What kills a valve?

[john_the_bass]

So the problem with my amp was apparently the output tubes - it seems one (or more) was a problem which caused them to overheat and stop working.
My understanding was that when valves failed, they usually blew the HT fuse (which didn't happen).

My knowledge of this sort of thing is quite limited but I have been told that the valves could overheat due to a short across the sockets or an open circuit. I don't really know what this means, but I would like to try and find out what the common cause of valve failure is, how it's identified and what I should do about tubes which have failed within 2 months from new.

It's an all tube amp btw - 4 preamp and 4 output tubes.
Ta

[TheRinser]

Were they properly biased?

I know about as much (if not less) than you, but I've heard that word bandied around before...

[john_the_bass]

I was told that the bias was pre-set by Laney and it was, to all intents and purposes, self-biasing. The tech I took it to post-fail, largely agreed that was the case.

[Dubs]

[quote]What kills a valve?[/quote]

A hammer

[size=1]I'll get my coat...[/size]

[BOD2]

A "short circuit" is effectively a wire right across the the two contacts of the output jack. This is unlikely although it could happen if you had a duff speaker lead in which the two wires touched each other.

An "open circuit" is the opposite - the amp turned on with no speakers attached at all. This can happen by accident if you forget to attach the speaker cab before switch on. You'll usually get away with it for the time it takes you to notice but you never know with these things.

Other than that you do get faulty valves. They're a lot like light bulbs - some will last for years, others will blow very quickly. If you get one faulty one it could damage the others.

To prolong valve life -

1. Ensure the valves are correctly biased (if the bias is adjustable)
2. Always follow this switch on sequence
a. Check speakers connected
b. Switch on power
c. Wait a minute or two
d. Switch on Standby

3. If you leave the amp for a period by want to use it again - switch OFF the standby

[Telebass]

Valves are particularly sensitive to impedance mis-match. Unlike solid state, you cannot plug an 8-ohm cab into a 4 ohm output and just lose some power. It MUST be matched. While it will work for a while, both the output valves and the output transformer will suffer.

[Tinman]

Not being allowed to cool down(after heavy usage) and then taken into a cold environment. That did for mine once

[Telebass]

Indeed, and careful handling while warm. 10 minute cool-down before moving, if possible.

[man_at_arms84]

[quote name='Telebass' post='323350' date='Nov 6 2008, 12:41 PM']Indeed, and careful handling while warm. 10 minute cool-down before moving, if possible.[/quote]

I have to say I find this a right pain. When I'm on tour I very rarely get to let my VBA 400 "cool down" for 10 mins. Most times we have to be in a massive rush to get our gear off so the next band can play or if we're headlining, bloody venue staff , bouncers or (and this is the worst) club night promoters telling us repeadly to get out. I usually have to get into it's flight case and out back into the van as fast as possible.
Also our practice/storage room is right freezing this time of year.
Am I right in think all of this could lead to a premature death for my valves?

[Dubs]

[quote name='man_at_arms84' post='325820' date='Nov 10 2008, 11:51 AM']I have to say I find this a right pain. When I'm on tour I very rarely get to let my VBA 400 "cool down" for 10 mins. Most times we have to be in a massive rush to get our gear off so the next band can play or if we're headlining, bloody venue staff , bouncers or (and this is the worst) club night promoters telling us repeadly to get out. I usually have to get into it's flight case and out back into the van as fast as possible.
Also our practice/storage room is right freezing this time of year.
Am I right in think all of this could lead to a premature death for my valves?[/quote]

If you’re putting it into its flightcase before it’s exposed to the elements then it should be fine, I’d guess so anyway as the flightcase will keep the warmth in for a while and will shelter it from the cold, if you get me…

As for the practise room situation, I have no idea. I’d guess that it’s fine to have it in a cold room, but the problems are with the glass valves suddenly expanding or contracting, but mostly contracting – i.e. going from a hot stage environment straight out of the back door of a venue to be taken to the van that’s parked 20m down the road and it’s a lovely -5C January night – the hot valves will contract suddenly when they’re hit by the freezing cold air and could possibly smash/blow etc…

Edited November 10, 2008 by benwhiteuk

[Telebass]

It's not just a temperature thing, letting them cool down. The heater filaments are MUCH more fragile while still hot, and more prone to just breaking. Any physical shock while still hot risks this. It's why car bulbs are made so tough. And expensive. Or projector lamps. Very fragile when hot!

Edited November 10, 2008 by Telebass

[Dubs]

[quote name='Telebass' post='326305' date='Nov 10 2008, 09:27 PM']It's not just a temperature thing, letting them cool down. The heater filaments are MUCH more fragile while still hot, and more prone to just breaking. Any physical shock while still hot risks this. It's why car bulbs are made so tough. And expensive. Or projector lamps. Very fragile when hot![/quote]

Ah, yeah I did hear this, but conveniently let myself forget all the advice I’d been given about valves once I sold my valve head…

Good advice though.

[Telebass]

I'm an electronics tech trained on all things thermionic, from basic audio amps to whole radar systems. I like valves!
But compared to a good solid state amp (all considerations of tone aside), they are inefficient and unreliable, and there is, quite literally, no way to change that. If you must have 'that' tone, fine. But expect to a) pay, and keep on paying. No-one who regularly gigs a valve amp should expect to revalve it less often than once a year. In practice, they do last longer if looked after, but that's what you HAVE to plan for. The amp tech's mantra here, for a 100W Marshall, is a fiver a gig at two gigs a month, or one gig a month for a 50Watter. In terms of reliability and cost, solid state is having your cake and eating it. It cannot ever be that way for tubes.

