In this series of articles, we will talk about various aspects of A2 operation. This is the purposeful biasing of a tube so that its grid is positively biased. We will discuss why one might do this, what the advantages and disadvantages are, and various aspects of design that are somewhat unique to this kind of operation.
For this series, we will use the 841 directly heated triode as an operating example.
The 841 is somewhat similar to the 10 or 801, except it has a mu of 30 instead of 8. Here is a set of tube curves for the 841. The 841 has a maximum plate (anode) dissipation of 15 watts, and 450 volts maximum ratings. The filament, like a 10 or 801 is 7.5 volts at 1.25 amps. Base is 4 pin "4D" connection. It is a W-Th filament DHT device.
I have already drawn a (green line) load line that we will use in this series. This is not to indicate that this is the only or even best operating point for this tube. However, for our example, the operating point chosen is 450 plate volts and 33 mA. We will provide +/-30 volts (60VP-P) swing into the grid, causing it to vary from +7 volts (quiescent) to +37 volts and to -23 volts. This will provide a 700 volt swing on the plate into a 10k loadline, which means the power output can be about 6 watts.
What A2 Allows Us To Do
When you operate the grid in the positive region, it draws some current. The amount of current it draws is a function of both the grid voltage and the plate (anode) voltage.The lower the plate voltage, the higher the grid current for a given grid voltage, and the higher the grid voltage, the higher the grid current for a given plate voltage. Sometimes you can find a set of grid current curves for a given tube. For the 841, assuming an instantaneous plate voltage of at least 2.5 times the grid voltage, the grid looks like a slightly non-linear resistor of about 2.5k ohms or greater. For instance, at a positive grid bias of 40 volts, the grid draws about 16mA, at 20 volts it draws about 8mA, at 10 volts, about 3.5mA.
The reason for considering this kind of operation is actually twofold:
The disadvantage of this kind of operation is, as mentioned, that the grid draws current, so you have to supply some power to the tube. This can be done in a number of ways, as we will discuss in the section below. One definite thing to notice, is that the driving circuit has to be low impedance, since the grid impedance does change over the operating range. Said another way, in order to "swamp out" the effect of the varying grid resistance, the driving impedance has to be much lower than the grid impedance. There is another subtle disadvantage. Since the grid is often driven in positive and negative region, only the positive region draws current. This means that you can't effectively use any form of RC coupling, since the "rectification" will change the bias point depending on the signal condition.
There is another subtle advantage to be gained in A2 operation. Usually the tubes that operate best in A2 operation are relatively high mu devices. In A1 operation, one problem with high mu devices is the relatively high input capacitance that you have to deal with (Miller Effect). Since A2 operation already pre-supposes a low impedance driver, the input capacitance is of little or no consequence, and there is no problem getting extended high frequency response.
Lets see if we can list these advantages/disadvantages:
|A2 ADVANTAGES||A2 DISADVANTAGES|
|greater power output||grid draws power|
|can use high mu parts||more complex bias/driver|
|linear plate resistance curves||driver must be low impedance|
|hi mu=lower amplification needed||can't "capacitively couple" output stage|
|can get good frequency response|
Driver / Bias Circuits
Here is a circuit of a very simple amplifier. As indicated above, the biggest "issue" is how to bias the tube, and how to provide a low impedance drive. I've shown the output circuit and 4 potential "drive" circuits.
The output section is relatively common. Note however, that I have chosen to use DC heating for the filament. When you're traversing through the "0 volt" bias point, this is often the best thing to do in order to avoid hum modulation. For A2 operation, there does not appear to be any sonic penalty for DC filaments that seems to occur in A1.
The first "attempt" at biasing and driving is shown in the "A" box in the schematic. There are several things wrong with this approach. First, since the grid is actually drawing substantial current at +7 volts bias, the voltage divider shown won't actually produce 7 volts on the grid. Also, since the current changes with applied signal level, it is only the "average" current that will cause bias. This means the bias shifts all over the place with applied signal. Bottom line: Nope-won't work. What we need to "learn" from this attempt is that we need a stable "DC" operating bias point; implying a low DC source resistance. The next three possibilities do just that.
In "B", I've used an inductively loaded cathode follower. For a 6H30pi, with 143 volts plate to cathode, a 270 ohn resistance in the cathode circuit provides about 26 mA, and therefore +7 volts quiescent. This is just perfect for biasing our 841, even considering the couple mA the grid of the 841 consumes at 7V bias. The very low impedance of the cathode follower allows us to drive a very clean signal to the grid of the 841. The disadvantage is a relatively large choke. In the next part of this series, we'll also talk about what to do with the grid of the 6H30pi.
"C" uses a very similar approach, except the choke has been replaced with a "large" resistor connected to a high negative voltage instead of the choke. Operation is substantially the same as "B", with the cathode follower providing a low source impedance to the grid of the 841. This, of course, trades the choke for a power supply. This may or may not be easier to do, depending on what you happen to have on hand, and whether you have "other" uses for a negative supply voltage.
Notice that B and C both rely on the low source impedance of a cathode follower to provide the "power" to the grid of the 841. Notice that the cathode follower has to have the capability of "sourcing" over 40 mA in our application. The 6H30pi chosen can easily do that.
The other approach we can take for bias and drive is shown in the "D" box. This is a transformer drive circuit. Since we have a good source of "stiff" 7.5VDC (and requiring it to be well filtered), the transformer secondary can be driven from the +7.5V "filament" voltage. The IR loss in the transformer secondary then delivers the proper +7VDC. The impedance transformation indicated then delivers a low source impedance AC drive to the grid. To provide the needed 60V peak to peak AC signal, you would need 270V peak to peak at the driver plate. This is achievable using a number of different driver tubes, but it does increase the overall voltage amplification needed in the system. The 270V peak to peak into a 10k load would pobably mean you would need a 250V B+ voltage on the driver tube (or even 300V). If you chose this approach, you would need to draw a load line on your intended driver components just like you would for an output stage.
In the next article, we'll put together an A2 amplifier based on the 841, just to see what can be done with this kind of operation. The approach I will take is an entirely "DC coupled" amplifier, up to the output transformer. THAT will be further expanded upon in part 3.
goto part 2