Which power amplifier, tube or transistor, is the right one?

If you are planning to purchase a power amplifier (transmitter amplifier), you will soon be faced with the question of whether to use a PA with tubes or transistors. In this article, Ekki Plicht, DF4OR, looks at the advantages and disadvantages of these two technologies.

Power amplifiers in comparison: tube technology vs. transistor technology

The traditional technology for transmitting amplifiers for shortwave and VHF uses tubes. The reason is quite simple: for a long time there were no (affordable) semiconductors for high power. As a result, a wealth of experience has been built up. There are no longer any significant teething troubles with tube amplifiers. In recent years, however, more and more inexpensive and robust transistors or amplifier modules have come onto the market that can also generate sufficient power. Commercial technology now only uses semiconductors across all power ranges. One exception is the very highest frequencies with high power requirements.

Advantages and disadvantages of tubes vs. semiconductors for transmitting power amplifiers

Special case: Mobile power amplifiers without a power supply unit

Before comparing size and weight, you first need to check whether the power amplifier has a built-in power supply unit. There are many PAs in the lower power range that are designed for mobile operation at 13.8V and are therefore supplied without a power supply unit. These power amplifiers are exclusively transistorised devices. The question of the pros and cons of transistors vs. tubes does not arise.

ACOM-1200S

The standard: Power amplifiers with integrated power supply unit

Lightweight vs. powerhouse: Which power amplifier format is the right choice?

Transistor power amplifiers are usually lighter and more compact, especially in the lower power range (up to approx. 500 watts). This is due to the simpler (switching) power supply unit, which does not have to generate high voltage. However, smaller housings are not so easy to cool. So you have to expect more noise background from fans. If you plan to take the power amplifier with you when travelling, transistor PAs are usually preferable in the lower power range due to their smaller size and weight.

In the upper power range (>750 watts and more), power amplifiers with tubes dominate. These usually require very high operating voltages, which can only be generated efficiently with a large and heavy transformer. Semiconductor PAs are available up to around 1500 watts, but they are hardly any smaller or lighter than devices of the same power with tubes. So if weight plays a decisive role, transistor PAs usually win in the lower and medium power range.

Tuning of tube and transistor power amplifiers: A comparison of benefits and disadvantages

Like every device in ham radio, a power amplifier also works with an impedance of 50 ohms. But not every antenna offers this impedance over the entire frequency range. So we use tuners. Although this is a compromise, it is better than no aerial at all.

Tube output stages must be tuned to the transmission frequency for operation. This is done via an LC circuit in the output stage of the power amplifier. Conveniently, this circuit (usually a so-called pi element) also acts as a tuning device, i.e. like a tuner. This means that tube power amplifiers with a tunable output circuit (which is practically all of them) can compensate for certain mismatches even at high power.

Transistor power amplifiers do not need to be tuned, which is their great benefit. They can be used immediately over the entire specified frequency range - provided the antenna always has the correct matching impedance. If this is not more or less exactly 50 ohms, a protective circuit immediately reduces the output power considerably. This is because the mismatch means that the power is not radiated by the aerial, but is mostly reflected. And thus comes back to the output stage, where it is converted into heat. The semiconductor component in the output stage can only tolerate this for a very short time. If the power is not automatically reduced within a fraction of a second, the output stage will inevitably be damaged. This is why practically all such transistorised power amplifiers have such a protective circuit.

This can only be remedied by impedance matching aerials or a high-power tuner located between amplifier and antenna. The supposed benefit of the transistor power amplifier, namely that no tuning is necessary, is then lost again - you have to tune the tuner. The purchase price and the additional complexity of the station must also be taken into account if you want to use such a tuner.

Tube output stages therefore have a clear benefit with less than perfect antennas.

Comparison of the longevity of tube and transistor power amplifiers

The market for transmitter tubes has changed considerably in recent years. Some traditional suppliers have left the market because the expense was no longer justified. With tube power amplifiers today, you have to check exactly which type is used and where you can obtain this tube as a replacement. In most cases, power amp manufacturers do offer them, but it is worth looking for alternative suppliers.

Semiconductors as a more modern technology are usually easier to obtain. They are also virtually indestructible, provided they are operated correctly with good matching and moderate power.

The best tip for a long service life, whether tube or transistor: reduce the power! If you always operate a power amplifier, regardless of the type, at around 60 to 80 per cent of the nominal peak power, the power components are virtually indestructible. Tubes lose some power over time. This becomes apparent very quickly if they are always operated at the specified power limit. Especially with operating modes such as FT-8 or RTTY, it is advisable to reduce the power significantly. 50 % is often enough here. Setting all controls to the right stop often means that the output stage will fail quickly.

