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Valve amp heat

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Hi,

Im new to valve amps and im concerned about the amount of heat im getting from my new Ampeg V4B. It might just be my inexperience but its getting very hot at gigging volume. Even a slight burning smell. Iv checked that the power supply is correct so its not that . I have also cleaned the inside out so i know the dust is not burning.

Could i have the impedance switch set to the wrong one? Its currently at 4ohms running two 8ohm cabs in series.

Any light that could be shed would be very much appreciated.

Cheers

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Are you sure you're running them in series? It's most common to run cabs in parallel I think. It would require some rather unusual cabling to get them in series.

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How hot are we talking? Can you touch the metal of the amp afterwards?

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[quote name='BassBalls' post='56747' date='Sep 6 2007, 11:02 PM']Hi,

Im new to valve amps and im concerned about the amount of heat im getting from my new Ampeg V4B. It might just be my inexperience but its getting very hot at gigging volume. Even a slight burning smell. Iv checked that the power supply is correct so its not that . I have also cleaned the inside out so i know the dust is not burning.

Could i have the impedance switch set to the wrong one? Its currently at 4ohms running two 8ohm cabs in series.

Any light that could be shed would be very much appreciated.

Cheers[/quote]
Big valve amps do get very hot mate. If it's troubling you and the dealer says it's not overheating, get an internal fan fitted like trace Elliot did on the V6 and V8 (two fans in the V8!).

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I have an internal fan in my Trace V4, too ;0)

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Hi,

it already has a fan installed so that cools it a little. I wire my cabs like this; head, speaker cable, cab, speaker cable, cab. Is 4ohms the correct ohmage i should have it on?

It gets very hot, the valves themselves have blemishes where the heat has been on them. I didnt touch the metal of the amp but i put my hands over the front and rear vents and the heat coming from them was hot enough to cause me some concern.

Now this may be all normal with valve amps but because i have had zero experience with them, i need telling. :)

Thanks for the advice already given.

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Hmm there shouldn't be any visable hot spots on the valves themselves. If you have two speaker outputs then I would reccomend connecting a single cab to each of these rather than daisy chaining them. Ohmage isn't so much of a problem on valve amps and it should drive a 4 or 8 ohm load without any problems, but if your cabs are rated below a total of 4 ohms (i.e 4 ohms each) then you might be sucking too much juice from it. The valves might be running 'hot' because they are not biased properly, too.

Now wait for an expert to be of more help!

ped

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The speaker cabs are wired correctly, you just got the terminology wrong. "Daisy-chaining" cabs in the way you describe will actually connect them in parallel not series. The important thing, however, is that two 8 ohm cabs in parallel give you a 4 ohm load and the impedance selector on the amp should be set to 4 ohms.

Note that if you ever use just one cab you must change the impedance selector to 8 ohms.

The only potental drawback of daisy-chaining the cabs is that all of the power goes through the first cable. If thats a good quality heavy cable then that should be no problem. If you have two speaker outputs on the amp and wire one output to each cab then the power is split equally between the two leads. Again, this shouldn't be an issue with good, heavy speaker leads.

Valve amps do run hot. Exactly how hot depends on how hard they are driven, how well they are cooled, and how hot the ambient temperature.

You should check that the fan is working ok.

If the valves are not biased correctly this could lead to them running hotter than normal. I'm not familiar enough with Ampeg amps to know if this is likely to be a potential problem.

If the amp is new then the "slght burning smell" might just be things settling down.

Did you buy it from a shop ? If so I'd ask them about it just to check that it's ok. You could also contact Ampeg directly and ask them if they think there might be anything worth checking out.

They have a [url="http://ampeg.com/support.html"]Support Page[/url].

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Technically if the amp was designed without a fan it it shouldn't 'need' the fan to cool it BUT if one has been retro-fitted into the amp are you sure this has been done correctly. I'm no expert on things 'lectricky' but has it been wired in correctly. The other thing to consider, if it is fitted correctly is it exhausting from the casing adequately, not much point having a fan in there if the heat isn't being dissipitated!

