The amplifier design below deals with high voltages — LETHAL voltages. If you are not comfortable or qualified to deal with these potentially deadly voltages, please do not attempt to build this circuit. Proceed at your own risk, expense, and responsibility.
This amplifier circuit supports the use of three Novar (9-pin Compactron) tube types; 6GF7A, 6KY8, and 6LR8. With some creative wiring, the 6LU8 may be used as well, as the 6LU8 is merely a 12-pin version of the 9-pin 6LR8. The tube versions with 12.6 V or 21 V heaters are also supported. Depending on the tube type used, this amplifier produces up to 6 W into 8 ohm.
The goal of this design is to provide an amplifier with good performance, that can be built for less than $200.
Unpopulated circuit boards for this amplifier are available for purchase below. The board measures 3.8 × 5.8 inches (97 × 147 mm). The board is double-sided FR-4, all holes are through-plated, and the layout has been optimized for signal integrity (i.e. low hum and high immunity to electromagnetic interference). The component footprints have been custom designed to minimize the chances of pads lifting from repeated soldering and de-soldering. All this to ensure success – even in the hands of a relatively inexperienced amp builder.
The board is accompanied by a 26-page Design Documentation document containing full set of schematics, bill-of-materials, links to projects set up with Mouser Electronics, circuit description, and assembly guide. This design documentation contains the most up-to-date component values, and is only available with purchase of the Novar Spud Board.
An unpopulated board is shown below.
The board is designed to have the tube sockets mounted on the bottom side of the board. The idea is that the tubes can be shown on the top of the chassis while all the other components remain below deck. The prototype build is shown below.
I have received several requests for a circuit board for a sweep tube based amplifier. Most of these center around my 6LU8 Spud, but the board for that amplifier would need significant rework to be in shape for professional fabrication. So I decided to take new look at the various options for a sweep tube Spud. The 6LU8 is a nice tube. It’s has a triode and a power pentode in one bottle. In addition to the 12-pin Compactron version, it’s available in a 9-pin Novar version labeled 6LR8. This tube is an excellent candidate for a Spud amp.
Another Novar triode-pentode combo tube is the 6KY8. The max plate dissipation of the 6KY8 is a bit lower (11 W) than that of the 6LR8 (14 W), so the 6LR8 will provide more output power. But it opens up some options for tube rolling. The 6KY8 and 6LR8 share the same pinout.
Finally, the 6GF7A is a Novar tube with a small signal triode and a power triode in one bottle. The pinout of this tube is the same as for the 6LR8 and 6KY8 – except for the screen grid pin, of course. In the 6GF7A the pin that’s used for the screen grid in the triode-pentode tubes is simply left floating. Convenient that…
Suddenly, the idea of a board which supported all three tube types emerged. As these tubes are incredibly cost effective (read: inexpensive), using any of these tubes also makes it possible to build an amplifier with excellent performance for very little money. In addition to designing an amp that will support the three tube types, minimizing cost has been a design goal from the start. It is possible to build a fully functional amp with good performance using this design for less than $200 (not counting the chassis/enclosure and shipping fees).
So why ‘Spud’? Well… Back in the heyday of tube amplifiers, the number of tubes in the amplifier was often used as a selling point. This was actually carried over into the early days of transistor amps as well. Today, amplifiers and radios may contain thousands of transistors inside integrated circuits, so this part of the stereo equipment numerology is rather meaningless today. But back in the day, tube amplifiers with one tube were referred to as one-tubers – or tubers for short. A tuber is the bulging part of the root system of certain plants – potatoes for example. Potatoes are also known as spuds. Hence, the term Spud refers to a one-tube amplifier. I have extended this to include stereo amps that have one tube per channel.
The term Novar refers to the tube type used in this amp. Novar tubes have a base with nine 0.038″ diameter pins describing a 0.687″ diameter pin circle. Note that the Novar socket is rather different from the European Magnoval socket which uses 0.046″ diameter pins. It’s also not to be confused with the much smaller Noval socket – such as that used by the popular 12AX7 and the like.
The various Compacton-type tubes lend themselves really well to Spud designs. Many of them contain everything you need to build a full amplifier. For reasons mentioned in the Motivation section, I chose to work with the 6LR8, 6KY8, and 6GF7A. With a bit of creative wiring, the 6LU8 may be used as well. The 6LU8 is identical to the 6LR8, except the latter uses the Novar (9-pin) pinout.
Input stage: Cathode biasing is used on the input stage. The cathode resistor is divided into two parts. This is done to allow for one part of the cathode bias resistor to be bypassed by a capacitor, while the other part is not bypassed. This can be used to provide a bit of local negative feedback to the input stage, resulting in slightly lower distortion at the expense of slightly lower sensitivity. If resistive cathode biasing doesn’t suit your fancy, LED biasing is supported as well.
Output stage: The output stage also uses cathode biasing. This type of bias has survived the test of time and is a very robust circuit. It also, greatly, simplifies the design of the power supply as all bias voltages are derived with only one B+ voltage. In addition, it eliminates the need for bias adjustment as resistive cathode bias is self-adjusting.
