I wanted to share this article I found it pretty funny in some parts.
Is That Amp Really Worth It?
Over the years, there have been some really well designed power amplifiers, some of which have stood the test of time. The Heathkit SB200 comes to mind. While it doesn't directly support the WARC bands, it is nonetheless serviceable. The 572B finals are still available, as are upgrade kits to modernize the keying and rectifier circuitry. If they haven't been abused, or improperly modified, they're still a decent buy. Street prices vary across the board, but are typically $250 to $350 in good working condition.
A few, like the Dentron MLA2500b (and its little brother the MLA1200) weren't worth the effort the day they were introduced. For example, the meter circuitry was suspect, and it didn't have a tuned input unless you purchased the ATI-6. If you own(ed) one of these, and the bleed resistors haven't flamed, count yourself lucky! What's more, it had no grid protection for the rather expensive 8875 finals (even back then). The last NOS (New, Old Stock) 8875s sold for nearly $650 each, and nowadays, they're extinct! Yet, I've seen MLA2500s being sold for nearly $1,000.
Dentron, along with Swan and a few others, made amplifiers which used inexpensive sweep tubes as finals. Whether the finals were 6KD6s, 6LQ6s, D50As, or what have you, the finals for these amplifiers are nonexistent. That is to say, I haven't seen any NOS units for several years. Even if you pay $100 for one of these relics, you've thrown your money away!
That last sentence sort of explains the subject of this article; Is That Amp Really Worth It? Well, I suspect that's in the eyes of the beholder, but if you don't know what you're up against, you're liable to get took! I certainly don't have all the answers, but a few things sort of stand out if you do a little research. As I alluded to above, probably the most important is the final tubes in question.
I have no idea how many different tube types have been used as finals in the myriad of commercial
HF amplifiers sold over the years. A few of the more popular ones included; 4-400, 4-1000, 3-400, 3-500, 811A, 813, 572B/160L, 8875, 8877, PL172, 3CX800, 3CX1200A7, 4CX250B, and even the 1625. I know this, some of these types haven't been made in 50 years!
Here's one
site you might want to visit. Dr. David Kirkby, G8WRB, has put together a list of the more popular finals, albeit some of them have never been used (legally) in any amateur
HF amplifier. Unfortunately, it doesn't list their current availability, or if they've been discontinued, recently or otherwise. It doesn't include any sweep tubes, but suffice to say, they're long-gone-johnson anyway.
The bottom line here is glaringly evident; Amplifiers which use discontinued finals aren't worth the effort, no matter how good a bargain they appear to be. You may argue that your MLA2500b (or what have you) is still operational, but when it does fail (it will sooner or later), you're sunk (pun intended).
I can hear it now; I modified mine to use XXXX finals. All this means is, the amplifier has been modified. And that too, can be a really big problem. Contrary to popular belief, there is more to substituting finals than replacing the tubes and/or sockets. Adding insult, most of the modification articles posted on the Internet are suspect at best.
I recently worked on a 10 year old AL1200. One of it's previous owners had replaced the meter lights with LEDs. Nothing wrong with that, had it been done correctly. It was obvious that a soldering gun had been used, and the LEDs were mounted on swizzle sticks! The various connections were insulated with Scotch mending tape! Yes it worked, but how much longer is the question?
In all fairness, I have seen some really good mod jobs (no pun here). A couple of years ago, I purchased a non-working SB200 for $125. The capacitor bank had been replaced with a very-well made home-brewed PCB, and the cooling fan had been replaced. The only problem I could find was an open grid resistor. The finals appeared original, and it easily delivered 600 watts out once repaired. The handicapped amateur I gave it to has used it nearly everyday since. Caveat Emptor.
That last sentence tells it all. Unless you know exactly what you're doing, and you have the tools and knowledge to use them, buying a questionable amplifier isn't advisable. If you're not in the aforementioned category, here are a few tips.
1). Don't buy any amplifier with discontinued finals, even if it comes with spares. Remember this; glass enveloped tubes do not store well unless they are used regularly. Even some of the ceramic ones are notorious leakers.
2). Don't buy any amplifier you can't play with first hand.
3). Don't listen to the seller's description of how good it looks and/or operates. Reread number 2).
4). Don't buy any amplifier that doesn't have a tuned input. I don't care if your transceiver is a TS520. Some day, you might want to upgrade your radio, and solid state transceivers don't like reactive mismatches.
5). Don't buy any amplifier without a manual. If it comes with a xeroxed copy, make sure you can read the fine print on the schematic. Fact is, some of the pdf, on-line versions can't be read either.
6). As a general rule of thumb, anything made before 1995 is suspect. There are a few exceptions, but once an amplifier reaches middle age, it may be more costly than just the selling price. Again, refer to numbers 1). and 2).
7). Always ask if the seller smokes. You'll already know this if you paid attention to number 2). If you yourself are a smoker, and you're selling an amplifier, have the courtesy to tell a potential buyer.
8). Any amplifier made before 1995 may or may not have low voltage keying. If the one you're interested in doesn't have, or your radio doesn't have a built-in keying relay (
IC-706, FT857, etc.), factor in the cost of a keying interface (about $50).
9). Whatever amplifier you buy (new, used, or home brew), don't try to drive it past its nominal input (typically 55 to 85 watts of drive). This is especially important if you didn't pay attention to number 1). If you want more power out, buy a bigger amplifier!
10). Don't pay too much. You can buy a new, tube-type amplifier for about $1.25 per watt output. Top of the line ones, can cost upwards of $2 per watt. So the question remains, why would you buy a used one for the same amount? Especially when you didn't pay attention to number 1)?
11). Before you spend your hard-earned money on an amplifier, take a long look at your antenna system. Dollar for dollar, you'll get more out of a decent antenna, than you'll ever get with an amplifier.
12). You'll probably need more than just the amplifier, as I mentioned in number 8). If you don't have a dummy load and requisite antenna switch, a decent directional wattmeter, a few coax jumpers and interface cables, and perhaps a 240 volt outlet, maybe you should invest in those first.
Digressing for a moment or two. It is difficult to say how much longer some tube types will be with us. Eimac is still making 8877s and 3CX1200A7s, so you're fairly safe with them. Certainly 572Bs, 811As, and 3-500Zs will be around for some time to come, but they aren't made in the US any longer. Svetlana has stopped production on some of the GU styles, and at least one GS style. Whether any of this presents a problem, remains to be seen.
Certainly the future is in solid state amplifiers. Currently, they're selling for $2.50 to $4 per watt, but that price line will eventually fall. Some of the older solid state amplifier finals are in the discontinued category, and the same caveats should be followed, especially number 9). Finally (no pun here either)...
Patients is a virtue not all of us have, but you need it if you're in the market for a used amplifier. The world's oldest marketing adage is very apropos at this point; if the deal sounds too good to be true, it most certainly is.
IS THAT AMPLIFIER REALLY WORTH IT?? 
U 212
Linear Mode
