Important tube question?? Tube experts wanted

About to buy a set of expensive NOS power tubes for the amp. My question is...Do I absolutely need to buy a set that is formal "matched quad" or should I be purchasing two sets of "matched pairs"??? I'm not an expert by any means yet but I am learning as I go so I was unsure how the amp recognize the power tubes in regards to the TSC system. Greatly appreciate anyone who can shed light on this or has already done this and has experience

I'm not the guy you will really want to answer this as I have a specific position about this issue because I come at the whole of musical sound reproduction from an engineering perspective.  What you call (I mean "one calls" not specifically you) Mojo is just engineering that you don't understand yet!  I can be a pain in the arse about these issues but it is because I look at them logically and honestly, not through a pair of swirly coloured mojo tinted spectacles.  And I'm not selling you anything or making money from you in any other way.  I hope someone else will come in and give you the answer I suspect you are really hoping for on the other side of the issue.  Let me give you my answer with a few questions you can think about if you want to save yourself some money and gain real experience which will help you in the future as opposed to gathering "common knowledge" which is mostly incomplete and flawed.

Why do you want NOS valves?  No, think hard about that, why do you REALLY want NOS valves!  Who has recommended them to you?  How much of it is so you can say "I have NOS valves" to others who also have NOS valves or who really want them (for the same reason )?  I have worked all my life with NOS components of all sorts around me.  I have never needed to use them.  They are generally tired, of inferior manufacture, are wildly inaccurate and have been superceded by much better versions in much better materials made with much better processes.  That said, valves are not completely in the same category almost being as much mechanical as electronic, but most of today's are just as good as the older ones.  There are differences which can be shown on good quality test equipment, but those differences are minimal and most are outside of the parameters which are important to us in guitar work (if you really know what you are doing ).

Are you aware of the massive difference in using valves in a hifi situation, (where NOS valves may sound different), and a guitar one?  Does 0.001% distortion in one against 5% distortion in the other seem to be equivalent?  Do you think that a small difference introduced by a different make of the same component may change your sound massively in amongst high levels of deliberate distortion, (even on clean sounds), in the same way that it would affect the 3D soundstage of a hifi setup?

Have you heard these valves in your GM36?  Have you heard them in anyone else's GM36?  If not, do you think you should before you part with a lot of money?  If you have, were they using the same cab as yourself or at least the same speakers and were you playing your own guitar?  Why are these questions important?  Would you buy a car just because a salesman told you on the internet that it is the one which will be perfect for you?  If he told you it would do 145mph and give you 45mpg would you just believe him?  If you saw a shaky blurred internet vid of the car tearing down a motorway would that convince you?

What is it that you are disatisfied with in the sound of your amp as it is?  What makes you think that it is the valves, particularly old ones, which will improve this?  What have you already done to deal with the things you aren't happy with in your sound?  If your hifi sounded rough as a gorilla's scrotum, would you first consider changing the amp or speakers, or would you spend £150 on a pair of solid silver cables?

The killer!  How many people who fork out their hard earned cash on things just like this which are to die for in their sound, rave about them at the time as the perfect solution then are back asking for advice on an alternative a couple of weeks later?  If these valves are so great why do they look for a change further down the line?

We have a generally pretty pragmatic and practical approach to this sort of thing amongst the people in the GM36 forum.  We don't always agree but we have open minds and we lean towards intelligent thinking not just acceptance of what the self styled and taught "techy experts" tell us.  We are all aware that there are a lot of industry "recommended practices" out there which have grown up over the years, and some of us know too that too many of these are just plain myths.  There is a lot of genuinely accurate information nowadays on the internet from high level people and companies which is debunking this sort of thing.  We also have advice from professional musicians who live and work with these questions every day.  One of our members in particular is extremely helpful to us all with his advice, based on first hand experience.  As he says, the difference between valves is "subtle at best", even extremely expensive ones.  I would seriously read some of the valve advice here, it is the best most honest you will get anywhere, and think hard about it.

Now to the second part of your query, it isn't necessary to get a full matched quad, two matched pairs will be fine as long as you keep them in the correct positions to complement each other as pairs across the output stage.  In the GM36 this means the outer pair and inner pair need to be matched.  One reason for matching is to balance up the standing (bias) current in each valve across the transformer so as long as there are balanced opposing pairs in there the DC bias currents will cancel out accurately.  The Tx does not like DC through it if you want to minimise distortion and maximise output power.  There is even an argument which says that unbalanced valves will introduce even order harmonic to the output and saturate earlier on one side of the signal and this can add to the valve sweetness that is so sought after.  All that said, it usually doesn't cost any more to get a quad, valve for valve, so that would be the more convenient way to do it.

