Urban myths about valves....

Arkless_Electronics

It's the title of an article I wrote for the journal of the institute of sound and communication engineers (ISCE) several years ago that I reckon would be of interest to people here on Fretboard.... it's deliberately kept fairly non-technical... It's in next post in order to fit it in!

Arkless_Electronics

Whist the majority of ISCE members will rarely (if ever) come across the use of our old friend the thermionic valve, there is an ever increasing demand for them in the live music, recording and Hi-Fi industries and it's always wise to be prepared for anything.....
With this in mind, I would like to lay to rest some of the more common “urban myths” about valves currently doing the rounds on internet forums etc.

1/  “You can always spot a dud valve because the filament will not be glowing”
 90% of the time the heater will still glow when a valve is dud. If it isn't glowing then yes it could very well be a dud but it's just as likely that you will find a problem with the heater supply to that valve.

2/  “If you tap all the valves with a pencil then the dud one can be picked out by the different sound it makes”
You can’t test for a dud valve in a powered down amplifier merely by tapping the glass with a pencil and listening for if it tings or pings differently from the rest. With the amplifier switched on it can be a good way for checking for microphonics in earlier stages and can sometimes provoke an output valve to go altogether (often in a “flash over”), thereby revealing itself as the culprit, but that can have other knock-on consequences

3/  “I've just had Acme Repairs fit a new set of output valves and check the biasing on my amp and it failed halfway through the second number of our set! They are obviously cowboys’ etc etc blah blah..”
Just because a set of output valves is brand new does not, unfortunately, guarantee that they will be reliable. Valves, like all electronic components, follow a “bath tub” curve when failure rate is plotted against time. The first few minutes to hours of use are the time when they are most likely to go (yes, really). These early failures are known as the “infant mortality” part of the curve. If they are fine after say 6 hours and a couple of gigs then they will probably go on to have a normal life span. However, valves are more prone to this early failure problem than any other electronic component with around 10% of  (modern) output valves failing shortly after being fitted!

4/  “I can see a blue glow inside my output valves so they must be duds/about to go”
It is normal for output valves to have a pretty blue glow around the screen grid (the spiral of thin wire nearest the anode) or the beam forming plates in the case of Beam Tetrodes. This is caused by all valves having a less than perfect vacuum and the molecules of gas being ionised by the high voltage. They then glow in the same way that a neon sign does. The intensity can vary, depending on the quality of the vacuum and how high the voltage is, BUT if it starts turning pinkish and brighter (and especially if it is tending to leave the confines of the screen) that is bad. It means the valve is going what's called "soft" and losing too much of its vacuum. Failure can then be expected.

5/  “I CAN'T see a blue glow inside my output valves so they must be duds/about to go”
If this glow is not there it does not always mean that the valve is a dud (but yes, it can mean this). There are a variety of other things that can cause it to stop glowing blue such as a failed H.T. or screen grid supply, open circuit screen resistor etc just for a start.

6/  “The heaters of some of my valves glow brighter than others so they must be duds”
The heaters (filaments) of the valves may well not all glow with equal brightness. This is quite normal and can be especially apparent if the valves are not all of the same make.

7/  “I can see a red glow that's not the filament but it's still playing OK”
Nothing but the heater filament and the cathode (the tube up the middle that the heater warms up) should glow red. If any other part glows red (screen grid and /or anode) you have a problem. Switch it off now before it causes more damage.

8/  “An output valve has failed in my amp but it still seems to work. Can I continue to use it?”
An amp can sometimes still work with one dud output valve, especially if it has four output valves,(they function as two push-pull pairs) but this will only happen in those cases where the valve fails harmlessly (open circuit) and behaves as if it just isn't plugged into its socket. There will be less power and more distortion under these conditions. It will not be good for the remaining valve on that side that's trying to do all the work though.
Even when an amp uses only two output valves there can be a surprising amount of volume from the Watt or so of class A output from the solitary working valve!


9/  “You can usually tell a dud valve from its appearance”
It is quite normal for a dud valve to look perfectly normal. If the heater fails open circuit and doesn't light up then that's obvious enough. Also if the valve has even the slightest crack/leak in the glass then the silvery black part of the glass (called the "gettering") will turn white. In either case the valve won't work...obviously, but only maybe 2/10 times will there be anything this visible.

10/  “I've heard that if a fuse goes during a gig  you can wrap it in foil as a temporary measure”
NEVER, EVEN IF THE A&R MAN FROM SONY MUSIC HAS COME TO YOUR GIG, WRAP FOIL AROUND A DUD FUSE AND PUT IT BACK IN!!
Fuses are there to prevent an equipment failure from turning into tomorrow’s headlines about a venue burning down. At very least the repair bill will be vastly higher if you need a new mains transformer and/or output transformer. You have been warned...

And on to a few more generalised points arising mainly from internet based misnomers....

