The Tube Swapping Thread

So you don't unscrew the springs, you lift it off the tube, so (here is comes) obviously you do have to unscrew the shield on V1, correct?

namklak wrote:So you don't unscrew the springs, you lift it off the tube, so (here is comes) obviously you do have to unscrew the shield on V1, correct?

No you push down on the shield a bit and then rotate (I think counter clockwise) a little bit and it should come off.

streuth wrote:

namklak wrote:So you don't unscrew the springs, you lift it off the tube, so (here is comes) obviously you do have to unscrew the shield on V1, correct?

No you push down on the shield a bit and then rotate (I think counter clockwise) a little bit and it should come off.

Oh, cool, excellent and Thank You.  I don't see the Give Thanks button on this forum...

New owner. I play in an Allman Brothers tribute band.
I only had to unscrew the bottom screws three turns to get the cover off.
JJ EL84s in the output is a definite improvement.
For the preamp,I changed V1 (the tube with the cover?) to an EH12AX7. For the other two,I felt swapping out the Chinese tubes took some of the life out of the amp. I tried JJs, Sovtek, EC83,81,803S,5751, 12AT7 - and the Chinese tubes in V2,3 won out for me...
Great amp!

I am not a tube expert so bear with me. After owning my GM36 for several months I've finally gotten around to trying some new tubes. Honestly I don't think the stock Chinese ones are all that bad, but after reading reviews about swapping them out and experiencing an improvement, I thought I'd give it a shot. I used to own a lot of Fender amps and I always swapped out the stock GrooveTubes with JJ's and I never had a bad experience with JJ's, so that's what I went with for my GM. A set of JJ EL84 power amp tubes and a set of JJ ECC83S preamp tubes. While I was waiting for them to arrive I talked to an old friend who is a tube aficionado with a massive vintage collection... I told him about my GM36 and he recommended an old vintage Holland ECC83S preamp tube for the V1 spot... at the very least, it's a tube that will probably outlive me, unlike typical modern tubes. I have to say that I like this tube combination... to my ears, the GM is by default a 'modern' and 'hifi' or 'hi-tech' sounding amp (I mean that in a good way!), but leaning ever so slightly toward being 'bright' and perhaps 'fizzy'. This new tube compliment (JJ's, with a Holland in V1) have reeled in that brightness without changing the amp's overall character. It's a subtle difference, but I like it!

Changed. I had an EH12AX7 in V1, and left the Chinese tubes in V2,3. Turned out the EH was noisy. After an hour of swapping I felt it was V2 that calmed the gain. So all tubes are now JJ. V1,3 are ECC83S and V2 is an ECC81. Low noise, better control of the saturation on Lead and Ultra, and decent warmth. I have some gigs coming up, so...

Nice combination! ECC81 in v2 between two ECC83s sounds interesting.

My current setup is:

V1: TAD 7025 WA (RT080)
V2 , V3: TAD ECC83 WA (RT008)

Driving my lovely Mullard EL84 Reissues!

I swapped my preamp tubes back to ECC83 because my new guitar has classic 57 pickups which are too "clean" for the 5751 tubes I used before with classic57+ pickups.

I've only been using my SG with a Pearly Gates in the neck and a JB in the bridge, both moderately hot pickups. My Strat has low output pickups, but I won't be using it on this first gig with the GM. I guess it'll be interesting when i add the Strat into the equation ...

I may have overthought this, but after my gig this weekend I am going to put my best tubes into the GM36:

V1:  Raytheon smooth long shiny black plate 12ax7
V2:  RCA long grey plate 7025/12ax7
V3:  Tung Sol USA long grey plate 12ax7

Power tubes will be Sylvania 6bq5 from Baldwin organ that test just below NOS

Honestly I think it sounds very good as is with the Chinese tubes, and I may end up not liking the old USA tubes.

I did a similar swap with my B-52 AT100 head and actually preferred the Sovtek power tubes and for the stages other than the first gain stage I preferred the Chinese 12ax7.  The Sovteks and Chinese tubes had a nasty bite/aggreession to them that I thought really suited that amp.