I am a s/w eng currently contracting at an RF company.  The guy in the cube next to me is a long time RF guy and a ham radio geek.  I am paraphrasing a 5 minute conversation with him, so keep in mind you are hearing it second hand as interpreted by a s/w eng who was a analog hardware engineer in a different life....
1.  If T5/6 are matched, and T4/7 are matched but different from T5/6, one pair will do more "work" (sink/source more current) and as you reach saturation, you'll get odd harmonic distortion as the harder working sets becomes non-linear and the other set starts to contribute.  Hmmm, this might be tasty!  My source said this will probably not cause any damage, as the TX will not see a mis-match, and tubes aren't prone to "thermal runaway".
2.  If the T5/6 and or T4/7 are mismatched (in other words, the two tubes within those pairs are not matched) , or if T3 is not a matched triode, you will get even harmonic distortion.  This too might be yummy, if mixed well with odd harmonic distortion.  This will also cause some DC to flow thru the output Tx, causing it to get warm.  A little is okay, a lot might decrease the life of the output Tx.  This could spawn a whole discussion about how well matched T3 should/shouldn't be...  Keep in mind, a guitar amp is specifically NOT a hi-fi device, so some mis-match within the two triodes of T3 are okay or potentially even desired.

So it comes down to which type of distortion you prefer and how much of each you want.  I have been considering for a while trying  a known/measured mis-matched 12AX7 in T3, to hear how much I like the results.

Please also consider the GM36 is running the EL84 plate voltage near 400V, and EL84s are designed with a max plate voltage of 300V, so well built EL84s (like JJs) will last about a year.  Others might be more delicate and not last as long.  Note that most guitar amps that use EL84s use a 400V plate voltage.  On this line, I'd be hesitant to use NOS EL84s - you're dumping $$$ into a planned-obsolesce part.

I haven`t posted here in forever and a day as I`ve been buried in the recording studio for a while now and honestly don`t feel like looking at the internet very often between working on WAY too many projects, but I can offer a unique perspective here:

I use two Gm`s in stereo. One has pretty ordinary JJ tubes at about $75 a full set. The other has some very high end audiophile level tubes that sell for roughly $100....not for a set, but EACH. I can say with absolute certainty that while they do sound a tick different, the one with $700 worth of tubes in it does not sound $600 better. And sure, when there`s a $6,000 studio microphone on the amps I can hear the difference, but for live shows it is, as previously stated, subtle at best.

That's going to be inscribed on your tombstone Jeff.  "Here lies Jeff.  He was always subtle at best."  






(EDIT: Hope the studio work is going well and you're keeping your chin up. )

bordonbert wrote:That's going to be inscribed on your tombstone Jeff.  "Here lies Jeff.  He was always subtle at best."  






(EDIT:  Hope the studio work is going well and you're keeping your chin up. )

Ah of course, and thanks. It`s been a challenge as I`m doing ALL of the writing myself (drums, bass, guitars and keyboards). Didn`t plan on nor want it to go that way but it did, so I`ve gotta run with it. Started a new live improv series, too, which as really been an experience.

With the higher plate voltage run on the power stage would a 7189 tube a better choice since it's rated at 400v plate voltage? I've seen other members say that because h&k pushes the voltage higher on the el84's that maybe they don't live as long as they could if run a little colder? That's why the 7189 seemed logical to me and would be more durable in this application

As you are new here let me point out up front that my own personal view is a little bit slanted on this sort of thing.  I absolutely mistrust the advice given by valve sales companies and "tech experts" on forums on this sort of issue, and when it comes down to "guitar techs", too many of them (not all by any means) are self taught opportunists pushing the same industry myths as the average punter guitarist.  I feel the hype about the wide range of "character" of different valves is nothing but snake oil and most issues like this are Emperor's New Clothes.  Others will disagree!

