How to select an HF tube amplifier ?

After the vacuum tube(s) the next most expensive component of a power amplifier is the high voltage (HV) power transformer. If the use of russian tubes can decrease the price of your amplifier, technically speaking it is very hard to reduce the price of wires, coils and magnets, and therefore the power supply unit (PSU) remains the stumbling block of your budget.

The amplifier PSU has to be designed to provide the required power. The requirements and therefore the design of the amplifier will differ from small audio amplifiers specs to power amplifiers (kW-class).

The power efficiency of such an amplifier being important, this factor has effects on its sizes, weight, power consumption and dissipation. This last requests special devices for cooling and to respect the thermal stability of the amplifier.

As shows very well the picture of the Emtron DX-2SP displayed above as well as closeups displayed below, transformers are the most cumbersome elements of an amplifier, the heaviest too.

Such transformers are about 20x15x10 cm in size (8x6x4") and their weight exceeds 20 kg (40 lbs.) ! Therefore some manufacturers send the PSU separate from the amplifier itself, to the amateur to made the assembling.

A transformer is just a piece of iron with a pair of wires coiled wounded around it - one with many more turns in the coil than the other. The coils of wire are not physically connected. In the largest power transformers the iron core can be immersed in an insulating oil bath which does not conduct electricity well.


Two models of power transformers. At left the ones installed in a QRO HF-2500DX amplifier. At right the one of a Commander HF-2500E.

How works a transformer ? Basically a conductor, such as a copper wire, is sitting in a magnetic field that is changing, and thus a current will flow in the conductor. This current will not be steady but will also be changing. Alternatively, if a changing current is present, it will produce a changing magnetic field of 50 or 60 Hz.

A transformer works only with alternating current (AC) circuits. The AC current enters the primary coil. A magnetic field is produced that is concentrated in the iron core of the transformer. A secondary coil of wires (also conductors) is wrapped around the iron core, not physically touching the first set of wires. The changing magnetic field produced by the first coil is experienced by the second coil and current begins to flow in these wires as well. The second coil has many more turns of wire and offers a higher resistance to the current flow than the first coil. The greater resistance means that a larger voltage drop (than is present across the first coil of wire) is produced from one end of the coil to the other. Therefore a low voltage enters the transformer and a high voltage exits, or vice versa.

Most transformers operate at high efficiency transmitting about 99% of the power that enters them. About 1% of the power is lost in heating the transformer.


Power transformer	Toroid transformer	Inductor/Choke transformer


CV Ferroresonant transformer	Control transformer	Class 2 transformer

There are many kinds of transformers : audio, class 2, CV-ferroresonant, power isolation, control, flyback, pulse, encapsulated, sense transformers, autotransformers, switchmode, inductors/chokes, toroids, current transformers, 3 phase transformers, power factor correction, high voltage, and more. In radio applications the ones that interest amateurs are mainly inductor/choke transformers, toroids and power transformers. Their specifications vary depending the application and power requirements. Don't be surprised to find in kW power amplifiers one or more toroid transformers of about 10 cm diameter and 4 cm high or a big one 22 cm diameter and 8 cm high. The "small" one weighs about 5 kg, the larger exceeds 12 kg.

An HF amplifier used at legal limit needs a distinct plate transformer separate from the filament transformer and control power supply. The plate supply should be able to handle 1.5 kW continuous in any mode. In full key-down CW ouput for example, a 3 kV plate supply should be able to sustain 700 mA. A plate supply capable of this DC current in a B+ line should have a filter capacitor bank of about 60µF, with a surge voltage no less than 1.41 times the peak plate voltage.

All a compartiment, so half the volume of this QRO HF-2500DX amplifier is dedicated to the power supply and its circuits boards. At left, inserted between the high grade electrolytics capacitors and the control boards is the HV rectifier board. We can see the twenty N5408 rectifier diodes that provide 5000 PIV capacity for DC smoothed and filtered by the eight electrolytic capacitors located below. At right, above the HV rectifier is the control board. It provides LV bias, screen trip and control circuitry. The smaller circuit board mounted at right is the screen supply board.

