The Tube Swapping Thread

So, without a schematic, there's some guess work here.  What do V1,2,3 really do?  If they are numbered in signal flow order (most common), then we can take some guesses.  One guess popular in this thread is that at least one side of V3 is a phase inverter for the output stage.  Does it use a single triode out of V3 to just invert for the "other" side of the push-pull output stage?  This is common and with only three pre-amp tubes, that might be a fair assumption.  If that is what is going on here, putting a tube with gain lower than 100 (lower than a 12 AX7) would probably cause an unbalance in the output stage, eh?  This won't hurt anything, and might actually sound cool...?  But is it doing what you want?  If both triodes of V3 are used in a balanced driver stage for the output stage, isn't all your doing is lowering the gain of this stage?  Personally, I like my preamp triodes to saturate (even in this stage), and I want my EL84s to roast, so I don't think I want to reduce the gain of this stage.  I have JJ ECC83S in V3 and put my master at noon-1.
It "sounded" to me that at least one triode of V1 is influencing the gain of the Clean and Crunch channels.  I like the gain of those channels as is - in fact, I generally run the Clean channel at a gain of 255 (using the numbers of 1-255 from the app).  What I had a hard time controlling was the  gain of the Lead and Ultra channels.  So I experimentally (isn't that we are all doing?) put a JJ ECC81 (yes, the much maligned on this forum 12AT7 sub) in V2 and found I could get more low side gain control range on the Lead and Ultra channels.  This allowed me to more accurately dial in the JCM2000 and Soldano saturated tones I was looking for out of these channels.
This left me with JJ ECC83S in V1,3 and JJ ECC81 in V2. And of course all EL84s are JJ.
Again, without a schematic, this is all empirical and just a discussion...

And again, over the tubes, the speakers had a major role in tone with the GM36.
I would like to test with greenbacks for classic rock/hardrock tones. I have the Eminence Governor and Cannabis Rex in my 2*12 and it's far better than previous combo I had. The CRex add a darker flavour that tames the GM's highs.

I must admit this is turning into a very interesting topic for me. As an engineer I have been interested in how H&K have set up the GM since I bought it, but of course they are very close mouthed about it and opening my own GM up to explore isn't high on my list at the moment. I think your post has raised a very useful issue Namklak. So much so I'm going to open a new thread with this question as its point. This one can remain for its original valve swapping thoughts and I'll post my own answer and open it up to schematic thoughts elsewhere.

very nice topics guys.
Appreciate these exchanges and having the scheme would be of course of great interest, to push ahead some light mods, but leading to great impacts on tone, such as the capacitor on the input guitar slot.

one"simple" mode we should condider is the choke mode which yields some good results soundwise. Also when I changed the stock cheap capacitors in my deluxe with some serious Japanese ones then the amp has been completely transformed.

We shouldn't talk about that here, it will hijack the thread from valve changes.

How about posting this info on the thread I've just started to do with the design of the amp in general?

Yes U're right, let's create a new thread.

Here`s a demo I just did for the Wathen Audiophile tubes. It made a pleasantly subtle difference in the top end and generally warmed up the tone altogether. It`s just preamp tubes right now, but I`m interested in doing the power tubes next to see what might come of it.

It`s NOT intended to be an advertisement for Wathen tubes, nor an exhibition of my playing. I tried to make it as purely A/B as possible. Read the description for details on the setup:
https://www.youtube.com/watch?v=h1ZGSVRA67E

(oh, and if you`re listening on a mobile device the subtlety may escape you. It was there on my laptop (barely) and blatant through my studio monitors)

exactly that Voodoo: this is with the lead tone (ultra channel in my case) that I noticed a significant improvement in a creamier way.
Of course the crunch and lead channel are improved too ...

Great my friend I think there is a noticeable improvement , if you change the power tubes too then I think it will be even better .

Hi I'm new to this group and will be receiving my new GM36 in a couple of days. I do at present own a TM18 head and love it to bits.
I can see and hear (having viewed Voodoo Jeffs video) that swapping tubes/valves will make a difference to the sound of the GM36 but through experience have found that different speakers and cabs make a huge difference, especially the cabs design,closed back,open back,ported,depth, dimensions,materials etc.
I own a few cabs and the difference between a Harley Benton vintage 2x12 (Thomann brand v30 cab) and an old Marshall 1936 2x12 with G12 75s is huge,so much so that you could be fooled to think on a blind test that you were listening to a different amp. I sometimes gig with an old 1x12 cab that I found on the tip which I loaded with a 16ohm G12 75 taken from a Jcm 900 cab, and it sounds awesome but completely different to the other cabs. Also as mentioned in earlier posts are the differences that different guitars and pickup’s can make. I own around 25 guitars and some have the same pickups and sound completely different.
Which in my view can only be a good thing So what makes the most notable difference? Cabs,valves,pickups,guitar materials? I would say Cabs,pickups and guitar materials.But that’s just my opinion. Sorry to waiver off topic slightly.

