TriAmp Mark II buzz on gain channels

I recently bought a used TriAmp Mark II. Now, channels 2A, 2B, 3A, and 3B have a loud buzz whenever a guitar is plugged it. Things I've noticed:

- The buzz volume changes with the amp volume or guitar volume.
- The buzz does not change when I touch the strings, pickups, etc.
- Changing instrument cables didn't help
- Different outlets didn't help
- Outlets test OK for grounding
- Lifting the power plug ground didn't help
- I swapped preamp tube positions (don't have any extra tubes on hand) with no change in the buzz
- Note: all tubes are back in their original positions
- Playing a chord or note *appears* to change the buzz until the note fades a bit

Any ideas on what I should look at next?

Thanks,

-tom

Hi there, welcome to the forum, we'll do our best to help but there aren't too many Triampers here as you can see. I don't have any experience of using the Triamp, but I do have access to a schematic so maybe that can help! Forgive me for such a basic question but by '2A, 2B, 3A, and 3B' you are talking about 'Crunch1, Crunch2, Lead1 and Lead2' aren't you?

From what you say it sounds like the amp doesn't buzz without anything at all plugged in, is that right?

Also you say the buzz changes when you hit a chord then comes back as it fades.  Do you mean it changes in character and can still be heard maybe behind the chord, or it actually stops buzzing completely?

The next thing I would ask you to do is to try all of the various outputs you can and see if they all include the buzz. Obviously the output to speaker will as it includes all stages, but does the DI output? Can you check the signal from the Fx Loop Out. Also check the Preamp Out signal to another amp.

This is a really odd one just looking at the schematics as the amp is organised in a very logical way. There is an input buffer triode specifically for the Lead channels and another shared by both the Clean and Crunch channels. After the Lead buffer it goes to the Lead preamp which uses all of its own stages and tonestack exclusively. After the Clean/Crunch buffer the signal is split to the Clean preamp stages and the Crunch stages which are also exclusive to their channels and their exclusive tonestacks. The power supply lines are also shared by the Clean and Crunch channels with Lead having its own. So far there is nothing shared by both the Crunch and Lead channels, which is where the problem shows up, and not including the Clean where it doesn't. If there is genuinely no sign of a problem in the Clean channel then we must assume it must be after the preamp stages but how it would isolate the Clean channel I can't guess.

Can you just confirm that the Clean channel definitely shows no sign of this issue while the Crunch and Lead all show it clearly?

Hi,

Thanks for the suggestions. I did some more testing today and after turning up the volume on the clean channel, I hear the same buzz. So it is on all channels.

I also used another amp's preamp out into the Triamp power amp in. There was absolutely no buzz. Switching those around, the Triamp preamp to other amp's power amp had buzz on all channels. I don't have a way to check the DI out.

So it is in the preamp section, but on all channels.

Maybe the input ground became detached?

It could be an input ground problem but it's unlikely in the amp circuitry, those "dry joints" you hear about on the input socket just don't happen. (A joint which is damaged as in cracking by leverage and other physical mishandling is NOT a dry joint! It's a buggered one and it's the user's fault not the manufacturer.) In that area it would be much more likely to be a cable or guitar problem and you seem to have ruled them out. It could be a power supply problem or it could be a problem in the first valve affecting both triodes in there, (a heater problem for example).

I have no idea how experienced you are with valve amps so forgive me if this is too simple. Are you sure this isn't just natural buzz caused by turning the Gain up too far? In all valve amps and even SS ones to a degree, high gain settings will automatically cause the guitar to pick up noise and it is common to find new valve users thinking this is a fault. It gets worse the nearer you take your guitar to the amp or other source of radio or mains interference. Does this occur if you try to find a clean setting on the Lead channel by turning its gain down quite a long way then turn the volume up, or does the buzz go away under those settings?

Also what guitar are you using when you get this problem, single coils will naturally be a bit worse than humbuckers in this respect.

When I'm back from my trip, I'll open up the amp and check the input ground. I was thinking that same thing.

I'm sure that it is not just regular hum/buzz. It was much quieter when I first had the amp at home. This buzz is quite loud....too much.

I'm using a Les Paul. The buzz is slightly quieter with the selector in the center position (both humbuckers), but only slightly.

Ok TPA, good stuff. Glad you can definitely recognise it isn't the normal background or "noisy room" buzz and the fact that it has become louder since you got it points to it not being normal.

If I can make a suggestion here, whatever you try, try ONLY ONE THING at a time. In other words, if you were to rule out the input socket by reflowing the joints with an iron do only that task and then retest it. That's the only way you can get a fairly definite picture of what is actually wrong which can help you in the future, (and even then it isn't 100% sure because it may have just been something which was fixed unknowingly simply by handling the unit. A lot of dirty connections are as delicate as that. Touch them and they begin to work for the time being.) If you "just have a go at this as well while I have it open", you can become convinced it was one thing when it was really another and when it reoccurs because it wasn't properly addressed you are already on the wrong path. Unfortunately that's how most people do it and they then start spreading the rumours of problems which don't really exist in any big way which others take as gospel and we all spiral downwards.

Also make sure to test it just by turning the gain right down on the lead channel. You should be able to dial in a quiet clean sound, and with the gain right down any natural hum or noise would be absolutely at a minimum.

Have a good trip and get back in touch when you return.

Back from my trip...glad to be back.

I opened the amp. No ground issue at the plug. The bypass caps visually looked OK, i.e. no bulging. Nothing else obvious visually.

My neighbor has a tube tester and we looked at all of the tubes...no issues seen. We tried the amp (same power cable, instrument cable, my guitar, but his cab) at his place after that and there was no buzz!!!

Went home with the amp and the buzz was back. I tried a different wall plug through a "filtered" power strip and still had the buzz.

