avoriaz
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Post by avoriaz on Jul 11, 2007 18:54:16 GMT -5
I have a Marshall Valvestate 8040 40 watt amp. It works fine when initially turned on but after about 5 or 10 minutes the sound starts cutting in and out and then cuts out completely. If I turn it off and let it cool down, it works fine again for another few minutes.
I have only just acquired it so I don’t know its history. It is probably about 10 years old at least but it looks in almost new condition and has clearly not been mistreated or heavily gigged. I think it probably lived in someone’s bedroom for most of its life.
Ignoring the “No user serviceable parts inside” warning, I opened up the electronic section and I cannot see anything obviously wrong. I have used switch cleaner on all 10 pots and that has made no difference. The circuit board looks very clean and there are no obvious bad solder joints though of course these are hard to spot anyway.
Being a Valvestate, it has a single valve in the pre amp section. I know very little about valves, but this one looks a bit old and tired. It is partly blackened on the inside at the top. If I were a gambling man, I would guess that a new valve would cure the problem. The sound cuts out on both the clean channel and the boost channel, and the valve is common to both.
Does anyone agree or disagree with me or can anyone offer an alternative diagnosis?
And yes, I already know that Valvestates are considered to be rubbish amps by some people but I like it when it works. ;D
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Post by Ripper on Jul 11, 2007 23:18:26 GMT -5
avoriaz...
I too have a Valvestate ( 2000 AVT50w ) I believe the model you speak of is at least 10 years old. I had an 8080 about hmmm?...1992/93 so I know the amp you speak of.
Ill bet dollars to donuts all you need to do is change the 12Ax7 valve and shell be as good as new. I must admit the sound from my 2000 AVT Valvestate is superior to the older style from 10 years ago ( Technology has grown in leaps and bounds ) but its still a great amp you have. I remember many nights working that 8080 hard and being quite pleased!
btw...All those tube worshippers out there ( You know who you are )....If they are being honest, cannot tell the difference between new hybrid amps, and all tube. Believe me, I have fooled many!....They are adorable though arent they? ;D
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Post by sumgai on Jul 12, 2007 13:03:46 GMT -5
The Doctor says:
A new tube a year, let's you play without fear.
;D
HTH
sumgai
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avoriaz
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Post by avoriaz on Jul 13, 2007 10:01:56 GMT -5
Thanks for your replies. I will buy a new valve and hopefully that will fix the amp. I bought the amp on ebay for £40 (about $80) and the seller has said he will either take it back and refund my money or pay whatever it costs me to replace the valve so I am not losing out and I have probably got a bargain. Valves are available on ebay for well under £10 ($20) anyway. Thanks again. I am not a regular poster but it is nice to know I can get good advice when I need it.
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Post by UnklMickey on Jul 14, 2007 6:12:25 GMT -5
Replacing the tube (valve) is an obvious first step, because it is easy. Besides, you should always have spares on-hand, so it that isn't the problem, you have a ready spare for later, if it fails.
Because the problem sound thermal in nature, I suspect finding the real culprit might be difficult. If you run the amp outside of the cabinet, it will be able to "breathe" and may not achieve a high enough temperature for the culprit to fail. If this is the case, you can take a heat gun, and warm different regions of the circuit, until you find the general region where the offender is. You can further narrow this down, by placing a hot soldering iron near, but not touching components, until the one causing the problem is located.
If, on the other hand, the amp still generates enough heat to fail outside of the cabinet, you can use a cooling spray. This is available at better electronics outlets. Cool individual components, until the sound returns.
