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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: What specs when looking for a "good enough" substitute? posted by Mark Tinordi on May 14, 2007 at 15:35:37:
A few comments.First, some of your specifications are off.
The maximum PEAK heater-cathode voltage is 200v but the average value is 100v.
The maximum cathode current is 20 MILLIamps, not 20 amps. Off by a factor of 1000.The heater current draw of the 12AV7 is .225 amps at 12.6 volts, or .45 amps at 6.3 volts, which means in the same circuit it draws about 50% more current than a 12AU7, not 3 times more. Most designs have enough spare capacity in the heater circuit that that bit of difference can be easily accomodated unless you're replacing a whole bunch of AU7s with AV7s. In other words, if you stick in a 12AV7 it WILL draw .225 amps at 12.6 volts, just as if you screw in a 100 watt bulb in place of a 60 watt bulb, it will draw 100 watts. No big deal, unless you substitute so many 100 watt bulbs in place of 60 watt bulbs that it overloads the circuit and blows a fuse.
Sonics are subjective, of course but generally speaking a designer will optimize the circuit for the tube specified, and the tube specified was chosen by the designer to do what he or she wanted it to do. Meaning that the "perfect fit" is the tube that the designer specified. Substituting a tube with different characteristics may sound "better" to you, whatever that means, but will likely NOT be operating at its best - the operating point may be off, the quiescent current may not be optimal, etc. It's always safest, as well as best, to stick to tubes that are electrically equivalent (get a tube substitution manual, that's why they are written) so that nothing blows up, burns out, etc., etc. Of course, "safe" tube rolling using tubes with the same specifications (e.g. substituting an Amperex 12AU7 for an RCA 12AU7) will likely produce more subtle changes in the sound then sticking in a tube that is grossly mismatched, but if you do otherwise you really are on your own and if the unit burns up you have nobody to blame but yourself.
Sorry, but the fact is that GOOD tube rollers stick with substituting equivalent tubes, IGNORANT tube rollers substitute whatever will physically fit (this is not just my opinion, it's also the opinion of David Manley among others, he's the founder of Vacuum Tube Logic, a well known tube equipment manufacturer). If you don't get the sound you want with substituting equivalent tubes then the best advice is get another unit that WILL give you the sound you want. But from your last comment it sounds like you don't want to hear that advice, So, if you're a newbie, and you don't want to accept good advice, what does that make you - if not a fucking idiot? Sorry about that, couldn't resist. :-)
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Follow Ups
- Re: What specs when looking for a "good enough" substitute? - JimL 05/15/0722:46:35 05/15/07 (0)
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