by bordonbert Sun Apr 24, 2022 9:20 am
Hi ECM. The basic average number of flashes is absolutely irrelevant in itself. Whether your valves are 3 flashes or 18 flashes each is neither here nor there and no indicator of whether a valve is on the way out or not. The number of flashes does not indicate an ageing valve. Some are just at 3 flashes and some at 18 right from the production line. The matching process performed when new simply sorts them into groups of the same level. What matters is the matching between them in circuit as pairs. As you have it, 3 being perfectly matched and one varying by only 1 flash means you have a great set of valves there. There is absolutely nothing you will improve by changing them.
If you look at the user manual table for the valve ratings you will see that the number of flashes shows how much bias voltage the valve needs to settle at the correct bias current. Any tweaking this area is completely out of your hands with the exception of making sure that each pair of valves match correctly. As you spotted, H&K say that up to four flashes difference is fine for tone. They also say that all four valves should be matched together and that would not hurt in any way. But there is an argument that says that is not necessarily correct. Remember, those valves are in pairs because one amplifies the upper half of the signal and the other the lower so we want them to match each other in every major parameter we can. The bias (standing/idling) current of each valve is set so they are almost turned on but not quite. (This is NOT how the 12AX7s in the preamp work! They don't work in pairs so they are permanently on and set to a "middle ground" current value.) That way they pass over control of the signal from one to the other without too much of a dead spot between them. That reduces crossover distortion at the swapping point which is very harsh. As it stays a constant amount for all signal levels its proportion increases to very high and coarse levels as the signal level decreases. (Think "2" distortion with "14" signal is only 2/16 or 12.5% distortion in total, but the same "2" distortion with "1" signal is 2/3 or 66.7%.) As long as both pairs, that's inner and outer, match in themselves then each of that pair of valves will respond to the upper and lower halves of the signal in the same way. The fact that one pair may respond differently to the other pair, as long as it responds the same in itself, could lead to a pleasing addition of different character of distortion without any other reliability issues. It still means that the response of the upper half (one valve from each pair) is the same as the lower half (the other valves from each pair). Effectively, if we have a pair of "5 flash" valves and a pair of "10 flash" valves then we have one 5 and one 10 amplifying each side of the signal.
Your preamp JJs are fine in my book (not everyone's). I use them myself. I have always found them easy to get, economical, long lasting, and they sound good to my ears. But then I'm an engineer "valve difference" sceptic as it is easy to show the whole idea of grossly different sounding valve makes is a myth. It's simply differences within production tolerance between any valves being used to suggest that there is some sort of leaning towards a "manufacturer's character/tone". That's hype hype hype. A 12AX7 is basically a 12AX7 as long as it meets the spec. That spec does not change from manufacturer to manufacturer. It's an overblown issue used to sell more valves to gullible musicians and to give some of those musicians the status of "tone guru"! Just use your ears and don't be scared to say "no" in case you are missing something which you can't actually hear.
The only thing I will add to that is that you have a Direct Coupled Cathode Follower stage in your amp (second half of V2). This is a very hard task on that particular triode (a triode is half of a 12AX7 which has two separate ones). They suffer mistreatment and severe stress every time your amp is switched on and is warming up, whether the Standby is On or Off. It has been shown that Chinese 12AX7 types are hardier and resist this abuse better than others for some reason. However, if you don't mind occasionally changing out a valve in that slot when it goes a bit soft then it isn't a problem, just use anything you prefer. That said, preamp valves last for many many years (many many many many many!). I have some from the early 70s which are still going strong and sounding great. Don't just change them out "every service interval" of 12months, 18months or whatever is this weeks imaginary likely failure point. They won't necessarily need it. Take it from me, valves do not just "wear out" in use. They don't have coatings burned off through use, the energy levels inside are nowhere near enough to do that. They deteriorate due to physical damage usually caused by some sort of mistreatment or other. Mechanical damage to inside parts, overvoltages (as in the DCCF mentioned earlier is in this bracket), out of whack heater voltages, use of badly designed standby (the normal type not the H&K design), they all cause a valve's performance to deteriorate, but even then it is usually a very slow process of change. That is out of your hands but it may very well mean that you have valves which are not being stressed where and how you use them. Only change any valve when you can genuinely hear that your tone, with that valve in place, is changing obviously, and it's not just down to different speakers/room/pedals/guitar/volume level. If you have to ask yourself "is that sounding different" then it isn't!
But then, I'm an electronics engineer not just a guitarist. On engineering matters like these I use my test equipment and my ears, not my emotions, when I think about whether things really need replaced.
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