New member, prospective owner of the GM40 here -- a few questions

Hi everyone, new member here on the forum!

I actually registered just to get some input and advice from owners of Hughes and Kettner amps, particularly the GM40.

I'm a rather passionate hobbyist and play mostly hard rock, heavy metal, thrash, metalcore, downtuned progressive (aka djent) and a bit of everything in between with my band. I love heavy chugging down-tuned riffs and rhythms. My favorite bands include Metallica, Pantera, Chevelle, Slipknot, Deftones, Alter Bridge, Periphery, Lamb of God and Killswitch Engage. Recently however, I've been trying to expand my musical palette and have started listening to SRV, Hendrix, Red Hot Chili Peppers and the like.


I've been saving up to get a GM40 since I first saw demos of it on Youtube -- I love how it can save presets and how affordable the thing is. I love the versatility and the built-in effects as well.

Before I get the GM40, I do have some questions:

1) Do I need to change the stock Chinese tubes? I've heard from a few people that changing the tubes is essential to get the most out of the amp. What tubes would you guys recommend?

2) I plan on using my Amptweaker Tight Metal Pro and Wampler Dracarys distortion pedals since I already have them dialed in to my preferred sound for recording. How well does this thing take pedals? If I use the built-in effects on the clean channel, then run my pedals through that clean channel, will the effects affect the distorted sound from my pedals?

3) How well do the Lead and Ultra Channels handle down-tuned riffs? I've always liked how articulate the H&K amps sound, but since I play mostly high-gain stuff, I'm actually choosing between the GM40 and the EVH 5150 50W head. Can the GM40 keep up with the kind of distortion the EVH has?


That's pretty much all for now guys, would really appreciate your input and advice!

1. No. You don't have to change the tubes. I have swapped mine to JJ's. It makes a difference, but not a major difference. The stock ones aren't the epitome of crap. I noticed more of a change in the GM36 which was a brighter amp.

2. I can't help much with that.

3. Yes it can. I got to try a 5153 50 watt a few weeks back. I thought it was a great amp, but the GMD40 can nail those tones and a lot more.

Bish0p is the better guy to advise on the GM40 but for that question 2) the best I can offer is to say that the GM36 has its preamp section followed by the Fx Loop followed by the internal effects.  Any effects you put in the loop will be altered by the internal effects you choose and not the other way around.  So, yes, the internal effects will alter your own pedals no matter where you put them, in front or in the loop.

It is possible that H&K have revised this for the GM40, they got enough criticism for that decision in the GM36 when it became common knowledge, but my own view would be that it is unlikely they would have changed it for the new model.

My 2 cents..

I think the stock tubes the GM40 comes with are fine, I changed mine and didn't hear any difference.

I've only tried an overdrive pedal, not full metal distortion, but the amp took it very well. The effects will change your distorted tone, but isn't that the point of FX?! Think of a like a chain of pedals - here you are just putting the distortion pedal in front of the modulation/reverb/delay pedals (which usually sounds best anyway).

The GM40 does downtuned heavy sounds very well - it stays nice and tight, especially if you are using the boost. Whether you prefer it to the Peavey is a matter of taste really. But surely it doesn't matter if you are using pedals anyway? I expect the tube distortion from the GM40 will sound better and you will end up ditching the pedals anyway.

And as has been pointed out before, the GM actually has a great Tube Screamer type overdrive/distortion pedal mechanism built into the first stage buffer anyway. It takes a little faith but this is really effective if you get the confidence to control it using your guitar's volume control but so many people just turn that full up and forget it's there.

Yeah the boost is great, I use it on most of my distorted tones. It is especially good on the lead channel

That's not the Boost Jonny, it's an inbuilt overdrive pedal which is in place all the time on every channel, with or without the Boost.  At low guitar signal input levels it acts as a transparent input buffer.  With increased signal level it starts to clip gently on one side giving even order harmonic distortion, the "tuneful " kind.  Finally at higher levels it clips on both sides adding odd order harmonics, the rougher kind. So Boost pedals in front of the amp will affect it of course, but it only reacts to the level of the input signal at the input socket, (with one proviso I mention later on.).

The clipping mechanism is a number of series zener diodes in the feedback loop of an opamp, one in one direction and 3 in the other, so the clipping in the feedback prevents anything at all getting onto the signal from the opamp which can't hit the power rails internally.  I know there are people who will totally blank an amp simply because it sat next to a box of solid state components on the bench, but they don't really know anything about circuitry!

If a signal can be shown to pass through a solid state stage without any change to its frequency, phase or transient content, then I can assure you there is absolutely no difference to hear.  Nowadays if you can hear a genuine difference then you can show where it comes from on the bench!  The Boost circuitry does not affect it in any way in at least the GM36, that only alters things after it.  In the GM36 only selection of the Lead/Ultra channels jigs it so it clips at lower input levels.

As always, we don't have any info on the internals of the GM40, (yet), but the GM36 is an open casket now.  We have to assume there is a close family resemblance in most areas between the two.

Ah, ok. Could you explain that in English for us uneducated people?

Hahaha! Ok, point taken, no tech info this time. (But it is now there again for others who may actually be interested! )

The Boost on the GM is NOT the overdrive pedal. What I am loosely describing as the overdrive pedal is a separate solid state first stage which is always in circuit, all channel selections, all Boost states.

The Boost increases gain after the input stage and is totally separate from it.

The input stage has 3 levels.

With low signals from your guitar it has no effect at all. It simply buffers the guitar. Your pedals before the amp and the circuitry in the amp after the input stage make the overdrive/distortion tones exclusively.

With medium signals from the guitar it clips softly on one side of the signal giving a milder creamy bluesy sort of distortion.

