Welcome, all! This new blog is designed to address some of the many questions I receive regularly from musicians about technical issues. We’ll cover a variety of topics, and I welcome comments and questions.

First, a quick introduction. I’m Frank, and I am the founder, owner and president of Studio Sound Electronics.   I have nearly 50 years' experience in amp repair.   Although I no longer repair amplifiers myself, I want to share what knowledge I have gained along the way.   As a bit of background, I earned a bachelor's degree in electrical engineering in 1970, did graduate course work following that, and  spent 35 years in the telecommunications industry as a senior engineer at BellSouth and AT&T.   During that time, I also began to repair guitar amplifiers for various music stores in the Louisville, Kentucky area.    I started Studio Sound Electronics in 1997 in order to provide an online source for technicians and do-it-yourselfers to find the parts they needed to repair their amps.   So, let's get started ...

Is it Time for New Output Tubes?

A common question I hear from guitar and bass players is “How do I know when to change output tubes?” A very good question, since output tubes aren’t cheap, and while you don’t want to change them needlessly, you also don’t want to sacrifice tone quality and risk damaging your amp by using tubes that are on their last legs.

So, how do you know when it’s time for new tubes? The short answer is to change them on a schedule, based on how often you play your amp. If you play professionally two or three times a month, plus band practice, then about once every year or two is a good rule of thumb. If you play a heavy schedule (3-4 days per week, 3-4 hours per day or more), about every six months would be better.   If you just play at home occasionally for your own enjoyment, then tubes can last 10 to 20 years.    Remember, moving your amp to gigs a lot is also hard on tubes, because the loading, unloading and sometimes bumpy travel on a highway shakes the fragile internal components.    If your amp rarely gets transported, tubes will last much longer.

Now for the long answer – determining by certain symptoms when your output tubes may be running on empty. These tips are especially useful when you’ve just bought a used amp from a store, an individual or on eBay, and you’re not sure how old the output tubes might be.

Signs that new output tubes might be in order
1. Output is weak, breaking up or distorted.
This one is tricky because it can also be due to the output transformer. But, if only due to complexity, cost, and likelihood of failure, it makes sense to change the tubes first.
2. Tubes show signs of overheating, darkening of the glass, or the plates of the tube glowing cherry red (shut down the amp if you observe this last symptom).
This can also be a sign of output transformer damage or decay or just overbiasing. With any output tube change the bias needs to be checked but with this one it is especially important.
3. Internal arcing
Make sure and check the tube sockets and screen grid resistors if this happens. Usually accompanied by B+ or primary fuse failure.
4. Bad tone
This one is highly subjective and if your output is not less than normal it is likely that your tone issues lie elsewhere – the preamp tubes may be the more likely culprits in this case.
5. Looking for a different sound?
Again, pre-amp tubes are probably a better choice for this but if you have some experience and have heard a lot of tubes, the output section can be a place to tweak your amp. Remember, if you decide to change from 6L6 outputs to EL34 outputs and your amp is not equipped with a bias selector switch on the back, you will need to have the bias circuit modified by a tech.
6. Cracked glass – tube looks “milky” inside.
No choice here. Reach for new tubes. And always change the output tubes as a set – a matched set.

Things that do not directly indicate time for an output tube change
1. Blue light or blue glow in the tubes that pulses with your playing
When tubes are flashed, the residual impurities in the vacuum are more or less removed. The less perfect the vacuum in the tube, the more blue pulsing you will see. By itself this indicates almost nothing. Ignore it.
2. Dirty tubes
Wait until they are cool and clean them with a paper towel sprayed with glass cleaner. Your amp will run cooler and love you for it.
3. Humming, noise, hiss, etc
None of these directly indicate power tube failure. If you don’t know for sure and your tubes are relatively new anyway (say six months of moderate playing or three months of heavy playing) then taking the amp to a tech may be less expensive in the long run. Hum, for instance, could come from out of balance on the output section. But that could be coming from a problem with biasing or the OT or it might have nothing to do with the output section and might be a crossed heater wire in the preamp or an unbalanced hum rejection circuit for the heaters, failing filter caps or any other number of things.

Output tubes are the heart and soul of your amplifier. You change your strings regularly to ensure a good tone – don’t forget to change your output tubes when needed for the same reason.

Until next time, take good care of your amp and it will take care of you.