Don't forget Marantz... They made some great vintage amps with a strong very warm sound.
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The ST-70 is a good place to start. Very nice output transformers and simple enough that the driver and power supply are easily upgraded/updated.
35 watts is about as high as you are going to see in an integrated and is not enough power unless your speakers are 96 db or more (for my listening tastes anyway... :)
In seperates 50-60 watts is the most you will see for the most part. The Dyna MkIII is, relatively speaking, avoidable compared to the sound of the ST-70, so you will have to concentrate on other targets if you want that kind of power. Of those, the Citation II is perhaps that best platform to work with as it has excellent power transformers and enough capcitance and driver layout space to allow for proper refurbishment, updates and upgrades such that it will keep up with a lot of modern amps with ease.
The Citation V has better output transformers but they won't make quite the same power and the amp for other reasons usually does not wind up sounding as good.
Fisher, Marantz, MacIntosh, and Scott made larger 50-60 watt power amps. The Marantz (model 9) is legendary and impossible to find. The Mac 275s aren't particularly easy either but in recent years was repopped. The Fishers and Scotts are rare too. You pay a lot for these names although the Citation II is much more recognized as a giant beater now then they were 15 years ago. The Scott is probably the least collectable.
In all cases an 'attic fresh find' will need the power supplies completely rebuilt, and all the sockets, connectors and transformers will have to be tested for integrety. I advise you to avoid a unit that is know to have a bad transformer- finding a replacement can be heartbreaking.
All vintage tube amps are now so old that ALL the filter caps in them should be replaced in them regardless of how they measure!
Coupling capacitors, particularly in the output section, should be replaced as even a small amount of leakage on their part can cause the output tubes to destroy some rare transformers! Similarly the selenium rectifiers that are often found in the bias supplies of vintage amps should be replaced.
Whomever does the service needs to be very aware that the piece is collectable and that new parts should be installed with respect to this fact. Often you can wind up paying more for suck a piece than buying something new- so stay on your toes.
I have owned a lot of vinatage gear and can cheerfully say that most of it sounds hazy and fat (kind of like me). The ST-70, while beloved by many, is a marvelous example of this breed. Upgrading it is IMHO, putting lipstick on a pig. So where would I go? The Heathkit mono amps for a start. The made several models all with excellent iron, some with the great Acro transformers. Most are a little lower in power, using the 6L6/5881 tubes, but no matter, they are very stout and have excellent drive and clear sonics, sounding more powerful than higher rated amps. Speaking of Acro, if you can get one of the Acrosound amps they will have good trannies as well. Go a bit further back in time and the Stancor triode monos using the 805 tube with anode cap are quite nice and sound oh so modern. For a really cheap amp, the Scott 208 is the amp section of the venerable LK-72 integrated and is very clear. It is only around 22 watts and uses the 7591 tube which can be a problem. Good luck!
It really depends on what you mean by vintage! 30 years ago 50 watts was easy but almost all of it solid state much of it garbage. 40-45 years ago was the quote "golden age." I have a suspicion this is what you want. 60 watts per channel was almost unheard of except for the aforementioned EL34 based Marantz 9 which is a major target for Asian knockoffs not to mention the legitimate Marantz * reissue by VAC. Virtually every thing else was based on two tubes per side EL-84s whether it be the 6BQ5 (same tube) the slightly more powerful 7591, or the most powerful the 7868, they all ran under 30 watts a channel in real life. You simply need to get the right high efficiency speakers to use them. I own 2 Sherwoods with 7591s and 7868s they were a lesser known brand. Most people like the Scott variants and the Fishers, they rarely go for more than $500 except the exceptionally clean rarer models. If you want a real Marantz in great shape (mono blocks), they will cost you more than a very good modern amp, 8- 10 thousand. The Harmon Kardons were never that sought after but I don't know why, you might want to listen to some. Be careful about people who refurbish the amps so extensively they are really not vintage amps anymore, they have been modernized so you lose the vintage sound, if you really want it. I can tell you it has its moments but is not particularily favored in light of what you can get today sonically. Experiment, get one that works well and get some same era speakers and you'll know what it sounds like. Its somewhat dark sounding rolled off not much low bass (that takes power) it can be very sweet in the mids.
If you mean vintage like 30 years ago get the Macs they will drive almost anything. Or if you want the late 70s second age of major audio buying especially the big reciever or integrated amp try the Sansui, Kenwood, Pioneeer, Marantz (Japan), a really good example of a mid line one will cost you only about $300. By about 1980 Japan started outsourcing already to the rest of Pacific rim and quality dropped dramatically. Don't be mad if you find the sound not to your liking. The big problem with these amps and recivers is finding any left over or scavanged circuitboards.The older stuff was frequently hard wired butnot always. Repairs get expensive or are just not possible I spent nearly 6 months trying to replace a circuit board in a top o the line Kenwood and gave up. Therefore I highly recommend if your going this route let someone else do the hard work of getting the item into shape without detroying it's character.-
Most of the EL84 based amps (fishers, scotts, eico,heath and more) are 20 watts max. many will only put out 12 to 18 watts continuous. They sound great but don't have much power. The EL34 amps will usually go 35 to 40 watts. KT66/KT88 & 6550 amps will put out decent power too-at a price.
I stay away from dynacos due to overpricing. Also, the dynakits tended to be home built. Some people did great jobs others were ????
If you find a garage sale amp for $50-go for it. I'd be weary of someone else's upgrades.
If you want to check out completely upgraded vintage gear that sounds amazing and has a warranty, check out mapleshade.com. They love to rebuild old fishers.
Also, audioclassics.com specializes in vintage amps, especially mac. You will pay a lot but will get nice quality.
Personally, if you don't have much experience here, you may want to play with inexpensive new gear first to get an idea. Antique sound labs and Jolida have some inexpensive stuff used.
Dynacos are actually cheap compared to other popular vintages.
First of all, you need to figure out exactly how much power is needed. 35-50... I would pick 50 just to be safe.
2nd, depends on the speaker design, some amps sound better with enclosed vs ported or vise versa.
Unless it is super rare collector's item like Marantz 9/10B, I wouldn't worry much about the upgrades. Just make sure that you don't pay much higher price for them. Worst case you can replace to desire parts yourself.
What's the right budget? you decide. If you want to get a collectable unit then you got to pay the price. At same time, these units will hold its value unless you over pay too much.
For any unknown or less known, $200 to $600 is the most I would pay since you can't dump them for any higher price after you decide not to keep it.
For the power you need, Push pull KT88/7027A/6550 would be the primary choice. Some Quad tube EL34 amp (Marantz 9) would be nice but pricy. Other slightless less power ones like 5881 or 6L6GC would be 2nd choice.