POWER TUBES / AMP REPAIR??? help...Hughes and Kettner

I turned on my hughes and kettner triamp today and one of the power tubes lights up BRIGHT purple and makes a continuous, loud farting sound out of the cabinet. It sounds like it's going to explode, and looks like it too; not the outlet, I'm hoping it's just the tube and not the amp circuitry; It's like there's an electrical short or something, but does anyone know of this, or has anyone had this happen? please help...thanks

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tallguynathan wrote:
I turned on my hughes and kettner triamp today and one of the power tubes lights up BRIGHT purple and makes a continuous, loud farting sound out of the cabinet. It sounds like it's going to explode, and looks like it too;

I know how it feels, im sometimes like that after an evening in our local Bombay Raj outlet. (toilet roll in the fridge for the morning)

have you tried swapping the tubes out for a new set? how long have you had it? sometimes output valves go short circuit, you can get matched sets quite easily on the internet, worn out ones usualy go microphonic, they whistle and crackle, not heard of the 'farting' noise though.

Nathan,

Is it a 6L6 powered triamp? It sounds like one at least has gone. Lee gives a good description of bad preamp tube noise (from a 12AX7 or ECC83 etc) but your descn is much more like power tube failure.

When power tubes go pphhhhtt, I really strongly suggest that the amp is looked over by a decent tech. They don't charge all that much for the job they do. Incidentally, this job involves saving you from getting killed. If that doesn't convince you, plonking in replacements from eBay and switching on again can blow more components like transformers etc.

More info on request, but put down the screwdriver unless you're familiar with tube amps. Sorry to sound so patronizing, but there are some horrible things in there to catch out the unwary.

you could also try the old 'pull 2 tubes' trick, pull out the 2 outside power valves and then try it again, this knocks your output wattage down to 1/2 power, but its a good way to test for blown or 'micy' valves, i wouldnt recommend running your amp like this for any length of time, i think it effects your Ohm output rating, i think it doubles or halves (bass come in) but ive used this test for my old AC30.
I'd have to put my disclaimer on here too...

Good point, Lee. This can be done but remember Nathan, if any of this to follow makes you "Huh?" then enlist a tech. There is nothing penis-shortening about handing a croaky amp over to a tech. Plenty of life-shortening things if you don't. If it makes you feel better, I regularly hand over a duffer to the sparkies who know what they're doing. That said, here goes...

There are four power tubes in your amp, yes? The glowing one is either on the outside or the inside of your quartet. So, either pull out the inner or outer pair to remove your glower. Put another way, you can pull out 1+4 or 2+3 but don't remove 1+2 or 3+4. As Lee says, you've now halved the output watts of the amp (but NOT the volume, that's a different story). Again, Lee is right and the impedance (ohms) to the speaker cab changes. Keeping it simple; once you've pulled two tubes:

For a 16ohm cab, set the amp to 8ohm. For a 8ohm cab, set the amp to 4ohm etc. Once you've done this, things are alright - you can check things out for hours without risking any frazzles.

If you pull the pair that contains your glower and the fartiness disappears, then you've got somewhere. After a switch-off AND THE SAFETY ADVICE BELOW, you could move this pair from the outer sockets to the inner pair (or inner to outer, depending on the first step). No glow, no fart, means the circuit's fine and you have a duff tube. If the glow and fart resurface, then that's the circuit's fault, not the tube.

SAFETY ADVICE
1) Valve amps retain enough current to kill, even after switching off.
2) This is mostly down to the filter capacitors. Big blimmin' cylinders near the valves/tubes. Their job is to take in the mains AC, store it, and pass it on without the spikes and wobbles. They're very good at this job. They can hold whopping current for a long time after switching off. In short (pardon the pun), they're the killers of amateur amp detectives.

SO DO THIS
1) Pull the amp's power at the mains, then play a zillion chords. This helps to 'drain' the caps. Fizzle-fart, fizzle-fart with descending volume = good news.
2) The caps can be drained further. It involves a crocodile clip attached to the chassis, a suitable (100K) resistor, insulating tape, and a pointy bit of metal tapping on the top of the caps.
3) Point #2 is a drag, (look up the 'bare screwdriver' method :shock: on the net) so I often wear one of those pervert's, thick, rubber gloves. Two uses: it insulates against current (very well) and it insulates against power tube heat (quite well).
4) Use your strong (eg, right) hand to do all the work and keep your weak (eg, left) hand in the back pocket of your trousers. If anything does go wrong, you'll bypass the heart. It might hurt, and give you an electrolytic "back, crack and sack" wax, but you'll live to post about it :)

Sorry to sound so scary, but I always think it better to scare an owner into the loving arms of a tech, rather than encourage someone to "open her up and have a go." Michael would be the first to say:

"Ops expressed are nothing to do with guitarsite.com and any injury resulting from... are solely the responsibility of... so don't even think about suing us..." :)