Could end up the same way here, we'll see

Kurt

I finally got around to swapping the tubes, I think there is an improvement, although its not night and day, over the Shuguang tubes.

The old USA Tung Sol didn't work out , it read a "5" and "weak" on the Orange VT1000 tube tester I tested the tubes in.  So It ended up:

EL84:  Sylvania 1960's from Baldwin organ, all 4 tested strong and are matched within a couple ma according to the source
12AX7 #1 (with the cover):  Raytheon shiny long (17mm) black plate  Tested 10 for both triodes on Orange tester (1-15 scale)
12AX7 #2:  RCA long (17mm) grey plate  triodes Tested 13 and 14 on the Orange tester
12AX7 #3:  I wanted balanced triodes so the best candidate was an RCA short (14mm) grey plate that tested 7 for both triodes

First Impressions:

I was hoping the clean channel would magically become my 1970 Twin Reverb, but alas, it did not  I DO think it sounds a little better though, a bit rounder and a bit less sandpaper-y.  The difference is not huge though.

The crunch, lead, and ultra channels seem to have a bit more what I would call "traditional" american rock sound to them, perhaps a little less fizz.


Unfortunately I have not had a chance to crank it with the new tubes, so I haven't had a chance to hear the EL84s, and the preamp tubes may indeed sound quite a bit nicer once the amp starts working.


I did watch the youtube video about not completely removing the sides to theoretically keep the warranty tape intact, so in theory if there ever was a warranty issue I could reinstall the original tubes and return it to stock.  In theory.

Not sure if the tubes autobiased or not but I hit the little bias switch with with my pick anyway.

All in all a very simple operation, probably took me 15 minutes tops, and that was at a "careful" pace....

I've recently picked up one of these in the UK second hand, complete with foot controller and mid adaptor plus leads.

Initial impression was of huge disappointment. Big flabby bottom end, piercing high top end and everything harsh and brittle.

I read loads of forums, reviews, watched YouTube vids to get advice because logic was that this was not how this amp was meant to sound as in reviews people were consistently giving this thing glowing reports.

First I changed the pre amp tubes from the lousy stock Chinese ones and put in some Groove Tubes I previously used in a Mark III Boogie. Then I took out the EL84's and put in 4 that came from Marshall. Big improvement, more rounded warmer sound, but still a bit of the harsh brittle tone still present.

I then realised that tubes were a huge factor but it was not just a case of putting in good quality tubes, it was also a case of finding tubes that countered the amps problem sonics while giving the amp the sonics/tone that I wanted, ie fat rich, warm, soaked with great harmonics for anything from blues to hard rock, but not going into full on metal.

It seems that while a tube, pre amp or power amp may be great for one amp, it may not be for another. Also there seems to be a big argument for the first pre amp tube in the chain from the input to start off strong giving the characteristics you want.

So I went to Watford Valves. 4 quad matched JJ EL84's and 3 JJ gold pin ECC83's

Incredible difference. You would be forgiven in a blindfold test for thinking this was a different amp.

Big fat, rich, lush tone. Clean has a bell like chiming tone that is lush and rounded. Crunch is exactly that with a well refined warmth. Lead and Ultra can both go from a Billy Gibbons Blue Jean Blues to a singing lead that sustains beautifully to complete raucous mayhem.

This is all plugged into one Marshall 1912 cab, Les Paul, Strat or Tele, it doesn't matter. The Tele in particular become a big fat blues rock animal.

Those that say tubes are tubes and all this matching stuff is one big bluff are wrong.

I've learnt so much in the last few weeks and persistence has paid off. This thing is fantastic and the Mark III Boogie, hardwood cabinet/wicker grill may be about to go.

Enjoy

I have also found the the drive channels get the best results by:-

1. Running the master at noon and running the channel volume up against that. The amp benefits from the power amp being driven, as do all tube amps

2. Going sparingly on the EQ, bass, mid and treble all around the 7-9 o'clock region. That seems to smooth things out

3. Clean channel, bass and mid up full, treble on about 7 o'clock. Nice rounded bell like tone.

I run all JJs, master at noon (I have an external active power soak). "Brown" all day long. Everyone loves this amp - my bandmates, my buddies, sound guys, etc. I've done 3 gigs now, and I'm still on my honeymoon with my GM36.

