Valve Amplifiers Explained

14: Purchasing an Amplifier

Many amateurs, when in the market for an amplifier, feel lost and bewildered. It
is similar to purchasing any high-priced item, be it a car, television set, house or some other thing. It helps to make a list of things you would like, things that are essential and things it would be nice to have if the cost isn’t too high. If you are in the fortunate position of not having to worry about the cost you are very lucky! Sadly, most people have to watch the pennies carefully and it may be some time before you have the necessary capital to make the purchase.

Some things are easy to make a decision on: which band or bands it must cover and some idea of the output power. There are then several options to decide upon. Do you absolutely need a new item or can you work with a good used item?

Buying pre-owned

When it comes to used amplifiers it can get tricky! If you know the owner, and judge him or her to be an honest person, so much the better. Unfortunately, there are a lot of sellers who do not fall into this category, especially with the Internet sales methods common today. Caveat emptor is the phrase to keep in mind – ‘buyer beware’.

Although a used amplifier might look an attractive option sometimes it can be a nightmare. Try to establish why the seller wishes to part with it, is the answer you get reasonable? If you have ever been around second hand car dealer showrooms you should know the pitfalls: “Yeah guv, only one lady owner from new. Never been over 50MPH”. Sounds like Dell Boy from Only Fools and Horses.

If it is an older HF amplifier there are a few ‘gotchas’ to consider:

1 Are the valves still available and what will they cost?

2 Does the amplifier have the WARC bands?

3 Does it cover 160m?

4 Is it working to full specification on all bands?

5 Does it come with a guarantee?

6 Do you offer a money back option if I am not satisfied with the purchase?

All of these are reasonable questions to ask and if the seller is honest then you should get a sense if things look all right. Questions 2 and 3 are important.

‘WARC’ Bands

Some amateur grade linear amplifiers do not cover the WARC bands and some do. I have discovered that this can be a double-edged sword. Prior to the introduction of the WARC bands – so-called because they were additions to the original amateur allocations at the ITU World Administrative Radio Conference in 1979 – the only HF amateur bands were 160m, 80m, 40m, 20m, 15m and 10m. Not all countries had these and not all earlier amplifiers had all bands fitted, which I will cover later. When the new bands were allocated for amateur use most amplifier manufacturers rushed to find ways of adding them, so that they didn’t lose out on sales to their competitors.

When fitted with these new bands some amplifiers began to exhibit problems, the reason being that certain parts of the amplifier, particularly the anode DC feed choke, had spurious resonances. With the earlier, smaller number of bands, a trick was to shift these spurious resonances to bits of the spectrum that weren’t being used. Along came the new WARC bands, which slotted into the unused bits of spectrum, and now the spurious resonance reared its ugly head. In some amplifiers, which had simply been ‘dickeyed up’ as a quick fix, this was a major problem – and still is! Tales of anode chokes bursting into flames were common.

The venerable Collins 30L-1 amplfier, using four 811A valves in grounded grid, did not come fitted with either 160m or the three WARC bands (photo: Steve, PJ4DX).

The 160m and 10m Sagas

“Why doesn’t this amplifier have 160m or 10m?” In the case of 10m it was because Citizen Band operators bought amateur amplifiers and used them on 27MHz illegally, particularly in the USA. The FCC issued a ruling that no commercial amateur amplifier for sale in the USA may be fitted with the 10m band components, to try to stop this illegal use. But they were fighting a losing battle, it was easy to obtain the correct components and fit them, even if they left the factory with them missing. Only export sales were allowed to have the 10m band components, but this didn’t stop the CBers. They simply had the amplifier shipped to Mexico and then had it re-directed back to the USA.

The 160m band story is similar. In the days when UK and the USA had maritime shipping coast stations operating in the 1.8MHz to 2.2MHz spectrum, there was a strict limit on the power that amateurs could use on the 160m band to prevent interference with this vital traffic. In the USA if you lived close to the coast there was a day and night power limit. If you lived very close to the coast it was 100W during daylight hours and only 50W at night. The amateur allocation on 160m was on a secondary basis. In the UK the SSB power limit was 26 and two-thirds of a watt!

The Heathkit SB-1000 is another amplifier without the WARC bands but, being more recent than the Collins 30L-1, it did have 160m fitted. The SB-1000 uses a single 3-500Z triode (photo: Steve, PJ4DX).

Because of the licence conditions it was illegal to have an amplifier that could exceed the power limit and so 160m was never fitted to amplifiers marketed for the USA or the UK, until much later when the maritime coast stations closed down and moved to other frequency bands. Consequently, there were almost no commercial amateur amplifiers of that era that had 160m and 80m was the lowest frequency band fitted.

As with the WARC bands, when the restriction was lifted it saw a rush by the manufacturers to fit 160m. And you guessed correctly, the anode chokes went up in smoke again, as well as some pi tank coils! The reason was that as the lowest frequency used to be 80m, the choke was made only with enough inductance to cater for this band. 160m requires an anode choke of at least twice the inductance and there simply wasn’t enough space to wind on the extra turns or fit a larger coil. Some manufacturers took a gamble and lost: they hoped that the 80m-dimensioned anode choke would be good enough. Well, they were dead wrong.