Good luck, Unk
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Post by sumgai on Jul 14, 2007 15:33:34 GMT -5
unk, Something I don't ordinarily do is to advise a player that he/she replace a tube before it's obviously bad. In this case, I do so because some, if not most, Vavlestate models have the 9-pin mini locked down with a clamp that requires a screwdriver to remove it. If things go south in the middle of a set, and you suspect it's your (valve|tube), this is not the time to be monkeying aboot in the nether regions of your amp! Exceptions are. But good point about using a heat gun/cooling spray, that's one of my favorite tricks, too. sumgai
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avoriaz
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Post by avoriaz on Jul 17, 2007 7:34:34 GMT -5
I have now replaced the valve but the problem still persists. Replacing the valve is easy. There is no screw, just a wire clip. When first turned on with the new valve, the sound was distorted, though I suppose that might be the valve needing time to warm up. After 10 to 20 seconds or so it does sound good but the sound is still drifting in and out after playing for a while. It does not cut out completely after a few minutes as it did before. It will now play fine for maybe 10 minutes but then the sound starts to fade in and out. It fades for a few seconds and then comes back on, which suggests there is some thermal issue as UnklMckey suggests. I have been playing it with the amp outside the cabinet so I assume that it will be worse inside. I am at a loss what to do next. If the problem is simple and easily fixed without spending too much money I would do that. I can easily buy another one for similar money but this example has clearly been well looked after and is in excellent condition, with the rather serious proviso that it doesn’t work properly. I think you know what I mean. A component has failed for some reason despite the previous owner looking after it carefully. The case etc is in virtually new condition, unlike most amps which have suffered from being heavily gigged and bashed around. I will try some of the above suggestions, but I have little experience of working on guitar amps. I know the basics and I can identify pots, capacitors, transformers, diodes, resistors etc but I have no idea how to trouble shoot them. There is a black finned heat sink with what I presume is the output transformer on it. This and the valve are chucking out a reasonable amount of heat but nothing I would consider exceptional. The only thing I can’t identify is a six inch long aluminium extrusion with 3 long springs, all covered by a black plastic shield. I have no idea what this does. Reverb maybe but that is a pure guess. The valve does not glow very bright, just a faint red glow. Is that normal? I am not used to valves. If you have any more suggestions I would be appreciative, otherwise I think I should maybe return it and get my money back. Thanks for reading.
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Post by sumgai on Jul 17, 2007 12:36:51 GMT -5
av, The fact that you can get your money back speaks to the quality of the seller, and you might be able to use that to your advantage. Try this: ask the seller to fix it,and the two of you split the cost. Most people would see that as fair. If the seller doesn't want to go that route, then you either need to find a technician (that you trust), or give it back. Problems like this (intermittent) are the worst kind to find and fix. But one thing you said does stand out.......... the heat sink for the power output transistors (there's no output transformer in a solid-state power amp) should not get very warm, unless you are driving it to the maximum. When you say "This [is] chucking out a reasonable amount of heat but nothing I would consider exceptional", I get to wondering. If you let the amp sit idle (on, but you're not playing through it) for a half hour or more, put your hand on that heat sink. If the heat sink is too hot for you to leave your hand there as long as you wish, then there's a problem. It should not even be uncomfortable, it should be only a few degrees warmer than the surrounding air. Your definition of exceptional is the crux here. A tube is supposed to get hot - 30 seconds after it turns on, it should be too hot to touch at all. But heat is the enemy of a transistor. The gut rule-of-thumb is the test I just mentioned. Only after you've been playing hard for quite awhile should it be too hot to touch, and even then, it should cool back down rapidly. (Aluminum's greatest feature is that it can absorb and dissipate heat rapidly, with very little physical distortion.) Unless you're a wild and crazy guy, then you should stop right here, and either take it back to the seller, or find a good tech. That may be the chicken's way out, but it's also the safe way out. And safe players get to be old players, if you get my drift. HTH sumgai
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avoriaz
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Post by avoriaz on Jul 17, 2007 13:52:37 GMT -5
Thanks Sumgai,
I did mean to write output transistor, not output transformer. As I said, I am not an expert. I am a hi-fi enthusiast so I know just enough about audio electronics to talk about it but not really enough to diagnose and fix anything much more complicated than a blown fuse or a bad connection.
You are absolutely correct about intermittent problems. I have had my share of them over the years.
I will talk to the ebay seller. He told me he was selling it on behalf of his brother. Fortunately he seemed a nice guy when I collected and he lives only a mile away from me so it is not too much hassle to return the amp.
I will consider getting a technician to look at it but the cost of that is likely to exceed the value of the amp. I don’t mind spending my own time on it as I enjoy a challenge, but I don’t want to throw too much money at the problem. I will try some of your and UnklMickey’s suggestions.
Thanks again.
PS, any idea what that 6 inch aluminium extrusion with three long springs does? I’m just curious as I have never seen anything like it before.
Avoriaz.
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Post by sumgai on Jul 17, 2007 17:30:23 GMT -5
av, Oooops, sorry, I meant to deal with that question too, last time out. Two questions back at you: 1) Where are the three springs located? Inside of a cavity? Outside? Are they used to mount the extrusion to the chassis or cabinet? (A picture would help tremdously here. ) 2) Are there two cables going from the amp's chassis out to the extrusion? sumgai
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Post by UnklMickey on Jul 17, 2007 17:44:50 GMT -5
Two questions back at you:
1) Where are the three springs located? Inside of a cavity? Outside? Are they used to mount the extrusion to the chassis or cabinet? (A picture would help tremdously here. )
2) Are there two cables going from the amp's chassis out to the extrusion? yeah, I suspect it's the reverb tank, too. ;D
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Post by sumgai on Jul 18, 2007 2:34:35 GMT -5
unk, I was gonna tell him to apply some percussive maintenance to the object in question, and see what comes out of the speaker. Then I decided to let you do the honors. ;D sumgai
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avoriaz
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Post by avoriaz on Jul 20, 2007 8:51:58 GMT -5
I am now uncertain what to do. The ebay seller has confirmed that I can return it and get my money back which is good.