Just as an update, I've cracked it.

I've noticed that my lower output guitars, ie Tele, 62 reissue Strat, had better tone than higher output Les Paul or super Strat HSS. This got me thinking about comments I've read about those using a 5751 instead of a 12ax7 in V1. Lower gain at the input stage seems pretty crucial to get a good tone and tame the high gain this beast produces.

So, JJ 5751 gold pin in V1, JJ EC83 gold pins in V2 and V3. Then quad matched JJ EL84's in the power stage.

This thing sounds fantastic. Don't worry about the 5751 lowering the gain by 30% or whatever at the V1 stage, there's still plenty of drive for the extreme metal heads out there. The clean chimes like a bell and crunch and lead are warm round and "brown". Thus is a far far cry from the stock Chinese tubes I started with.

Incredible, versatile piece of kit.

Hope that helps

There are some excellent comments in a discussion about tube quality and swapping on the H&K facebook page.

Worth a look and H&K seem to welcome user feedback on products

https://www.facebook.com/hughesandkettner?fref=nf&ft[tn]=kC&ft[qid]=6132370524175108438&ft[mf_story_key]=883526291205835879&ft[ei]=AQJhFZd1AGgLD2fjhXsOPEE7SHfBX-ueBWmLVpg9kkVu1djcGs1TBjniXhtRfXliQacZ76rwo2zOxd0DeGDaplsb6hvFCIuMbV_soZghyAAamwB_V8YsYwSVLjh7Rsac87tntXgMuz4pNiB-m1Nj6QrmNHxELe7G0swtTj1raENn4aCtE18VRoZ_xPvgwzcjY-GzeO8k8LxmLknFvB6e2BZGy4tPq09zd7XEoo91ljXH9IZpvcHpkuHP9q2vADyrCDp_0MxTxCn5YgMqVrwkYevF2sozTXxT8Dzu35nevLnyWHnCBF1Ej6HzvAXlRVveI2PvD8Ob1JGcD-FYwnkq330ZGHZ49hnNq28-BkIToxo16q-xWZ9Ot07hdByqQTl-vtZQobKehL8nOii9ORqXiTwqpw98AoMGt5ul_ZQLW0q9i1r9poueaRdGj8pzDNW13NViz3-N0pAbGjc0UfMw1xk4y5ODBaz7CZ04oCGjTdAt53Rll9AsjeyZHprelU6pYmenbKfeedshqf-Yna5FoKXDat2U8ByrEEg85lBi4L3TBgFp6CbP9Gvs2Ckacd3-dQqMITnXiggp7JyT-h1yMInK5_MwoItJbGZEXrHMTvIym6c-Mrj_UrMgGOHm76P2hC5Xx0XwfDm65BAbZKrs-tL9w4r2_pjGWy06iRHS1aVVAxQmP5FiyjUGr1LPSPsVUJkcnpCa_994SXrMWsMFepl251Ob5ntZJWRT7sh--ibz4RLhHFLqomzWv_fTkNdzxAUKuXaORqR0F_Qowqd5N_ujnSUT63rPqxwyafddhv9R-aaVtbLO5-j5wvNiOZy6BfLeHjNBEDkupPKdICXFPbHJzN2kkmIfZ34ht92ChshsDhNFwXZ2ipvZTceeP1L6ew_vL-dw-9lKm35LcycD2GAUHclBwCFB5EbUfseDReYd01RDeu4oWsoTfY1Ni12BHKX0coXpCXDRYmJprjnSe9W5K6I4yDR6JpBdL3vRlKW-RreMfKz6EKOoHjkFI8nCOPxh309B5sf7Xt8rJSAJEACraJreogeu9Wzdqv9Z2PfZl3xhAxC01BWX00pLf1rkbUHaWSMt6dipwRjrJ_YVGiO4ORuUGfD2voMP7Vm9xxbe221OeMc2yDp7NOCcV9kTx2d6OpQJcPmsab-2T9wIi6J52_9sBte_h7ZjQ9UQS1uk75doli9bBXI6jI-OQTxr-MHzadlQWEw1RcvR8Y3QSBNJT63TL4oSg79-oheFnT5pg33JcfQJKYQMf8m5DsI9h4XmxhxZp_eL9BcnqjsIGY_eQgFjCtctkxj613kfjQctaS3BqJpS5NFpJAjKBT-9OZPzWx6jm7izqIcooLp1BmBhuc3OZGQZHcF_goUcuQhKQ_p19buEIsIo94eU41n0yDM-KTiXSQx0txIsFRBURIBXN02xpn3l05td6mXBL9EkOhlJxhG8au3y1cU5l6-aI3rohSdmvTI1VHftLs18ZSvPe9gmKg1HQP_5_cc4ZZTj5GKoH9h0trF1Oz0eG4w1twWzHtVgDkr7EcFZyUyQ5-Jqze4fEAPSGbCjJD95EBQiDmvq3iAPbeqLOCxzI7u3QL95WyHyte3ly1B8gwAEmPg0MjIYXmsGfZAcHPgeHw1P6gE_QoSw2ZLLipp10vubYMmK9N5y95t3kZJooeY4bK-BM2X9yjJdqiyFeapkOeRjTvdo4sMhNhJH2a-TgdS8WuQy71T5cv9x8iGRgzWZCxUMa3aPdBihHttrbBazKZUNLkSxAO29n2FEmVnm22d4rTnuG9zJN4617DZZEtat4DoIz0hkAttDFF42NvyjSiEgsJAam1ebxBhy3leaOEtGjeAdS51INk4l-PwTHQ6KezzDI2aMCK_wTAqZcpry0XnJQ1w3QCKG8D_BviiLy5-z6_AqVC6XV0uXDyloDQv0iNws_qrwSJ8Tz1c_4zKKaKXD6WPBfSdT64Oeai7jcRWcmfuR6S-jfJDPDZSEKTnzajdZxhMPGyK_txjh-Y7Sh0PjfeSlPLHhknrGLnpgYjPxGl4AinT_4FILb1fDTinAetOVIkLM1IvJjX4nwD7RCApzQRqtNAmwSrngK0Qotpn4n0C3upsyeqQ0LdXWsS1GszgLveAHgTzejGqISjj9qAUZT4ZD8TTfa3FUp-3_1aAcfSmXrYLlTc_Zg8lGOK6r3ZrltAGxBip7u5KqOM_qn30s71Cbt_19lxKBa6HviX88A0WZRWo1VqwbAm2TKaA8sMjWI215qNGJNM0CmgOY6ibtuwiUzW34U2u_yxhthPgItUahEg82aYVglaFtupNreWD3hc_jAlf2HibDajPtmEMfvjGw9u1R1AxnUGz7hvj0nwARx9x7U4dqHtrC5JHK0pxq1jl0jlW7tV4oKCAE2LItNXDq9q7zs80mvGivVhiNZ7U4Hd7K2ESaqemqxzosK4iHxyMz3-maESB9P5FG2Cg-SwVwXt3xlFFHNFf4CMFOikN0R2F0Ea0RIJgFsBFo8a59HPq2z91WryX2AYJQ_VEFVoB8YE3iDZjM3gktVb1pDUKccNo6sWOXePGfQs0HF1xPzwKcEdYrnYE8CF2UhHCpUWqNP3R4FHECQKl_GpkVS12AKzGumRoJlMpQEz8LYP_CWSQrxLx6sck0xYksZq0AIvgxYj48aBRiHAWN9Kp_RuaexUlncwuVJlC0flUC74vt4zTrxi9fROF5TAiylcpEIC9YZ4kpvYHPkuX-wGBHSnb6ny0XFVoSgL-g9USoffTriWI7V7Lx9Ss7vI4dymwjz2sLq8wyI4KKdJ8V5Sj74tlPePEEjUhlUAf6j7Ibrt7mEjGicpKnWDbdiUW-yDt2nR3NOlqVNCT-09xLarILrH6F7sJPdWnE3YHXNsHm08eOaUj4OzurQDaVbxTwzSeNtECDuimXecbN8s4VQcAc1CGIbp0dwJ37X-1g_e-fAybZMC1uF7A4LeJpjDFwzxp_NwugmCsVulQ_KPl4DS8_IyZVIGLLGZeIjiqPNZ_7_G7fJ677R76J95KvRpd3Mwf7hkbMfiNPxSVghY-QmT7icUyDqZR2wkmkmaJ6ujlSbaQoFmcH-Y70SmtxQUFg4Bnika1TWS30YekLxVhaN_8GWYE9zcCmpC84hvcbakgseODPtNGwAJTdMrH6vfR5tSKlMJvIKOIwPLCjQR8tHGAvYWtgPyTepqXvxTam9Tej4IKcud_QTO2ziqaTYfT-AoUu2NJzPtvb4WAhZAExkBqpLHf6hIsRTRReURNrczf10KeU_PcD-8R9OI-RFp4LjqNk1V8bTSNZRbMj-HoAHmBFf7GW_7G1sL3w9rWj1TF0xEIX&ft[fbfeed_location]=1&ft[insertion_position]=14&__md__=1

