Valve amp gone up in smoke

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Ady

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So I turned on my 845 amp and it went through it's start up procedure, the music started playing via my streamer and then after about 2 minutes the sound went off. I was momentarily in the next room and just heard my wife shouting that the amp was smoking. I immediately turned it off via the power switch by which time the mains RCCB had cut in and cut the power off. The amp was emitting quite a lot of white smoke for a few seconds. 

So, any idea what may have caused this? My wife said when she looked up the GZ34 valve was red but the other valves were not lit up. 

I will obviously be taking back to the dealer on Monday but was interested in what may have caused this? 

Cheers Ady 

 

audio_PHIL_e

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Could the power supply circuit have failed? Not surprised if the HT fails but the supply to the valve heaters is also not working (as attested by your wife). The smoke may well have been from the electrolytic capacitors in the power supply. If the mains RCCB cut out then maybe you have a short in the power supply caused by the failing component(s). Hope you get it fixed soon, it may be a good idea to keep a spare amplifier which you can press into service at times like this.

 

Juancho

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Could the power supply circuit have failed? Not surprised if the HT fails but the supply to the valve heaters is also not working (as attested by your wife). The smoke may well have been from the electrolytic capacitors in the power supply. If the mains RCCB cut out then maybe you have a short in the power supply caused by the failing component(s). Hope you get it fixed soon, it may be a good idea to keep a spare amplifier which you can press into service at times like this.
As the man said who beat me to it!

 
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Jules_S

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So sorry to hear that Adrian, what rotten luck.

I have my spare Pioneer A-400 here I can happily courier up to you to use in the interim if you need something just to fill the gap and make some sort of noise while your Icon is being repaired. It's hardly in the same class (in any sense of the word!) but hopefully better than silence...

 

Ady

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Could the power supply circuit have failed? Not surprised if the HT fails but the supply to the valve heaters is also not working (as attested by your wife). The smoke may well have been from the electrolytic capacitors in the power supply. If the mains RCCB cut out then maybe you have a short in the power supply caused by the failing component(s). Hope you get it fixed soon, it may be a good idea to keep a spare amplifier which you can press into service at times like this.
@audio_PHIL_e many thanks for that input. Hopefully I'll get it back to the dealer on Monday. Unfortunately the spare amplifier I did have I gave to a friend 6 months ago so he could get into vinyl. 

 

Ady

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So sorry to hear that Adrian, what rotten luck.

I have my spare Pioneer A-400 here I can happily courier up to you to use in the interim if you need something just to fill the gap and make some sort of noise while your Icon is being repaired. It's hardly in the same class (in any sense of the word!) but hopefully better than silence...
@Jules_S I am really grateful for your offer which is very generous. I'm going to see if Icon will lend me something on Monday, if not I will take you up. Many thanks Ady 

 
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rabski

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Thanks for that. The word 'biggie' is a bit worrying though 🤣
But the word 'not' is more important 🤣

Without seeing it, it's impossible to guess. However, smoke and fuses may be spectacular, but are usually a straightforward fault. It's things like odd noises, distortion or channel imbalance that can rack up the time on the bench.

 

audio_PHIL_e

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I have a Vox AC30CC2X. For them that are not familiar with Vox guitar amplifiers, they have a GZ34 in the power supply, a number of ECC83s in the pre-amp, and the power stage is a set of 4 EL84s. I turned mine on (with the standby switch on standby) then after a short interval, switched from standby to on. Result was a fart in the speakers and no guitar sound. They've put the standby switch in between the GZ34 and the smoothing caps so that the inrush of current on switching from standby to on kills the GZ34. If you're not going to modify the circuit then don't use the standby switch (keep it permanently set to on). My AC30 now has the standby switch after the smoothing caps to minimise the chance of that happening again.

I know the Icon Audio amplifier isn't an AC30, but I wonder if the GZ34 (especially in modern incarnations) has a weakness that is exposed under power-up conditions?

 

Ady

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@audio_PHIL_e unfortunately I would have no idea if the GZ34 is involved however you may have a good point as on inspecting the valves afterwards it's only the GZ34  that has a dark mark around the red base although this is the first time I have ever removed it from it's socket. 

 

rabski

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There will be almost certainly some HT delay in the 845. In general, 'standby' switching on directly-heated triodes can cause issues. You don't want to leave them with heaters on and no HT for too long, as you'll do the cathode no good at all. On the other hand, you also wouldn't ideally want HT and heater applied to a cold 845. In this case, I assume the GZ34 rectifies the HT for the driver stage(s). It won't provide the HT for the 845, as the 845s will be running (or certainly should be) with 900 volts or so anodes, and that's way more than a GZ34 will take. That means like the vast majority of 845 amps, the HT for the power valves will be solid-state rectified. With no delay at all, that puts 900 volts or so on the anode of a 'cold' 845. They're actually tough as hell, but that's far from good practice.

