hearingisbelieving

Valve amp help please (blown/dead valve/s???)

39 posts in this topic

Hasnt been my week with audio this week. Having had problems with my active speaker recapping I thought things couldnt get much worse. Alas, my Audio Note Oto SE is now playing up.

Upon turning it on, with volume set to zero, there was a loud humming(that grew in volume as the amp warmed up) coming from the speaker/s(didnt leave the amp on long enough to check whether it was one or both channels).:nup:

I took the lid off to see if there was anything obvious but there wasnt - pushed all the valves down to make sure they were seated properly. Tried again but same result. I then tried with just one speaker connect and this time the amp started to make a noise like an old police siren:shock:. obviously i turned it off sharpish.

The good news is that my speakers are fine and my trusty ol Pioneer A400GTE is providing me with some music.

So any ideas about whats causing it? My thought was maybe a dead valve or two in one of the channels - though not sure how to test that -can i just swap them into one channel and leave the other channel without valves or is that a big nono?

Any advice gratefully received:^

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swap left to right in pairs and see if the problem shifts. Personally I'd take it to a tech as you dodged a bullet with your speakers.

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swap left to right in pairs and see if the problem shifts. Personally I'd take it to a tech as you dodged a bullet with your speakers.

I would be surprised if the amp had damaged the speakers.

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Me too. The output transformer can't pass either DC or high frequencies at very high power levels.

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Hi

As said above, 99.9% of valve amps drive the speaker via an output transformer, so only AC can pass to the speakers which will not harm them, most solid state amps do not have output transformers, and so when they go wrong can pass lots of DC through the speakers, which cooks the voice coils.

From your description it could be a power supply problem or driver valve...... try turning on with just the output valves in place

Richard

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I didnt think that through, I just had a knee jerk to when a lowther I had blew up. :roll:

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thanks guys for the input. I will try swapping the valves about tomorrow to see if i can find the culprit.

just to make sure - if i take all the power valves out of one channel(to test the remaining channel) and turn the amp on it wont harm the amp?

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Er, it might. I don't know the circuit of this amp at all but in the worst-case scenario it may be that the designers are using the current drawn by the power valves to hold the HT voltage down to a certain level. If you remove the power valves then you will not draw that current and the HT voltage will rise. The capacitors that have to withstand the HT voltage may do so when it rises :), or they may not :shock::(. I think it would be safer to swap the valves across as SMEagol suggests in post #2 i.e. don't leave ay out even though you suspect some might be defective. I'd also recommend connecting some cheap speakers (if you have access to some) for these tests and switching the amp off just as soon as you have discovered whether the fault has followed the valves or not. Obviously checking which channel the fault is in (or maybe it's in both ?) is the name of the game here.

You mentioned a humming when you switch the amp on and say that this gets louder as it warms up. Does this mean that there is some humming immediately when you switch on ?

VB

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OK, I had a hunt round on the internet and it seems this unit uses a solid state power supply. If this is simple, as the circuit diagram I found suggests it is http://www.audioasylum.com/forums/audionotekits/messages/6652.html, then the worry about over-volting the HT capacitors is somewhat reduced bacause the full, unloaded HT is applied to all the capacitors at the instant of switch-on and it's only pulled down when the valves start to warm up. If the caps can survive this big hit at switch-on then they should be able to survive for a few seconds at least while you have power valves removed. HOWEVER, if the circuit is more complex than the diagram suggests and there is some delay circuitry which holds off the HT until the valves are warm then everything I've just said doesn't apply and you're back to the situation I described in my previous post and testing with valves removed could be risky.

If there is humming immediately at switch-on then this could be due to a shorted valve, or some other shorted component. In that case the safe thing to do would be to power the HT line from a separate current-limited source. That way you could find the short without blowing anything up. But while a supply like that is fairly easy to construct (basically it's a normal HT supply with a suitable resistor in series) you're now beyond what an inexperienced person could reasonably be expected to do. Messing with high voltage is something that needs to be learned in a safe environment, not on your own and/or under stress and/or without the right kit to hand.

VB

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Hi VB thanks for the detailed reply. Just to clarify, the humming noise doesnt start immediately, it takes maybe 5 seconds to start then raises in volume as the valves come to life.

I will try swapping the valves over and see what happens. thanks.

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Had a similar issue with my Prima Luna when I returned from hols. I had turned everything off when I went away. I dropped a CD in the player, just to see if it remained when playing music and lo and behold the problem seems to have gone! Must have been some interference from the CD player (not that I know why!). Worth a try...

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I've had these symptoms in the past when one of my power valves has gone. I was easily able to tell which one it was as the internals of the valve glowed a real bright red!

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well didnt have any time to play around with the amp last night except for a quick switch on without any speakers connected to check for any bright glowing valves. The 12AU7's(ECC82) all glowed brightly briefly when the amp is powered on but then quickly fade so i think that is normal.

What I did notice is that both of the output transformers started ringing which was quite alarming! Also, when testing with my multimeter I noticed there was continuity between the pos and neg binding posts of both channels - is this normal??? :?:doh:

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I wouldn't run the amp without speakers connected if it's in a state where it can output any sound at all. It can be bad (perhaps very bad) for the output transformers and for other components too. Yes, there should be continuity between the binding posts. The DC resistance of the output transformer secondary may well be less than an ohm.

VB

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Some general advice.

Walrus who sold me a Jadis amp & later Linear A stresses that output should never operate w/o load.

Since the noise happens with vol at zero suggests it occurs in the power stages. The ringing could be due to unstable oscillation!*! Leaving out any valves or component could cause damage.

My linear A o/p xfers are isolated from each other & as well as grnd. The Jadis orchestra had both +ve o/p connected to grnd! (so if using a subwoofer using -ve as grnd- which is the logical thing to do then the o/p would get shorted).

I'd let Peter at Walrus sort it out. He's very good & not expensive labourwise. However he's pretty busy (fixing crofts to kondos). Their tel 02077247224.

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