steve15366 wrote:Hi I'm new to this group and will be receiving my new GM36 in a couple of days. I do at present own a TM18 head and love it to bits.
I can see and hear (having viewed Voodoo Jeffs video) that swapping tubes/valves will make a difference to the sound of the GM36 but through experience have found that different speakers and cabs make a huge difference, especially the cabs design,closed back,open back,ported,depth, dimensions,materials etc.
I own a few cabs and the difference between a Harley Benton vintage 2x12 (Thomann brand v30 cab) and an old Marshall 1936 2x12 with G12 75s is huge,so much so that you could be fooled to think on a blind test that you were listening to a different amp. I sometimes gig with an old 1x12 cab that I found on the tip which I loaded with a 16ohm G12 75 taken from a Jcm 900 cab, and it sounds awesome but completely different to the other cabs. Also as mentioned in earlier posts are the differences that different guitars and pickup’s can make. I own around 25 guitars and some have the same pickups and sound completely different.
Which in my view can only be a good thing So what makes the most notable difference? Cabs,valves,pickups,guitar materials? I would say Cabs,pickups and guitar materials.But that’s just my opinion. Sorry to waiver off topic slightly. :
roll:

Hi,
the cab is the most important. With the GM36 that I found too brigth (see other threads), I put in my 2*12 cab an eminence CRex and a celestion G12m. This the clearest difference one can ear.

Guitar and pickups of course ! huge differences.

After the tubes (and before I would say the whole electrical scheme of the amp !!).I aws just spending some more time this week end on that topic: love to play with tubes !!
Again I modified the preamp tubes:
- V1: JJ 5751
- V2: 12AU7
- V3: RCA 12AT7
sounds good to me for all channels. I can play in my home studio with 5w presets from clean to ultra for leads. Creamy as can be.

I would like to modify a little the scheme... see other thread on that.

In fact put the 12AU7 in V1. Sound better for disto and pedal in front if I want.

I just purchased and installed a standard Retube kit from Eurotubes which included quad matched JJ EL84s and three JJ ECC83S with one balanced for V3. Like others I agree the change in tone is subtle. To my ears it's ever so slightly warmer. I seem to notice the biggest difference on the ultra and lead channels. Happily my ears detect less "shrill" at higher pitches. I seem to notice a little more on the bottom end, but not enough to write home about. Now I just need to play better. Lol.

For kicks I also ordered a JJ5751 to try out in V1. I've been having so much fun I haven't yet tried the 5751. Will report back when I do.

Raf, I share your observations on the differences between the stock tubes and JJ's.

I have JJ's in my GM, from eurotubes. Great guys there, I've been a customer for many years.

Same here Raf as I've said many times.  I'll be VERY interested to hear your impressions of the 5751 swap.  You've chosen the right way to go with that alternative.

There was a suggestion a while back that the GM may have an integral solid state Tube Screamer type of overdrive setup before the valves which is used as an integral part of the voicings.  This has not been confirmed or refuted in any way as far as I am aware.  If that were to be the case then reducing the gain of the valve stages could create a significant shift in the balance of the distortion creating mechanisms.  There would be less valve overdrive available before the SS clipping kicked in which may make for a very different sound through the onset of distortion.  It may not be an unpleasant thing, (or it might be!)  It may also be that that is effectively the "Boost" option, an additional solid state circuit which only engages with the Boost button, effectively giving us a boost/overdrive/distortion pedal setup within the amp. That's a very clever approach if it is true. It's effective, exactly like we do already, and hence it doesn't break the "amp must be all valve or it sounds crap" rule! It is all valve but it has a standard addition before it.

Anyone else have any knowledge of this area, or any ideas as to whether it makes a difference?

bordonbert wrote:

Anyone else have any knowledge of this area, or any ideas as to whether it makes a difference?

I will see many of the people from H&K Germany in about a week and a half. I can certainly ask if I get time with an engineer.

That would be great Jeff.  Tread lightly so as not to worry them! Bear in mind that the H&K team are a bit mystified by anyone's desire to modify their baby in this way.  When I originally spoke to them about valve rolling and its implications in the GM36 circuit they were very kind in confirming that the usual 12AT7 and 5751 swaps would not cause any undue distress to the amp but, as I said, they were puzzled as to why we would want to.  They had voiced it to perfection!

I find I am increasingly turning to higher power soak/lower gain sounds to get near to where I want to be and this will be another variable to help find my own sweet spot.  (Or maybe just cloud the issue even more.)  I think it may have been yourself who suggested this as another approach to try out.  It is certainly opening up for me.  And if it turns out to have a SS preamp boost/distortion stage, lower input signal and gain should take away some of the effect of that.

bordonbert wrote:Same here Raf as I've said many times.  I'll be VERY interested to hear your impressions of the 5751 swap.

I apologize as I've not yet had a chance to swap in the 5751. A few other projects have been eating into my playing time so I've been content just to play. I am really enjoying the swap if for no other reason than taming the treble. My ears thank my wallet daily. I hope to get on with this later in the week. I also have a TM112 speaker swap I would like to try, but I'll post that in the appropriate thread when I get around to it.

