JJ el84s or Mullards?

I was thinking of upgrading my GM36, i think it sounds a little steril and to much treble, can't stand that treble sometimes I thought maybe changing tubes might help a little and i was wondering if anyone have tried either JJ el84s or Mullards in it?
Heard that JJs were a little brighter than Mullards. And then i heard the reissue version of the Mullards isn't as good as the original.

Curious to hear if someone has tried swapping tubes to one of these brands

Have a look lower down at The Tube Swapping Thread just below this one.  In particular instead of reading through the whole lot make sure you look at Page5 from post #124 and look at what VoodooJeff has to say about valve swapping concerning his $120+ Wathen valve set.  (That's $120+ for a single 12AX7 not for a set!)  It's "subtle at best".

You won't get a lot of tonal change with a valve swap between manufacturers, you never do.  The reports of "it gave me a new amp" are grossly exaggerated if people are honest.  Once you have invested in teh new valve set you are under real pressure to convince yourself you hear something which makes your money well spent!  We have all done it.  We buy something "knowing" it will be great and then - - - well - - - it 'aint so great after all.  Do we admit we have wasted our money, or worse, we have been hoodwinked by listening to others?  Not usually.  It takes a lot of honesty to admit we were just plain stupid.

You will get much more obvious genuine change with speaker swaps and that should definitely be your starting point in sorting out the tone.  That fizziness in the top end is a very common characteristic of the amp and it is always improved more by changing to more suitable speakers than valves.  For my money it can be exaggerated by Vintage30s, some love it for that, and it can be reduced by G12M Greenbacks as I found, but I play only older classic 70s/early 80s rock so of course your tonal needs may be different to mine.  But I will say it can't really be removed no matter how old and "pure tone" your $50 NOS Mullards are.

And my own recommendation as an engineer who knows what is going on inside each of the amp stages is steer clear of any 12A*7 type swaps even though "gurus" tell you they are amazing, they always screw up the designer's careful stage setup.  Those valves are NOT "equivalents", they are anything but.  They just happen to mostly be able to cope with the conditions a 12AX7 valve stage sets up without damaging themselves or other components around them, (usually but not always).  If you really must go down the path of "reduced gain", (there is no such thing in valve terms!), go for a 5751 which is very similar in characteristics to the 12AX7 but with an "amplification factor" or μ "mu" which is around 30% lower than a 12AX7, (remember that is NOT the "gain" of the valve, it is more complicated than that as it is tied in with two other parameters).  The gain in use is not necessarily the same as that amplification factor.  The stage design outside of the valve sets the gain and the amplification factor determines how closely that gain can be reached along with all of the valve's characteristics.  You can't change the gain of the stage with a 12A*7 swap without changing many other aspects of the stage behaviour, not all of them pleasant or reliable. Reducing the gain of a stage with a different valve type is a bit like making your car run slower by permanently levering on the handbrake a touch and putting in a bolt to prevent you engaging top gear. It works but it is so-o-o-o-o-o inefficient and unreliable.

Anyway, enough preaching, I would suggest you have a scout through that thread and let us know what you think then maybe others will come in to make their suggestions.

As from reading other posts and watching videos on YouTube, the GM36s are shipped with bad Chinese tubes(why). I changed mine out for EHX 12AX7 and GrooveTubes EL84s. Took some of the highend fizziness out. Changing tubes out is not a silver bullet. That is why we buy some many pedals to flavor our tones. I love my GM36.

MesaMonster wrote:...the GM36s are shipped with bad Chinese tubes(why).

The official H&K answer to that is that nowadays everyone "knows" that valves make such a huge difference, ( ), that they are already planning to swap them out for their own particular honey tongued variant before they even receive the amp. (And I have found there is a lot of truth in that.) In the light of that fact it makes no sense for them to use a more expensive valve just to have them thrown away as a matter of course. Remember that the few dollars extra cost for valves at H&K multiplies up and up when you consider all of the markups and taxes along the way from the factory to your home. They say the eventual cost difference to the customer would be noticeable if they used more expensive valves so they just put in Chinese ones with that in mind. What do we care really as most of us are going to throw them away anyway no matter what they are because someone online on a forum with the title "guru" told us that the Japanese hand made platinum pinned teflon coated Yamamoto Ichibans that he is the sole supplier of have the silky smoothest top end and the most precious midrange of all the very different variants? And he wouldn't lie would he?

It's also worth noting that Chinese valves are noticeably better at standing up to the abuse that a DC coupled cathode follower will put them through with its unique distortion mechanism. They last a lot longer than the more delicate golden voiced valves. And guess what there is in a lot of the higher power H&K amps!

This is how different they all are.  Many makes of the same 12AX7 slotted one by one into the same Fender style preamp. Perhaps people would like to post their thoughts on where the differences lie?

Hi Dave.  Good to see you back.