Hi Benny (Christian?), welcome to the forum. Here is more technical info than you would want to hear on valves. Many people disagree with this. They are generally guitarists not engineers. Engineers generally keep out of discussions on this topic as they know that, once the idea is planted in a guitarist's mind it is impossible to shift. "Don't try to teach a pig to sing. It will drive you mad and it only annoys the pig!" The guitarists' views are worth listening too but, what they can't take is that everyone's view, including theirs, must be questioned and criticised in the light of factual information and rigorous engineering testing. If you can't see it on a test bench you can't hear it with your vastly inferior ears. You can't demonstrate differences in aspects like a valve's frequency response in a real life valve stage as they don't exist. I can point you to info which shows that clearly if it is of any interest.
This topic of valves has been hotly disputed here for many years now. We have a massive "valve swapping thread" here on the forum. No doubt you have already found a plethora of advice online about which brand sounds how. This emanates from people making a lot of money selling you the latest hot valve on the market, (the one you don't currently have
), and the priesthood of "the enlightened" in the guitar world jumps onto the bandwagon to keep their places as the leaders in the field. Caveat Emptor! You will also find pseudo-technological arguments posed as to why they sound different. As a senior practicing design engineer in electronics for a whole career until retirement I can categorically say that those arguments are bunk and none of it stands up on the test bench and the equipment used there to check things out is decades more sensitive than your ears ever will be. It is absolutely objective, reliable and repeatable. Your ears are anything but that.
So what IS different then if anything. Well as I said, each valve type is very well specified and every valve of that type MUST meet those specifications no matter who it is made by. The kick is that the specifications only hold within certain defined limits. The way we use valves in guitar work can deliberately force the valve outside of those limits, severely at times! There are differences in valves when they either bottom out or switch off because that area is not laid out in the specs. If the signal fed to them is large enough (positive) they will pass so much current in one direction that the voltage on their anodes will drop to a point where it can go no lower and the bottom of the amplified signal is clipped off. If the signal drops low enough (negative) the opposite happens and the valve ends up passing no current at all so the voltage can go no higher and it clips off the top of the amplified signal. These two states are not symmetrical, they have different "shapes" as you approach them so the valve overdrive/distortion sound is sweeter with more even harmonics than say symmetrical diode clipping from a pedal which is a similar effect. These areas can differ between valve makes - BUT - the effect is very subtle if it is audible at all and it is NOT the "black and white" feature that people claim it is. Don't trust anyone's advice, including mine, without objectively testing the idea for yourself. Use your own ears and be objective when you are being told this.
So yes, they will sound different in overdrive conditions but that is nothing to do with their frequency response which is in the MHz region for all healthy valves or their "gain" which is largely made irrelevant by feedback within a stage design. When played clean they have exactly the same characteristics controlled by the other components around them in the stage. And on the subject of healthy, don't be conned into changing your valves every year or 18 months or 2 years as everyone "knows" you must to avert "tone suck" or whatever that effect is called this week. Your preamp valves will stay usable for many many years. I have a Marshall SL100 which I have owned since the late 1960s and that is still using some of its original preamp valves without any degradation of sound.
There are exceptions however. Your output valves will need replacing earlier than any preamp valve. They don't "wear out in use", they just suffer from different effects because of their different job. One tip here is, in a normal classic amp and not your GM40, never use the Standby switch, it kills valves by a process called cathode poisoning. (Even Sweetwater God bless its little cotton socks is now aware of this and warns against it). The GM40 has a different way of approaching Standby which eliminates this problem. Other amps may simply switch the HT off to the output valves and sometimes others too, which leaves them heated and emitting electrons with no anode current passing through to sweep them away. Bad practice! The other exception to the "valves can last a lifetime" rule is if you have a DC coupled cathode follower stage in your amp, (sometimes described as a "DCCF"). I believe the GM40, like the GM36, does have that as one of its V2 triodes. These stages are put under a lot of severe voltage stress when the amp fires up from cold because of the way they take their grid voltage from the anode before them. They can literally give up the ghost because of this. These valves are not huge contributors to the overall sound and the effects of them going a bit soft are not necessarily obvious. One way to help alleviate this is to use a Chinese valve as your DCCF as they seem to have a higher degree of resilience to the over-voltage problem.
To your other point about charge holding. Yes, valve amps do hold a lethal charge. In a well designed amp there are methods applied to quickly drain off this charge at switch off. The H&K amps are all "well designed" amps. Give them a minute or so and there should be no dangerously high voltages left inside. If you follow the correct procedure to change your valves there is no danger anyway as the high voltages are not accessible from the upper chamber of your GM40. When changing valves DO NOT FULLY REMOVE THE BOTTOM SCREWS ON THE END COVERS. Only remove the top pair and the rear middle one on each end, the front middle only holds the handle on the end cheek. You then simply loosen the bottom pair to allow the end cheeks to flop outwards and the top lifts off. There are anti-tamper devices stuck on the bottom screws which will indicate you have removed them and technically your warranty could become null and void. Be careful with the cable to the LEDs which sits under the lid on the top of the perspex front. Make sure to fit it accurately and clip it in place when you refit the lid. And no, you don't need to turn the Standby to on or connect the amp to the cabinet, there is no value in doing that at all. The problem is the power supply capacitors and they have drain resistors fitted directly across them.
My advice is not to panic. Valves are not going to go away. They are not going to become impossible to get. They may be short for a while but don't create the shortage by just buying them up because you have heard there WILL be a massive problem. When you can, just grab a couple of 12AX7s and get a pair or maybe a set of EL84s as spares. Good old JJs are as good as anything else out there despite the crap talked in high (expensive) places. Then forget about them and just play. Don't become a "valve cork sniffer"! Don't sit obsessing about whether the new valves will sound different to your current ones, they may if the current valves are genuinely going soft otherwise they won't. When a valve is going down you will hear it, if you are straining to make your mind up whether it is faulty or not, it isn't! You are then trying to convince yourself there is a problem and that indicates there is nothing wrong. Personally I use
Karltone for my valves when I need them. He is no fuss, offers great advice on anything you care to ask him and is very reasonably priced. Watford Valves are ok but are not the cheapest and do feed the hype with their marketing spiel which I don't like.
Now, it's down to you. Can you use your own ears to genuinely listen to the claims made without being influenced that there is a "certain delicate edge to that top end isn't there?" and have the confidence to tell that guitar guru or valve store salesman "I can't hear anything!" You can see out there how few people have that ability.
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