Well my 5751s appeared at the door today.  I opened up the GM and had a look and, what do you know, I had my NOS Mullard 12AT7 still in place as V1, and a Fender 12AX7 in V2.  I don't remember doing that....

Anyway, I swapped out V1 for a 5751 and turned on.  It's instant.  I loved it.  So I went the whole hog and swapped out both V1 and V2.  I loved it even more.  I will be using this setup for a while now in order to burn them in and see where it goes but I can't seem me going back.

Everyone here will be bored to Hell with my bleating on about the 12AT7 being a s**te choice as a substitute for a 12AX7.  They are so far apart in many things that really matter it's untrue.  The average Joe can't see that, while it seems to be a "good thing" to drop your "gain" (mu) from 100 to 60, the problems caused by other parameters changing is more profound.  If you haven't already seen it, have a look at page 4 of this thread.  At the top I have posted a simple comparison of the 3 valves.  The parameter Plate or Anode Resistance, (Ra or Ri depending on where you live), has a tremendous effect on the sound.  To change this by a factor of 5-6 times without knowing the consequences is stupid.

The output of an amplifying valve stage has an output capacitor and this sets the low frequency breakpoint for that stage along with the overall anode resistance.  If it has a 100k resistor in the circuit and a Ra of 80k (12AX7), then the valve is really seeing those two in parallel (100k//80k) as its load, that's 44k.  The capacitor will be chosen to give a cutoff not too far below the bottom E string 82Hz, say 40Hz for easy calculation, (it isn't a bass amp and too much low end extension sounds bad for guitar).  Changing the 80k for 15k (12AT7) means it now sees 12.6k.  That means the low frequency cutoff for that stage has just been raised to almost 4 times its original frequency, about 139Hz!  That's potentially much more of an effect than lowering the gain a tad.  With a 5751 the difference is 34k and 51Hz.  Much closer to the original so the tonal characteristic will remain much as it was while the gain changes.

So to the differences.  Well the gain has obviously changed, 100x100=10000, 70x70=4900, not that simple in real life but we can guess it will be pretty much halved.  The Gain control now spreads its effectiveness over a much greater travel.  I love it!  It means I can now more easily control those voicings where I want to use say the Lead channel with a cleaner sound.  The Crunch channel has become more sensitive and easier to control too and the sweet spot is easier to find.

The bottom end has changed and filled out.  That damned 12AT7 must have been thinning things out way more than I realised.  My latest speaker mod of my Mitchell foam rings, (reported here on this forum), is now almost too much over the top.  If I can't learn to compensate to live with it I will have to go back and remove some of that.  I use a 1/2" and a 1/4" thick pair for each speaker so there is plenty to play with.

The tone controls at both ends, bass and treble, have kicked in a little more, always a good thing with the GM.  I wasn't expecting the top end to change but there seems a little more sparkle to it without any added harshness hence the treble perhaps has more to work with.

Another unexpected one, even though the gain may have gone down, pinch type harmonics have now appeared on higher but not extreme gain settings.  They are not my finest feat I'm afraid.  I'm no metal player so have never really looked for them in my playing but now I can get say ZZTop type sounds more easily.

It's definitely a success in my opinion and blows away the 12AT7 substitution, NOS Mullard or not.  That valve was recommended by an independent valve expert as the one which would offer the least thinning out of sound, so God knows what you guys using any old 12AT7 are hearing!  It's much more a case of getting the benefits I wanted from reducing the gain while keeping the best characteristics of the amp with its design 12AX7 bottles.  And don't be frightened to swap out both if you have the chance.  I can't speak for the hardcore metal freaks who want so much rasp they can't identify the pitch of the notes   , (see what I did there - rasp?), but for all else you do not end up with too little gain to develop a full range of overdrive/distortion tones and feel.

Bordonbert thanks for such a thorough review. It's to hear you're enjoying your new tones! You mentioned going whole hog. Question- did you swap both V1 and V2 with a 5751?

Yes both V1 and V2 are now 5751. My repertoire is mainly classic rock from when rock was really classic! Very little from new bands after about '80. So the amount of overdrive I feel I need is not the same as a lot of more modern stuff. Even with the reduced gain two 5751s gives me, I can't imagine anyone except the most flesh shredding extreme black death doom speed thrash metal exponent would find something missing.

I really need to start proofing my posts!
Sweet! You may have motivated me to perform the swap tonight.

Has anyone tried TAD el84's? I've got JJ's in there now and they seem a popular choice.

Thinking of ordering some of these, but opinions would be appreciated in case anyone has had experience of putting them in a GM

http://www.tubeampdoctor.com/en/shop_TAD_Premium_Tubes_TAD_Power_Tubes_SELECTED/EL84_Cz_TAD_PREMIUM_Matched_2831?selected_slave_id=353

Hi BB,

You say that you have 5751's in V1 & V2 but what do you have in V3? Apologies if I missed it.

Gary