Thread: Tube saturation
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Old 2007-03-15, 02:17
Necrovore Necrovore is offline
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Join Date: Aug 2004
Location: Texas
Posts: 81
The tubes themselves are another factor. KT66's will distort faster than EL34's which distort faster than E34L's/6L6's/5881's, which will distort faster than the 6V6, then 6550's, and then KT88's. These are the most commonly used power tubes in guitar amps. I listed them from least power handling to most.

Amp example would be a Marshall JCM800 2203 Master Volume amp. The only changes would be the bias resistors needed to bias the Pentode type tubes(first 5 listed) vs. the Tetrode/Beam Tetrode tubes (last 3). All other componants are identical(transformers etc...). In this case I do not believe that the 6V6 can be used in this amp do to the pin configuration 9 pins vs. 8 in all the others, so lets leave that one out for the time being.

The KT66 tube has less power handling capabilities than the EL34's which are rated for an average of about 35 watts, meaning less headroom before the tube distorts. The E34L, 6L6, and 5881(military variant of the 6L6) will handle a slightly higher output wattage rating than the EL34, but average usage is pretty much the same. They are cleaner sounding tubes though. The E34L is nothing more than an EL34 that uses 6L6 architecture. In other words, a tube that sounds like an EL34, but can handle a bit more output power before distortion.

The 6550 and KT88 are pretty much identical tubes. The KT88 is the Brit version of the 6550 but was designed to handle monstrous amounts of voltage, up to 800 volts as opposed to the standard 500-600 that the 6550 is rated for. Essentially what this would mean in the example amp would be tremendous amounts of headroom, or clean signal before distortion. Either one of these two tubes would allow the user to get pretty much all of the preamp tube tone that the amp has before the power tubes even started thinking about distorting.

Using a standard Marshall amp a very simplistic rule of thumb would be like this:
KT66 = classic rock tone from 60's and 70's but at lowered power output
EL34 = classic rock/metal tone. You have heard this on pretty much every rock/metal album from the mid 60's through today.
E34L/6L6/5881 = Classic to somewhat more modern tone being a bit cleaner(not too much). The 6L6 is the standard power tube for Fender and Mesa amps. Newer Marshall's use these as well.
6550 = Mid 80's to today more modern tone high output volumes, more preamp signal.
KT88 = Like the 6550 but loud enough to kill the unborn children that reside in your sack.

6V6 somewhere in between the 6L6 and 6550 types. I have only seen these tubes used in older Peavey amps from the mid 70's.

Preamp tubes are rated fairly easy. 12AT7, 12AU7, 12AX7 from least hot to most hot respectively. Each kind will have its own voicing as well as gain hotness from brand to brand. I will not begin to discuss which preamp tubes are best as that is totally a personal descision to each guitarist. In my Marshalls I use NOS tubes in the preamp section. A Bugle Boy in V1, an GE-JAN in V2, and a Tesla military unbalanced (2400-1875) for V3.

I also do not match my power amp tubes. Well not totally true. I match only to achieve a 7-10 ma difference from one end of the power tube array to the other. This allows for more 3rd and 5th order distortions to come through the signal which are the organic distortions that guitarists want. The 1-2ma matching that most guys use these days work, but the amps sound stiff and more hi-fi to my ears. We play dirty guitar amps, not clean hi-fi stereo equipment.
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