do have an amp at present.
checking the physical things that can disconnect is a good first start. All connectors, tubes seated in sockets, fuse, solder joints look ok - as has been said.
The schematic that was linked is a gold mine of information. If you have the experience to start checking live circuits, then start by verifying power supplies are working. you have 6.3, +15, -15, +2.5, +3.3, +5, +34, -34 all with test points indicated on the schematic. If these are working, some of the LED should be lit.
BE AWARE that you will be probing a live circuit at this point and there are high voltages for the tubes that you will not want to get a knuckle across. BE CAREFUL!!
Indeed. It’s my understanding that tube circuit voltages can be fatal. Even when the power is off, capacitors can stay charged up…for a few minutes at least.
No risk to replacing fuses and power cords, but anything more should be done by someone with experience.
This one is only about 70V, so it will bite, but not likely to do more. The biggest problem is flinching and dragging the probe across two places you don’t want it touching. Been there done that.
Caps are small on this as well as the tube voltage, so just wait a few moments and it should bleed down pretty low.
This does not seem to be a very high voltage power supply (70v max), so the most danger is around the input AC voltage from the wall. Not the 400v dc in a stereo tube amp. (It also matters the available current and therefore power, I have felt 20,000 volts, however the power was minuscule. Still was a surprise…)
Regardless, voltage can kill so unless comfortable with it don’t mess. At all.
That said, since nothing happens when the switch it turned on, it has to be on the source side of the transformers. No reason to sweat the rest of the circuit.
Ian,
That particular Vox has a 12AX7 vacuum tube for amplification. That may be the issue. Try changing that first before you dig any further. If that doesn’t do it let me know and I’ll research for a schematic and we can go from there.
Best of luck,
Mike from Pennsylvania
One simple thing to do would be to make sure there is power from your wall outlet. Sounds dumb, but try plugging something else into that outlet. It may just be the circuit breaker kicked. Sometimes the most complicated problems have the easiest solutions!
Mike
I’ve never seen one of these amps but I did check out a picture that I found online (The VOX Showroom - Vox VT50 - A Look Under the Hood) and a couple of things came to mind.
First off, keep in mind you are dealing with electronic stuff, it only takes about 5mA going through the wrong parts of your body to stop your heart. Make sure the amp is unplugged and has set long enough to allow the bleed off resistors to discharge the filter caps (as someone else mentioned above). 70 volts DC is not to be considered safe.
Others have mentioned power cords and plugs, all possibilities, but not likely to fail with the actions of just the powering off and back on.
There are two things that I see from the picture that I would consider high probabilities;
1, The power switch. It sees wear from regular use and it’s mechanical parts, that drive the contacts, or the contacts themselves, could have failed during the act of powering off. The safest way to check this would be to unplug the amp, give it time to allow any voltage to bleed off, take a resistance measurement across the switch using a meter.
2. The tube, a 12XA7, has a heater element in it that is very similar to an incandescent light bulb. Just like a light bulb, the act of powering it on can blow out that filament/heater. If you have a pin-out of the tube could could pull it and measure the resistance through that heater filament, (again make sure everything is powered down) or just buy a new tube and try replacing it to see if that works. I see that those tubes can still be found online.
Been there done that.