I have researched various methods of controlling noise/hiss in high gain tube amps. In this case, a Peavey 5150 series. Considering your amp is working properly, proper cables, proper tubes, and solid gear, the next tool is using a noise gate effect/pedal/device. The effect itself and what it does seems rather simple, but there seems to be a wide range of products that attempt to fill this need. What is the big difference between many of the popular noise gate products and do any of them have anything that puts them above the rest beyond simply setting a threshold and letting it do the work?
More or less as you specified a noise gate is a set it and forget it kind of effect. The important factors to consider come from questions such as: "Where in my chain should it go?" Honestly that's a good question to ask when purchasing any new effects pedal and the subject of much debate. First, let's explore where hiss comes from.
Your guitar. Hiss and hum here is often derived from poor components, cold solder joints, poorly wound and shielded pickups, or the fact that you happen to be using single coils. If you want to make your rig quieter, start here. Make sure that you have a proper ground path in your components, and that all the solder joints look smooth and mirror like. Now move on to the next point.
Your effects chain. An effects chain is the aggregate of a lot of circuits that were all built by different people. Each box has it's own personality, but hopefully the builder followed some basic best practices to ensure that their pedal is as quiet as it can be. Sometimes, however, when you work with boosts or anything that is designed to increase the volume in your chain then noises from elsewhere will be amplified. This is a reason why people are usually advised to place their compressor at the beginning of the chain. The nature of this effect will amplify any noise from any other sources--and if you minimize those sources (i.e. just the guitar) then you are guaranteed to have less hum. Proper order of effects here will reduce hum, as will the use of what's known as a true bypass looper.
Your amplifier. Your amplifier wasn't built perfectly. Especially if you are using a tube amplifier, your tubes can introduce hum and hiss into the equation due to microphonic noise, poor design, wear, etcetera. One place where hum is produced that most players don't even consider is at the connection from your amplifier to the speaker terminals in your cabinet or combo. These connections are usually made with a spade connector, which forms a very poor connection in 99% of the cases. Lots of players solder their speakers in ensuring a more stable connection and reducing the hum and hiss of their amplifier quite a bit. I performed this modification on my combo and I have never looked back.
Cables. Cables matter. The cables you use to build your board, the cables you use to run your guitar to your board, the cables you use to run your board to your guitar; everything. They all should be high quality audio cable. This doesn't just effect hum and hiss, but also tone. Some people don't believe this until I tell them to play their rig straight in and then with a couple hundred feet of cable in between their guitar and their amp.
So, with all that said, why do you need a noise gate? Well, if you wish to tackle problems numbers one, two, and four simultaneously. Problem three will remain unless the noise gate grounds the line out to the amplifier when it is engaged, but that isn't typical of the effect. What would I look for in a noise gate? Configurability--since it's just simply a threshold designed to kill your signal when the input volume is below a certain value. Whatever unit you pick, you should be able to change the threshold easily and I'm sure there are some out there with lots of more whiz-bang features with other applications. I would also look for transparency--as you want the gate to color your tone the least bit possible. Know that due to the nature of the effect all noise suppressors will be digital, or at least I don't know of any analog ones nor how to construct one off the top of my head. Why does that matter? Your guitar signal will have to pass through an ADC and then a DAC, the first in and the second out respectively, and that may effect your tone a little.
Where should a noise gate live in my chain? That's likely subject to debate, but I believe that the obvious yet naive answer is: always make it the last effect. Why? Because of problem number two. If I have my noise gate directly before a very noisy effect, then the gate isn't doing it's job properly. There are also advantages to placing it first, as it will tame the hum and noise that comes from your guitar before it gets fed into amplifying effects (such as a compressor), but I would rather take care of the root of the problem in that case: the guitar. Take all this with a grain of salt though, as the position of any new effect should be experimented with when added to your particular chain. It may sound better somewhere else. I can see some uses of placing a noise gate in the effects loop of an amplifier, such as keeping pre-amp gain from drastically amplifying any noise generated from problems one, two, and four, but that's not nearly enough evidence to specify that as a hard rule.
I think the main thing is that you want to be careful that the noise suppressor you use does not change the tone of your guitar. I am partial to the Boss NS-2. I have experience using it live and in recordings (where having a quiet signal really counts!) with a Strat through a Mesa, Marshall, Fender, and through a bunch of different distortion/od/boost pedals and it works exceptionally well in every situation. Also, in terms of cost, convenience in carrying around, & simple user friendly controls, the NS-2 was a clear winner for me.