I have only had mine about 3 weeks, played it live twice, so I am still tweaking.  That said, I believe I am close to the base clean, Crunch, lead and Ultra tones, settings on Stompbox mode.

A couple of comments - of course it will likely make a very large difference in the tone you are getting, depending on the pups you are using.  Some people are mentioning it, others not.  Might be good as of course as a humbucker vs single coil vs p-rail etc. , dark, bright, will be quite different.  My PRS MT408 has a very high gain and bright bridge pickup and I do find the amp to be a little bright by default.  I found it very similar with the Tubemeister 18 I had previously.

So I have the following coming to test out the outcomes for my guitar, that you guys have been coming up with

JJ ECC83-s 12AX7 for V1 and V2
JJ EXX81 12AT7 for V3

All with matched balanced triode sections

I play at 18 watts live with Master at around 9-10 o'clock so I have not ordered power tubes yet.  Will start here and see if I can get the master up some using the ECC81 in V3.

The amp is quite a feat of engineering and it may take a little time to get everything out of it, get it all programmed etc.  But hey, that's part of the fun too, although playing it is more fun.  My bandmates liked the Tubemeister but this one allows for more dynamics and headroom, just using the basic Stompbox mode.  So I am very pleased so far.

UPDATE!

Tubes are in - easy as pie - do not have my cab at home right now so I am listening through headphones or  monitors.   So I won't really know until I play it live this weekend.  So far though it seems to be smoother with better, more even distortion.  Less bright but still plenty of juice.

UPDATE!

Played this past weekend.  Yes there are differences, and all for the good.  The JJ's are first of all smoother, a little less harsh you might say, and a tad less bright as well.  By putting the AT7 in the V3, I can put the Master up to about 10-11 now and still keep the Volume up as well.  Might experiment with the even higher master settings as was suggested next time.  But right now I am running at 18 watts for everything.  Haven't tried to get 36 working.  That would definitely be too hot for clean, but might make it with the other 3 channels due to the increased compression.
JJ in V1 definitely breaks up sooner than the Chinese one.  I had to cut the gain quite a bit on clean.  But that was no problem and what is left is breaking up if you dig into it on say 11 o'clock and that just means the Crunch and Lead (and even Ultra) are deliciously creamy now without having to use the gain 3/4 to full out and lose some articulation due to the compression.  I think the FX now sound better and less thin as well.  The neck pickup tone, single or hum bucking setting, sounds fantastic.  All settings benefit but this one especially and the bridge is really good too.  All without the harshness.  These are 83s so no gold,,,, work really well.  Next time I will upgrade the power tubes but I want to enjoy this upgrade for a while first.

UPDATE!