I think we can safely say that 'hughes and kettner' is on its way to an amp techie, but i think an important point here, is not to be afraid of changing out valves, they are user changeout items, they dont have that great a life and to take your amp to a techie every 2 years for a valve change is not good. I have a THD Bi-valve 30, its a 30watter with changeable preamp ant output valves, (dont try this unless you have a THD amp) i have a large selection of different types of valves that i change in and out, by the way this cannot be done with any other make of amp, so for heavens sake dont go changing out your EL34's for a quad of KT88's cos your amp WILL blow, but i regulary change out valves, i even pull them when they are hot (with oven glove).
But you must have a a healthy respect for the dangers in there too, like Bass says those large capacitors carry a lot of charge and can certainly give you a killer zap even after the thing has been unplugged for a week, but the killer stuff (bare connections) is usualy on the underside of the amp so dont go there unless you know what you are doing, but dont let this put you off changing out valves.
Bass, what the *&^% is the rubber gauntlets all about?? i cant remeber the last time i saw an electrician, amp tech, rocket scientist, or even the fuse changer in the local power station use them! except on the weekend of course.
Can you imagine going into your local B&Q and asking for a pair of those!!

Lee, you have a THD Bi-valve? I would love one of those wallet-emptiers, even if they are tricky to justify for the bass player. As you say, they are supremely set up (you might even say their raison d'etre) for quick and hassle-free change of power tube. They are not the typical, push-pull amp and are designed for experimentation. Chinese EL34 and old KT88? No problem - stick 'em in. As you say, the layout is very good - built for oven-glove tryouts.

Unfortunately, most other amps are not like this. Tubes need to be matched and, unless there's something new to me, this involves running the amp out of the box (I know about BiasRite etc). I make no apologies for erring on the scary side. If someone reads all the stuff and is confident, then go ahead and replace the power tubes and make the tweaks. If you are not quite sure, then have someone else do it.

It's simple; techs are there and charge about $40 plus tubes. With good tubes, this won't need doing again for some years depending on usage. Money EXTREMELY well spent. I worry about the guitarist with a wife and two kids who reads somewhere that they ought to pull out the glowing Sovteks and replace them with (say) JJs. Fizz-bang, one widow and two orphans because someone cheapskated on a tech or got egged on that it's easy and anyone can do it.

If you wouldn't open up your VCR while it's running, definitely don't arse about with your valve amp in the same circs. It is considerably riskier than the average electrical goods.

Oh, and to answer the "pervert's glove" Q :) ...

The simple answer is I'm not an electrician, electronics expert, etc etc. I only have three valve amps so I'm not diving in them every week like the proper techs. I can make a mistake. Just tonight, Mrs-Bass filled my head with crud and I had to edit a post because I made a basic error.

Probably not one that a day-in, day-out type would make. Seeing as it only takes one lapse in concentration, I wear a pervert's glove on my strong (left) hand and keep my weak (right) hand in my back pocket. So far, it's been overkill but that's fine with me. I may yet post the story about when I was glad I did.

Good post Bass, i suppose i cant assume that everyone has the same respect for the old 240v (or 125v for US) as i do, obviously if you are a little unsure then get your amp in to the local repair man.
The bi-valve is great amp, i spoke to Andy Marshall too (inventer of the THD range) at a yearly guitar show in Birmingham, (west midlands not Alabhama!), he's a great guy, the Seymour Duncan of the valve amp world, i rcently took the amp along to band rehearsal just to turn it up a bit, and it was awsome, our Bassman Gary who is a valve amp nut, and is not easily impressed with my purchases was blown away, i just had 2 EL34's in there. I also have a Hiwatt 100 custom from 1970, it was given to me by my old Bass player Mick who used it in 77' for playing bass in a punk band and had kept it in garages and lofts since then, i had to get that a complete set of vavles pre-amp and output, and then found it needed the big caps replacing, i took this to a techie to have done, mainly because the amp needed re-biasing too, and i didnt fancey my chances with it. I also have a VOX AC15 with the blue speaker, it's a 1999 reissue and sounds fantastic, when i got it there was an annoying buzz that only happened on a certain string on a certain fret, it took me 2 weeks to track it down to a bad bit of machining on the spring reverb casing, easily repaired with a piece of 'blue tack'. It would have cost a fortune to have an amp tech to fix that one. Thats it on the amp front, but i have had a good few in my time, i dont like Fenders, they are great amps but they just dont suit my style of music, Marshalls are great, ive had a few of those in my time, 800 range is stunning, but they are getting a bit expensive for what they are, ive been looking for a Guytone for some time, one just came up in the states on ebay but it's looking a bit expensive.

Touché with the great post, Lee. Funnelly nuff, my band's guitarist goes by the name of ... Andy Marshall. Unfortunately, he's neither the head-honcho of THD nor heir-apparent to "Father of Loud" Jim's empire.

Avoiding a hijack, I think I'll post an amp-brag thread elsewhere (I gather the Bi-valve with 1xEL34 and 1x6V6 is something else).

Back to the theme. Unlike the Bivalve, most amps are Class A/B and require a bias if the power tubes are replaced. If you're thinking, "What is a re-bias?" then I strongly suggest using a tech. The plate voltage of many an amp is ca 450V but, to paraphrase Goldie Lookin' Chain, "Volts don't kill you, current do" and some hefty current is required to shift those lazy electrons around. Please do treat them with respect. Changing preamp tubes is a piece of the proverbial, though, provided some common sense is applied.

'Common sense' ?? was'nt they that 80's band that played in a thunderstorm in the middle of a field with no cover??

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