The 300B tube is a popular tube, and no wonder — with its lush, full mid-range, music played through a 300B attains a superbly realistic sound. It has long been considered the ultimate in power output tubes, and yet its popularity has been hampered, largely by two things: its lack of highs and lack of lows. However, after extensive experimentation, we have discovered a way to overcome both these problems in the circuit design.
The Lack of Lows
The lack of lows, as it turns out, is primarily due to poor quality driver design. This is not such a problem in other tube types; for one thing, most other tube types need much less driving voltage. Anymore, a 300B driver usually consists of two sections of a 6SN7 directly coupled to each other, so that the designer can boast “only one coupling capacitor!”
However, what the tube manufacturers fail to mention is that the second 6SN7 stage is biased with a massive cathode resistor to make up for the rather high positive voltage on the grid imposed by the previous stage. In other words, the cathode resistor is not only providing the biasing voltage for the tube, but also compensating for the, say, 100V plate voltage of the previous stage.
The cathode resistor is bypassed by an electrolytic capacitor to prevent so much degenerative feedback that this stage would have no gain. And this capacitor negates any improvement in sound quality gained from having only one coupling capacitor. In fact, the end result is probably worse than just using two coupling capacitors. It’s the price we pay for marketing hype!
Furthermore, poor power supply design means smaller values of coupling capacitors; otherwise the amplifiers will “motorboat” (low frequency oscillations) due to the high impedance of the power supply at low frequencies. The end result is that it is little wonder there is no bass!
You can read more about electrolytic capacitors and coupling capacitors in general. As these articles point out, the bass response of a 300B amplifier, or any amplifier for that matter, can be greatly improved by much larger values of coupling capacitors than are typically used. However, to make this work, a low impedance power supply is needed. Regulators may be necessary, but a choke input filter can also do the trick….
Lack of Highs
As far as the lack of highs is concerned, a large portion of this problem can still be traced to the driving stage, requiring careful design there. However, much of the problem can be eliminated by using a cathode follower buffer directly before the 300B. This eliminates the difficulties with the 300B’s rather high capacitance at the input of the tube—and most notably, the high capacitance between the grid and the plate.
Using a buffer stage makes a huge world of difference. In fact, by adding even small values of resistance between the buffer and the grid of the 300B, we have found that the detailed highs that we were hearing faded away. With the buffer directly attached to the grid, however, the resulting highs were outstanding — even better than those on our KT88 amplifier!
A final tip: After extensive experiments we have discovered that, as a general rule, the buffer tube had best be of the same type as the driving tubes. This tube matching consistently results in an incredibly full and robust reproduction of the music being played in our tests.