Ok, so to the issue at hand.  This is a genuine point you have made as it is down to the technical aspects of using the valves.  I'm not a valve expert in the day to day sense but I do have a background in electronics design so the principles of their operation are fairly clear, I can interpret reasonably accurately available data and other peoples' advice.  The GM36 spec actually shows an anode supply voltage of +390V.  Yes, that voltage would seem to be above the valve spec of 300V but there is more to it than that.  What is actually specified in the valve data is the "maximum anode voltage" as measured with respect to the cathode and in the case of our valves this is a "design centre rating".  This means that a significant amount of headroom is built into that spec to accomodate mains voltage rises and component variations.  We don't deal with industrial class valves with "absolute ratings" in guitar work.  Also, that is the voltage from cathode to anode not the supply.  Effectively the valve sees "(supply voltage) - (voltage dropped across the anode load) - (voltage at the cathode)" which is lower, how much depends on outside factors.

The limiting factors for a valve are not just the Anode voltage in isolation.  If you draw a graph of the valve's anode voltage v cathode current curves and plot on it the three parameters, max anode voltage, max cathode current and max anode power dissipation, you get the attached plot.  As long as the valve stays within the shaded area things are hunky dory.  The valves is set up to work on a chosen straight(ish) line set usually by the value of anode load from the y-axis to the x-axis.  Even with a 400V supply the worst case for anode voltage occurs when the current is at a minimum and vice versa.  Even at maximum working anode voltage, i.e. lowest cathode current with that valve not working for that side of the signal, there will still be voltages dropped across anode and cathode components as the valve has bias current flowing even though it is not doing any work on the signal.

The designers of the amp will have taken all of this and much more into account and testing will have been thorough.  There are absolutely no reports of valves dying in droves or even "running in the blue".  I think you could swap out for those valves if you really were curious but I would honestly say there is no need to do so based on practicalities and realities of what we find in using the amp every day.

Allow me to be blunt for a moment: If you read 73 reviews of a given tube and people love the way it sounds in amplifier X, it`s a good bet you`ll like it, too. If you read one review of some oddball cat who put 63 year old tubes in a three month old amp and it sounds like God himself farted out rainbows, he`s a whack job with no real sense.

No matter what, tubes are not going to change the fundamental character of an amp. Not gonna happen. And as far as factory bias putting a certain tube to task over another...also rubbish. Tubes absolutely in no way burn out or have the abbreviated lifespan they are claimed to unless something is wrong with them. I ran my previous amp at nearly DOUBLE the recommended bias current and the tubes lasted me three years before I sold them amp. As far as I know they are still in it a year later.

I don`t mean to come off as brash or aggressive, but the great tube debate has gone on for years. And years. And years. And it`s really all based on some poozer`s ego, claiming he knows the super big secret to great tone that thousands of guitarists and amp techs before have never been able to find. The Indiana Jones of valve guru-ism, if you will. It`s crap. You put a set of brand name replacement tubes in place of the barely-meet-the-standards stock chinese tubes the amp sounds better. Brand A will not sound better than brand B. The amp, at stock bias, will not burn through one brand faster than another provided it`s the right tube. If you go putting in alternative models of tubes, you kinda get what you earn when they don`t do right. It`s simple.

As far as tone goes, there is no "magic pill" that holds the key to that magic sound. It`s in your hands. The people who don`t believe that are the ones who want to be able to bypass the work and just buy the sound. Likely the same folks who never practice enough to actually be the kind of player they wish they were and blame it on "not being born with the natural talent of Hendrix or Van Halen".

Sorry guys, I kinda went on a rant there. I`m in a bit of a mood today. It wasn`t aimed at anyone in particular, more just me venting about something that irritates me like a pimple in the buttcrack.

        Teeheeheehee!  Duck for cover guys, it's HulkenJeff.

Good words for us all to bear in mind Jeff.  You have an articulately artistic way of putting into words what I struggle to find the cold tech terms to say.  Stuff like this will always need to be repeated as new people come here and ask the same questions, it's the nature of the way knowledge works.  And the message is not lost by an occasional rant, everything you say there is very logical and based on your own real experience at the sharpest tip of this guitar thing.  (...especially that stuff about blaming our poor playing on not being born with enough talent...  )  I now quote your "lives full of meaning" motto to my friends when I catch them out being negative.

Now if you were to put this all into a country & western lyric, ("The 12AT7 to Tonetown"?) we could just play it to new people when they ask and you wouldn't have to be constantly repeating it.