In terms of differences between products, some noise gates are made strictly for noise reduction, and others can be used more as an effect. Some have been known to mildly affect the tone of the guitar. I've used the NS-2, but I'm partial to the ISP Decimator II (it's better suited to pure noise reduction).
I like placing a noise gate after my reverb pedals. It gets rid of the reverb tail, which gives a tighter and somewhat bigger sound i.e. not as washed out. It's a subtle effect, but it's definitely a use case.
So much depends on the pedals. There are 8 pedal groups. O.dr/ dist. 2. Modulation 3. Ambience 4. Pitch altering 5. Sound altering 6. Filtering 7. Effects delay (compression/sustain, loopers & noise suppressers) 8. Tuner pedals. I use #'s 2,3,6,& 7. W/TC Helicon V.H. Singer & A Digitech Trio. My MXR N. SUPP. Is in the center of my P. Chain. after my modulation (phase) & Ambience (Delay). Tuners should always be first in the chain, before the N. Gate/Supp.
I have used the Boss NS-2 for a long time. I currently use 2 noise gates. The NS-2 before the amp input, The TC Electronics Iron Curtain is at the begining of the fx loop. I did a number of things before deciding to do that. I have 3 pedal boards. A: main pedal board that is using a SKB Stage Five pedalboard case. I reorganized the power layout, and improved my cabling. At the time, I started building my pedal board (A) I didn't have a multi-channel amp. I added a Boss MS-3 Switcher, which greatly improved my board. I was able to remove my NS-2, EQ, TU-2, and compressor. Which later ended up on another pedal board (C).
The Boss MS-3 added stereo fx, and a NS-2 within the unit. The amp like OD pedals are in 3 loops, that are programmable now. This setup is run all in line after the MS-3 Switcher. So, the MS-3 gives me some "AUX" fx. Momentary fx are assigned to the channel button. Also, in manual mode, other fx are assigned, across the other buttons. The MS-3 that controls all the gain overdrives, quiets the overdrives and gives me an auto compressor on just the clean channel. The stereo outs go to my first series of stereo fx, TC Electronics Nova Dual Modulator, then into a Stereo Eventide Pitch Factor, then stereo TC Electronics Nova Delay, then finally a DigiTech Polara Reverb. The SKB Stage Five has a nice patchbay built in, along with other nice features: Cable tester, headphone monitor, several ways to organize you signal going to fx loops, etc. I send a stereo cable to a rack mount converted into an amp head style box. A stereo tube preamp, analog stereo speaker simulator and a MosValve stereo power amp. I was getting a hum, so I use a Nady Hum eliminator stereo box that is patched at the inputs to the tube preamp. Now, all I hear is the fan on the amp.
My other pedal board (B) was built with out the need for multiple amp like OD / Distortions. I only use a BD-2 and Xotic SP compressor to push the amp, gently. I mounted a TC Electronics Iron Curtain Noise Gate inside my Mesa Nomad 100. I have dual lock Velcro to keep it in place. This starts the signal at the fx loop. Then, I use an Empress ParaEQ located on top of the amp, to sculpt out some of the excessive mids and allow fine tuning of adding tighter bass and additional brightness when needed. Recently, I mounted my NS-2 in the Mesa Nomad 100, for knocking out any noise going in to the amp after the fx in front of the amp. I use a mini volume box, after the EQ. This is where the FX send goes to the first of a series of stereo fx: modulation, pitch, delay, then Reverb. Left output of fx goes to Mesa Nomad 100. The Right output goes to a 2nd amp. (Musicman HD 150 or Orange Micro Terror). I use a Two Note Torpedo Captor 8 (reactive load)speaker simulator and a Hughes and Kettner Red Box Classic. The 2nd noise gate was a great option and makes Mesa Nomad 100 Rig quiet. The pedal board (B) is just a DigiTech Drop, wah, and DigiTech FS3X (remote switch for Eventide Pitch Factor. I also squeeze in a TC Body Rez, Parametric EQ, and DI box for piezo signal to PA.
I have tried many other noise gates. The old Boss version of Noise gate, Rocktron The Hush, Rolls Noise gate, Behringer NS-2 style plastic junk! I have been interested in the ISP Decimator, and TC Electronics Noise Sentry. One time I thought my NS was broken when it was a faulty power supply. After that experience, I decided to review my power needs. I found some affordable options. I like the Vitoos DC 8 isolated power supply, and a Dinosaur power supply. I use that for my Mesa Nomad 100 fx at the amp. I have only had to Y a few devices. Some 18 volt fx and 12 volt fx. I wish I could afford a power regulator. I have a couple of Monster Home Theater surge power protectors, that work very well. I also need to get the EBtech Hum X, that safely removes ground.