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Life span of a tubes(both pre-amp and power-amp tubes) depends on amp "usage" And/Or how often you turn on and turn off your amp. But I need some clarification definition on the word "usage"

Because I just discovered something new, There is a dummy load on the speaker out of my amp, And It only gets hot when I actually play the guitar and gets cold when not played.

So can we make a safe assumption that pre-amp and/or power-amp tubes do not or less wear out when the amp is on but not played because amp is technically not "used"? If so I can leave my amp open for long time and don't worry about tube life.

Or the word "usage" directly correlates to how much leave your amp "on" but then why dummy load does not get hot when not played?

P.s My marshall amp has no stand-by, just on-off

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It should be clear that the most wear on tubes are when sound is being produced, so by definition it's less wear when no sound is produced. It should also be clear that there will be more wear when the amp is on than when it's switched off, otherwise the amp could be left on at all times. I've changed the title to capture that there is a tradeoff concerning the length of the break. –  Meaningful Username May 31 at 21:55
    
@Meaningful Username tnx, when we say "most wear" what percantege roughly we are talking about? And is it equally true for pre and power tubes? –  Spring May 31 at 22:10
    
@Meaningful Username and What would be the answer to your new title? what is the trade off time limit that you should power off; 1 hour, 3 hours? –  Spring May 31 at 22:20
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The trade off time would depend on the circuit design, layout, components used, ambient temp. etc. Remember tube failure is mostly due to fatigue from cyclic thermal expansion & contraction. Like many appliances & machines a significant portion of total wear occurs during the start up & shut down periods. I personally set the trade off time at around two hours. –  Fergus Jun 1 at 0:25
    
@Fergus does your 2 hours time is based on an amp with standby mode or just on? –  Spring Jun 1 at 10:00

2 Answers 2

There is no constant current component over the speaker. When the speaker works, current flows through it altering direction roughly as many times per second as the main frequency the speaker is emitting. When the speaker is silent, there is no current at all, and any load on the speaker will stay cold.

However the filaments of the vacuum tubes must stay hot for all time, as the cold tube would not be able to amplify the sound signal when it finally arrives. As the filaments mostly fail while heating from cold to hot, it makes sense to avoid too frequent turning off and on.

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I'd like to emphasize one point: it is the heating/cooling of the tube filaments that are the primary cause of wear that leads to tube failure, not whether there is sound coming out of the speakers. Using the standby switch (which keeps the tubes hot) or turning down the master volume (which should do the same) has the same effect in terms of avoiding additional heating/cooling cycles. –  Dave Jun 2 at 14:01

Vacuum tubes or valves (UK) have a number of factors that contribute to their life span:

1) integrity of design

2) choice of materials

3) integrity of manufacturing

4) cooling

5) physical shock

6) electrical stress

7) number of hot to cold, or cold to hot cycles

Integrity of design factors should be considered, tubes are optimized for performance or optimized for longevity.

Materials factor as matter of cost vs performance.

Manufacturing practices factor in, as you might have the best design, and the best materials, but the assembly is not optimized. Examples can be found by comparing vintage NOS tubes like RCA, Genalex, Mullard vs. many overseas brands made today and this goes for design and materials as well.

Cooling. The internal temperature of many power tubes is close to 800 C which generates a considerable amount of heat on the exterior side. Consider early VOX amps of the 1960's with a cabinet/chassis design that failed to provide adequate cooling and hence started nasty fires on stage. Consider how vintage 1960's Fender Black Face amps had wide openings near the tubes to allow for air flow. I believe there are a number of later model amps such as the Mesa The Lone Star® that use a fan.

Physical shock. Pinging a tube too much may damage some of finer internal attached materials, some tubes are by nature very microphonic as well. Always test this before purchasing by a light tap with a chop stick to see if it can recover with out ringing. I once tested a set to RCA Meatball logo 12AX7s preamp tubes (circa 1955) that were so microphonic that one actually acted more like a ring modulator.

Electrical shock. Exceeding the spec sheet on a tube's B+ can kill or severely shorten its life. Consider one of the key differences between Leo Fender's "tweed" amps (1950s) vs. his "black face" amps (early 1960's), which was the B+ voltage.

The "tweed" amps had their B+ supply at about 270 to 350 vdc, while the "black face" amps were around 350 to 480 vdc. This was done to extend the head room so the "black face" amps tend to play clean all the way to diming them out. Leo's tradeoff was that to get the head room he had to put the power tubes on the upper margin of their spec. In the 1960s, utility companies in the US were typically setting the outlet at around 110 to 115 VAC, but today this is closer to 120 to 125 VAC, hence the power supply for the B+ is even higher. Cautious players of these vintage amps will put a variac between the outlet and the amp to keep the input voltage closer to a 'vintage' value to save the tubes as well as best match the optimal tone.

With all this said I have not answered your question yet. I just wanted to start with a look at the bigger picture. I personally love vacuum tube amps because of the rich, warm, and vibrant tone. This works particularly well with Blues as for the way tubes produce a more pleasing set of overtones than solid state amps when driven into saturation/distortion.

I understand that your Marshall amp does not have a standby switch. All the vintage Fender amps that I have restored already have a standby switch. In these designs, the standby simply shuts off the B+ while leaving the 6.3 vac heater voltages live, allowing all the tubes to stay warmed up. I generally allow 2 minutes before engaging the B+ this helps save the tubes. Remember tubes are highly inefficient with regard to power, they release 70 - 80% of the energy they consume as heat.

(My Answer). I recommend that once you turn on your amp, leave it on until the gig is over unless you have a 2 hour gap (this is an arbitrary value, in a 1 hour gap I might turn it off, I have no reliable data to support 1 hour or 2 hour). Be sure the part of the amp that has the tubes is adequately vented, if you have a small fan that is not going to interfere with the music this will help keep things cooler and the tubes like that part.

Other things you can do. Purchase the best tubes you can afford (most vendors of vintage tubes will have tested matched pairs and will take them back if these fail on delivery), make sure the power tubes are properly biased*. Don't store your amp in the trunk of your car other then to transport it. This will assure that the tubes do not suffer undue physical shock and extreme temperatures. Likewise, don't store the amp in a place that gets extremely hot or cold.

At the gig, measure the outlet voltage or mains to be sure it is an acceptable value, e.g. above 100 VAC, and below 125 VAC in the USA. Also make sure that the venue has grounded outlets and that your amp has a chassis ground with a 3 prong (ground, hot, and neutral) plug for safety reasons.

  • biasing power tubes can kill you if don't know what you are doing. There are lethal voltages in a tube amp. This is true on many amps (all Fender vintage amps) even when the power has been removed. Take your amp to a qualified amp tech to have the tubes biased and checked once a year.
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