Ultra-Linear or Triode option: OPT1 is a wire link enabling the selection of either Ultra-Linear (UL) operation or Triode operation. This is only relevant for the 6KY8 and 6LR8 tubes. The highest output power is obtainable in UL mode. This does, however, require an output transformer that has a UL tap on its primary. In triode mode, the UL tap is not used and the screen grid of the power pentode is connected to the plate. This provides slightly lower output power, but some (myself included) prefer the sound of triode amps to that of pentode amps. I recommend using an actual wire link for this option rather than connecting it to a switch due to the voltages involved.
The expected output power for the different tube types and configurations are tabulated below. The power figures are measured at the onset of clipping (3 % THD). The plate dissipation listed, is the power dissipated in the output section of the tube.
|Tube||Output Power, Triode||Output Power, UL||Plate Dissipation|
|6GF7A||2.3 W||N/A||11 W|
|6KY8||3.1 W||3.2 W||12 W|
|6LR8||4.6 W||5.5 W||15 W|
The plate dissipation may be slightly over spec in some cases, but these tubes are pretty rugged. I’ve seen no signs of redplating even after hours of operating under these conditions.
Heaters: The tube heaters are connected in parallel and are brought directly out to a terminal block. This allows for the use of the versions of these tubes that have 6.3 V, 12.6 V, or 21 V heaters. It also allows for the use of a DC heater supply if desired. If using this amplifier as a headphone amplifier or to drive very efficient speakers, I highly recommend using a regulator, such as my Universal Filament Regulator, for the heater supply.
No global negative feedback: This circuit is free of global negative feedback. However, should a little GNFB be desired, it can be supported by tapping into the cathode of the input tube.
Needless to say, this board allows for quite a bit of experimentation if desired. It can be an experimenter’s jungle gym if needed. Or it can be a plug-n-play solution if that’s more to your liking.
Power Supply: After much experimenting, I opted to use a silicon bridge rectifier and CLC filtering for the power supply. Short of using a regulated supply, this provides the best performance. It also happens to be the most cost effective solution. The supply is intended for use with an Antek AS-1T250 (AS-1T200 for the 6GF7A) power transformer. Fast-recovery diodes were used to eliminate the inductive buzz when the diodes turn off. This was done to minimize hum. It also eliminates the need for snubber circuits.
B+ Regulator Option: If a regulated B+ is desired, this circuit includes an option for adding a high-voltage regulator. If you are using this circuit as a headphone amplifier or to drive very efficient speakers, I highly recommend implementing a regulated B+ supply using my 21st Century Maida Regulator.
As mentioned, one goal of this design is to provide good performance at a reasonable price point. The largest variable in the parts budget in the cost of the output transformers. The cost of the remaining parts is mostly constant. The total parts budget breaks down as follows:
|Resistors, Capacitors, Diodes, Inductor, Terminal Blocks||$60|
|Tubes & Sockets||$15|
Above budget reflects the minimum build cost using new components and Edcor XSE output transformers. I would consider this a minimalist build. For better performance, I suggest upgrading the output transformers to either the James or Edcor CXSE models (see Resources below). Using the bigger Edcor or the James transformers, the total parts cost is still only about $300.
All components on the board, except tubes and tube sockets, are available from various component distributors including Digikey, Newark, and Mouser. The board includes a bill-of-materials showing part numbers for Digikey and Newark. I have nothing against Mouser, I just tend to shop mostly at the other two.
Sockets: I got mine through ESRC. Make sure you get Novar sockets (9-pin compactron) for use with 6GF7A/6KY8/6LR8. You can find ESRC on eBay as well. The PCB footprint for the socket is shown below. The diameter of the pin circle is 1000 mils = 1.000 inches = 25.4 mm.
Output Transformers: If you’re price conscious, Edcor or James are probably your better options. Specifically, the James JS-6113HS ($75) looks like it would be a good fit, though, I have no personal experience with that particular transformer. Occasionally, TC Tubes in the US has them in stock. The DIY Audio crowd reports that James is friendly to deal with directly as well. The Edcor CXSE25-8-5k or CXSE25-4-5k ($90) is another well-suited option. Personally, I use the CXSE25-8-5k during development. Those transformers provide incredible value for the money. In the low-cost end of the spectrum, there’s the Edcor XSE15-8-5k ($20). Yes! Twenty dollars! If you’re on a tight budget or building a compact amp, those Edcors are definitely worth considering. The sound quality does clean up quite a bit with the much bigger CXSE25 transformers. But … $20… For those who are looking to eek every bit of performance out of the circuit, I recommend having a pair of transformers custom wound by Electra-Print. You’ll need output transformers that are specified for 10 W, 60 mA, and 5 kΩ primary impedance.
Power Transformers: The torridal transformers by Antek Inc. are very hard to beat when it comes to good performance for little money. If you’re planning to use the 6GF7A, I recommend getting the AS-1T200 for a rectified B+ of 250 V. If you’re using the 6KY8 or 6LR8, I recommend the AS-1T250 for a 300 V B+.
DIY Audio Build Thread: Novar Spud :: A $200 sweep tube Spud with PCB.