Added 2 new outside power tubes - (I only play at 18 watts right now) JJ EL844, that is right 84(4), The extra 4 denotes a lower power tube. Matched pair. I popped them in last weekend and they performed just fine. The tone is a little rounder and smoother as I was able to turn up the Master to about 11:30. I will add another matched pair for the inside next and see if having all the power tubes lower power will allow me to run at 36 watts, with the master in a decent position. That would be cool. Anyway - the lower power JJ's are a nice option for those of you who do not need all speaker cab output or run the Red Box exclusively. Give's a nice brownish sound and break up when you dig in, but also clear up nicely with a light touch and/or rolling off the volume a little. If you like ultra clean and while loud these will probably not be your cup of tea. I get clean channel break up with volume and gain set on 10 and digging in.... but boy it sounds awesome. Kind of like a Brown-ish VOX.....!

Hey stapletongas r you still happy with your setup I'm thinking of going the same way as you tube setup wise and marshall 1912 I play blues and hard rock wat do you think
Cheers luke

Please, while I have got into the same way of thinking as yourselves on the value of this, just a cautionary note here. The idea of swapping 12A*7/ECC8* types around has been around for a long while. So much so that people have got into the way of thinking that they are completely equivalent. Sometimes it's ok to substitute but sometimes it most assuredly is not! There is more difference in these valves than just the gain. Circuits which are designed properly for 12AX7 do not automatically suit 12AT7 and will never suit them perfectly. Permitted voltage levels are different, anode load resistances are different, current levels are different, and frequency responses will change along with gain in the same circuit.

Look up online for the 12AT7, 12AU7, 12AX7, 12AY7 swaps and actually read some of what the technically knowledgeable posters say rather than looking for "someone" with an "expert" posting status who tells you its ok so you go ahead. And for Pete's sake take it seriously. It's dangerous to choose to believe the guy who says "well I just did it and it sounds great for me". Firstly his ear may be severely flawed and completely different to yours. He thinks it sounds great while the rest of the world and all of its cats and dogs are holding their ears and howling. Secondly, one month down the line he may be looking at a smoking pile of components, as could you!

Here is just one example. Try to sort out the advice from those who "do it" and those who "understand it".
http://www.diyaudio.com/forums/tubes-valves/211582-12ax7-12at7-recommendations.html

For the record, I'm running a NOS Mullard 12AT7WA in my V3 PI slot to drop the gain a tad, and I took pains to talk to someone who knows a lot more about specific valves than I do. This is the only one he would recommend. But I'm watching the situation closely for any signs of stress on the amp and I'll go back to my JJ/Electro Harmonix if I spot any.

Hi bordenbert cheers for the heads up I had no idea at all that different valves could stress the amp I'm a total noob when it comes to amps played guitar for 10 years but know nothing about this sort of stuff I was guna order them tomorrow I don't know what to do naw I do really want to bring the gain dawn to a more workable level but don't want to break the amp are the valves your using more inline with what this amp can handle and have u found you r getting the change you hoped 4 I no every ones ears r different but it's all iv got to go on at the moment because I no so little I no I want to bring the gain dawn a bit and get the amp sounding less fizzy if it is at all possible but after what you've said I'm thinking of emailing hk or the shop that I plan to get the valves from for some more information

Luke5678, all good but I'm making some changes for the next gig in a few weeks.

V1 and V2 will be JJ12AX7 while V3 will be a Mullard 12AT7.

I found that also the master was up around 1 or 2 o'clock, I needed that extra drive

See how this goes

I really don't mean to put this whole issue into a red alert state or to suggest anyone's opinion is stupid but as I said, there's more to it than just swapping equivalents.  I'm not a valve types expert like some who have spent many years working with them, but I do know my electronics so the principles behind this at least are clear to me.  There is nothing hard in understanding the technical side if you really want to, but there are so many threads out there just the same as your own where this sort of question is posed and the advice is always the same, "go ahead and do it, this one will sound great, this one will sound rough" and so on.  The voices who say "if you do that you may lose bottom end because of a shift in the frequencies of the various time constants" or "that valve was never designed to do that sort of job, it doesn't have the plate voltage to take it reliably" or "that valve will run with too much current if put into the biasing circuit of the original" are never listened to.  Everyone wants to listen to the expertise of the non-techy guy who has just plain gone ahead and done it.  No one wants to listen to the tech savvy guy who knows what can or even will happen.