Regarding all the talk one hears/reads about different makes of valve. There are only a few companies actually making valves in this day and age..... Sovtek, Svetlana, JJ (formally Tesla), Ei and a few Chinese makes (mainly Shuguang) manufacture 95% of the valves you will come across (Sorry to any brand I've missed out).
Virtually all other brands such as Electro Harmonix, Groove Tubes, Tube Amp Doctor (TAD) and Marshall, Fender, Boogie own brands etc etc, are re-badged versions of the above makes.
Sometimes you are paying more for these “posh brands” because they have been better "burnt-in" and tested so you are less likely to get a dud....mostly though you are just paying more because they have rubbed "Sovtek" off the glass and printed "Fender" (for example) instead. Yes you can get them colour coded into sets by Groove Tubes etc but you can buy standard Sovtek in matched sets from many suppliers anyway.
It is true that “new old stock” (NOS) valves from yesteryear and manufactured by once famous names such as Mullard, Brimar, GEC and Mazda etc are of much better quality, reliability and longevity than modern brands. In the authors opinion however, they are not worth the 10-20 times as much as a modern valve that they sell for.... and can even be fakes! Caveat Emptor.
The following may be rather a pedantic point but it is usual to hear most non-technical types refer to all the smaller valves in an amp as being in the pre-amp (“pre-amp valves”). This is not the case. There will always (unless it's a single ended class A amp and probably about 5 Watts) be a phase splitter valve, sometimes a driver valve/s after this and often a valve driving the reverb line. Some models such as the VOX AC30 have valves in the tremolo etc. All the above mentioned functions will most usually be carried out by ECC83 (12AX7) or ECC81 (12AT7) or ECC82 (12AU7)...all so called “pre-amp valves” but not only found in the pre-amp section.
Apologies to our more technical readership that will be all too aware of those areas which I have simplified or glossed over, but I hope that this article will be found useful by many readers who are less familiar with the warm glow of the valve.

jpfamps

SED (Svetlana) and Ei no longer produce valves.

Arkless_Electronics

It was written about 2006!

hywelg

11/ An amp is a perfect place to stand a beer during a gig.

Sporky

12) Valves sound better.

pintspiller

13) having a valve amp will get you laid (provided you want laid by another guitarist)

Sporky

14) Valves don't sound better.

Drew_TNBD

Sporky said:

14) Valves don't sound better.

Yes they do.

Sporky said:

12) Valves sound better.

Agreed!

)

Sporky

Wrong way 'round there, chief - these are the myths.

Flippancy aside I was getting at the myth that anything with valves is better than anything without - there are plenty of absolutely crap bits of kit with valves in.

Drew_TNBD

Sporky said:

Wrong way 'round there, chief - these are the myths.

Flippancy aside I was getting at the myth that anything with valves is better than anything without - there are plenty of absolutely crap bits of kit with valves in.

Yeah totally, I'm just playing.

What about pedals that have 'lightbulbs' in them pretending to be valves! Nice one Behringer! )

Sporky

I have a T-Rex reverb that has a valve in it. If you take the valve out it doesn't work, but they still put some teeny LEDs under so that it glows.

Drew_TNBD

S'all about that proper LED warmth.

p90fool

Sporky said:

I have a T-Rex reverb that has a valve in it. If you take the valve out it doesn't work, but they still put some teeny LEDs under so that it glows.

I once fixed a Hartke head which had a 12ax7 in the preamp, it also had an orange LED underneath it which lit up slowly when you turned the power on.
Devious!

Sporky

Drew_fx said:

S'all about that proper LED warmth.

But LEDs are quantum, which is digital, right?

ecc83

Sporky said:

I have a T-Rex reverb that has a valve in it. If you take the valve out it doesn't work, but they still put some teeny LEDs under so that it glows.

This criticism has been levelled at the HT pedal range. Fact is the (orange, not "valve-like" red) LED is on the power PCB as an indicator and shines up thru the valve which is on the PCB above it.

I cannot recall if the pedals will pass a signal thru in bypass (TL0 in TL0 out), it may do as a failsafe but the minute you stomp on a drive switch you ARE going thru the triodes.

I also have an idea that the Behringer valve is in PARALLEL with the signal path and therefore will not kill it completely if pulled. Bellringers are still shits though, seen the HT-5 and HT-20 rips?

Dave.

Richardj

In isolation a valve amp possibly sounds better than a solid state one. BUT, at a decent sized gig, in an acoustically imperfect space, when the audience has had a few beers, very few people give a monkey crap if you have a boutique head wired with unicorn hair or a twenty year old Peavey Bandit as long as it sounds good and they are enjoying it.

ICBM

ecc83 said:

I cannot recall if the pedals will pass a signal thru in bypass (TL0 in TL0 out), it may do as a failsafe but the minute you stomp on a drive switch you ARE going thru the triodes.

I should hope so, the bypass path is independent of the valve!

A sound myth:

15) Overdriven valves produce even-order harmonics which sound good, whereas overdriven transistors produce odd-order harmonics which sound bad.

A practical myth:

16) Running a valve amp into a lower impedance than it's meant to be will damage the output transformer.

Sporky

ecc83 said:

Sporky said:

I have a T-Rex reverb that has a valve in it. If you take the valve out it doesn't work, but they still put some teeny LEDs under so that it glows.

This criticism has been levelled at the HT pedal range. Fact is the (orange, not "valve-like" red) LED is on the power PCB as an indicator and shines up thru the valve which is on the PCB above it.

I don't mind it at all. The valve is doing something and the glow is nice.

ICBM

Richardj said:

In isolation a valve amp possibly sounds better than a solid state one. BUT, at a decent sized gig, in an acoustically imperfect space, when the audience has had a few beers, very few people give a monkey crap if you have a boutique head wired with unicorn hair or a twenty year old Peavey Bandit as long as it sounds good and they are enjoying it.

A twenty-year-old Bandit sounds a lot better than quite a few valve amps I know of, at least as good as many others, and isn't likely to stop working any time soon…

I would own one, except that for some reason I tend to have acquired the smaller and bigger models in the range at various times… currently a Special.

Arkless_Electronics

IMHO valves do tend to sound better for guitar amps (shock horror revelation stuff eh). For other uses it's not so clear cut! If this were a hi fi forum this is a subject that would go on for pages... many say valves are better for hi fi. Many others disagree. It's largely down to how you use them and in what circuitry rather than whether valve or transistor...