A full wave bridge rectifier board in the plate supply is a must. This HV rectifier board contains some dozen rectifier diodes (N5408) in a bridge arrangement, providing for a 3kV plate PS a total voltage rating of 5 kV PIV for DC smoothed, and filtered by several high grade electrolytic capacitors.

The best electrolytics are the ones that can endure years of heat in an amplifier. So they can be top grade, as the ones used in computers, or even oil filled models. Like tubes, it is essential to place them in a well ventilated cabinet fixed on an insulated bracket.


At left, inside a Ranger 811H amplifier we discover the PSU in the upper right part showing the EHT capacitor stack mounted on a board. Below are two toroidal transformers of about 5 kg each. At center the filter capacitors (116mF, 3.6 kV) attached to the PSU of an Emtron DX-3 amplifier. At right, in this QRO HF-2500DX the electrolytics capacitors are located in the left compartment.

A bleeder resistance is a resistance connected across the output terminals of the power supply. Its functions are to discharge the filter capacitors as a safety measure when the power is turned off and to improve voltage regulation by providing a minimum load resistance. The time lapse required to drain the high charge of the power supply can vary from a fraction of a second to several hours in worst cases... Contrarily to the past where amplifiers used filter chokes and large bleeder resistors drawing 10% of full load current on the power supply to dissipate all the heat, modern amplifiers needs only a bleeder able to draw barely 1% of the load once the amplifier shut down. To meet this requirement a resistance of 100-500 KW across the entire filter bank will do that job. There are two possibilities : either using individual resistors ranging from 68-75 KW 3-5 watts each or a single power resistor of 100-200 watts depending on the resistance.

The glitch resistor installed in a SB-200 amplifier.

The glitch resistor installed in the Heathkit SB-200 power amplifier.

At last to prevent any overheating and shorten the capacitors life span, it is recommended to mount the bleeder resistors away from the capacitors bank.

As all powered devices, an amplifier can always be subject to a surge. AC line fuses are far to offer sufficient protection against current limits. Therefore a glitch resistor is recommended in series with the HV lead, even is some amplifiers have an electronic plate over-current circuit. Made of vitreous enamel insulated wire-wound type this resistor acts either as a "fuse" and will explode in opening the circuit to protect the power supply or as a barrier against current. It the first case it is recommended to use a 0.68 ohm 1-2 watts resistor in serie in the B+ line and in the second case a 50 ohm 50 watt power resistor to limit the B+ current; this latter has for function to not "fuse".

At last, examining how wires are attached and how connections have been soldered in an amplifier you can immediately recognize the sign of a quality material manufacturer. Avoid amplifiers in which wires looks like a mesh or which circuits show dark or too large soldering, extended-wrap connections and wires of too small size, sign of amateur work. On the contrary choose a model in which wires are thick, and the wiring circuit well laid out, solderings bright, tie-wrapped. The connection to the other modules should be made via a wire loom, and grounded to the chassis.

For security purposes wires constituing the AC primary should be at least #12 AGW in size (inner Ø2.07 mm), while the HV wiring should be made of heavy Teflon or silicone rubber insulated, and no less than #18 AGW in size (inner Ø1.16 mm).


At left, the wiring in the power supply compartment of the QRO HF-2500DX Mark III amplifier. This section is totally separate from the RF compartment with a stainless steel pane. Teflon dominate in this high quality amplifier. At right, the wire loom of a Henri Radio amplifier. Does your actual amplifier shows a similar quality ?

All sections of an amplifer, the PA stage, Pi-network, controller circuits, power supply unit, etc should be totally insulated and enclosed in individual shielded compartment within the amplifier chassis. Power lines and control leads should enter these shielded enclosures via feed-through capacitors.

In the RF section all interconnect wiring should be made of coaxial or shielded wire. "RF" and "non-RF" wires have also to be separate and never be found side by side in the same area. "RF" wiring for example should be isolated in its unit away from the other wires, like should be the full Pi-section to avoid side effects due to RFI. RF interconnecting cables should have their braiding grounded at shield entry points. Lead dress should be used.

These protecting measures will maintain RF integrity, minimise internal RF feedback, and ensure regulatory electromagnetic compatibility (EMC) compliance.