Tube CompatibilityThe table below compares the key tube characaterstics of heater current and amplification factor for the 6H30 and the 6Dj8 family of 9-pin tubes.
|Tube||nominal heater current|
ma @ 6.3 V
|amplification factor (Mu)|
No AmplificationA key characteristic of all tubes is their ability to amplify voltage. A tube’s amplification factor is defined as the ratio of change in output voltage per change in input voltage (at a given plate power supply voltage and current). This amplification factor is referred to as the a tube’s “mu”. But lets remind ourselves that the TPB.V1 is a buffer with a default gain of 1x. A gain of 1x, or unity gain, means literally zero amplification from the tube in this application. True, the user can opt to increase the buffer’s gain by +3 dB (gain of 1.4x) or +6 dB (gain of 2x) but these are very modest gains for a typical tube aplication. The key takeaway here is that a tube’s amplification factor (inherent gain) matters very little to the overall performance of a given tube in a zero/low amplication buffer application. So why use a tube here at all? To capture the sonic euphonics that only tubes can provide…of course!
Low Voltage OperationAn interesting consequence of zero/low amplification is that we don’t have to power the tube plate (anode) with the high voltage (100-200 volts or more) that is typical of most tube applications. In fact, the Tortuga tube buffer operates with a plate power supply of only 30 volts. Ideally, tubes must operate over a largely linear range of their performance curves in order to minimize distortion. With zero/low amplification most tubes do in fact remain very linear even down at very low plate voltages while operating with input signal grid voltages (Vg) near zero volts. This is evident in the performance curves shown below for both the 6H30 tube and the 6DJ8 family of tubes and is borne out in practice. The advantages of low voltage operation include:
- Ultra low-noise power supply (Belleson SuperRegulator)
- Direct coupled signal input to grid without coupling caps or voltage offset
- Immunity from tube microphonics (tube acting as a microphone)
- Less heat/thermal stress (longer tube life)
- Compatibility with a wider range of tubes (enabling tube rolling!)