Valve Question

[Sondekker]

As a relative newcomer to the world of valves since buying a Croft 25R & 7R, I have done a little tube rolling usually by buying an occasional tube via ebay.

I have always made sure that I purchased valves that have been tested but what I'm not sure about is whether there is any risk to the amplifiers themselves if I were to purchase and fit an untested valve if it was in any way faulty?

Would it just not work correctly or would it potentially damage the amplifier?

[walpurgis]

Would it just not work correctly or would it potentially damage the amplifier?

Either.

[struth]

potentially both as Geoff says.... switch on and watch with finger close to off switch.... it anything looks wrong then power down...watch, listen, smell

[cyclopse]

Do the instructions state if there is any form of protection circuit for your Croft amplifiers?

[Sondekker]

Instructions....??

I'm not sure Glenn believes in such things! :)

[Firebottle]

There is a simple 'clamp' circuit across the speaker output in the Croft 7 to prevent speaker damage in the case of a fault, however a faulty driver valve will not have any effect on the output.

The 25R has regulated power supply rails so it is unlikely to be damaged by faulty valves, plus the valves in the 25R are not big power valves.

:)

[Sondekker]

Thanks Alan and the others.

Based on these replies and Alan's in particular, it would appear that whilst the risk of damage is still present it would appear to be very small.

As it happens, my 7R is one of Glenn's special 'Tannoy' versions with further upgraded valves which according to Glenn were as good as you can get. I do not wish to change any of these.

My real interest is in rolling the 12BH7 valve that's in the line stage of the 25R. I would like to try a few different versions and whilst browsing for options there are quite a lot of valves that I think might be worth trying a number which are untested. As they are understandably cheaper than tested versions I simply wanted to know if I could risk trying them at low cost to help with finding something I like.

I think I feel a bit more confident now that I could consider this.

John

[Arkless Electronics]

This old article of mine may answer some questions on valves.... It was written far a technical journal but kept at a low technical level for folks relatively new to valves. It also focuses mainly on guitar amplifiers but that makes little or no difference to it's message and meaning...


"This is a bit of a rant in which 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.

This article was first published in the December 2010 issue of the Journal of the Institute of Sound and Communications Engineers and is copyright © 2010 by Arkless Electronics."

[Barry]

Excellent thread Jez, and one that will continue to be of interest to Members in the future. As such, would you be willing to have it placed in The Knowledge? You would of course be acknowledged as author.

Thanks

[Jimbo]

Thanks Alan and the others.

Based on these replies and Alan's in particular, it would appear that whilst the risk of damage is still present it would appear to be very small.

As it happens, my 7R is one of Glenn's special 'Tannoy' versions with further upgraded valves which according to Glenn were as good as you can get. I do not wish to change any of these.

My real interest is in rolling the 12BH7 valve that's in the line stage of the 25R. I would like to try a few different versions and whilst browsing for options there are quite a lot of valves that I think might be worth trying a number which are untested. As they are understandably cheaper than tested versions I simply wanted to know if I could risk trying them at low cost to help with finding something I like.

I think I feel a bit more confident now that I could consider this.

John

Hi John I have rolled the 12BH7a in my line stage (Modified Croft 25R) and use an RCA NOS. GE NOS were also good but a bit light and airy. Glenn also recommends Westinghouse NOS 12BH7a

[Arkless Electronics]

Excellent thread Jez, and one that will continue to be of interest to Members in the future. As such, would you be willing to have it placed in The Knowledge? You would of course be acknowledged as author.

Thanks

Yeah no problem Barry. Hope it's of use to people :)

[Barry]

Thanks - that's great.