dbw said:
know basically nothing about tube amps.....understand more or less how a transistor works
So, do I want EL34s or 6L6s? What's the difference? Why would one switch them out? And just what the frick is bias, anyway??
In the silicon world there are basically PNP and NPN transistors. You either tickle them with some voltage to get 'em to conduct, or you take some voltage away to get 'em to conduct. Its about that simple. You have a collector, emitter, and base.
In a transistor - the base does the throttling of current that flows from collector or emitter. Silicon, being a bit special, is configured as PNP or NPN depending on the relationship of current flow, and base voltage. Rather than get into that, lets say... that you either make the base higher voltage to get the current moving from collector to emitter (PNP), or... make the base lower voltage to get the current moving in the reverse direction (NPN).
A tube, in an amplifier, is very much like a PNP transistor. For the transistor you have a collector that wants to allow flow to the emitter, if only it could. Tickle the base with some voltage and the flow starts once the threshold is reached. Once you're at that threshold, you can further alter the voltage on the base to get more or less current flow - a throttling arrangement. The current flow can go up and up... until the transistor self destructs. You have to arrange the voltage on the base of the transistor such that the current flow is limited to acceptable ranges from "off" at the low end, up to someplace under its maximum rating at the high end. If you apply a variable signal to the base, it throttles, or varies the current flow through the transistor which is useful. If things are right with the current flow and the rest of the circuit is in balance, you can have that current flow through a resistor. When that happens, a voltage drop will occur. The more current, the more the voltage drops. In this way, a tiny voltage, controlling a big current, can flow that current across a resistor, which in turn creates a large drop in voltage across the resistor. That voltage drop is large, because the current is large. In that way, a small voltage is "amplified" by the transistor. If there is no resistor... then the transistor is simply a current flow device, which might be found driving a speaker load, for instance. Transistors can drive speakers directly because of their impedance, but we wont go there in this discussion.
A tube works the same way. You have a plate (collector) cathode (emitter) and control grid (base). Make a current want to flow though the tube from plate to cathode and you can throttle it with the grid. The rest works all the same... with some additions on fancy tubes, like output tubes.
On output tubes, you have whats called a screen grid, and a suppressor grid. These are there for specific reasons having to with electrons flying around inside a fire hot vacuum bottle. The purpose of the screen grid is to lower (halve) the capacitance that exists between the control grid and the plate. This capacitance, called the inter-electrode capacitance (fancy term) can mushy up the circuit, for lack of a better term, and makes linearity of high frequency response suffer. The screen grid between the control grid and plate makes that two capacitors, and two capacitors in series are less than a single capacitor, so the inter-electrode capacitance is roughly halved. I'm being simplistic on this... don't skewer me. The purpose of the suppressor grid is to contain and redirect the electron flow within the tube. In very simple terms, it keeps splattering electrons from creating too much current flow through the screen grid.
All of that stuff has been worked out about 60 years ago or more... and all audio amps pretty much use only a few variations on a theme. Keep in mind too, that all tubes have whats called a filament - or heater - to heat up the cathode, so that electrons will flow within the tube.
BIAS - often misunderstood and scary... It needn't be, as its setting is less critical than you'd think, and mis-setting it doesn't immediately result in flames and smoke and time rifts through inter-galactic space. Bias, by definition, an offset. We have applied "bias" to location of a hole in the guitar body... its been offset. We have a robot do a bias search routine for location of formed DIP components during module insertion (robotic electronic assembly). We provide a bias voltage to the grid, so the tube's conduction of current remains where we want it to be.
And where might THAT be?
Well... we want the peaks of our audio input to turn the tube on... and in the case of preamp tubes and some output tubes, we want more than that. We want both the peaks and valleys of the signal to fully control the tube (which is Class A operation, in case you wondered). So figure this with me.... say our input voltage varies from .1 to -.1 volts (as in a pickup... remember minus too, since its AC). In a tube if the gird is at zero volts, there really is gonna be a LOT of current flow, and we don't want that. Instead we want to offset the voltage on the grid by some amount thats a little bit less than zero volts (as compared to the rest of the circuit). So instead of .1 to -.1 volts being on the grid, maybe we will arrange it so we get -1.3 to -1.5 volts on the grid. See? Same .2 volt range. Just "offset" or biased to make the range useful. Thats bias.
There are a lot of ways to achieve "bias" or offset on the grid. All of which are for another time....
Keep in mind, you CANNOT go willy nilly and change EL34's and 6L6's and 6V6's, and 6550's unless considerations have been made for it. The considerations are - whats connected to the pins in the amp... particularly pin #8 on output tubes. And, also the bias level needed. And also the amount of current the filament, or heater, uses. Slap a big draw heater in a low capability amp and woops... gonzo is the transformer. All those tubes use drastically different heater current. They also have different bias needs, and different screen grid needs, suppressor grid needs etc etc. There are ways to "convert" using electronic surgery, but even then you need to have a robust enough transformer to get it to work reliably afterward.
Tone wise - 6L6s offer a nice middle of the road round tone, with good highs and very soft saturation. Tubes hit the saturation point (where they cannot conduct any more current) in a soft manner. They ease into it. Transistors hit saturation like a brick wall, btw. EL34s saturate a bit more edgy, and a bit sooner. The 6550 takes gobs of current before it saturates and does so about like the 6L6. The 6V6 to me, is nothing more than the kid sister to the 6L6, and it has a softer and more easily broken up tone. ALL of that, depends on the rest of the circuit - generalities cannot be made, that apply to every amp everywhere, all the time.
And since your brains are now full, I shall take a break.