But they do sound good!

[Dubs]

[quote name='Telebass' post='327084' date='Nov 11 2008, 08:31 PM']I'm an electronics tech trained on all things thermionic, from basic audio amps to whole radar systems. I like valves!
But compared to a good solid state amp (all considerations of tone aside), they are inefficient and unreliable, and there is, quite literally, no way to change that. If you must have 'that' tone, fine. But expect to a) pay, and keep on paying. No-one who regularly gigs a valve amp should expect to revalve it less often than once a year. In practice, they do last longer if looked after, but that's what you HAVE to plan for. The amp tech's mantra here, for a 100W Marshall, is a fiver a gig at two gigs a month, or one gig a month for a 50Watter. In terms of reliability and cost, solid state is having your cake and eating it. It cannot ever be that way for tubes.

But they do sound good![/quote]

I guess having an amp with a decent solid state power section and a great tube pre is a reasonable sacrifice with all things considered. That's the way I've gone anyway, and it weighs sh*t all!! (Shuttle 6.0 – Class D solid state)

[man_at_arms84]

Ahhhhh I dont get the unreliabilty of valve amps. My first amp was valve. A 1970's Marshall Super Bass. Still goiong strong now. Every other amp I have had before has been Either Solid State or Hybrid and they all bust. All of them proved super unreliable. So I've come full circle and gone back to my first love, Valves. I havent had one problem yet. (touch wood)
Im the least tech minded person in the world, but seems to mee too much bells and whitsles on Solid states now. Too much to go wrong. I'll stick to simple made, tube valve amps from now i think.

[Telebass]

Agreed on the 'too many bells and whistles' front. 'S what I like about the LMII - simple and bombproof.

There are always some of anything that simply don't go wrong...long may your amp survive!

[john_the_bass]

So if a valve was faulty and perhaps overheated, taking the other valves with it, what would be wrong with that valve or valves in the first instance (hypothetically).

My amp is cathode biased I believe and I also am under the impression that the bias is also fixed, or perhaps in part, pre set.

[bumnote]

Its a long time since I had a valve amp, but I gigged pretty extensively in the 60s and 70s with vox, marshall, selmer and other various valve amps, and in all that time, I only ever had 1 valve go which was replaced with another one, and I dont think anyone even knew what biasing was.

I dont recall any real problems with any of our gear, [or anyone elses I knew] which included a valve pa, and a bloody great hammond we dropped on a regular basis when taking it up or down stairs. Gear was taken staight out to an old commer van or similar, and left there rain, cold, ice and snow.

Remember valves were used in tanks, guided missiles, radios dropped behind enemy lines, and combat aircraft, [I read somewhere that they were used in the MIG 29 becuase they were less susceptible to EMF], so valves are not that fragile.

[Telebass]

No, not [i]that[/i] fragile but vastly more so than modern solid state.
As to the Mig-29 thing...The EMP from a nuclear burst will instantly destroy all solid state devices not explicitly designed to cope, or without some external protection measures. The high voltage induced in valve circuits tends to simply arc across the valve plates and dissipate through the circuitry, which still needs to be 'hardened' a bit, but will otherwise have a good chance of survival.

If a power valve fails due to incorrect biasing, it's usually because the bias was set too high, or a resistor value has changed somewhat due to heat. The valve then tends to suffer thermal runaway as current increases cause further current increases. Quite often, the valve envelope can melt down, and the screen grid resistor burns out. This can throw a load onto the other valve in the pair, sometimes causing that one to suffer the same sequence of events.

[john_the_bass]

so if I have an amp which has a set bias and hypothetically was running at the correct bias - could there have been something wrong with the valve to start off with which could have caused it to fail and knacker up the others?

[alexclaber]

MiG-25 not MiG-29:

"The majority of the on-board avionics were based on vacuum-tube technology, not solid-state electronics. Seemingly obsolete, vacuum tubes were actually more tolerant of temperature extremes, thereby removing the need for providing complex environmental controls inside the avionics bays. In addition, the vacuum tubes were easy to replace in remote northern airfields where sophisticated transistor parts may not have been readily available. As with most Soviet aircraft, the MiG-25 was designed to be as rugged as possible. Also, the use of vacuum tubes makes the aircraft's systems more resistant to an electromagnetic pulse, for example after a nuclear blast. "

Anyway...

Alex

[man_at_arms84]

I'm definetly finding this thread intresting.
So on the subject of what kills valves, How do you know when your valves need replacing. What are the tell tale signs of a dying valve.
What should I be looking and listening out for to know its time to replace them (other than the obvious ones of a valve actually physicaly braking or blowing!)

[escholl]

there seems to be this black art mystique surrounding valves, and tbh it just sort of makes me laugh...as mentioned previously, valves have been used in tanks, aircraft, etc. Just because they're made of glass doesn't make them fragile, yes there's a heater element however it's not like a lightbulb where the filament is loose, valve filaments are pretty rigidly fixed in place. The technology behind them is so simple, and so proven, that it's very unlikely for the average valve circuit to have reliability issues.

In terms of reliability, I'd much rather have a valve head than any head with an SMPS in it.

What kills a valve? Well, in this case, probably just one valve that had a QC issue from the start, and took some others out with it.

[Telebass]

[quote name='john_the_bass' post='333169' date='Nov 20 2008, 02:54 PM']so if I have an amp which has a set bias and hypothetically was running at the correct bias - could there have been something wrong with the valve to start off with which could have caused it to fail and knacker up the others?[/quote]

As escholl said, yes.