I traced this saga back over the years by looking at one of the amplifiers that had the biggest problem. I won’t divulge the maker or model to avoid legal problems! But if you examine the itemised parts list for the anode choke for the non-WARC model it has a certain part number. Now look at the WARC band version and you will find it has exactly the same part number. Now check out the version with 160m now fitted, and guess what, it still has the same part number!

The cure is actually very simple if you have one of these amplifiers. You lift the bottom of the normal anode choke wire off the DC decoupling capacitor and insert a larger choke in series and connect the end of it to the original DC decoupling capacitor and voila! – problem solved. The new choke can be wound on a ferrite toroidal core of about 27mm diameter and doesn’t occupy much volume. As it is already in series with the big anode choke it needs to be about the same value and getting sufficient inductance on a ferrite core is easy. The RF voltage across the new choke is a fraction of that across the older wound choke so breakdown isn’t an issue. Several amateurs have invented other fixes, the best engineered I saw was a new choke stuffed full of high permeability ferrite rods and encapsulated. I made one for a friend’s amplifier and it worked a treat.

So if you want 160m and 10m, you may be disappointed in your choice of amplifier if it is a vintage model. However, those older amplifiers were built to last, and they were often built like battleships compared to today’s attempts by some manufacturers. The only things likely to go wrong are the valves, the electrolytic smoothing capacitors or the anode tank circuit band switches. If you come across one of these old ladies they are definitely worth considering as the basis for a bit of a revamp using more modern valves. They are becoming much sought after items and a good clean working model can fetch serious money by a collector today.

Those Names and Power Rating

Another thing to be aware of is how amplifiers were named and rated in earlier days. For example the British company, KW Electronics, made an amplifier called the KW 1000. You may think this was rated at 1kW output, but you would be wrong. The ‘1000’ refers to the DC input power in watts. Hence, it would deliver a bit less than half of this number in output.

The British-made KW 1000 uses a pair of 572B valves (photo: Steve, PJ4DX).

This was the norm as the US licence in those days was in DC watts input. The original US licence had a limit of 1kW DC input. Later it was raised to 1.5kW DC input and only later did it change to the existing power output measurement. In the UK, it was 150W DC input to the final amplifier stage for all modes except SSB. When SSB came along it was raised to 400W PEP output, which is the same as a 100W AM transmitter fully modulated.

The Yaesu FL-2000 was a 2kW PEP input amplifier but in fact it struggled to get anywhere close to this. The later FL-2100 series also aren’t 2.1kW input. The 572B valves are hard pressed to exceed about 1.5kW peak input, the latest sales literature quotes 1200W PEP input and are pretty much flat out at this level.

It is a bit like audio amplifiers with their mythical ‘PMPO’ (Peak Music Power Output, a fictional power multiplier rating) – the thinking being that an audio amplifier capable of, say, 100W RMS should be able to run at instantaneous peaks to some higher figure, often 40 times higher. When I was involved in public address amplifiers, the only real test was what the amplifier could muster with a 1kHz sine wave input signal and measured as the RMS power into a load. If the amplifier were rated at 100W RMS output it would give this at some distortion level, usually below 5%. At 80W RMS the distortion dropped to around 1%, which the average human ear cannot hear.

Some of the older amplifiers used large colour television sweep tubes, typically the 6KD6 and others, and today these are pretty much unobtainable. You may not be able to find new valves but the carcass is ripe for a bit of revamping. I have seen some superb rebuilds of these basket case amplifiers. When the owner discovers that they can’t get replacement valves the value drops considerably. If you are prepared to put in a bit of effort and do a lot of reconstruction work inside, a newer Russian triode will really make it fly. The quality of the mains transformers and tank circuit components used in these older amplifiers was surprisingly good, even by today’s standards. I estimate that for about 25% of the cost of a new amplifier you could have a real pile-up breaker.

Other ‘Gotchas’ with Older Amplifiers

One of the frightening things about older amplifiers from the USA and other countries is the apparent lack of safety features. There was one famous brand of HF amplifier that nearly killed me, and I only had it plugged into the mains and not even switched on. They fitted capacitors between the live and neutral to the metal chassis. This particular amplifier belonged to a friend and he asked me to give it an overhaul and to fit some new electrolytic capacitors. Also, he thought it might need some new valves, but wanted to know before buying them.

So before starting I decided to flash it up and check the power output to see if the valves really did need changing. When I went to connect the coax from my HF transceiver and put the coax plug into the back of the linear it bit me – hard. I was thrown backwards and fortunately landed on the carpeted floor. Shocked by this – literally – I unplugged the linear from the wall socket to see what had caused it. I discovered that what appeared to be a nice thick three-core mains lead with a three-pin plug was in fact only a two-core lead with no earth wire. The capacitors from the live and neutral with no earth wire caused the metalwork to sit at half the mains voltage. In this country the mains is 230V, so it had 115V on the exposed metalwork. The first thing was to fit a proper mains earth wire by adding a separate wire to the mains plug and bonding this to the big wing-nut on the back of the amplifier. That tamed it.