Next steps (in order):
- Try a totally different house circuit in a different part of the house
- Get something like a HumX to see if that helps
- Get a set of preamp tubes because there may be a tube with slight short or beginning to fail

-tom

Woah! Don't like the sound of that at all!!!

While that could be down to something like a weird problem in your speaker cab, (maybe a blown driver amongst other good ones dropping the overall impedance to an overly demanding level), it sounds more worryingly as though it could possibly be a domestic supply problem. I'm not American so I don't really know the intricacies of your supply setup but I would assume you are running your amp from a 3 pin Type B plug with live, neutral and earth connectors. I would guess so as you said you used the same cable for your neighbour's test which showed no problem. Are you sure that you have solid live and neutral connections and that they are the right way around in your domestic outlet? Also, check out the integrity of your earth connection in the wall socket. If that has become disconnected or the main physical earth connection is badly corroded or has dried out then you have no screening on anything and more worryingly your metalwork, the stuff you are touching and which is connected to valve equipment, could be unearthed!

If you are on a big property then is it likely you have a local ground connection the likes of a large metal stake in the earth? If these dry out then the ground connection suffers. This may all be only my "worrywarting" but I would make sure that this is not the issue. It will kick into other electrical items and could be quite a dangerous situation if left.

The solution is to run the supply to your amp through a RCD device which should detect fault conditions.

I will be checking the outlet.  I am using a three pronged plug.

What is an RCD device?

RCD - Residual Current Device, not sure if it is called the same in America so you may already know it by a different name.  It's a safety device which sits between the supply and your unit(s) and constantly compares the current in the live and neutral lines.  For normal action in any domestic device these should be exactly the same of course.  Current drawn from the supply through one wire must pass on and return via the other.  In the event of a fault condition where some of this current is drawn away to the ground/earth wire possibly through you, the RCD detects that the live and neutral currents no longer match and it trips out a very quick circuit breaker removing the supply.

RCD Device

These devices sense small current differences, 30mA is common, and they act quickly, in about 30ms.  That's small enough current for your own body to be protected and fast enough to be able to save you.  They do not protect you from electrical shocks where you short the live and neutral with your fingers! With the amount of electrical equipment we use and the varying standard of supplies in pubs and clubs that bands play in, it makes sense to have one of these protecting each person's distribution box Where it plugs into the supply.  Mine once indentified a serious wiring problem in a school supply where wall sockets close together but on different loops were out of whack enough to give me a stinging shock on the lips when I touched my mike.  My amp was fed from one and the PA from the other.

Glad you're going to check the outlet, just think it might not be you who suffers from any fault!  Hopefully it will be an easy spot and a quick fix and the amp will be fine again and will have luckily pointed out a problem to you.

Hi,

I understand. In the US these are call GFI (Ground Fault Interrupt) or GFCI (Ground Fault Circuit Interrupt).

Checked the outlet with my multi-meter. It looks OK. The polarity is correct (120V is on the "hot"). The ground is OK too (hot-to-ground shows 120V, hot-to-common shows 120V, common-to-ground shows something very, very small. Freq measured at 60Hz and the duty cycle was 49.9%. I don't have a scope that can handle that voltage, so I can't look for noise on the line.

I tried a 2 pronged plug and flipped it too, with no change.

Decided to check my guitar ground. Checked the continuity between ground on the cable (plugged into the guitar only) and pretty much every metal part on the guitar. It all was good.

Let the amp sit unplugged all night and all morning (let the caps all discharge) and then opened it up to check the ground at the input plug. That connection was good. I plugged in the guitar and checked guitar metal parts to the amp chassis for connectivity. That was good also.

I visually inspected all of the big caps and didn't see any bulging (not really expected as the amp is only 5 years old).

Running out of ideas, I decided to replace the 1 failed blue lamp and then put the chassis back in the head. I borrowed a neighbor's Squire Strat for another guitar. I tested that amp and....no buzz...with my guitar (or his). I'm happy, but unsure because I didn't fix anything other than the lamp. Could it have been the lamp? One wire had pulled out of one side of the lamp. I put in a new lamp.

Just made a really dumb post through looking at the wrong schematic! Ignore it anyone who saw it!!!

Which of the Blue LEDs did you find a fault with? I can't see any reason why that would cause the problem but if you had say a loose wire from one which was shorting to ground then it could pull the power line down and that may have an impact on other circuitry that works from that same line.

And glad to see you have a healthy attitude towards those caps! You're supposed to change them out every year or two nowadays, (apparently ). [Just in case anyone reads that the wrong way I was being sarcastic there with that piece of knowledge!!!!]

They aren't LED lamps, they are cathode fluoride lamps. There are two and are each powered by their own driver boards, which are mounted on each side of the chassis.

It doesn't seem likely that this was the problem, but I can't explain how the buzz went away.

Thanks for all of your suggestions.

Gotcha with the lamps, hadn't realised they were fluorescents. They don't show on the schematic that I have for the Mk1, (I guess that would be different to your own version too), but it doesn't seem too obvious an answer unless there is some sort of RFI issue with the state the light and driver were left in when it went. It is possible but I can't see it.

Anyway, good to hear it's gone now. Let's hope that was the fix. Make sure to keep us up to date with a sit rep in a week or two as to whether it stays ok. Good luck.

Well, it is still good. The amp is working well.

I wish I really knew what changed to "fix" it, but at least now, I just get to play it.

I'm really liking this amp.

Thanks for all of your help.

-tom

No problem Tom, you're more than welcome. Yes, it does leave an "unfinished" taste in the mouth when it fixes itself and you don't know why. Don't worry though, that situation is more common than amp techs will ever let you know! Hope it continues to give you that same enjoyment for many years. In the meantime, you know where we are.