Rectifier Tubes Part 2
As an amplifier designer my main objective when selecting a Rectifier Tube is to select it based on it's physical characteristics (which I will outline) as well as its own filament voltage/amperage requirements. I will give examples as I go to better help explain the design decisions I have made from a purely practical perspective. ALL of this blog entry will speak to what TYPE of rectifier is selected and the next entry I will provide a short discourse on sound quality and which specific brands/tubes I prefer.
Lets get started! Generally speaking, Rectifier Tubes come in two basic types; Full-Wave and Half-Wave. A Half-Wave Rectifier is a single Diode in a glass envelope. A Full-Wave Rectifier is a Dual-Diode in a glass envelope. I only concern myself with the Full-Wave type as there are no currently produced Half-Wave rectifiers, and, the added complexity of having to allocate resources (space and an extra tube socket) to employ two Half-Wave Rectifiers to achieve the same goal as using a single Full-Wave Rectifier seems rather indulgent and wasteful.
The Full-Wave Rectifier is available in many different physical sizes and shapes...however, these are not the "characteristics" that determine which Rectifier is selected for a specific power circuit. The characteristics which are MOST important to an amplifier designer are the following: Voltage Drop, Max Voltage, Max Current Handling, required Filament Voltage, and lastly required Filament Current. These are the characteristics that ultimately (along with the selected power transformer) determine whether or not the designer is able to provide adequate power (voltage @ rated current) to the various stages of the amplifier (this is collectively known as the amplifiers' "OPERATING POINTS"). Simple enough.
Therefore, it serves to reason that the power supply circuit (along with the necessary Rectifier Tube) is designed AFTER the desired Operating Points of the Amplifier Circuit are established. The Amplifier Circuit itself then becomes the primary dictator of which Rectifier Tube is selected. I'll provide an example: My new NIRVANA DHT Stereo Amplifier (which can use either the Type 45 OR 2a3 Output Triodes) requires (collectively) a Maximum of 305v @ 125.6mA of "Rectified" DC to function at my designed Operating Points. This provides me with the minimum voltage and current that my Rectifier Tube must be able to achieve. Looking at several "TUBE DATA SHEETS", I am immediately able to narrow down the list of possible choices to only those that exceed the minimum base-line. For arguments sake, lets say that I have narrowed down my choice to either the (available in current production) 5Y3GT, OR the not-currently-produced Type 80. BOTH of these Rectifier Tubes have nearly IDENTICAL ratings and will both do a spectacular job. However, although the 5Y3 is in current production (quite inexpensive as well!), it does NOT have the same "panache" (nor "period-correctness") as the Type 80 in this particular amplifier. Additionally, because I have designed this particular amplifier to function with either the Type 45, as well as the 2a3 output tubes, I have designed the amplifier to run at TWO different OPERATING POINTS that best take advantage of the differences between the two Output Tube types. The Type 80 Tube uses what is known as the UX-4 base and as long as I select another Full-Wave Rectifier with the same "Pin-Out" and Base requirement I shouldn't have any trouble. The adjustment to the new "OPERATING POINT" Via a change in Rectifier Tube Type is attributable to the differences in "Voltage Drop" between the two rectifiers. I have selected the Type 83 rectifier as the alternative to the Type 80 as this Mercury Vapor Rectifier drops LESS voltage than the 80 allowing for a higher OPERATING POINT. I have designed the NIRVANA circuit to leverage this difference when running the 2a3 output tubes. Arguably, this could also have been achieved with the different voltage drop between the 5Y3GT (for the Type 45) and switching with a 5AR4 (for the 2a3) using the same conventional "Octal" 8-pin socket (AND, these are both current production Rectifiers), BUT, the sound quality isn't even close.....stay tuned for my next installment....Cheers! Matt.