Gradual HT ramp up

[bibi]

Bibi were talking about a gradual ramping of of the HT supply not a delay ,
what tube amps do you know of that are reliable with solid state rectification and no standby ?

Ramping up of the B+ supply is certainly a way of delaying the voltage applied to the tubes, but I understood what you meant.
I'm not sure your question is serious actually. There are innumerable hi-fi amps, test equipment and studio gear built since the advent of solid state rectification that don't have reliability issues and they aren't built with a standby switch. You are trying to prove is that there is a problem that certainly would have been known when tubes were the only method of amplification readily available yet designers and manufacturers have always avoided using a simple switch that would have "significantly increased" the life span of the tubes. This seems such a strange hill to die on.

In fact, if anything, I would say that I have seen more problems associated with standby switches that were incorrectly implemented, solid state rectification or not. The reissue Vox amplifiers spring to mind, the standby switch actually stresses the rectifier tube enough to significantly decrease its life.

 

[reinw33]

I think it is unfortunate that people perpetuate this myth of standby switches prolonging tube life. It has gotten to the point where people regularly ask for one on amps that were even designed without them. When asked, I always put this question back to them: can you name a single piece of professional test equipment, military equipment or hi-fi equipment that included a standby switch?
If delaying the B+ voltage truly extended tube life to any significant degree, even marginally, it would have been omnipresent on the best equipment where longevity and reliability were paramount over cost.

That's just an assumption against an assumption. Nothing more.

 

[bibi]

That's just an assumption against an assumption. Nothing more.

One assumption comes from examining the manufacturing practices of professionally built equipment for military, studio and high fidelity use for over 50 years.
The other assumption comes from experience with the reliability of a relatively small number of guitar amplifiers.

By all means feel free to add a standby switch to every piece of tube equipment you own if it makes you sleep better at night, but I personally will stick without.

 

[Tubetec]

A tube rectifier with a standby , thats utterly pointless ,
Rectifier tubes are not meant to be hot switched , yet even Fender did it in a few amps .

Ive repaired and serviced just about every major brand of tube amplifier over the past 35 years ,
and I consistantly see amps without standby shorten tube life , this is not based on assumptions , its based on my real world experience .

You also didnt answer the question Bibi ,
What amps do you own with solid state rectification and no standby that are reliable ?


Id say nearly every tube amp Peavey ever made had a standby , the only amps that I know of that dont are the Classic 30 and the Nanovalve 5W and they'll both **** out brand new power tubes in as little as a few months .
The classic 30 also has series connected power tube heaters , thats a really bad idea for several reasons., and its an issue thats made considerably worse without a standby .

Ive come up with a simple solution that avoids the need for a standby , it replicates closely what a tube rectifier does , spares every component in the amp stress and wear , sure the bimetalic switch will eventually fail , but without any danger of harm to tubes or amp when it does and its cheap, easily available and quick to replace.


What tube guitar amps have mosfet based HT regulation or capacitance multipliers , I know of none , and I wouldnt want it in there .

Tube mics and preamps are a slightly different situation , most likely you'll have several smoothing caps in a chain with either hundreds or thousands of ohms of resistance between them , that gives a short but useful ramp up .

So much modern electronics is designed to fail just outside of warranty , you only have to look at the pile of electronics waste weve generated , and its getting worse by the day .

 

[trobbins]

So much modern electronics is designed to fail just outside of warranty , you only have to look at the pile of electronics waste weve generated , and its getting worse by the day .

I'd suggest the more common situation is that manufacturers' aim for the design/parts not to have any failures inside of warranty. Like your bimetallic switch example, it is known to fail, but statistical testing for the range of rated conditions would push the onset of failures out past the warranty conditions, and so give confidence for the manufacturer that they would not lose money and reputation on significant number of warranty returns. The customer then has some confidence of no faults during warranty - after that then it is consumer experience that either informs the customer that they need to allocate some annual funding for maintenance/repair, or just 'wing it'.

 

[Tubetec]

I have a large mains isolation transformer , 1kW
normal circuit breakers were no good for it and it tripped often at power up .
I needed a type C breaker ,designed for heavy inductive loads ,usually used with motors .

A tube heater cold measures only a fraction of what it does heated , in some high quality designs Ive found small value series resistances included in the heater chain , there to lower the inrush current at start up .

Thermistors can indeed blow in spectacular fashion for no apparent reason , there made of a crusty metalic substance inside a blob of epoxy , my guess is thermal cycling causes the epoxy to eventually have hairline cracks , moisture get in , and the thing goes out with a bang .

What I actually see is they only want the device to last until the new replacement version hits the market , what weve also seen is updates that cause your device to slow down or compromise battery life , this is purposfully done , then they offer you a concession on the new product .

