Configurations - The Output Stage
The SAM-A1 can be configured to use triodes or pentodes or pentodes as triodes. With triodes the SAM-A1 will deliver between 1Wrms and 7Wrms*1 in to 8Ohms in:
• Pure Class A all the way - Class A input, Class A output push-pull,
• Class A input, Class A to AB output push-pull, or
• Class A input, Class AB output push-pull.
With pentodes, the SAM-A1 will deliver between 8Wrms and 12Wrms in to 8Ohms depending on configuration; Class A, Class A to AB or Class AB Push-Pull all in Ultra-Linear mode.
With pentodes as triodes the SAM-A1 will deliver between 6Wrms and 10Wrms in to 8Ohms depending on configuration; as pure Class A, Class A to AB, or Class AB output push-pull.
*1 These figures depend on the configuration of the output tubes, their bias setting, the specific quad of valves used and its age. As valves age they lose emission capability
and require lower grid voltages to achieve a given bias point this limits how hard they can be driven before the onset of distortion. (see Output Bias Setting below).
Output Bias Setting
The output valves in all configurations are all controlled by fixed bias (the correct way to set up an output stage!). The bias is easily adjusted (with the cover on) so that should you wish to,
you can set how the output valves are biased.
(Should you be in ANY doubt of your capability to perform this task safely, please do not attempt it; refer to our manual - link below). This adjustable bias means that absolute matching of the quad
of output valves is not critical and we match our quads on what is important for achieving the best results in our amplifiers, using our own automated, computer-controlled valve tester.
Our default bias is about 55% of the maximum power rating (anode dissipation) for any given valve type, what is known as a relatively 'cool' bias.
There are may theories and more opinions about whether or not the valves should be biased hotter (closer to maximum power) or cooler
and their relative merits in terms of the resultant sound quality. What is a fact, is that if you run your output valves at 100% of their maximum power rating, all the time, they will wear out
significantly faster than if you run them cooler. The only advantage in running the valves at 100% power bias is that both sides of the push-pull output stage will always be operating in pure class A.
Biasing at a lower power quiescent point means that the amplifier still operates in pure class A at lower volumes, migrating seamlessly to Class AB at higher volumes/output powers.
Running the output valves at maximum power also reduces the maximum power output before significant distortion sets in. The grid voltage is reduced to make the valves pass more current
(dissipating more heat/power) this means that a smaller signal can be applied at the grid before it clips, casuing distortion of the signal.
We believe that running the valves cooler prolongs the life of the valves, reduces wasted energy and still gives the purest Class A sound at lower volumes (when you really need it) and acoustically,
we cannot distinguish the point at which it migrates to class AB at higher output volumes.
Of course, provided you do not ask us to run the valves at greater than 100% (we won't!) you may ask us to set the bias point wherever you would like it to be set.
All of the output valves listed in the Compatible Valves List have been tried and validated against the design and can be interchanged.
The bias will need to be reset each time a new set of valves is fitted.
View all SAM-A1 compatible valves
Configurations - The Input Stage
The input stage is operating in an open-loop Gain configuration, so the two triodes used for the left and right channels should be similar in terms of their gain in the circuit;
this does not mean that they are required to be perfectly matched, as overall gain trimming is possible via adjustable, overall feedback). We match our input valves accordingly
using our in-house computer-controlled valve tester.
The second half of each triode is also not important from a matching point of view, since in both inverter configurations this is used at unity gain.
The input stage can be traditionally biased, known as automatic bias, using a cathode resistor, or use LEDs to set the bias. The latter means the bias current remains largely the same
as the valve ages, but the grid to cathode voltage reduces; the former means the grid to cathode voltage remains more stable but the bias current reduces - this is seldom an issue since
the input valves are used at very low powers and have a very long useful service life. LED's are said to offer a better sound quality by being a more perfect bypass for AC signals - hence
this is our defualt configuration.
The inverter stage can be configured as a Paraphase inverter or a Cathodyne inverter and both are adjustable to ensure both halves of the output drives are perfectly matched.
The input stage is designed to allow a range of double triodes to be used, depending on what you want in terms of input sensitivity and on your preference for sound (and how it looks!).
All of the double triodes listed in the Compatible Valves List have been tried and validated against the design and can be interchanged, allowing you to experiment and try different valve types.
If a type of valve is not in the list and you want to know if it can be used, let us know and we will investigate. If we think it will work, with or without modification.
Please note that valves should always be replaced as a set i.e. both input valves and/or all four output valves. Whenever valves are replaced we would always recommend that the amplifier
is re-calibrated or 'tuned' to ensure the best performance from your valve selection.