Mains Leads and Fusing

The other thing you can come across in the older US amplifiers, and a lot of the early Japanese manufacturers who supplied most of their production to the USA, is the use of the American mains lead wiring colours of black, white and green to denote the live, neutral and earth wires. Only in recent years have they changed to the European standard of brown, blue and striped green / yellow and started to fit three-core leads.

In America, which has 115V two-pin wall sockets with flat pins, the plug could go either way into the wall socket and it does not normally matter which way it is inserted.

It is common to find another horror story. A lot of this earlier equipment has the mains on / off switch in the live side but the fuse is in the other leg, the neutral. As far as AC mains is concerned, it doesn’t care which is live and which is neutral, except if the fuse should happen to blow. If you have an amplifier such as this, please rewire the switch and fuse so both are in the live leg, and fit a three core mains lead with a three pin polarised earth plug. It could save your life!

Keying PTT Relays

Another common problem is that the earlier amplifiers often used the –100V grid bias supply to operate the various relays for antenna switching. With older HF transceivers this wasn’t a problem as they had big chunky relays in them and they simply laughed at –100V. The newer all solid-state rigs mostly use wimpy little relays or, worse still, transistors to key an external linear. And they expect the PTT line from the linear to be a positive voltage of not more than 15V and not requiring more than about 50mA to be sunk. So if you hook one of these older amplifiers to a modern transceiver you stand a very good chance of doing some serious damage to your modern transceiver.

These types of amplifiers need an interface relay keyed by the transceiver to key the –100V relays in the linear. I have had to repair several modern transceivers because of this. The newer types use surface mount transistors which literally explode if you try hooking an older linear to them.

Purchasing a new amplifier

A new amplifier, purchased from a reputable agent, should be covered by a warranty that covers any manufacturing faults. Some manufacturers have exclusion clauses, for example they may not cover the actual valve(s) and as this is one of the major bits that can go wrong, this needs to be clearly known before you part company with your cash. Manufacturers are aware that valves can be abused by the operator and, like certain parts of a new motor car, they are sometimes not covered by a comprehensive warranty. In motor cars, things like tyres, clutches and fan belts are not normally part of the warranty, unless specifically included in writing. An amplifier isn’t like buying a car with a five-year no-mileage-limit warranty or a maintenance plan. When you walk out of the shop clutching your new toy you can be pretty much on your own!

The Alpha 9500 is a top-of-the-range American-made amplifier capable of the full US legal limit of 1500W output. It offers fully-automatic tuning (photo: Steve, PJ4DX).

The best advice is to do your research carefully before making the big decision to buy something. If the amplifier is a new model, does it have a test report published in a respectable magazine such as RSGB’s RadCom or ARRL’s QST? Both of these societies have experts who write reviews on new equipment, often prompted by the manufacturer or the agent to sell new products. If it has been reviewed, obtain a copy of the report and study it carefully. Most reviewers are impartial and tell it like it is. If it is really new on the market, and hasn’t yet been reviewed, perhaps it might be better to wait and see what the reviewers have to say – it might prevent you buying a lemon.

It may well be that you know someone who has already bought one and you can ask for their honest opinion, good or bad. It helps to know of any potential problems so you can avoid them if you decide to go that route.

Some operating manuals are a nightmare, whereas others are quite easy to read and understand. Have a look through the manual, if it is at a trade show, and decide if it is adequately written and covers all the things you would expect to want to know. If it is an older model that has been on the market for some time, often someone has put the manual on a website you can download and study at your leisure, not with a sales person pressing you for a sale. Never rush into a purchase on a whim; know what you are buying!

The Bulgarian-made Acom 1500, using a 4CX1000A, is another 1.5kW output amplifier but, unlike the Alpha 9500, is manually tuned (photo: Steve, PJ4DX).

Where to Buy a New Killer Amplifier

You might assume that the USA is the place to go shopping for the largest amplifiers, but you would be wrong. There is another country that has an even higher output power than the USA limit of 1.5kW PEP. And it may surprise you to find out that country is Canada. There the licence allows up to 2.25kW PEP output and there are a number of specialist manufacturers of bespoke amplifiers who will happily build you a monster PA. (However, the UK power limit remains at 400W PEP output and, if you should consider relocating to Oman, there you are only allowed 150W PEP.)

OM Power amplifiers from the Slovak Republic, on sale at the Friedrichshafen Ham Radio trade show (photo: Steve, PJ4DX).

The other places to shop for a monster amplifier are the former Soviet states and surrounding countries. I visited the Friedrichshafen Ham Radio trade show a few years ago and I noted some companies selling some very nice bits of equipment using Russian valves. Talking to the owner of one Hungarian company, who spoke excellent English, he informed me that they can make amplifiers of up to 10kW for HF and 2.5kW for 2m and 6m should you desire, and the price in euros was reasonable. They sell amplifiers for HF and VHF with or without a power supply, which is a separate item. Looking inside a few examples, with the covers off on their display, showed good engineering and the right sort of components. So if you prefer to purchase, this may suit your pocket.

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