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Old April 20th, 2009, 01:41 AM   #1 (permalink)
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tube differences? Soft, Med, Hard?

What do these differences mean really? Gain? I want to buy a new set of EL34 power tubes that are matched.

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Old April 20th, 2009, 05:41 AM   #2 (permalink)
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My understanding is, (GrooveTubes as an example) soft tubes break up the earliest/quickest, medium tubes are what most manufacturers put in thier amps as a "middle of the road" break up and hard tubes give you the cleanest signal/max headroom before break up. If I'm incorrect about this, someone wil set things right.

When I replace tubes, I always go for the hardest/cleanest tube I can get, but that's just me-I like a very clean signal for what I do.
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Old April 20th, 2009, 11:58 AM   #3 (permalink)
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When power tubes are matched, they need to be matched for current draw, so it'll take approximately the same bias current to run them in the right range. If you put one tube with a high current draw in the same amp with another tube with a low current draw, unless the amp has individual tube bias adjustments (rare), one tube will run too cold and one will run too hot - if the hot tube is hot enough to redplate, that can cause a very expensive amp failure, burning out at least some resistors, and maybe taking out your output transformer. To measure the current draw the tubes need to be put in an amp, or a tube tester that runs them at full voltage with a load. There can be quite a variation between tubes of the same type and manufacture. Ideally when you buy new power tubes for a fixed (i.e., adjustable) bias amp, you measure the plate current with the tubes in question in place, then set the bias current to bring the tubes to the proper operating range at idle (target is often 70% for fixed bias push pull amps, can be 90% for cathode bias amps). Once you've got the amp running in the proper range with a particular set of output tubes, if you had another set of output tubes that were matched with the first pair for current draw, you could swap them in without needing to rebias. That's the point of the "soft, medium, hard" or numeric ratings that some tube resellers use. If you replace a "hard" set of tubes with a "soft" set, you will be running them hotter, which will give a juicier sound, but if it's outside the acceptable range of bias, then the tubes will redplate and destroy some important parts of your amp.


Preamp tubes in most amps don't need the bias to be adjusted, and there can be quite a difference in gain and tone between individual 12ax7's of different manufacturers, and between individual tubes from the same manufacturer. If you're trying to make your amp sound gainier or cleaner, trying a few different preamp tubes in the position the guitar input feeds to first can make a big difference.
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