Bear in mind that the TSC only takes care of the bias of the power valves, the 3 preamp valves are in positions where there is no auto biasing function so their bias will depend on their own characteristics and these could change drastically for different valve types, even ones usually considered close like 12AX7/12AT7.

The answer is simple:

1)  Start off by sticking to the advice of people who will tell you WHICH 12AX7 you should use, not which OTHER valve type you should replace it with.  This is the safest option and therefore a best starting point.  If you can achieve the sound you want this way then it's the best possible solution.  I know from my own experience that you can make all of the changes you need to remove that harsh top end "fizziness" you describe, (which others have reported as well as us), by swapping out the correct valve types for other makes and getting your speaker choice right.  (Read the threads on speaker choice in both the GM36 and TM36 forums, that is the most important thing to cover).

2)  Make sure you only take advice based on YOUR OWN amp.  What is the point of someone telling you what difference a valve swap made in a Marshall if your question was about a H&K?  I've even read posts where people swap out "the valve" in a solid state modelling amp and advise on the basis of that change about valves in a full valve amp.  There is no guarantee you will get any of the alteration they report.  It's like saying that a Ford car responded well to changing the suspension geometry setup so your Peugeot would too.

3)  Make sure you do not just change the valves out and then automatically tell yourself that it sounds better!  I've been in the audio electronics game for many years now in both guitar and hifi design and this is the most common error we all make.  It's in our nature to be unable to respond to a change honestly and critically even to ourselves once we have paid out money for it and spent time making it.  We WANT to find it makes a big improvement therefore we HEAR one.  And we advise others on the strength of that and send them down the same path as ourselves.  If this is the case it doesn't last!  How many people have you come across who make the most astounding improvement to their sound with a change of some sort, then they are back in a month or two asking about more changes of the same sort?  Give yourself a significant time using the amp before you make a final decision as to whether the change really made an improvement.

The real answer to that one is blind testing.  That is where you have the change you are testing, (swapping a valve for example), set up so it can be swapped easily and invisibly so it is the only change that could occur, (not easy for the valves).  An operator sets it up for one or the other at random and so the test subject cannot see which is in use at any time.  Then he plays it for a while and says which he thinks it is.  He is given no info about whether he is right or wrong.  The results of a largish number of test guesses are then analysed to see whether there was any degree of correctness in the guessing.  If there is no difference things should work out close to 50:50 of course and the more tests you take the better these results become.  47:53 would not really be significant, but 38:62 may well be.

The perfect issue for this approach is guitar tone capacitors.  You take a magic mojo capacitor and measure its value very accurately.  You then find a plain old cheap everyday type which measures up at exactly the same value.  You solder them both into the tone circuit of a guitar with a small switch to swap between them and make sure the guitarist subject cannot see it in any way.  This way the only thing in the whole experiment which varies at any time is the capacitor type.  You toss a coin where the subject can't see it and if it comes down heads the switch goes one way, tails the other.  You always make a play of changing the switch even if you don't!  You let the guitarist play until he has decided which one he thinks is in circuit.  When he gives his result you give him no information as to whether he is correct or not.  After a large number of results you look to see how many times he was right and apply a couple of formulae to see if it is significantly off the beam one way or the other.  You know what?  When this is done as I have described no one can ever....  Well you know what I'm going to say don't you.  And it would even be possible to prove this way that the cheaper option is preferred even by those who initially thought the special option was better!

