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Why?

lenbone said:
Thanks Jeff ! I'm sure my tech buddy will take care of my tube needs,And yes I promise I will put them in the proper sockets !! lol I did notice last time I checked them (pre-amps) are kinda tricky to put back in, you gotta be "Gentle" Thanks again ! --L.B.
Hey Mr. Bone, are you sure you need "fresh" tubes?

Reason I ask is that preamp tubes are voltage amplifiers and deal with small currents. They don't wear out like you may think. True, tubes are glorified lightbulbs and can burn out anytime but like a lightbulb a preamp tube will work "brightly" until the end of its life.

How long is the typical life? I've pulled tubes over 40 years old from some amps that still are working just fine!

Not always of course and some do fail earlier but I've found that replacing them without actually checking doesn't give any value for the money. When in doubt the easiest check is to replace the tube with a new one and listen for any big and obvious jump in gain. If there's not much difference then just put the old tube back in.

The 12AT7 reverb driver does work with power and wlll often grow weak faster than if in a regular preamp application. The tube is used to generate a watt or two of power to drive the reverb springs. In effect, it's running like a small Champ! You could take the cable connecting to the input of the reverb tank and put it into a small speaker and it will work just fine. In fact, that's the easiest way to check if the reverb driver stage is working. Anyway, again it's easy to tell if this tube is ok - is the reverb working or not?!

One preamp tube problem that can happen regardless of age is "microphonics", where the tube becomes vibration sensitive. If you tap the tube you'll hear it amplified in the speaker, just as if it was a microphone! Sometimes if the tube goes really bad in a combo amp with the speaker built-in you can get acoustic feedback squealing with the bad tube picking up the signal from the speaker, just like a microphone too close to the PA speaker.

My point is that I hate seeing guys paying $50 or more for a set of preamp tubes for no good reason, just because some minimum wage music store saleskid thinks he's a technician or simply wants to clip you for the money. 80% of the time there's not a damn thing wrong with them!

Now output tubes (that's the BIG ones!) are a different story. They DO handle lots of current and will wear out far more quickly than preamp tubes!

How quickly? Depends on the tubes and how often you play. That's why we techs have tube testers. As a rough guess, if you wail at a good volume at least 3 nights a week then after a couple of years you may be needing a fresh set.

Most important, output tubes usually need to have the idling current set with every tube change. This is called a "bias job" and a good tech will charge maybe $40-$50 dollars to set the amp up for the new tubes. Many guys like myself will also do a quick overall check while we've got the amp open - sometimes such an inspection will catch a developing problem early. Can't hurt!

I've posted a long-winded explanation of biasing output tubes here before so I'll give you the short version. New tubes vary in characteristics, even of the same brand. When you stuff 'em into your amp it's a crap shoot whether or not the bias adjustment is "close enough". If the idle current is too low the amp will have a "thinner" more sterile tone and less power. Idle current running too high will be warm and loud but the tubes will have a much shorter life - like in extreme cases maybe days! So it's worth it to have them set to the "sweet spot".

Another benefit to buying your tubes from a good tech is that he can mark the old ones as to "good for a spare" or "NFG" (abbreviation obvious).

Just FYI and IMHO!
 

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"Exactomundo!"

Ripper said:
I agree completely with Bill, far too much hype, "catch phrases" and misinformed audiophiles out there. Preamp tubes can last forever. Your tech buddy is right, the sizzling could be a preamp tube, or a cap, or transformer. Don't jump on the change every tube in the amp scenario right off the bat. True enough the preamp tubes and change the sound, but I've seen it many times when one tube makes the difference.
Ripper's right. Changing every tube as the FIRST action is like paying to change your car's engine when it only needed a tuneup. Great advice to give if you're the one selling the tubes instead of the one buying them!

Believe it or not, some of us techs charging you for repairs and tubes like to sleep guilt-free at nights!

Sizzling can be a tube but also a whole bunch of other things, like a resistor breaking down. I'd try taking ONE new tube and swapping it with the other tubes one at a time. If a tube is "sizzling" then you'll find it real quick! Why pay for all of them if the problem comes from just one? Or none!
 
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