This shows that even in countries with a power limit of 750 watts, for example, it can make sense to buy a PA for 1500 watts. If such an oversized PA is always operated within the legal range, i.e. up to 750 watts, then it will last for several decades, regardless of whether it is a tube or a transistor. However, the many years of experience of WiMo's repair workshop also show that tube PAs can cope better with short-term misuse than transistor PAs. Semiconductor output stages can quickly be damaged if they are operated with gross mismatching and continuous wave operation.

Cooling of tube and transistor power amplifiers: Comparing the challenges

Whether transistor or tube - both principles of power generation are not perfectly efficient. Heat is always generated as an undesirable side effect and this heat must be dissipated. Transmitter tubes usually have a heat sink already fitted for this purpose. This is blown into an appropriately designed cooling channel by one or more fans. A transistor module is usually screwed onto a large and solid heat sink, which is then also actively cooled by fans. Incidentally, this heat sink contributes significantly to the weight of a semiconductor output stage.

In terms of heat development and the necessary cooling, no technology has a noticeable benefit.

Active cooling, ideally temperature-controlled, represents a not inconsiderable noise burden. You therefore have to consider installing the power amplifier a little further away in order to shield the noise. Of course, this is not possible if you have to tune the power amplifier frequently, i.e. with tube power amplifiers. In this case, the PA must always be within reach of the operator. Some transistor power amplifiers have a benefit here, as they are often available with a remote control. This means that the PA can be set up in an adjacent room where it is insulated from noise and everything can be conveniently controlled remotely.

Power supply of tube and transistor amplifiers in comparison

As we are only looking at power amplifiers with a built-in power supply, the technology (tube or transistor) is irrelevant. The efficiency of transistor power amplifiers with a well-designed switching power supply is slightly better, but where power is to come out, power must also go in. It comes from the AC mains. You always have to make sure that the power supplier provides enough power at the point where the PA is to be installed.

A tie, with a slight benefit for transistor PAs due to their better efficiency.

Pre-distortion for clean signals: high-end function for modern power amplifiers

Capable amateur radio stations, whether for contesting or DX, utilise many automation options. Power amplifiers with CAT control therefore have a benefit here. Whether it's permanent monitoring or automated antenna selection - modern power amplifiers can do it. Of course, this also comes with a corresponding price tag. As automatic tuning is more complex with tube power amplifiers (motorised tuning), semiconductor power amplifiers have the edge here.

Very modern transceivers make it possible to adjust the transmission signal via a feedback signal, picked up after the power amplifier. A sensor in the power circuit measures the distortion level of the transmit signal and reports this value back to the transceiver. This can now create a so-called "pre-distortion", which results in the transmission signal becoming much cleaner overall. This is a very effective way of avoiding spurious emissions that can lead otherwise to QRM and TVI. Only a few top-of-the-range power amplifiers offer this feature. Of course, the communication between the power amplifier and transceiver must match.

Price-performance ratio of tube and transistors

If you only look at the ratio of "watts per euro", both tube and transistor power amplifiers offer almost the same ratio. This certainly applies to the upper power range from 1000 watts upwards. Below this, there are individual models with tubes that offer a certain benefit in terms of price-performance ratio. The reason for this is the use of simpler and cheaper tubes such as the 811A.

Tube vs. transistor: Which power amplifier is the right one for your amateur radio station?

Tube or transistor in the transmitter output stage - both have advantages and disadvantages. The choice is heavily dependent on the station as a whole, especially the antennas used (see matching). Another important aspect is the location. Do you only want to operate the PA at home or in the clubhouse or also when travelling? See size and weight. The more modern, but also somewhat more fragile and demanding technology is that with semiconductors. The traditional design with tubes is tried and tested and somewhat more robust, but always requires experienced handling of the tuning. Automation? There is automation for both technologies, but increasingly for transistor PAs.

The decision is not easy. WiMo will be happy to help you decide on the right power amplifier for your station. The large selection and many years of experience of the radio staff will be a great help.

My personal favourites:

An Acom-1200S would be my first choice, assuming good matched antennas are available. If not, the ACOM-2000A would be my dream amp.

ACOM 1200S Transistor-PA (1200W, 160-6m)

ACOM 1200S Transistor-PA (1200W, 160-6m)

Acom 2000A (1500W, 160-10m)

Acom 2000A (1500W, 160-10m)

OM-2002W 2m-Transistor-PA, 1800W

OM-2002W 2m-Transistor-PA, 1800W

For VHF I would like to put the OM-Power OM2002W on the table.

Elad

Amplifiers by Elad

If you want a portable power amplifier for shortwave - the Elad "DuoArt" models not only look good, they also work perfectly.

AL-811XCE (600W, 160-10m)

AL-811XCE (600W, 160-10m)

And if the budget is the determining factor, my choice would be the Ameritron AL-811XCE.

4O3A Signature Power Genius XL

4O3A Signature Power Genius XL

vy 73,

Yours Ekki, DF4OR

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