How to select an HF tube amplifier ?

Due to the high energy flowing in a power amplifier, several protections have to be implemented to prevent any damage due to a malfunction or a bad manipulation from the user. For short, any power amplifier should be at least equipped with the next four safety systems : a step-starter, that starts the power supply gradually (in about 2 min), in order to protect electrolytics' load and rectifier diodes. Such mechanism can also be found in the filament transformer; a high tension control that prevents the amplifier to work until turning on the high tension button; a stand by knob that, sometimes, cannot be activated before having pressed the high tension button and an interlock relay that disable the power supply in case of the cabinet is opened. In addition several other devices that we are going to discuss are welcome.

Some kW amplifiers request a warming that can last 2 minutes, what could surprise novices. In power amplifiers step-starting is a must. This is required for the plate supply. If a voltage adjustment resistor is not used in the filament transformer primary, the filament supply should then also be equipped with a step-starter.

What's the matter if you open by curiosity the cabinet of an amplifier when it is powered up ? Statistically speaking you have all chances to get... a fatal accident. I remind you that the DC voltage exceeds 10 kV and some dozen amps. So to avoid to be electrocuted, any good amplifier must be equipped with a fast relay able to disable the power supply if the cabinet is opened while the unit is switched on. This is the function of an interlock relay that must always be enabled to prevent any accident if by mistake you touch a device under high voltage. So be very careful if you have to open an amplifier powered up. Usually this action must never be carried out at home but only at the dealer workshop or in the company of a radio technician.

This circuit is a must that we should find in any amplifier using power tubes. Its function is to protect the electronic in case of overdrive, high SWR or if a mis-tune happens by mistake. It disables the amplifier until it is reset and the problem is corrected..

An Automatic Drive Level Circuit, ALC for short, is designed to limit the grid current. It prevents overdrive by applying a negative DC voltage back to the exciter, what reduces its output level. In high-ends amplifiers the ALC is able to sense as low as 50 mA of grid current.

Most kW amplifiers require a drive level between 50 and 80 W (or 80-120 W if you use a Kenwood TL-922) to achieve full output power. The ALC circuit can be internal or external, manual or automatic. In all cases it is recommended to select an ALC offering an adjustable threshold control of the output power. If external ALC is not used, the output of the transmitter must be reduced using the power output control to avoid damaging the tubes. Some ALC circuits (i.e. Emtron) also produce a 1-10 V negative going voltage, proportional to the grid current, but only when overdrive is present.

Note that not all transceivers have a direct ALC EXT/RL CONT output on the rear side. The Kenwood TS-570D(G)S for example replaced these terminals by a single 7-pin DIN plug that plays the same role; called REMOTE, it must be connected to the amplifier ALC/RL terminals thanks to a dedicated cable provided by Kenwood. On the contrary, the Yaesu FT-1000 MP Mark V for example has a built-in ALC cinch among others terminals.

To get permanently a status of current flowing in your amplifier, you need a display including some meters, among which the most useful are a dedicated grid current meter and a plate current meter. You can optionally use a high voltage meter. If you work with tetrode or pentode amplifier you should add a screen current meter position.

It is recommended to select an digital bar graph metering if you search for a fast reading. But usually its resolution is lower than a good analog meter.

These meters must be protected from parasitic or flash-over in installing back-to-back diodes and RF by-pass capacitors across the meter. This last prevent random RF from damaging the meter movement.


At left, the Emtron DX-3 digital display. Plate current, plate voltage, output power, reflected power, screen grid current, SWR, overdrive, ready, transmit and fault information, for short all parameters are displayed on this sophisticated solid state display panel utilizing colored LEDs. A right, a classic plate current meter. If you prefer a mix of both displays, may be will you be interested in this Ten-Tec 422 Centurion hybrid.

This metering displays plate current, plate voltage, grid current, forward or reflected while the LED bargraph displays the peak power.

In a power amplifier we find many components and devices radiating heat, beginning with the tube(s) and to some extent transformers, and largest capacitors.

If you want to cool these units with efficiency a wheezy fan is useless and your system will power off after a few minuts of work or worst, the tubes will break. What you need is a power blower rotating at high speed which specifications are in respect with the volume to cool or the manufacturer requirements.

To get an idea of the heat dissipated by a tube amplifier, an old model like the Kenwood TL-922 sustaining 2 kW PEP uses a blower model Centaur CT3D55F using a ball bearing and powered on 100V for a power of 8W at 50 Hz. In operation it blows an air so warm to the outside that after some minutes of use you do no more need of a heater in your hamshack ! Indeed it has to reject half that power out, and is thus well equivalent to a 1 kW electric heater ! The chassis located the nearest to the tubes and the fan are so hot that it is practically impossible to let your hand on them... Of course, due to this heat the space behind the fan must be free of objects on a distance of about 30 cm (1 ft). This demonstrates that to cool the tubes of an amplifier you fan must answer to accurate specifications.

The main problem that you could experiment using undersized fans is the low intensity of the air flow. The second problem, in using larger models is that they can produce much more noise. At last the blower rotation speed must not be too high to prevent an overcapacity and too much noise too. This latter effect can be substantially reduced in using a large ball-bearing fan/blower. All these constraints lead to select high grade blowers able to handle manufacturers specifications.


