> responds really well to pick attack....cleans up with a light pick, then distorts when dug into! Also responds well to the guitar volume, and it's a "smooth" type of compressed overdrive, but still retains definition....I like it alot!
Assuming normal picking and pickup, and roughly 250V supply: a triode will not overload (much), a pentode's higher gain probably can overload. And for most biases, the pentode's overload is more complex than a triode.
> I believe there was some "pentode high impedance" thing involved in this.
"Pentode" covers a wide spectrum. For maximum low-frequency voltage gain you work very high impedance and low current. For driving loads, or maximum high-frequency bandwidth, you can work a large pentode with a low impedance albeit at high current.
So "it depends".
> what I've read is that the "target" ballpark is keeping the screens @ 2/3 voltage of the plates or less.
Well, the get-it-working easy path is:
Pick a plate DC load resistor based on your audio load impedance.
Note the Typical ratio of plate/screen current, usually 3 to 5.
Find a screen resistor 3 to 5 times bigger than your plate resistor. (You see why 220K+1Meg is a common setup.)
Start with 1K cathode resistor and adjust so that plate sits "about half" of B+.
That works, is stable and consistent with supply or tube changes, will give good gain and output (after you bypass screen and cathode).
In gitar amps, "good" may not be "best" so you will be, as bluesbear wisely says, "just fiddling around till it sounds right.".
Tubes:
6J7/6SJ7 is the classical audio pentode, and IMHO 6J7 is mighty hard to beat. True, it has no glow (metal).
6AU6 is the classical post-war all-purpose sharp pentode. It is however a bit bigger than we need for most audio chores (300mA heater).
5879 is intended for audio gain applications. 150mA heater. It claims low microphonics. I've worked with a lot of 5879 gear, neither great nor crap.
> an "off the wall" pentode
Bah. 5654 is just a premium 6AK5, which happens to be (IIRC) the very first of the wretched "miniature" tubes. The "mini" idea was partly about size, partly cost, and was essential for getting gain at very high frequencies (dozens of MHz)... trying to work an Octal at 20MHz is like tapping a drum-roll with a 20-foot fishing pole. Huge quantities made for OEM and service, still available cheap ($5), and really a fine little bottle.
"Pentode microphonics" is partly about complicated internal structure, partly because they are hard to make well never mind "solid", but do not forget that pentodes can give HIGH gain per stage, and if you stacked good triodes to the same gain you might have the same microphonics.
Pentodes will always give higher hiss than a similar triode; this may be a non-issue with modern pickups.
Topic: HoSo56 amp project...design input sought (Read 127987 times)