 

[merlin]

Id say nearly every tube amp Peavey ever made had a standby , the only amps that I know of that dont are the Classic 30 and the Nanovalve 5W and they'll both **** out brand new power tubes in as little as a few months .

The Classic 30 cooks output tubes because it runs them on the edge of destruction with hardly any screen protection, it's not the standby (or lack of it).

 

[Tubetec]

The classic 30 has almost identical voltages and circuit values to the AC-30 , yet it doesnt destroy its tubes the way the classic 30 does .

 

[merlin]

Not sure I agree. The AC30 is notorious for eating tubes, and the reissue model that had a standby switch fitted ate the rectifier tube too! (the originals had no standby)

 

[Matador]

Ive repaired and serviced just about every major brand of tube amplifier over the past 35 years ,
and I consistantly see amps without standby shorten tube life , this is not based on assumptions , its based on my real world experience .

Can you explain the underlying mechanism inside the tube(s) that is failing due to the lack of a standby switch? Not a single tube datasheet says anything about power sequencing requirements between filaments, and cathodes/anodes/grids, other than a spec not to exceed a specific voltage between filaments to cathodes.

You see old tube radios often had similar output sections as guitar amps, and they were made in numbers several magnitudes higher than guitar amps, and I haven't to date found a single one that implemented a standby switch. In fact, a tube radio between 1940 and 1960 was probably power cycled far more often and by far more people as compared to guitar players, and still you don't see standby switches.

 

[Tubetec]

Sure the AC-30 biases hot , and that in itself does contribute some wear on the tubes ,
the facts are an AC-30 runs identical voltages to the Classic 30 and despite class A bias , doest eat tubes the same way as the Classic .

Your correct about that Matador , that corresponds exactly to the point silicon started to take over from thermionic and metal oxide .
https://www.radiomuseum.co.uk/metalrec.html

 

[Tubetec]

As I said previously in more than one solid state rectified tube amp without a standby , I found a degradation of either the heater cathode insulation or the cathode coating itself , its often visible as white powder residue inside the envelope ,
Its caused by uneven heating of the elements within the tube ,

Even under ideal conditions with tube rectified supply with graceful ramp up and controlled heater inrush , an EL84 exhibits a vast amount of random pops, clicks and bangs in the first 20-30 minutes after power-on as the structures within the valve settle . The reason I know this is I use it as a preamp tube

If you can direct a laser thermal probe at a tubes cathode and look at the variation between , instantly applying LT and HT , LT first then HT on , ie like a standby switch , and controlled ramping of both LT and HT rails you;ll see exatcly whats happeneing thermally , if you happen to have a FIR camera you can do us a comparison video.

 

[Matador]

Its caused by uneven heating of the elements within the tube ,

So holding the filaments active (hot), and bringing voltages to the other elements "later" stops this from happening?

 

[Tubetec]

instant application of LT and HT causes the entire tube to undergo stress , the cathode especially ,

Im starting to wonder if half the people jumping in on this thread have even read what I said , because Im having to repeat myself over and over .

 

[Tubetec]

Later is a delay Matador , CONTROLLED RAMP UP

 

[Tubetec]

 

[trobbins]

With heater turned on well before HT, the cathode temp would imho hardly increase when HT is then applied. The cathode temp is determined by the hotter heater temp, and by radiation to mainly the anode, however the anode is significantly cooler than the cathode, and the T to the power 4 aspect of thermal radiation from the cathode implies the cathode temp would only increase by a titch as anode temp increases from pre to post HT being applied.

For sure, heater to inner cathode region is subject to movement of the heater along the length of the cathode tube, and to itself, and that can cause changes to heater-cathode resistance and capacitance, and I guess microphonic output (which would be more noticeable with no feedback around a valve).

 

[Tubetec]

Yeah the general advice with standby switches is give it a minute to warm and dont leave in in standby for extended periods of time , the entire structure within the valve changes temp when HT is applied and has a settling time measured in minutes .

Going back to metal rectifiers ,
these devices disspate a considderable amount of heat , as current draw through the load increases the metal oxide encrusted disks physically press closer together , so it inhibits the spike that you see with silicon .

 

[Tubetec]

If the bimetal strip is sacrificial to preserve the life of the other componentrs down the line thats a good deal ,
The thermal switch could easily be made a user servicable part .

 

[trobbins]

instant application of LT and HT causes the entire tube to undergo stress , the cathode especially ,

I'm just pointing out that any form of application of HT - fast, delayed or soft - including from valve rectifier, or standby, or secondary NTC or soft-start, is likely to have very insignificant impact on cathode stress, which you coupled in directly in post #54.