How do you replace preamp and power tubes in a guitar amp?

March 13, 2007 7:06 PM

How do I replace my preamp and power tubes?

I have a Crate V3112. Unfortunately, it seems that my stock preamp tubes have died (the gain fades after a few minutes of playing and there is extra humming). I figured I'd replace both my preamp and power tubes.

I ordered 3 Tung-sol 12AX7's and 4 JJ EL84's to replace the stock Sovtek 12AX7A's and EL84's. Being that this is my first tube amp, I have no idea how to replace the preamp tubes and power tubes.

I understand that all amps are different, but does anyone have detailed instructions on how I can change the tubes myself? The only amp repairman I know of in San Francisco has a 2 week turnaround time.

I don't know if this matters but the new Tungsol tubes are 12AX7, while the stock Sovteks are 12AX7A. Does the extra "A" mean it's incompatible?

Do I need to get the amp professionally rebiased even if I'm replacing the tubes with the same kind?

Here are some specs in case you need them:

OUTPUT POWER RATING
30 Watts RMS @ 6 % THD, 8 ohm load, 120 VAC

POWER REQUIREMENTS
120 VAC, 60 Hz, 35VA
100/115 VAC, 50/60 Hz, 35VA
230 VAC, 50/60 Hz, 35VA

Bonus points if you know a website with pictures or diagrams explaining the installation process.

posted by atmu to technology (13 comments total)

rebias? professional? 12AX7A?

Nah, you're good. Grasp the base of the old tube. Firmly pull upward. Check out the arrangements of the pins as the tubes come out.

Take the replacement tube, arrange the pins so they fit in the socket like the old tube did, then firmly press down.

Repeat for each tube.

My guess, by the way, is that you blew a power tube, not a preamp tube. Preamp tubes almost never die. You could test this by replacing the power tubes first and then trying the amp out. If it still doesn't work, replace the preamp tubes.
posted by ikkyu2 at 7:19 PM on March 13, 2007

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 usually turret mounted, you just twist one direction or the other and they'll come off. The preamp tubes won't necessarily be covered, but it is common on guitar amps, because the tubes are often oriented down, and we don't want them to fall out.
posted by RustyBrooks at 7:43 PM on March 13, 2007

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.
posted by magikker at 8:34 PM on March 13, 2007

Found it

I recommend this.
posted by magikker at 8:36 PM on March 13, 2007

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 last longer, and age far better.
posted by paulsc at 9:08 PM on March 13, 2007

His amp is actually a class A amp.
posted by RustyBrooks at 9:41 PM on March 13, 2007

His amp is actually a class A amp.

To be pedantic his amp is almost certainly not class A. Cathode biasing does not make an amp class A in and of itself. If you're interested read Randall Aiken's the Last Word on Class A.
posted by 6550 at 10:00 PM on March 13, 2007

The Crate website says class A, but who cares? What matters is whether this amp is fixed or auto bias. Crate's website and the manual are not helpful. Since the manual, which only talks about tube amps in general not this one in particular, spends so much time on biasing I can only guess that it is fixed bias in which case you should have it professionally biased after you retube it, although it's not like you can't play it in the meantime. Just don't wait too long because if it is far off it will really stress the tubes.
posted by caddis at 10:34 PM on March 13, 2007

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.
posted by paulsc at 11:02 PM on March 13, 2007

Schematic

It uses auto bias.
posted by caddis at 11:23 PM on March 13, 2007

"It uses auto bias."
posted by caddis at 2:23 AM EST on March 14

If that is the schematic for the amp, as it appears to be, I agree that bias is not adjustable. Change whatever tubes you like, until it sounds like whatever you want it to sound like. Start with the EL84 output tetrodes.
posted by paulsc at 12:20 AM on March 14, 2007

Nobody mentioned this.

WEAR. GLOVES.

Oil from fingerprints can create a hotspot on the glass envelope of the tube (esp. if your running the power tubes hard.)

*Every* fast failing power tube I've seen had a fingerprint on it. They're not as bad as theatrical lamps, where a fingerprint can result in a lamp failing in minutes (then again, they draw 400-2000W, not 12W), but fingerprints don't help.
posted by eriko at 4:45 AM on March 14, 2007

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.
posted by Chuckles at 9:00 AM on March 14, 2007