chev, it's not the KT88 that's working against the distortion, it's the amplifier's circuitry itself. It works the same way with 6550's. However, as the tubes are slightly different, the results may be slightly different as well.
+1
Another thing to remember is the era when these circuits were designed, tubes weren’t considered the magic mojo holy grail glass bottles they are now, they were in freakin’ EVERYTHING. Until the advances in transistor technology in the early - mid 70’s or so vacuum tubes were how you amplified signals, from radio stations to television sets, portable radios (including car radios); including of course MI amplifiers. Even the computers of the day used vacuum tubes.
The point of this is that tubes were pretty much considered just another component rather than being thought upon as some miracle tone changer like they are today. I think this is for a few reasons, most notably the fact that they are now rare and expensive, and also because they are fun to talk about and an
easy modification for the neophyte “techie” musician to do themselves. It’s far easier to swap out a tube and feel like you’ve done something to modify your amp as opposed to modifying the circuit itself and
actually making a significant tonal difference in the response characteristics.
I’m not suggesting that there is no tonal difference between tubes; there is. Different tubes have different tonal characteristics, but as far as the design specification for any particular amp; even more so when referring to the era of amps that are being discussed here, the only real critical numbers were plate dissipation, voltage requirements/restrictions, etc. rather than the now popular discussions regarding
which 12AX7 sounds better, or whether a 6550 or a KT88 is better or distorts more. As far as the tubes being mainly discussed in this thread, 6550’s and KT88’s are pretty much interchangeable in any circuit designed around either tube, the nuances between them are almost negligible to the ear, provided of course that the tubes are equally “fresh” and up to spec.
NOT nonexistent; “almost negligible”.
I have 2000S heads that are
identical except for one using GT KT88’s and one using GT 6550’s, same SS rectifiers, same filter cap mods, same grounding circuit, etc., even both biased the same. Guess what? I can’t tell you which one I’m playing without looking. The KT88 is louder, but; when I swap out the tubes between them the 6550 one becomes the louder amp, so IMO it’s attributable to the nuances inherent between any hand built PTP type amps.
Now, for anyone interested in a more in depth explanation of what’s going on in your amp, as well as a better illustration of why the circuit rather than the tubes have to major influence over the distortion characteristics of the majority of amplifiers discussed on this forum, here’s some more information.
I’m not a huge fan of Wikipedia as a reference, but this article seems to be a relatively easy to grasp description of the function of “negative feedback” in controlling the non-linearity (distortion) of the amps being discussed here;
http://en.wikipedia.org/wiki/Negative_feedback_amplifierAs Isaac mentions, the circuit
itself fights the distortion that is seemingly being sought after in this thread, and as I posted earlier in this thread distortion was the enemy in the design specification of these amps from the very beginning; when they were initially designed to be as
clean as the technology of the day could make them.
Here’s some more. The reason that this is all in the forefront of my mind is that as I type this I’m right in the middle of testing a “dirty” bass amp design for Reeves Amplification. Their C225 bass amp is
exceptionally linear (clean), maybe even
too clean for a lot of players who prefer the grit of an SVT or Orange AD200, so I currently have a stock C225 with some adjustable features added that allow me to “dial a dirt” on the fly using pots to vary the amount of feedback and additional front end gain in the circuit itself.
In reference to this thread and more specifically Isaac’s post, over the last couple of months we have pretty much exhausted trying various KT88/6550 output tubes, biases and voltages to get the amp to “dirty up”.