The heat marks suggest something around the GZ, but it could simply be that's where the smoke came out. Really, without seeing the circuit or the amp, it's all guesswork. Icon are pretty decent though, so I'd not be overly concerned.

Indientally Phil, the standby on the Vox is a known nasty. The switch basically shouldn't be there at all and it's pointless. The GZ has a reasonably slow warm, so all the valves will get HT after filament current anyway. Guitarists think it's somehow good to turn the Vox to standby when it's on and not being played, which does more harm than good and can shorten the power valve life. The last place you want an HT delay is between a rectifier and the reservoir cap, for exactly the reason you've noted. Leave it on and just power the whole thing from cold. A GZ34 won't mind HT before heater for a second or two.

 
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audio_PHIL_e

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@audio_PHIL_e unfortunately I would have no idea if the GZ34 is involved however you may have a good point as on inspecting the valves afterwards it's only the GZ34  that has a dark mark around the red base although this is the first time I have ever removed it from it's socket. 
It may be that other components around the GZ43 failed, which then caused it to fail. I've known a GZ34 fail without taking out other components around it. Best wait for the official diagnosis. All the best with getting it fixed.

 
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pmcuk

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If the GZ34 is rated by Mullard standards - pretty tough - then a substitute may be over-stressed. But as said above, the 845s will have solid state rectification so if they went out the trouble might be upstream of the GZ34. When and if you get the amp back, check to see how much the GZ34 is stressed and which versions are as tough as a Mullard. 

 
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Ady

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@rabskiI  believe the GZ34 is a rectifier for the driver stages. The smoke mainly came out from the underside of the amp through the air vents but some some did come from around the GZ34 which would account for the marks like you suggested. I'll hopefully get it sorted Monday but many thanks for your knowledge and input. 

@audio_PHIL_e thanks, I'll update when I know what actually happened. 

Cheers Ady 

 

rabski

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Very much off topic Phil, but you'd like what I need to get round to. A pal dropped off a much battered AC10, which turns out to be a very, very early super-reverb twin, but in a single cab. Even the VOX showroom only has pics of it as a head unit. Probably been butchered, but certainly extra-rare. Unfortunately, mind you, definitely been butchered internally, so it's going to be a right pig.

Anyway, as to the 845... It's a valve amp. There is absolutely nothing that can't be fixed on a valve amp. Nothing.

 
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Chivas

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Reading between the lines you seem to be using an Icon Audio 845 amp. Do you know if you have the Jensen caps? If so, they have a bad tendency to go faulty and leak DC which sends the 845 valves into red plating.

Apologies for any technical nonsense, I’m not technical but speak from personal experience.
It’s happened to me twice, once I fried an output transformer, the second time only a few resistors.

The good news is that Icon Audio are very helpful and sort you out in no time, and give you a loaner amp in the meantime. David Shaw really went out of his way to sort me out

Best of luck, I hope it’s not too serious


Sent from my iPhone using Tapatalk

 

Ady

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Hi @Chivas thanks for that.  The amp is an IA 845pp integrated about 18 months old and at the time of purchase David Shaw wasn't using Jenson caps for that very reason so I had Mundorf silver/gold fitted instead. Although I'm not technically minded at all I did take off the bottom plate to see if I could see anything obviously blown inside but to my untrained eye there was nothing obvious. One small resistor was coloured slightly brown but that's all.

As you say all should be revealed tomorrow. 

Cheers Ady 

 

audio_PHIL_e

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Indientally Phil, the standby on the Vox is a known nasty. The switch basically shouldn't be there at all and it's pointless. The GZ has a reasonably slow warm, so all the valves will get HT after filament current anyway. Guitarists think it's somehow good to turn the Vox to standby when it's on and not being played, which does more harm than good and can shorten the power valve life. The last place you want an HT delay is between a rectifier and the reservoir cap, for exactly the reason you've noted. Leave it on and just power the whole thing from cold. A GZ34 won't mind HT before heater for a second or two.
It is, because its inhibits unwanted noises from the spring reverb tank if you move the amplifier and the results of (un)plugging instruments, but I agree it does do more harm than good, plus I can think of other ways to solve the unwanted noises problem but these other ways do depend on the user having more "presence of mind" than some guitarists seem to have (the standby switch is easy to use and its purpose would seem to be fairly obvious). [FWIW did you know the standby switch on a Selmer Treble'n'Bass doesn't disable the HT? It only disconnects the HT from the phase inverter leaving the rest of the HT rail live and dangerous!]