Photo by Andy Ellis.
I always enjoy reading your column and I have a suggestion for a future Ask Amp Man. The subject would be the care and feeding of your tube amp—or something similar. I started working with tubes in the early ’60s in high school. I’m now a retired electrical engineer but still enjoy tinkering with and repairing tube amps (and of course playing through them). I still have my old RCA and GE tube manuals and two ancient tube testers. While there’s a lot of information available on the web, there is a lot of misinformation as well. Some questions you might answer:
How often should I replace the tubes?
Does the phase inverter need to be replaced when I replace the output tubes?
In a 100-watt amp, do I have to replace all four output tubes (or six, as in my Peavey Mace) with a matched set if only one tube is bad?
Does the grid bias need to be reset every time I replace the tubes?
Do I need to replace them with the same brand tubes?
How can I tell if I have a cathode-biased amp?
Is measuring the control grid voltage an acceptable way of setting the bias?
Most guitarists I know play their amp until it sounds real bad or the fuse blows, and then turn it over to me. A little knowledge might help them avoid some trouble.
Keep up the good work,
Thanks, Dave. I selected your question for this issue because I thought it might be timely. Some players may have gotten a new or used tube guitar amp from Santa, and it could possibly be their first foray into the wonderful world of tubedom. Knowing what you have and how to treat it can be a key factor in having the best possible experience, so with the help of your questions I’ll try to impart some good information for those in need. Let’s tackle them one at a time.
How often should I replace tubes? This is a great question that has no absolute answer. Regarding output tubes, I generally tell players the typical tube life for the glowing bottles produced today is approximately six months to two years, depending on use. How long and how loud you play will affect tube longevity, and some brands more than others. Larger bottle tubes—such as the 6L6, 6550, KT66, KT88, and EL34—tend to fall on the longer side, while smaller tubes like the EL84 and 6V6 tend to wear out sooner. It’s always a good idea to carry a spare set with you, as you never know when a failure may occur. With regards to preamp tubes, I feel it’s okay to run them until they start making noises that no longer resemble a guitar, but if you get to the point where they’ve been in the amp for five or 10 years, you might want to consider replacing them, too.
Does the phase inverter need to be replaced when I replace output tubes? Personally, I say no. The phase inverter does drive the output tubes, but it doesn’t suffer the wear and abuse that output tubes endure. I treat it like any other preamp tube. If it’s problematic, I replace it.
Do I have to replace all output tubes in a 100-watt amp with a matched set if only one tube is bad? In a perfect world, I’d say yes. But I’m a practical guy and I always look at this on a case-to-case basis. If the tubes in the amp aren’t too terribly old or don’t have too many hours on them, you can opt for just replacing a pair. Just remember, most amps have what’s known as a push/pull output stage, where each side (either one tube, two tubes, or even three tubes in the case of your Mace) is amplifying half the signal, so you should replace the bad tube plus one from the other side, in order to keep the output stage as balanced as possible.
Does the grid bias need to be reset every time I replace the tubes? If you’re replacing all the output tubes, then yes, I would highly recommend it. Properly biasing output tubes will let your amp sound its best and can also extend tube life. If, as in the above answer, you’re replacing only some of the output tubes, then biasing all the tubes the same may not even be possible, as there is generally one bias adjustment that controls the bias voltage to all the tubes. As with any output tube replacement, I always recommend matched sets, as this helps keep the output stage balanced and will yield the least hum from the output stage.
Do I need to replace old tubes with the same brand? If you’re replacing only some of the output tubes, then I highly recommend trying to find the same type of output tube that’s currently installed in the amp. This will give you the best chance of the new tubes running close to the bias current of the installed tubes. If you’re replacing all the tubes, then absolutely not. There are quite a few selections for each type of output tube, and tubes produced by different manufacturers in different countries sound different. If you have the luxury of auditioning different types of output tubes, do yourself a favor and experiment. You might find something that better suits your taste or expectations.
Photo by Andy Ellis.
How can I tell if I have a cathode-biased amp? Just by looking at the amp, you can’t. However, if you’re handy with a multi-meter, there is a way. You can measure resistance between the cathode pin of one of the output tubes and ground. On large octal sockets for most standard output tubes, this will be pin 8. On a 9-pin socket for an EL84-type tube, this will be pin 3. If you measure zero resistance, the amp is grid biased. If you measure in the area of approximately 50 to 250 ohms, the amp is cathode biased. Unless there is a problem with the amp, this should hold true, with the exception of some late-’60s Fender amps, which exhibit both grid- and cathode-bias characteristics.
Is measuring the control grid voltage an acceptable way of setting the bias? No. Some schematics do show a bias voltage reading, but in almost all cases this is merely a reference for troubleshooting in order to determine if the voltage is in the ballpark. It’s always best to measure the actual bias current flowing through the tube at idle. Quite a few companies offer bias meters that include an assembly that is inserted between the tube and socket to measure the actual bias current. This procedure is irrelevant on cathode-biased amps, as these are also known as self-biasing and are generally not adjustable.Well, PG readers, there you have it. There can be exceptions, exclusions, and further explanations, but I hope this helps some of you understand a bit more about what to expect from your tube amp and what it expects from you. Let’s keep those tubes glowing!