I'm starting this thread to introduce the wonderful world of tube amplifiers to everyone.
I've been using tube amplifiers since the early part of my audiophile days (once I had enough money to move on from the popular British budget amps of days gone past). Tube amps are not for everyone though, but I will explain why I like tube amps. Put simply, they are able to convey a liquidity and dimensionality that is hard to find in solid state designs. Enough so, that I willingly put up with practical issues such as heat, re-tubing costs and limited power.
Here are some frequently asked questions and my personal take on them,
1. Should I choose a single-ended triode ("SET") amp, or a push-pull design ?
It really depends on how efficient your speaker is, and the type of music you listen to. SET amps generally have very limited power (think single digit watts !). From a purist approach, most audiophiles believe that the design of a SET is more "pure". I can tell you from experience that there are plenty of bad examples of both SET and push-pull amps out there.
2. I want an amp based on x tubes because I've been told that it has the most magical midrange / high frequencies / staging etc * (*choose whichever applies)
While the tubes used have an influence on sound, there is a ton of other factors such as the circuit design and parts used. 300b based amplifiers do not all sound the same, and this applies to any other tube out there.
3. Why does a tube amplifier have speaker taps ? I have a 6 ohm speaker but the tube amplifier I am looking at only has 4 and 8 ohm taps.
Almost all (you do have special designs that don't have output transformers) tube power amps have an output transformer to couple the power tubes to the speaker load. You should choose an amp with speaker taps that are the same as your nominal speaker load. That being said, nominal impedance is just that - nominal. Impedance will fluctuate at different frequencies. If you have a 6 ohm speaker load, it could very well be 4 ohms or 8 ohms at different frequencies. Choose whichever tap sounds best to you in such a case.
4. My tube amplifier uses tube x. Can I substitute tube y ?
You always get this temptation, especially after reading the internet about some random guy that has tried 6550 or KT88 tubes in his EL34 amplifier (all 3 tubes use the same socket). Always get clearance from your manufacturer before trying this. Even if the tubes work properly initially, it could shorten the life of your amplifier or even damage it. When the KT120 tube was launched, many people with 6550 / KT88 amps wanted to give it a shot. The KT120 uses more heater current. Your manufacturer should know whether your amp's heater supply is up to the task or not.
5. Can I buy tubes from anywhere, or must I get them from the manufacturer of my amp ? Do I need matched pairs / quads etc for power tubes, or preamp tubes with matched sections or low noise ?
You generally can get them from any reliable supplier. However, some manufacturers have selected tubes that are required for their equipment to perform optimally. In some high gain designs for example, the manufacturer may have selected tubes for low noise / microphony.
If you can get matched pairs / quads for power tubes, by all means get them. It is no guarantee though that they will remain matched in your amp over long term use ! If you have a fixed bias amp that only has one bias trim pot per channel (for a push pull amp), or heaven forbid, one trim pot for the whole amp, then you will need matched power tubes since you can only adjust bias in for a specified group of tubes.
Low noise tubes, or matched sections for preamp tubes are only necessary if recommended by your manufacturer. Otherwise, you just end up wasting money for nothing. I have some finicky equipment that are very selective on tubes. Many tubes that are too noisy for this amp work just fine on other equipment.
6. How often must I replace my tubes ?
Your manufacturer will have a recommendation on when this needs to be done. It depends very much on how hard the tube is being run, and the ventilation you have. Preamp tubes generally last a lot longer than power amp tubes.
7. How often must I check the bias of my power tubes ?
There is no real need to check the bias of your power tubes religiously, nor a need to bias them to atomic clock precision levels. I always check the bias of my power tubes when I replace them, and I'll follow up to check it after a few days of use, and maybe a month down the road. After that, I'll just check it once every 4-6 months. In fact, checking them too often and fiddling with the bias pots everyday wears them out.
8. Do all tube amps sound warm, syrupy and "thick" ?
Nope. I've heard solid state amps that sound like tube amps and vice versa. Most brands have a certain house sound and some brands have a more romantic sound, while some others really sound fast and linear.
9. Singapore has 230V mains voltage. Is it OK to import a tube amp rated for 220 or 240 V AC operation ?
Ask the manufacturer. Depending on how high / low the plate voltage is, and how the heater voltage is derived, this could be a problem. In a worse case scenario, your amp rated for 220 VAC could see incoming mains voltages as high as 240-250V (not uncommon in Singapore). This could result in higher plate and heater voltages and other nasty things. Not good for your amp.
In theory, a 240V model used with a 230V supply would result in the converse, overall low voltages in the amp. Also not good. However, in practice very few people in Singapore have voltages spot on at 230V or below that. Most are on the higher side anyway. In that sense, it makes sense to go for a 240V model over a 220V model.
10. I want a tube amp with balanced inputs and outputs. Why are there so few on the market ?
There are very few tube preamps with true balanced signal paths. A 2 channel preamp with a true balanced signal path is actually a 4 channel preamp (normal and inverted left, normal and inverted right). To reduce costs, many of the tube preamps that offer balanced inputs and outputs actually use converters (whether transformer or circuit based) to convert balanced signals to single ended, and then vice versa on the output.
11. I want to match a tube preamp with a solid state power amp.
Quite a popular approach. Very powerful tube power amps are expensive, hot and bulky. Why not combine a tube preamp and a solid state power amp ? You get the tone of tubes, and the power and slam from solid state.
Most tube preamps have a fairly high output impedance. When you couple a preamp with a high output impedance, with a solid state power amp with a not so high input impedance, you will get high frequency roll-off. You should aim for at least 50x difference, e.g. a tube preamp with an output impedance of 1K ohm should be paired with a power amp of at least 50K ohm input impedance. Depending on who you ask, you will get all kinds of answers, like 10x, 20x, 100x.
Tube preamps that do not have start-up muting circuits may also make pretty nasty noises upon turn on / warm-up, so you have to develop the habit of switching on the preamp first, waiting for a minute or so, before turning on the solid state power amp.
