Vacuum Tubes Part
Okay, so far we have an amplification stage with a modest amount of gain. That's a good start, but it is not yet a complete preamp. Why? After all, the voltage swing is there and the current is okay, so what's the problem? Well, the output impedance is high and that leaves it at a disadvantage when driving interconnects and such. There are several potential solutions to the problem, but the classic one is to add a cathode follower.
The amplifier circuit we've been looking at is technically
called a common cathode circuit. It receives a signal at the grid and gives its
output at the plate (or anode, if you prefer). The cathode follower (officially
known as a common anode circuit), like the common cathode, takes its signal at
the grid, but the output is taken at the cathode, not the anode/plate. This
leads to several differences. The common cathode circuit has plenty of voltage
gain, some current gain, and high output impedance. The common anode circuit has
no voltage gain (if you want to get picky, it actually loses a tiny amount of
voltage gain), but in return it offers current gain and a low output impedance.
I don't know about you, but the first time I looked at a
cathode follower, I was a little unnerved how does the thing bias? Won't
it... I dunno... blow up, or something? Not to worry, the cathode follower knows
what it's doing. Look at it this way if the grid is held at the plate
potential of the preceding stage, then you know that the cathode will be within
a few volts of that. In our current circuit, the grid's going to be around 150V.
Take the value of the cathode follower's resistor and plug it into the Ohm's Law
formula: 150V / 47k = 3.2mA. Which, interestingly enough, is the same as the
current for the first stage.
Before going any further, I'd like to note that I'm going to skirt another controversy. The classic cathode follower bothers some people because the resistor at the bottom is a passive component and doesn't contribute to the current flow at the output. All I'm going to say is that there are more complicated follower circuits that address their concerns, but that the vast majority of classic tube circuits use ordinary cathode followers and it hasn't hurt their reputation a bit; they sound just as good as ever.
While you can set up a cathode follower's grid with a DC
blocking cap and a pair of resistors arranged as a voltage divider, it's much
easier to bias it by tying the grid directly to the preceding stage's plate.
Simple, elegant, and it disposes of three parts. A win-win proposition.
cathode follower does have one quirk. Since its cathode is at a relatively high
voltage relative to ground, it can arc between the cathode and the filament,
which in many circuits is referenced to ground. There are two common solutions.
One is to reference the entire circuit's filament supply to some voltage other
than ground, using a voltage divider. The other is to use a separate heater
supply for the cathode follower (referenced to a higher voltage) and leave the
rest of the filaments at ground potential. I tend to use separate filament
supplies, which has the added benefit of spreading out the filament current
demands over more than one transformer. That can be a great benefit if you're
using more than one or two tubes in a circuit. Yes, it makes the circuit a
little more complicated, but it's not that bad. A 6SN7's filament requires 600mA
at 6.3V small transformers like that may already be in your junk box,
waiting for an opportunity to earn their keep.
So how do you proceed if you want to use the filament winding
on the transformer you already have, rather than run multiple filament supplies?
The first step is to look up the relevant spec for your chosen tube. In the case
of the 6SN7, the rated maximum heater/cathode voltage is 100V. The first stage
cathode is near ground, so we'll round that off to 0V. The second stage, the
cathode follower, has its cathode near the plate voltage of the first stage,
which we're going to say is around 150V. Let's split the difference and raise
the filament supply to 75V, which means the first stage sees 75V between the
heater and the cathode comfortably within range. The cathode follower's
heater will see the difference between the heater at 75V and the cathode at
around 150V, which leaves 150V 75V = 75V, also within spec. So, something
like a 51k resistor from ground leading to a 150k resistor to the rail will do
the trick. You can tie either side of the filament supply or the midpoint,
if you're using an AC filament supply to this reference. Easy. Just keep the
heat dissipation in the voltage divider resistors in mind and you'll be fine.
The 51k resistor will need to dissipate a little over 0.1W, so a 0.5W part will
be fine. The 150k part will take about 0.3W, which isn't too bad for a 0.5W
resistor, but I'd use a 1W, just to be on the safe side. You won't have a
problem in the first year or two if you use a ½W, but if you look at older
equipment you'll sometimes see scorched circuit boards from years of exposure to
high temperatures. Occasionally the resistor will burn out. Since we're building
these circuits for ourselves the extra few cents for a 1W resistor is trivial.
If this was a commercial effort the bean-counters would give us hell, but we'll
take the long view and thumb our noses at them.
