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Valve questions from a total numpty

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morris_minor View Drop Down
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    Posted: 01 Jan 2016 at 5:06pm
I'm getting together a little bedroom system and recently got an amplifier off eBay which did what I was after - two analogue inputs and Bluetooth connectivity.

The downside (or maybe upside!) is that this little thing has a valve pre-amp. The first time such an anachronistic device has set foot over the threshold . . Embarrassed

So my questions are:

1. Do valves wear out more quickly with being left on for long periods, or being turned on and off many times?

2. When one fails, is it a simple question of just turning the thing off and inserting a new valve? I read about setting bias, and valves having residual current (or something).

How can I prolong the life of the tubey things, and not kill myself when I need to change one? Shocked

I'll leave the issue of tube rolling to much (much) later . . . Wink


Bob

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jan 2016 at 5:17pm
Never had a valve amp but always found the early 80s Audio Research very inviting (albeit way out of price range)...

An extremely experienced and traditional audio engineer I use advised some time ago that valve gear tends to need 'tuning' it may be the bias you mention. I strongly suspect they may degrade over relatively short period.

Perhaps Graham can verify or otherwise?

Richard
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Chris Firth Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jan 2016 at 9:23pm
You should get a relatively long life out of your preamplifier valves, typically somewhere in the order of 10,000 hours as long as the valves aren't getting a caning from poor circuit design.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldagetraveller Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jan 2016 at 10:27pm
Just make sure the valves are there to carry out their intended function and not for effect!
I have seen reports that some so called valve pre amps from China have cheap valves fitted to show a nice glow but do absolutely nothing.
Peter

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 02 Jan 2016 at 11:04pm
Thomas Alva Edison was inventing the light bulb when he by chance happened on another invention which he eventually developed into the thermionic valve (tube).

He discovered electron flow in the vacuum of the light bulb envelope was plating the inside of the glass (much as an auto bulb does when your alternator regulator packs in), because he was using DC volts (quite popular in the USA at the time - and quite dangerous!).

After much development by those following in his footsteps reliable valves became a possibility. Reliable to a degree. If we understand that what is basically metal is being very-very-slowly deposited from one place to another it will eventually fail. Therefore between first switch on and failure its characteristics are changing.

Transistors got around this problem with electrons and holes where it's all "recycled" around the circuit they're in.

The above is intentionally simplified. The people reading this are intelligent but in their own field of work. And I don't feel like writing an essay.

For the valve to work and due to its high impedances (ac resistance) it needs a HT supply. HT is high tension -- high DC voltage. Most valve preamp circuits work at 100 - 250 volts DC, which is above the (75 volt) Low Voltage Directive (not that manufacturers care).

Signal modulated voltages greater than 75 volts are found in valve amps on the output transformer primary. Like 100V line used in PA if you touch it you can "feel the music". But that's a valve amp, not a preamp.

As you will get an electric shock from the mains supply, you will get a belt off HT, but it will have the effect of making your hand tighten around the conductor, and this is why DC mains was eventually banned in the UK -- because of all the fatalities. An ac shock because it alternates gives you chance to let go.

Capacitor discharge at such voltages isn't nice either, but not quite as bad as an EHT (extra high tension) discharge. You have little chance of accidentally killing yourself from EHT discharge, but you have more chance of being killed or seriously injured by the way it throws your body at high acceleration away from it. Luckily when it happened to me there was a soft sofa to reduce my impact with the wall behind it.

My thoughts on valves: I'd rather have my heart beating.

Why do I know so much about electric shock? Because I was trained in it and still have my certificate somewhere. Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 2016 at 9:39am
Hopefully the bed will stand in for the sofa . . . LOL

What I'd really like to know is does either frequent switching on/off or being left on for long periods have a worse effect on valve life? Or doesn't it matter?
Bob

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 2016 at 11:06am
Originally posted by morris_minor morris_minor wrote:

Hopefully the bed will stand in for the sofa . . . LOL

What I'd really like to know is does either frequent switching on/off or being left on for long periods have a worse effect on valve life? Or doesn't it matter?


If you leave it on then the tubes will obviously be burning like light bulbs, or to be more precise, the heater elements which are needed as a catalyst to get electrons to flow are burning just like light bulbs. The heater winding on the transformer was 6.3 volts when I was a lad, so the heaters are glowing a bit like torch bulbs in a 6 volt lantern. Some are DC and some are ac which are a bit more "hummy". I can't remember which type lasts longer and I wouldn't know what yours has. I don't know what's "trending" in valve hifi, I just know how to design using valves but I don't put it into practice.

So it's probably best to switch it off. Each time you switch it on each heater element has to go from cold to hot. Therefore it suffers a step-change. Obviously that is a designed-for occurence. But like a light bulb it has a lifetime. It's all basic physics. If you get some thin metal and keep bending it back and forth it eventually fractures. If you were to switch on and off in rapid succession the heater element would eventually fail. Valves are more subject to fatigue than solid state because they're more "mechanical".

In the case of those USSR valves which don't have a heater the above is academic.

Now, do you want me to explain how valve biasing works?
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