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Things to look for when purchasing vintage / re-imagined or hand built tube amps:
Everyone knows that tube amps are cool. Everyday, at least a 100 are sold on Ebay and other online sources. Some are sold to seasoned professionals who know what they are looking for, but the majority go to first time buyers wanting to get in on the tube experience. You first time buyers are the ones that I put this list together for. You want to buy an amp that is plug and play. You want an amp that is safe. You want an amp that will work with your input source (Phono, Cd Player, MP3 Player or what have you). You want an amp that will work with your speakers. You want some tips on the proper care and feeding of your first amp. If this is you, read on…
1) NEVER buy an amp with a two-prong power cord. A properly installed grounded power cord is the cornerstone of safety in an amp. The ground lug connects the (usually) metal amp chassis to ground. With a three prong plug, if something goes horribly wrong inside, the chassis cannot be energized to deliver a lethal shock to you. Most vintage amps originally were fitted with two-prong cords that do not provide this protection. Any reputable seller offering a vintage amp for sale describing it as 'turn-key' or 'ready to go' will have upgraded the power cord to a three prong. With vintage gear, you have the option of asking the seller to install a grounded plug - most will do it for little or no charge. If they balk at the request, don't buy.
Anyone selling an amp as their own - key word to look for: hand-built, custom, handmade, etc. should absolutely have a three prong grounded cord installed. If they don't, remember this: To this builder/seller, your life as a buyer isn't worth the $3.00 and extra 10 minutes it takes to install a grounded cord. Simple as that. If a builder takes shortcuts with simple safety items like this, you can be assured they are taking potentially lethal shortcuts elsewhere in the amp (more on that below). No matter how sexy a new amp looks on the outside, if there is a nasty brown two-wire power cord running out the back, DON"T BUY IT! You and your families' lives aren't worth the gamble. Honest builders never take shortcuts with safety.
Shady (or ignorant) sellers may try and tell you that it is okay to use a two-prong cord, as long as the amp has a fuse, or that you should just plug the amp into a computer surge protector. This is ABSOLUTELY WRONG! No commercially available fuse will act in time to prevent a lethal shock. Fuses are designed to protect electronics from prolonged overload conditions. The only purpose a fuse will serve for consumer protection is that it will probably prevent your corpse from catching on fire after the amp delivers a fatal shock.
Anyone selling an amp with a two-prong cord and claiming compliance to any modern code (UL, NEC, etc.) are just feeding you BS. Any item presented to the UL for consumer testing that has a metal chassis and ungrounded power cord would fail even the very basic initial visual test. Although the UL does not have a specific category for tube based audio amplifiers, the closest analogue are residential or commercial PV invertor units - the UL requires that these have the hot and neutral AC lines isolated from the chassis, that the DC components are grounded to the chassis , and that the chassis ground be tied to an earthed/ ground conductor.
2) Never buy an amp that the seller is not willing to disclose pictures of the insides. Honest sellers are open to full disclosure on vintage gear. Additionally, someone who claims to have built a custom amp, but won't show you pictures of the internal wiring, is either too embarrassed of their work to show you, or are hiding shoddy work. Honest builders are proud of how their amps are wired inside, and almost always include pictures of the internal wiring in their sales listings. There are lots of excuses provided by shady sellers such as: "I borrowed a friends camera for the photos", "I already have it boxed up for shipping." Or my personal favorite: "The circuit is proprietary / my secret design / special , and I don't want people stealing it". This one is an easy call to make - if the seller won't show you the insides of their amp, don't show them your bid.
3) This one should go without saying, but… For your first amp, never buy one described as untested or as-found or for parts or repair. Trust me, you won't get lucky and get something in perfect working order.
4) Hot Glue. Never buy an amp that has hot glue holding components in place. Hot glue was invented to empower housewives with worthless husbands to sort-of fix things around the house. It has no business on the inside of your amp. As its name implies, it melts when it gets hot. Guess what? The inside of your amp is going to get hot. That glue is going to melt and let high voltage components touch the chassis. If you have a grounded three prong plug, the amp power supply is going to short to ground and probabl y burn itself out in the process. If you didn't listen to me earlier, and you bought an amp with a two-prong cord, the amp is going to set there energized and ready to bite you with a potentially lethal shock as soon as you touch it. Big banks of electrolytic filter caps stuck down with gobs of hot glue are a recipe for disaster. Any hack that would build an amp using hot glue to hold down high voltage electrical components is one to be avoided.
5) Tubes. Approach any amp for sale without the tubes with suspicion. I recommend never buying an amp that does not include ALL the tubes needed for operation. People who 'short-sell' tube amps often buy an amp, pull and sell the tubes separately, and list the remaining carcass as a fully functioning unit, never having even powered the unit up. Custom amps using exotic tubes (300B, 2A3, etc.) may be an exception to this of sorts - but any reputable custom builder using commonly available tubes (6BQ5, 6V6, 6L6, 5881, 12AX7)will always provide a set of known good working glass with their amp.
