The easiest way to select component values is to remember that a tube can be regarded as a signal generator in series with resistor R2 and the same rules that apply to parallel resistors also apply to tubes - the total resistance is less than either resistance alone - and total current increases while total voltage remains the same. If the triodes inside 6H30Pi are connected as shown above, then we can regard the whole arrangement as a single tube with twice the current handling ability. We can easily represent this on the anode characteristics of 6H30Pi by doubling the anode current scale. If we had connected three identical triodes, we would triple the current scale, and if four triodes would be connected in parallel, we would quadruple the current scale.
A single 6H30Pi triode has maximum anode dissipation limit of 4 Watt, thus two in parallel should have a total limit of 8 Watt, since currents are doubled. Keep in mind that because the anode voltage scale and grid curves are unchanged, Mu also remains unchanged. However, for a given change in grid voltage, the change in anode current is now doubled, so Gm is twice its original value. And since
Mu/Gm = Rp
it also follows that Rp (which is R2 in our example) is halved, and this is to be expected since two identical resistances placed in parallel make a total of one half their individual tubes. If we had used three tubes in parallel, we would find Gm tripled and Rp (which is R2 in our example) would be one-third its original value, while Mu would still remain unchanged. Exactly the same principles apply to pentodes.
Note that because the current has doubled, if we wanted the same gain and bias from a pair of parallel valves as a single tube, then we would need to use half the anode load and bias resistances, and the cathode bypass capacitor would need to be doubled in order to maintain the same frequency response.
These design principles are adopted in our Frigate headphone amplifier kit.
Parallel tubes are commonly found in hi-fi amplifiers since they give an improvement in the Signal-to-Noise Ratio (SNR). This happens because the two tubes combined can operate at lower anode current while still achieving the same gain as a single tube. Less anode current means less flicker noise. In fact, it is a general rule of electronics that every time we double the number of amplifying devices in parallel, we get a 3 dB improvement in SNR, in theory at least. In practice the figure may be better or worse than this, depending on how other circuit parameters change.
For example, headphone amplifier kit Frigate, which is using a parallel arrangement, operates at 70% of the current of the single stage, so the total anode noise current of both circuits is the same. But because the parallel stage has a lower anode resistance Ra, its output noise voltage is lower, while its gain is higher. These two elements combine to give an overall improvement in SNR of 4 dB. And even more importantly, the parallel stage will give lower harmonic distortion and more output swing.