When to change a valve/tube in amp?

Some advice needed.

I only started learning the guitar in June 23. Knowing nothing about guitars or amps, I bought an old used Vox VT50 Valvetronix amp for 50 bucks. It’s a forerunner of the Vox VTX40 and introduced in 2011, I think.

It’s a sort of hybrid valve/solid state amp. It has one small valve/tube and this is what I read about it online:

“The concept of the circuit is to use a 12AX7 preamp tube in an unusual way - as the heart of a one watt power amp. This one watt power amp even includes a small output transformer to simulate the electronic interactions that happen in high powered tube output circuit. The one watt output from this 12AX7 then supplies a solid state power amp to achieve the final 15 watt (VT15), 30 watt (VT30), 50 watt (VT50), or 100 watt (VT100) output.”

I have read that valve amps need more maintenance than true solid state and that valves need replacing from time to time. As I said, I have no previous experience with amps and nothing to compare it to. It sounds ok to me and functions as it should according to the manual but should I replace the valve with a new one? I doubt the valve has ever been replaced before and it’s probably over 10 years old. As I said, I have nothing to compare it to so I have no idea if it could sound better with a valve replacement than it does right now.

Thanks for any advice.

Is it working?

Is it looking discoloured? Does it rattle or make zing noises when tapped?

Those 12AX7’s dont look expensive to replace tbh $30usd?

It’s working and it’s not discoloured or rattling. The amp works ok but I read that valves need changing from time to time.

I’m not an expert but it looks like 1000-5000 hours of use and pre amp valves last longer.

If you turn it to 11 and wail on it daily for a decade or so, you might damage the valve. That little valve it probably the most robust part in that entire amp. I would not worry about it.

If something goes wrong, and it isn’t obvious, the valve is the easiest part to replace and not expensive, so try that, but I would wager it won’t be the problem. If the valve fails, the amp won’t play and it will be pretty rapid.

If you live in the US and would like a spare , I probably have one in a box somewhere. I could drop it in the mail for you. I am no longer using them. Not 100% that I have a12ax7, but it is common, so I probably do. Or a substitute.

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Thanks for the offer but I don’t live in the US. It’s seems that they are quite cheap so I’ll probably just change it anyway.

Can’t hurt and then keep the old as a spare. You can look up substitutions and see if you want a slightly different tube. Tons of online information and several options.


Jamolay. Thanks the link to that website is really helpful as it basically contains all the information I wanted. Best, Ian

I wouldn’t replace it unless there is a problem with the amp. Take that $30 and experiment with various string types or picks, or buy yourself a Behringer pedal. I recommend the SF300.

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A 12ax7 will last 500-10000 hours, so no need to worry. If it starts to hum, get noisy, or microphonic, replace it. They are cheap, but don’t buy the cheapest Chinese ones, and yiu should be fine.

I bought a Marshall preamp (used) with 12ax7s back in 1997. Gigged it regularly for 10+ years. Still have it, and it still works. Had to change the tubes once, when one of the channels stopped working correctly.

These things last a long time :wink:

So don’t worry about it, that would be my advice…


Thanks to everyone who replied.

I also asked Vox directly and this is what they replied:

“The valve in the VT50 runs at fairly voltages and it is designed to last the lifespan of the amplifier. In other words, unless there was a specific problem with it, there is no need to replace it.”

So in in view of the replies on the forum and from Vox, I will leave it as it is.

Best, Ian