That said, I have swapped for JJs in the EL34 (edit: errm, EL84 of course) positions and that maybe smoothed out some of the roughness but it wasn't really blinding.  I have tried Electro Harmonix and JJ 12AX7s in the V1 position.  Of the two I prefer the JJ and will stick with that.  I have an Electro Harmonix in V2 at the moment and would like to see what a JJ would do there but I don't have another one to hand and it isn't a crucial issue so I can wait.  I have also gone for a NOS Mullard 12AT7WA in the V3 position.  (This is sometimes listed as CV4024 and sells for under £15.00.).  I was advised by a very experienced valve techy guy that this was a feasible swap and the job was suited to the capabilities of the Mullard.  A Mullard WA type was the only one he would recommend, he said that others sounded thin due to resistances changing and frequency response being altered.  I can't say it was night and day but the amp became a little easier to dial in "classic" sounds rather than full on "high gain modern".  They were still available, I just had to wind things up slightly more to get to them which gave me a little more control lower down where I wanted it.

To round off (at last), talking here to others about what they found when they swapped out valves is the start of your answer.  We are all GM36 users and our own findings are therefore all directly relevant to your own situation.  We can certainly report on what changes we heard when we did a change ourselves, but this is highly subjective.  One man's total lack of bass is another's balanced Heaven.  We DON'T know all of the results of swapping out valve types, no one yet has any schematic for the GM36 to study for any implications.  Bear in mind H&K say that they only guarantee the product if: "Assembly, extension, re-adjustment, modifications or repairs are carried out by the manufacturer or by persons authorised to do so".  You are not one of those persons and valve type swapping is not a sanctioned mod!

Keep us up to date with your progress, we all learn from it.  And don't stress over it, it isn't something you HAVE to get right straight away.  Hang fire until you are pretty sure of where you want to go.  The GM36 is a superb amp even with the original valves in there.

Don't know if these have been mentioned but I found this:

5751 is a GE "Five Star" version of the 12AX7. It is electrically very similar to 12AX7, but with slightly less gain, more rugged internal construction, etc. It's not much like 12AT7. It's really more like a variant of 12AX7. One can almost always directly substitute a 5751 in a 12AX7 circuit, but again, the gain (mu) is a bit lower (5751 mu = 70, 12AX7 mu = 100).

12AT7 is a quite different tube, with lower rp, higher gm, less gain (mu = 50), etc. You can put a 12AT7 in place of a 12AX7, but now you're into modification territory, and the circuit will now act very differently from the original.

I've had a quick glance at the datasheets for the 12AX7, 12AT7 and 5751.  Here are a few of the highlights:

The 12AX7 is designed as a "hi-mu audio voltage amplifier and phase inverter".  The 12AT7 is a "hi-mu grounded grid radio frequency amplifier or as a combined oscillator and mixer at frequencies below approximately 300MHz".  The 5751 is "ruggedised hi-mu adaptable to low level input and high voltage gain" such as "voltage amplifiers, phase inverters and multivibrators".  So which two are closest to each other there?

Valve:	12AX7	12AT7	5751
mu:	100 / 100	60 / 60	70 / 70
Plate R:	80k / 62.5k	15k / 10.9	58k / 58k
Plate I:	0.5mA / 1.2mA	3.7mA / 10mA	0.9mA / 1mA

.
The dual values are quoted at 100V and 250V.  You can easily see how much better a match for the 12AX7 the 5751 is than the 12AT7. Look at the difference in Plate Resistances, and that is an important parameter with respect to the bandwidth of the stage.

Here is a superb link for anyone interested in the technical side of this and wanting to understand the bigger picture.  Why do tubes sound different?  It clearly describes why Plate resistance is the important parameter for overall performance and not mu!  And I have also posted on the H&K Facebook site asking them for guidance as to whether tube rolling is a good idea for the GM36 from a reliability point of view.  They have passed the query on to the designers and promised to get back to me.  I'll post when they do.

Hey stapletongas cheers for getting bak to us and keep us posted on your new change I'm guna get the marshall 1912 and go with your first set up to start you know the 3 jjecc83 gold pin you got from watford valves wich versions were they there seems to be quite a few different ecc83 and I want to make sure I get the right ones thanks for your help mate

Cheers for in info bonded beret I'm guna play it safe and go with jj versions of what's already in cheers mate

That's a good decision Luke. When you get used to the JJs you can always borrow a couple of alternatives from someone to try out without lashing out more money that may just sit in a draw. I rate those JJs too.