The one remaining thing to cover is that the first stage of
this preamplifier the common cathode part inverts phase. The output of a
common cathode circuit goes the opposite direction of the input. If the signal
at the grid swings negative, the output goes positive and vice versa. The
easiest solution to this is to reverse the leads at your speaker. On the other
hand, if your power amplifier happens to invert phase, then the two inversions
will cancel out and all will be right with the world you can hook up your
speakers normally. In case you were wondering, the cathode follower does not
invert phase. A lot of circuits add a common cathode stage simply to re-invert
the signal. Yes, it works, but unless you need that gain, you're going to have
to burn it off again in some manner. All is not lost, however. There's another
circuit that will allow you to have your cake and eat it too, but we'll leave
that until later.
A couple of notes for those who want to experiment:
-- Higher bias (which you get by lowering the value of the
cathode resistor) is a good thing as far as sound quality and the ability to
drive other circuits, but watch the plate dissipation on your tube. Calculating
the plate dissipation is not hard. Measure the voltage drop across either the
plate resistor or the cathode resistor and divide by the value of the resistor.
This will give you the current running through the tube; it will be in the
milliamp range. Then measure the voltage from the tube's plate to cathode and
multiply that by the current you just calculated. I'd suggest staying below 75%
of the rated plate dissipation.
-- Be aware that the tube will push back as you fiddle
with it. Let's say that you're running a tube at 1mA and you want to double the
current to 2mA. You might think that halving the cathode resistor would do the
trick. It won't work out that way. You'll end up with something less than the
intended 2mA because the tube's characteristics will change.
-- Keep the plate at around ½ of your rail voltage. It's easy to forget that if you're increasing the bias, the voltage drop across the plate resistor will increase. Let us assume that you're running a 300V rail and a 150k plate resistor with 1mA across it. Then you decide to increase the bias from 1mA to 2mA. Let's do the math: 2mA * 150k = 300V. Yikes... you just put the entire voltage across the plate resistor there's no room left for the tube!
Of course, it doesn't work that way in the real world. As
noted above, the tube will push back and it, the plate resistor, and the cathode
resistor will reach an accommodation where some of the voltage actually does
appear across the tube. The real reason you want to run the plate at about half
of the rail is to maximize the linear voltage swing. Imagine that the tube took
up 295V of the 300V rail. As the output tries to swing positive, it will only be
able to go 5V before it hits the rail, but it has 295V of potential negative
swing. This asymmetry will cause distortion. You don't have to get too crazy
about it after all, you're not going to try to swing +150V at the
output of a preamp but it will matter a lot if you're trying to build a
power amp and want a clean output from your driver stage where the voltage swing
will be in the hundreds of volts.
-- A more subtle point is that increasing the current through
the tube will reduce the voltage spread between the grid (which is at ground
potential, i.e. 0V) and the cathode. The tube will behave wonderfully at idle,
when there's no signal, but once you start playing music through it, the signal
will drive the grid positive, at which point the grid will start to attract
electrons. This is called grid current and a small amount is okay in fact,
some portion of the electrons will always hit the grid no matter what you do
but it's not a good idea to intentionally push the tube into conduction at the
grid. Tubes have a limit as to how much of this sort of thing they can take and
it also tends to get non-linear pretty quickly.
Have some idea as to how big a peak signal you expect at the
grid and try to set the grid voltage at about half that value. In other words,
if you're expecting a signal with 2 volts peak-to-peak, then set your cathode at
least 1V (i.e. half of the 2V value) above the grid to allow the positive half
of the signal room to move.
-- What if you've got a junk box power transformer that you
would like to use, but try as you might, you just can't hit 300V (assuming
that's your target voltage); your power supply obstinately refuses to go any
higher than, say, 275V. So near and yet so far... arrrgh!
Do you have to drop the money to buy a new transformer? Good news the answer
is no. Tube circuits are marvelously forgiving. Go ahead and build the circuit
as-is. You then have three options: a) enjoy the music (er...that sounds
familiar...where have I heard that phrase before?), b) do a little fiddling to
optimize the circuit for the target (consider increasing the bias current a
smidgen to take advantage of the newly-available heat dissipation due to the
lower voltage at the plate), or c) go with (a) and listen to the circuit while
you save up for a new transformer. Your choice. I would not suggest going below
200-225V. The circuit will work, but if you're getting that low you might want
to think about using the 180V portion of the table. Again, do not exceed the
rated voltage for the tube. You can use lower voltages, but if you're thinking
about increasing the voltage, take a look at the tube's specs first.