Some amps are designed to drive tubes hard, some will drive tues so gently that they last forever. Generally I have owned amps by designers who don't run tubes hard and I've had very few tubes fail. I've been running 80 and 100 year old tubes lately and still haven't had one fail. I often have my amp on most every hour I'm home. I use it for music and also for powering the front speakers during TV/movies. If you're having tube failures, I'd look at my amp design. Jerry |
@xenolith pretty sure your balanced power supply doesn't regulate voltage. Regenerators are the only power supplies I know that do. My amp has adjustable heater voltage. I do have to adjust it for different driver tubes but I'm using a wide variety of type 10,10Y, 210, 801, and 801A tubes. Jerry
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@whart made a couple of points that I agree with. I check the bias frequently. I don’t have to use a fluke meter as my amp has meters for voltage and current for both the driver tubes and the output tubes. My amp is very stable but I have been rolling tubes a lot lately. I also agree that there is no need to use white gloves. I’ll admit I’m an outdoor guy who doesn’t use lotion so my skin tends to be dry. but most tubes run cool enough that a little skin oil it isn’t a problem. Thinking about this thread for the last day, I think the biggest factor in tube failure is running them at the wrong voltage or current. Most amps don’t have as much adjustability as mine. I’ve had amps with no adjustability (cathode bias) and they are designed to be easy on tubes--no action required. I had a Decware ZMA and it had a great biasing system that I really enjoyed. it accommodates a lot of tubes and the bias current for each tube can be different. to calculate the bias current for a particular tube, look up the maximum plate dissipation on the tube data sheet (watts). Measure the B+ voltage of your amp. The best way is to use some insulated probes clipped to the correct pins (tube slightly elevated) running out to a volt meter. Make sure you don’t create a short to the chassis, turn your amp on, write down the voltage, turn your amp off and remove the probes. This number is important to know. Sometimes OEMs will provide it. Divide your plate dissipation (watts) by B+ voltage (Volts) will give you amps. You’ll have a decimal. Multiply by 1000 to get milliamps. Now decide how hard you want to drive your tubes--usually a number between 60% and 80%- and multiply that times your current and that is where you set the bias. I have found recommended bias numbers well above what I calculated. I have also know guys who ran their bias purposely higher than recommended because they thought it sounded better. I think this is a myth, the "higher must be better" mentality. If you have auto bias, you’re out of luck. I’ll never buy an amp with autobias. Jerry |
Your bias current is usually measured across a 1 ohm resistor so that mV=mA. The meter measures volts but because the resistance is 1, then the number is the same. So a custom meter will measure volts, but it will say amps. if your maker didn't order a special meter dial, it may say mV, but is actually reading mA. There is a post above that contradicts this and I'm not sure what the poster meant. Jerry |
@inna I wouldn't say that I recommend choosing an amp that is easy on tubes so much as I recommend against an amp that is hard on tubes. How do you know which is which? I'd say avoid an amp that gets more power than normally attributed to that tube--unless I needed that power and was willing to buy tubes. And I'm fine with that. Personally I don't think tubes are expensive compared to the rest of the audio equipment we all have to purchase. I buy tubes when I don't need them. A nice pair of Hytron 801As showed up yesterday. There is a trend now to make higher power tube amps to drive lower sensitivity speakers. I like higher sensitivity speakers but if you're married to your 92-95dB speakers and want to drive them with a tube amp, you need more power. So they are making more push pull, parallel SET, and High power SETs with the higher output tubes. if you buy some of these, recognize you'll have to replace tubes more often. (not saying push pull amps are hard on tubes, unless designed that way). If I'm looking at a 300B SET and the OEM says 8 wpc, and 300b's generally do 6 wpc, I'm going to wonder about tube life. (or else the marketing talk is exaggerating and it really is a 6 wpc amp). I've found that if you go with an amp builder with a lot of experience and knowledge, they are likely to design their amps conservatively. And if you call Aric or Apollo and order and amp, if you tell them to use conservative parameters to promote long tube life, I'm sure they can do that. jerry |
Good points by @atmasphere and a couple I want to comment on. My current amp has a warmup relay to warm the tubes up before applying B+. What he suggests with a warmup switch is easy to do and a great suggestion. Avoiding high voltage is important. The designer was very careful to provide the correct voltages and if your voltage goes up 120 to 125 (mine varies from 117 to 124 and that is just what I have seen) then the increase goes right through the power transformer to every voltage in the amp. A good way to regulate voltage to 120 (I have mine set to 119.9) is with a regenerator. Jerry |