However it is now working perfectly. I have put the amp back into the cabinet and run it hard for over an hour a couple of times with no problems.
I think I will give at a long run at the weekend and, if it remains problem free, I think I will take a chance and keep it. I only paid £41, about $80, for it and similar amps are selling upwards of £60 in less good physical condition.
If it fails again I will get a friend of mine who is knowledgeable about circuit boards and handy with a soldering iron to help me troubleshoot it. Don’t worry Sumgai, I’m not going to electrocute myself. I wanna be a rock and roller but I really don’t want to die a rocker’s death.
I play at home only, I don’t gig as I am not nearly good enough yet, so I don’t have to worry about an on stage failure.
I have concluded that the extrusion with 3 springs is indeed the reverb mechanism. I have read that a Marshall Spring reverb is fitted to this model and tapping the cabinet with the reverb knob on full definitely produces some interesting sounds.
Thanks again guys for all your help and suggestions. I do appreciate you taking the time to share your knowledge.
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Post by Ripper on Jul 20, 2007 9:05:08 GMT -5
avoriaz...
Im happy things worked out for you. Remember, you cant kill a Marshall!
Though sometimes you want to! ;D
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Post by sumgai on Jul 20, 2007 13:24:06 GMT -5
Uh oh, don't look now, boys and girls, but I detect a Surf Guitarist in the making! ;D
sumgai
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Post by UnklMickey on Jul 20, 2007 15:32:17 GMT -5
Kick the reverbbbbbb.
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chagzuki
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Post by chagzuki on Mar 22, 2008 15:14:08 GMT -5
Can someone explain how to replace the valve in an 8040? I just bought one second-hand and I can't see any easy point of access for the valve. There's a screwed on metal plate by the mains input that I assume is something to do with the PSU. Do I have to take out the amp section entirely?
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septos
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Post by septos on Sept 1, 2009 17:33:18 GMT -5
A common issue with this model is oxidisation of the preamp-out and poweramp-in jack sockets on the front rhs of the amp... "When no plug is inserted the contacts should close and complete the circuit internally. When these socket contacts oxidise they would never quite close cleanly or tightly enough and my sound would cut in/out for a while then invariably stay mute. I never got around to cleaning the jack sockets but simply used a 6" jack lead and looped this straight out of [preamp out] and back in to [poweramp in]. Worked a treat for me If you use effects in this same loop then fine. Try it - it might be the same issue I had. At least it was a zero cost fix for me." see www.marshallforum.com/workbench/6940-8040-valvestate-problem-volume.htmlthe tube on this model is under a very light load, so its not likely to be the culprit: www.fixya.com/support/t461135-marshall_valvestate_8040_change_valveGeneric problem for all Amplifiers and Preamps Marshall valvestate 8040 - how to change the... chagzuki on Mar 22, 2008 " " The marshall handbooks don't mention this yet it's well known that valves need replacing every so often. What needs to be removed to find where the valve is located? S OK Solution #1 posted on Mar 22, 2008 Somewhat Helpful) robotek robotek Rank: Guru Rating: 91%, 1075 votes Hi chagzuki, you are right, but only when the valves are used as outputs, You amp has transistorised outputs, and a single tube used for distortion/gain. You will get a very long life out of this tube also because its supply voltage that is is used in the valvestate preamps is really quite low. Just enough for it to work really, so the tubes life can be expected to be much longer than normal. It is also a preamp tube, not an output tube, so they rarely need changing. If you ever did want to change it(dont bother), it is located on the PCB inside the unit. You have to slide the sleeve off to get at it. Get yourself a JCM800=the best guitar amp ever built(imho), then yo got pure Marshall tone, and output tubes that need changing every 6 months or so. Now go RIFF OUT and dont worry about your valvestates' "valve"
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Post by newey on Sept 1, 2009 21:13:11 GMT -5
Septos-
Hello and Welcome!
Thanks for the tip on the Valvestates. Although this is an old post, and I suspect avoriaz has either solved the problem or returned the amp by now.
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stu
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Post by stu on Jan 14, 2017 0:52:41 GMT -5
Hi All Just inherited a 8040 with this issue and used advice from septos and used a jack lead and looped this straight out of [preamp out] and back in to [poweramp in] and problem solved for $0! Thanks
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