[FullRangeMan]

I'll be a bit more clear. He's never had a 300B fail in 12 years, but he has rolled various 300B's thru the years. So it's not like he's had the same pair in service for 12 years. I believe he's tried 5 or 6 different manufacturers. The best (and most pricey) of the bunch has been his current Emission Labs. They've been in the amps for about a year.

Thanks for clarify;

[JoshK]

FRM,

I think you are confusing two very different things.   One is the ability of the amp to drive a low sensitivity.  Obviously a low watt amp won't do a good job here.   That is a completely different issue than "stressing" a tube and shortening its lifespan.  Lifespan is a function of, aside from quality control and design issues, of bias current.  Meaning how hard you run the tubes in the amp.   With all due respect, that is no fairly tale, its simple physics.

[geowak]

FRM,

I think you are confusing two very different things.   One is the ability of the amp to drive a low sensitivity.  Obviously a low watt amp won't do a good job here.   That is a completely different issue than "stressing" a tube and shortening its lifespan.  Lifespan is a function of, aside from quality control and design issues, of bias current.  Meaning how hard you run the tubes in the amp.   With all due respect, that is no fairly tale, its simple physics.

Frm please educate me here. Would a low wattage amp have to be driven harder, say above 1/2 of total volume, to begin stressing the tubes? Would it be different than say a higher wattage amp driving the tubes hard? This would be with the speaker sensitivity being the constant.

[JoshK]

Were you asking me or FRM?

The part that I am not 100% clear on the effects on the output tube is how clipping (say hard clipping) of the amp effects the tube's characteristics via secondary effects (via caps, resistors, etc).   This isn't very clear cut.   But in class A the tube can source X amount of current at Y amount of voltage based on the operating point, asked more and the tube compresses but doesn't source any more (except maybe to steal from the driver stage ala A2 operation, so it isn't really doing anymore work beyond what it can do. 

[FullRangeMan]

There are many ways to shorten the life of a valve, one is low sensitivity speakers, others would be:
Hot Bias
Run the amp around the clock
Lack of cooling or hot summer etc
Lack of soft start/warm up
Run speakers in hi SPL
Low impedance speakers
Vibration, physical shock

Some of these stressing factors create heat in the tube other had to do with the amp hardware/circuit;

[geowak]

Were you asking me or FRM?

The part that I am not 100% clear on the effects on the output tube is how clipping (say hard clipping) of the amp effects the tube's characteristics via secondary effects (via caps, resistors, etc).   This isn't very clear cut.   But in class A the tube can source X amount of current at Y amount of voltage based on the operating point, asked more and the tube compresses but doesn't source any more (except maybe to steal from the driver stage ala A2 operation, so it isn't really doing anymore work beyond what it can do.

Yeah Josh, my mistake, the question was directed to you. Thanks for clarification. I guess something that I am unclear about is this; I thought tube amps were supposed to run hot. I mean I have only SS stuff to compare which just get warm. My 216IA is running 4 Genelex Gold Lion reissues and 4 NOS Sylvania mil spec tubes and it runs hot, but never enough to make me worry. Do SET amps run hotter? I don't worry too much with my speakers ( 4ohm or 6ohm) drawing too much from my amp, it does not seem to strain at medium volume..

[FullRangeMan]

Sorry the unsolicited post:
Hot up to a point, if too hot the valve can be damaged, the thin metallic wires can evaporate and be collected by getter.
The first alarm sing is when the plate becomes red/cherish, it looks too much bias too me, but I can be wrong:

[JoshK]

FRM:  833s and many transmitter tubes (like the big EIMACS) are meant to be operated with cherried plates which isn't normal for most audio tubes and those tubes can handle it without loss of life.   There isn't a hard and fast rule, but for audio tubes it isn't typical to run cherry hot.

Geowak:  SETs run hot because they are pure class A (as in thermal class A, not fancy marketing talk).   Which means they put out something like 4 times the heat that they do in power.   SS amps today are mostly A/B which depending on their bias current means then can run cool.    Your 216IA amp is likely Class A/B (guess, I'd need to look up specs) which allows it to run cooler. 

FRM:   I agree with some of your reasons for shortened life except for the 3,  low impedance speakers, high SPL and low efficiency speakers which aren't supported with facts.   Do you have an evidence to support your claim?

[FullRangeMan]

Most or all small tube amps builders recommed use its amps with hi sensitivity loudspeakers.
Its much more easy to the amps.

[pehare]

Wouldn't it be a novel idea if a tube amp builder gave the buying public an approximation of how long a set of tubes lasted under normal operating conditions?  It may sound great in the reviews or showroom but without having some idea of expected tube life, the buyer has no idea of what they're getting themselves into $$$

[Captainhemo]

I'd imagine if you looked at  recommeded bias current setting   for  similar amps, it is going to give you some idea of  how hard an amp is pushing  its tubes. 
If you take 2 amps  using the same tube,  and one is  bias'd at a current 25% higher , I'd think   it would  have an effect on the tube life.

If I'm understanding what others are saying about high bias current....

-jay

[Ericus Rex]

Most or all small tube amps builders recommed use its amps with hi sensitivity loudspeakers.
Its much more easy to the amps.

I think that spec is given to best showoff the meagerly-powered amp's capabilities without clipping.  No manufacturer of a 3 watt amp is going to recommend 84dB speakers because those insensitive speakers will clearly show the amps limitations.  I've never heard tube life given as a reason to use high efficiency speakers.

[Guy 13]

I think that spec is given to best showoff the meagerly-powered amp's capabilities without clipping.  No manufacturer of a 3 watt amp is going to recommend 84dB speakers because those insensitive speakers will clearly show the amps limitations.  I've never heard tube life given as a reason to use high efficiency speakers.

Hi Ericus Rex.
I don't see any relation between tube life
and speaker efficiency.
You can only estimate tube life,
because of many factors, such as on-off cycles,
bias and even ambiant operating temperature.

Guy 13

[JoshK]

I'd imagine if you looked at  recommeded bias current setting   for  similar amps, it is going to give you some idea of  how hard an amp is pushing  its tubes. 
If you take 2 amps  using the same tube,  and one is  bias'd at a current 25% higher , I'd think   it would  have an effect on the tube life.

If I'm understanding what others are saying about high bias current....

-jay

It has more to do with total dissipation then bias current actually.   Meaning, a tube with higher bias but lower voltage might be operating at the same dissipation mark as the same tube operated at lower bias but higher voltage.  I realized that I was using bias a bit loosely, when I meant for a given voltage, so I thought I'd clarify.

In the tube DIY world, there is typically two groups, those that like low and hard and those that like high and lean.  All else being equal the prior has the ability to drive more current but swings less voltage linearly (amplification factor), the opposite for the latter.

Typically for long life, the rule of thumb is to bias (for a given voltage) a tube so it is operating at no more than 75% of the tubes dissipation limit.   The venerable 300B often is quoted as a 40w dissipation limit (although most modern tube apart from WE, EML and JJ can really handle that; by the way you can tell from the size of the plates(anode)) so you want to be operating at less than 30w dissipation if you want to run those tube for years to come.   See Morgan Jones for reference.

And I'll reiterate, low watt amps need high efficiency speakers to sound good and not distort to all hell but it has almost nothing to do with tube longevity.