The SAM-A1

The SAM-A1 from Fireworks Solutions Limited is a high quality, stereo valve amplifier, designed from scratch, from first principles, using modern components to get the best from vintage vacuum tubes or valves. The design is fully configurable so that we can configure your amplifier the way you want it - see configurations below. If you know exactly how you want your amplifier configured, we will build your amplifier specifically for you. Unsure? then go with or recommended default configuration, which we believe offers the best sound quality and power output, with a sensible quiescent power bias (saving energy and valve life compared with a full power bias) - read more below.

Our amplifier designs use bespoke, toroidal power and output transformers, designed to match the valves perfectly to achieve the unique, pure sound that is achievable with valve amplifiers.

Our enclosures are designed to be clean, elegant and attractive; supplied in soft black anodised, heavy-duty aluminium, with no transformers or capacitors sticking up or boxed in on top, so that just the valves that you have chosen to use are on display.

Picture of the SAM-A1 valve amplifier, a Customisable HiFi Stereo Valve Amplifier

  • Specifications

    Size: 382 x 341 x 77mm W x D x H(mm)*

    Mass: 7.75kg, +/-0.25kg

    Supply Voltage input: The input voltage is compatible with BS7671, requiring a range of 230Vac +10% -6%, 207V to 253V.
    The device will operate from 205 to 255Vac.

    Input Signal, impedance and voltage: 98-110KOhms impedance, 200-500mVrms for full power output depending on configuration and valve choice. (This will not work with a vinyl turntable without a pre-amplifier).

    Bluetooth connectivity: if you wish to connect your device to the SAM-A1 via Bluetooth, there is a large range of bluetooth receivers available on-line, see the owner's manual, link below.

    Output, impedance and power: 8Ohms impedance; Power Out is entirely dependent on valve choice and configuration (see below).

    Frequency response: To a lesser degree dependent on the valves, as measured with 6N23P i/p, cathodyne splitter and 6N6P triodes o/p:-3dB response better than 5Hz to 50kHz at maximum power.

    *Note the depth excludes the connectors at the rear. Remember that sufficient clearance is required above the valves; they get hot!

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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.

Designed to make the best of vintage valve technology

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Overall Feedback

We chose to use overall feedback and this can be adjusted allowing the ‘brightness’ of the sound to be altered but also lowering overall harmonic distortion, reducing hum and making it very unlikely that powering your amplifier without a speaker connected will lead to oscillation, runaway and the destruction of your output transformers and output valves; but you can chose to remove the link and not have it!

Mains supply

For Optimal performance the SAM-A1 needs to be set up for your mains supply voltage. If you know what this is, we can configure the power supply accordingly; if not we will set the SAM-A1 to its default 240Vac (230-250Vac in) please see the Owner's Manual for details.

The Output windings of the bespoke mains transformer provide AC power to the Power Supply Unit (PSU) and to the valve heaters (yes we use ac heater supplies ­ and no, in the SAM-A1 valve amplifier design, this does not cause hum!). Changing the choice of mains Transformer input winding also has a significant effect on the valve heater voltages with our preference being simply to stick as close as we can to the mid specification for the valves.

Power Supply

The Power supply; the heart of any good amplifier! The Mains Transformer is designed and rated to cover all variations of the SAM-A1 amplifier; it has been wound and screened to minimise magnetic fields and hum. All the supplies (input and output valve supplies and bias) are controlled by linear regulators to minimise the effects of incoming mains supply variations and again minimising the potential for hum. Even driving very sensitive speakers, you will be hard pushed to hear ANY hum (measured output at idle less than 3mV).

What this means is that the amplifier can be set up in many different configurations, to use many different types of valve, allowing the amplifier to be set up to your preferences to achieve the sound you are looking for! (If you don't know; then go for our defualt configuration - it's the one we use!)

Looking for more information? Read our User Manual