At left, the blower near the solid state relays installed in a QRO HF-2500DX amplifier. At right, the small fan installed in the QRO HF-2000 amplifier used to extract the heat radiated by the two 3-500ZG triodes (removed from their socket) located at the foreground.

The best blower must force air under pressure to move around corners, in both axial and radial directions. Such an exchange is hard to get in a cabinet containing separation plates or in which some very large components prevent to reach the smaller but hot one located behind them. Therefore sometimes several blowers are installed in order to reach all corners of the amplifier, together the tube(s) section, the RF tank and the power supply.

A blower should be mandatory in the power supply section of the amplifier where large components generate much heat. A smaller blower can be installed in the RF section to cool the heat generates in the tank. Some manufacturers install a chimney over the tuibes which is good system, but not mandatory from the moment when the exhaust is well separated from the air intake area.

Like all blowers - see blades of your computer one - they attract dust and it should be useful to install an air filter in the air intake. After one year you will surprise by its color.

At last the air exhaust size and position will influence the efficiency of the blower system. The hot air coming from inside must exit the unit quickly and following the straightest path as possible. But with this constraint to not blow in the direction of the user... This means that the exhaust must be located on rear or better on top of the cabinet, the air flowing through small openings optionaly protected by a light grid to prevent input of particles. Avoid to purchase an amplifier which heat is forced to flow inside, to other components before leaving the unit, or a model which exhaust is located in such a position (left or right seeing the amplifier in front) that it will blow hot air on other devices located near it.

I already hear some of you seeing a well designed HF amplifier saying "Whaow, what a beautiful amp". Indeed your first feeling seeing an amplifier will be its look, its design. Naturally everybody is attracted by pleasant and good looking things, leaving the "ugly" models on the side. However it should be not the first time that this ugly model should be a great performer. So don't trust too much in your first feeling and check well all specifications of the amplifier of your covetousnesses. You could be disappointed by your first feeling.

Some well-known kW tube HF amplifiers


Ameritron AL-572	Alpha 99	QRO HF-2000

Commander HF-2500	Ten-Tec 417 Titan III	Kenwood TL-922

Acom 1000	Emtron DX-2SP	Andrews Comm. DX-1600A

Ulvin Tremendus II	Tokyo Hy-Power HL-3KDX	Ranger 811H

The size of the amplifier can also be a major concern when one knows that its mean size that can reach 45x40x25cm (18x15x10") often smaller, plus some space left behind (30 cm or 1 ft) to dissipate the air blown by the fan. So measure your available place before purchasing such a gear. In addition don't forget either that due to the large power supply a kW amplifier will be heavy with a weight ranging from 30 to 50 kg (60-100 lbs). Placed on your desk it can be quite bulky.

Cosmetics size, select a model which body or front panel is hard coated of paint, rugged, i.e. with an epoxy base or baked on thick coating. This latter with last decades without suffering under the daylight or after years of manipulations. If I take mine for example, it is still like new although it was bought in the 90's. Pictures published herewith confirm its excellent state.

I cannot select an amplifier at your place as many parameters have to be taken in account, the financial one being not the least. Remind you only that a low-cost model will not last for years if you use it regularly at high power. If it uses low quality components tell you well that it will probably request more servicing than another model using high quality components. A badly designed amplifier, which blower for example doesn't extract all the heat or accumulate dust on components, will shorten the lifespan of the unit too.

Now if your wallet is quite slim, you can find excellent amplifiers below $1000 and even with some luck second-hand kW models 5 to 10 years old at $500, sold by amateurs whose regulation does no more allow them to work with high powers. A simple amplifier, taking advantage for decades of approved and well-known technologies can be your companion all long your life, maybe during much more time that the latest nec plus ultra solid-state model.

The Svetlana 3-500ZG triodes standing by in the author's Kenwood TL-922 amplifier. Clean, no dust, this rig is well maintained and works for years at its nominal power, 80W in and about 850W out.

To conclude, don't forget that a power amplifier is really a very special gear. First, to protect your health against EMI place the power transformer at a distance exceeding 60 cm from your body (it is usually in most shacks). Then remember that a linear amplifier is a dangerous peripheral, source of very high voltage. The current required to produce ventricular fibrillation is about 80 mA and the resistance of wet skin is about 500-1000W. In worst case, a possibly fatal voltage could be as low as about 0.08A x 500W = 40V DC or 32V AC (RMS). Knowing this, many domestic devices even the ones using transistorised circuits like TV, computer or PSU, exceed this value by a 10-factor ! Take care.

Depending how long and at what gain you use tubes, there is some chance that they blow up or failed for any reason, that there are manufactured in the USA or in the eastern countries. In input, to prevent overdrive or a failure of the ALC circuit, use preferably the nominal power (80W PEP) instead of the maximum (100-120W PEP) on your transceiver.

Trying to push your amplifier to the maximum at full power input (say 120W PEP in) is as stupid as wishing to push your car at full speed, what has for sole consequence to shorten its livespan if not damage it ! In fact, if you experiment shut down of your amplifier after one hour of operation, be sure that this is because you pushed it to far during contests or working pileups at full power years long... Unfortunately now your peripheral is worth half its price, and needs a serious servicing to restore its normal way of functionning hours long... at its nominal power. What you won in power will be now lost in money !

Then it is useful to temporary "retire" your actual tubes (say that you need two) from the amplifier on a regular basis, say once a year if you use it at least weekly, and exchange them for new ones. This rotation will keep your tubes in their best condition for a long time.