6) If you are buying a vintage or re-imagined amp, be sure that the seller has replaced the original electrolytic filter capacitors. These go bad slowly over time when in use, and very rapidly when left discharged and unused. Most vintage amps to be found are coming back from years to decades of dormancy. Standard best practice for a good seller is to have replaced ALL the original electrolytic caps prior to sale. Don't buy into the standard assumption, often asserted as fact by shady sellers, that just because there is no power supply hum the capacitors are good. Lots of folks swear that vintage unused NOS electrolytic capacitors can be reformed to new condition. I'm not going to disagree, but I find it easier to just buy new stock and avoid the hassle.
7) Input device and levels. Most amps are built to accept consumer line level inputs, meaning that you can drive them directly from your tuner, CD Player, or MP3 player. Most will not accept a phono input directly from a record player - for that you will need a preamp that not only boosts the signal level but provides the proper RIAA equalization correction. Discussions of these are beyond the scope of this primer, but feel free to email me for assistance. Lots of vintage amps came originally with spade lugs or weird brand specific connectors to connect to other components. For your first time amp, look for something that has a more consumer friendly input interface installed. RCA jacks are usually standard, but some builders like to use ¼" stereo phone jacks or even 1/8" mini stereo plugs, just like the ones you use for the headphones on your MP3 player. If the seller does not specifically call out line level inputs, ask them to confirm that it will work with your gear. Lastly, don't expect cables for connections to your source to be included - they usually aren't.
8) Speaker outputs. As with the previously mentioned inputs, the original speaker outs on vintage gear can be weird manufacturer specific connectors, flying leads, or lugs. Look for upgraded binding posts or screw down terminals that are clearly labeled for polarity or color coded. Tube amps generally have output transformers designed to properly load the tubes when connected to specific speaker impedances. Check to make sure that the speakers you intend to drive are at least close to the recommended load impedance of the transformer. Some amps will have multiple output taps to allow a variety of speaker impedances to be used. Common speaker impedance numbers for tube amps are 8 or 16 ohms.
9) Warranty. Someone selling you an amp described as being turn-key and fully functional should back it with some sort of return or repair policy. Reputable sellers put an amp thru at least a few hours test use prior to offering it for sale, and are confident it will not arrive in your hands in a 10-7 or DOA condition. Amps sold on an as-is / no-returns basis are best to be avoided for your first amp. Don't expect sellers to cover return shipping or warranty items damaged by the buyer. "Whoops, I plugged the tubes in the wrong sockets, and now it doesn't work anymore" is not a valid reason to return an amp for a refund.
10) Don't get hung up on a specific circuit (single ended, push-pull, OTL, etc.) or specific tubes (6BQ5, 6V6, 6L6, etc.). Pick an amp based on your listening needs, not the other way around. Well built amplifiers hold their value, so you can always trade up or sell what you have and try something different down the line.
Update 2/11/09:
Wiring Diagrams & Clarification:
Here are a set of three wiring diagrams illustrating the difference between a transformer power circuit utilizing a grounded electrical plug and variations of those that do not. Figure 1 shows a very simplified amplifier power circuit configured to the current UL requirements for High voltage AC and DC power equipment. The ground lug from the power cord is tied to the chassis, as is the DC ground of the power supply. The AC mains are fused and isolated from both the chassis and DC supply. This is as safe as you are going to get - an electrical fault on either side of the transformer is dumped directly to ground, and the chassis cannot energize to any voltage potential.
FIGURE 1
Figure 2 shows a very similar but potentially deadly amplifier power circuit configured to old(pre 1970's) UL requirements for High voltage AC and DC power equipment. There is no ground lug on the power cord. However, the DC ground is tied to the chassis. The AC mains are again fused and isolated from both the chassis and DC supply. A fault on the DC side of the power supply will energize the chassis, but this is generally a brief event as the electrical load of the short should blow the fuse in short order. A fault on the AC line side of the transformer will energize the chassis without showing any outward signs of danger, except for a noticeable hum from your speakers. This type of fault may or may not blow the fuse, allowing the chassis to set hot until you reach over to turn it off... Some vintage factory designs employed 'safety capacitors' across the line side or from the line side to ground. The term is actually a misnomer - these caps were designed to reduce AC noise interference, and were not in themselves a safety feature. Just Radios has a nice write up about them here: http://www.justradios.com/safetytips.html Dave and Babylyn at Just Radios are also a fantastic source for obscure value capacitors and resistors.
FIGURE 2
Figure 3 shows just about the worst you can do for safety. There is no ground lug on the power cord, and the DC ground is NOT tied to the chassis. The AC mains are again fused and isolated from both the chassis and DC supply. A fault either side of the transformer will energize the chassis, without blowing a fuse or manifesting any symptoms. The only way you will know the chassis is hot is when you touch it. Common sense says never to wire an amp this way, but some builders still do because this safety flawed design does offer improved AC line noise rejection. In my book it is a big price to pay for a little bit of hash elimination.
FIGURE 3
I can't stress this enough - If you are buying an amp, insist that it be wired per figure 1. Custom Builders can politely disagree about tubes, circuits and just about anything else, with the exception of safety.
Anyone that begs to differ on this point is either a jackass that doesn't care about your safety or too ignorant to understand the difference. You shouldn't buy from either.
I am always open for comments; good, bad or otherwise. Feel free to email anytime - jafix@comcast.net
Update 2/17/09:
Safety Links:
Here's a fun one from Thailand - not everyone in the world is hooked up to a great grid: