How do you replace preamp and power tubes in a guitar amp?
It’s possible his Crate is Class A, which makes biasing more, not less important. A tube stage operated Class B has something like a 50% duty cycle, whereas a Class A stage is always working. Definitely have the thing properly biased, and checked for peak output, clipping, and overdriven conditions, once the tubes have been changed as a set. In some amps, there will be gain limiting pots that can be set, so that the amp isn’t overdriven, which forces clipping, which is a kind of high order distortion that is generally not at all musical, and really hard on tubes and the speaker.
Rebiasing the preamp tubes is probably unneccessary. The rough and ready school of rock on amp repair also says you can probably swap the power tubes out too, and if they’re similarly specced you’ll probably be OK. But not definitely. I’ve built power amps, and when I build them I include test points to measure DC amperage flow through the power stages. I use this to determine at rest power consumption to determine whether I’m likely to burn out a tube. Whether or not this is important depends on how close to the limits of the tube’s specs the amp is pushing towards, and how marginal the replacement tubes are. Most likely, replacing a power tube with a very similar one will be OK. In all actuality the most likely problem is that it won’t sound the same if the bias point is different. Preamp tubes can be kind of hard to get out. Just wiggle gently and pull, they’ll come out. Make sure you remove any kind of metal cylinder or cap w/ springs holding them in first. The metal cylinders are
Guitar amps are generally pretty dirty, by comparison to hifi amps. Guitar players generally like distortion, and the “sound” of an amp, whereas hifi afficionados want “a straight wire with gain.” So, in guitar amps, the level of negative feedback is generally pretty small (maybe 1/10 to 1/1000 the level employed in hifi amps), and there are resultantly, fewer amplification stages (often just 2 in guitar amps, whereas most hifi amps will have 3 or even 4, to develop sufficient negative feedback). So, bias in a guitar amp has a lot more to do with tube life than it does in a hifi amp, particular in the final stages, where a guitar amp generally operates as a Class B device, while hifi amps will typically run as Class AB. So yes, if you replace your tubes, you should re-bias the amp, particular in designs where maximum interstage gain is used to keep the number of gain stages to minimum. Your tubes will las
Well, I’m a transistor guy, but.. Biasing is important because: if it is low, you will not be running in Class A or Class AB mode, on a push-pull amp you will be in Class B and on a single ended amp you will be clipping at very low volume; if it is high, you will over drive the output devices (tubes, transistors, whatever) you just put in, and they will fail much sooner. In transistor amps there is the additional problem of thermal runaway – when you run the parts a little too hot they have a tendency to get even hotter, and this spirals out of control until.. Well, as some like to say, until the magic smoke comes out. This all happens fairly fast, a lot of the time, so it is a case where you really can’t “try and see what happens” – it will be too late. Basically it is a big problem, and requires significant effort to mitigate. I doubt that it is as big an issue with tube amps, but I really have no idea.
I used to know of a website that I can’t seem to find right now, that would sell labeled tubes. So that you could buy a set of red labeled ones and get your amp professionally biased. Then as long as you kept replacing them with other red labeled tubes you could just pop them in and out with having to get the amp biased. Biasing is more important with the bigger older amps, you are probably fine.