With higher and higher levels of signal it clips harder on both sides and helps to add to the High Gain sound.

This is achieved with nothing but your guitar volume control. Try it on the Clean channel. Set it up with your guitar volume low and the amp volume high so you are just on the verge of getting a completely clean sound. Just increase the guitar volume in small steps and listen to the tone change. You won't get High Gain sounds but you should hear it gradually kicking in in a way most popular pedals don't. It's a great piece of design.

It isn't intended to be an "all out does everything distortion pedal", it's an "additional less full on distortion mechanism" to add to the other distortions you can coax out of the amp. It's one of the most important reasons everyone loves the GM's Clean sounds.

Ahh ok. I will say that I have noticed my GM40 sounds much better when I roll of the volume knob on my guitar, compared to when I am using my blackstar HT-Metal pedal (which is the best distortion pedal I've ever tried). This would also explain why I find it tricky to get totally clean sounds from my guitar without the gain way down. As my bridge pickup is really high output.

My neck pickup is only 4.5k when split though, and it gets a beautiful 'on the edge' clean sound that only breaks up when you pick hard. That's one of my favourite sounds from the amp.

I have been gigging with the GMD40 all summer long. Some of those gigs, the amp was in direct 43 degC California sun - and never overheated.

Before commenting on your questions, I will share why I bought a GMD40.
I was gigging with a monster rack - a Mesa Mark V with a racked pedal board and midi switcher. This is a rolling ATA rack case for the Mesa and the rack units, and I couldn't lift the unit by myself. Just got tired of hauling the beast around. I went looking to downscale in size and weight. I have two other Mesa combos, and as much as I LOVE my Mesa amps ... I was just tired of carrying around all this massive stuff. During my process of shopping, the GMD40 was announced. Tubes, MIDI switchable, programmable, FX, noise gate, Red Box, iPad app (for the WiFi accessory), power soak, blue lights - and 16 lbs (7.25 Kg).

I bought this amp having never played through one - and only with the knowledge gained through YouTube vids and this forum. It is now my main gigging amp.

1) I am still using the stock tubes. I have read a lot from others about how changing the tubes makes no difference, and I have not bothered to see for myself.

2) The amp takes pedals just fine. I run my pedal board on the front end, and have done everything from running the GMD40 distortion with no pedals just fine, to adding pedal overdrive to a clean channel, to running distortion to an already distorted channel.

A word about compression pedals: I found that to get the best clean sound, I had to find the right compression pedal. I could not get a super clean sound with any of my guitars that have a humbucker in the neck position without rolling the guitar volume back. For a while I was using the TC Electronic Hypergravity, but ultimately swapped it for the Compressor I used with my Mark V rig: Carl Matin Compressor Limiter. (I had modeled my rig mostly from this video, where it is highlighted: https://youtu.be/9QXLuyIoCtI?t=7m21s) I think it is the Limiter aspect of this pedal that works so well with the GMD40 - for me.

I run a Keeley Red Dirt as my main overdrive pedal. I am about to order a MoJo MoJo and will try running both of those on the board. Paul Gilbert has been using the MoJo MoJo consistently, and really want to try it and see.

I have a TC Flashback delay that I find I use more than the internal delay. A MXR Van Halen Phaser. And another spot on my pedal board that rotates (Dreamscape, SubnUp, Vicious Vibe). I also have a dunlop wah pedal on the pedalboard.

I have stacked FX from the board with the onboard FX with no issues. I can't really say that I have dialed in any specific patch where I intended to play live with the stacked FX. Just sometimes if I decide to create some nutty sounds live, I will turn them all on for some crazy echo-volume swell stuff. I have done this live on clean and distorted channels.

I can't really answer about your question on tuned down or heavy lead channel sounds. The closest I get to that is recently we are covering MJ's 'Beat It' - and for the singers to sing it I have to bring a special guitar set up to be tuned down do Dm. https://www.youtube.com/watch?v=Oo_jve-9NCQ

As far as channel switching, I have been using the Voodoo Labs GCP. I have this set to individually turn FX on and off, can turn the boost and noise gate on/off - and then select between 4 channels per bank. I really only use 4 channels live - so I have only bothered to program the first four channels.

I spent a lot of time programming the amp using the knob interface on the amp directly. This is pretty straight forward. Recently, I have been using the app more - and I find this much more convenient. So if you are going to get the WiFi adapter - as much as the price for the thing sucks, I would say that I am glad that I have it. One trick though, is that if you are using it - it does not work well at all with a 7-pin midi cable to a midi board powered by the cable. At least this was my experience with the GCP. I had to switch to a 5-pin cable and power the GCP with the AC adapter. Works great now.

The biggest issue I have found with the amp so far - is that it will start having issues if the line voltage level drops below some threshold. Most analog amps can just deal with the low voltage situation. But this thing is a computer, and if the available power drops - so will the amp. I discovered this at a couple outdoor gigs - where when it was daylight, things were fine. But then when it turned dark - and the outdoor lights went on ... the amp went wackadoodle. So, after that I bought a Furman power conditioner with the built in Voltage and Current monitors - so I can at least see what the situation is wherever we are playing. The live fix, is to take the mic cable from the mic mic'ing the cabinet - and go out the RedBox (since the amp will boot up and when it sees there is not enough voltage, the Power Soak will automatically switch to the "speaker off" setting for you). The Furman was handy recently at an outdoor show where during setup I could watch the voltage swing wildly between 90-115V, so we knew we had to find a second circuit to power the show. I have also done shows with the GMD40 where the show was being powered by a gas generator, and that was fine.

I still have my Mesa amps and LOVE them!
Only I love them at home now, in between loving my lightweight H&K amp at home and when playing live.