by bordonbert Wed May 16, 2018 6:29 am
Hi Ugabuga, welcome to the site. If you want details on valve stuff go to the GM36 forum and look there for the Tube Swapping Thread, you'll get all that you want and more. Make sure to read around the middle of Page 5 Post#122 onwards for a bit and have a look at Page 6 Post#141. There is a comparison there from VoodooJeff between JJs and Wathens. The Wathens are apparently made of Unicorn horn in Hogsmeade just down the road from Hogwarts School by qualified wizards from Harry Potter's class working under Hermione Granger and they cost currently around $140 per 12AX7!!!! (That price is absolutely true! Check it on the Wathen Valves site.)
The differences between valve makes are hyped and overstated. Period! It's marketing speak in order to shift more valves. Any differences between the same valves from different manufacturers are very subtle and can hardly be heard in a live environment. If you are a bedroom virtuoso then you may be able to hear a slight difference if you are specifically looking for it. Playing along with others, not much of a chance at all? What people fail to realise is that these valves were originally designed for radio work, not audio which they just fell into as the cheapest and most suitable available. They have characteristics to which they all must conform to be given their type designation and their frequency responses all extend up to the Megahertz region making some of the claims of differences in tonal quality a joke. Not much chance of much difference at around the 5-6kHz upper level we work to. If there is a difference it should be obvious when put on the bench and tested rigorously with good quality electronic test equipment. As yet, no one has to my knowledge managed that and if they had it would be front page news! Yet they all still claim it is like night and day. I and a few others around claim BS! My advice, don't be fooled, and if you compare types make sure you know the limitations of just swapping with a couple of minutes in between listening (10secs max and the ear and brain "recalibrate" and the test becomes nonsense as you hear what you want to hear) and be brutally honest with yourself. Don't listen trying to hear a difference, try it the other way around, listen to NOT hear a difference and see if it surprises you. If it is as obvious as people maintain then it should, repeatedly.
Balanced valves are only of use in the Phase Inverter stage of some amplifiers. They make no difference to the preamp input and amplification stages at all. The triodes in these are used independently in their stages.
The original valves in the H&Ks used to all be the same. They had "China" stamped on the front down towards their bases beneath "12AX7". Looking on the H&K site I can see what you are talking about, the GM40D seems to have a different set now. They are marked 12AX7B and are missing the Chinese designation. The "B" is supposed to denote a "Shuguang's better selection item versus their 12AX7 or 12AX7A" according to TAD. "It's better", very helpful and informative especially when it is listed in full but with the ordinary 12AX7 down at the bottom of a list of equivalents. The valve on the right is the input valves and is screened by a blue metal can to prevent noise pickup. The input stage is more susceptible to that than the others as the signal is at a lower level there. If you twist the blue metal cap round it should spring off and you will see the actual valve underneath. My bet is that it will be the same as the other two. If it is different in any way it will most likely just be a low noise type of the same 12AX7. That is the main area where there will be differences between valves, (but not necessarily manufacturers), some are less noisy and some are less microphonic. All valves are microphonic, some are just more susceptible than others.
The idea that there is very little difference between different manufacturers' valves of the same type is "Hint 1". You can't even test it properly without engineering lab conditions to make the change instantly switchable. "Hint 2" is one which you are unlikely to hear in many other places it has become so widely believed by the untrained. Don't be tempted to swap out for different valve types like the 12AT7 12AU7 12AY7. These are definitely NOT equivalent valves, they are very different and screw with every aspect of the stage design to the point where it may even cause damage, (though damage would be pretty rare). These are often pushed as a "lower gain equivalent". Anyone who quotes you the term "gain" doesn't know anything about designing with valves! The parameter does not exist. There are a number of interconnected parameters, Amplification Factor (The one that is often thought of as "gain" but which isn't), Anode Resistance and Transconductance, and a change to any one affects the others. You cannot lower the value of Amplification Factor without screwing the other values to places where they are an appalling fit in any stage not designed specifically for that valve type. It is much more complicated than just "lower gain". Why pay money for an amp which a good expensive designer has spent months setting up to toss his work out of the window by putting in the wrong valve type and messing up everything he was so careful to get right?
Anyway. You should be happy with your Electro Harmonix when they arrive, they are a decent valve. If you read the Tube Swapping Thread you will find that other makes like JJ are also good valves, in fact most of us here seem to use JJs and are very happy with them. The reviews are based on hype! No one ever compares the valves in anything like an accurate rigorous way and never in a real live situation. When you make money from selling valves or writing reviews you have to justify buying the valves or reading the reviews to your public. Have you ever heard a valve supplier or reviewer say "well, there's actually not a lot of difference I can hear"? It goes against the idea of swapping out your valves and that is lost revenue.
The "magic mojo" component doesn't exist! Not in any area.