EDIT

I may have misread or misinterpretted the fan thing. I assumed it was a retrofit; is it original Equipment?

Edited by warwickhunt

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If you've got spots on the valves, it sounds like they could be running too hot. Solution: take it to a tech and get it rebiased (as suggested above).

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Any chance you could take a picture of the output valves, so we can see this 'blemishing'? You don't need to have the amp switched on or anything: just take the back off and point a camera at them.

If it's just that the printing on the glass has changed colour, don't worry - that's quite normal with some brands.

Edited by Oxblood

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Here is a pic of the blemishes on the valves.

[attachment=2152:Photo_0316.jpg]

Im sure the fan is working. The impedance slider is also set to 4ohms too. Ill need to get another cable to have the speakers comming from a different output.

Thanks again for all the advice!

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[quote name='BOD2' post='56870' date='Sep 7 2007, 10:49 AM']The only potental drawback of daisy-chaining the cabs is that all of the power goes through the first cable. If thats a good quality heavy cable then that should be no problem.[/quote]

For the short runs and low power in bass amps the cable gauge barely matters, especially with the low power output of valve heads. Despite that I'd rather run from head to cab than daisy chain, it just appeals more to me - also if one lead starts acting up it doesn't affect both cabs.

Alex

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[quote name='BassBalls' post='57101' date='Sep 7 2007, 05:11 PM']Here is a pic of the blemishes on the valves.

[attachment=2152:Photo_0316.jpg][/quote]

Phew! Breathe a sigh of relief, BB. Those aren't blemishes: they're "gettering" - a perfectly normal metallic deposit that is formed when the valves are manufactured. In the majority of valves, you see this deposit at the top end of the valve envelope, but not always. With your valves, it's on the sides of the envelope instead.

What's gettering (I hear you cry) ?
When a valve is manufactured, it's essential to get as pure a vacuum as possible inside the glass envelope. They pump out as much air as they can, but even the most powerful vacuum pump can't get rid of every last molecule of the gases that make up our atmosphere (nitrogen, oxygen, CO2 etc.), so inside the valve, they coat an electrode with a small amount of a barium compound in powder form. Once the glass envelope is sealed, they then pass a large current through this electrode and the barium compound explodes like flashbulb, throwing a thin coating of molten barium metal onto the inside of the glass. Barium is a 'hungry' element: it grabs hold of any molecules of gas that might be left inside there and reacts with them to form other harmless compounds. In this way, the gas molecules are 'eaten up' by the gettering, so they can't pollute the vacuum of the valve.

Edited by Oxblood

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my 70s V4B was ht enough for you not to want your hand on the back for 5seconds. Could someone clarify whether daisy chaining or going straight from both speaker inputs of the head is best please..... with impedence selector on 4ohms of course. Cheers

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[quote name='fusionbassist1' post='57148' date='Sep 7 2007, 06:51 PM']my 70s V4B was ht enough for you not to want your hand on the back for 5seconds. Could someone clarify whether daisy chaining or going straight from both speaker inputs of the head is best please..... with impedence selector on 4ohms of course. Cheers[/quote]

[b]Best[/b], doesn't really come into it. I realise several people have kind of answered your question above and they've said either/or is OK but that is the case; there is no best both will work!

To put your mind at ease if you have a pair of 8ohm cabs and your amp is set to 4ohms, with 2 outputs from your amp then I think you'll find that most people will run one output to each cab. As Alex said if the lead going to your 1st cab acts up then the 2nd cab isn't going to be achieving it's full potential or may get no signal, added to which if the 1st cab develops an internal wiring fault you'll be pulling your hair out trying to figure out what is wrong with cab 2.

In summary; 2 leads (heavy duty speaker cable NOT screened guitar leads) out of your amp one going to each cab!

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[quote name='Oxblood' post='57138' date='Sep 7 2007, 06:42 PM']Phew! Breathe a sigh of relief, BB. Those aren't blemishes: they're "gettering" - a perfectly normal metallic deposit that is formed when the valves are manufactured. In the majority of valves, you see this deposit at the top end of the valve envelope, but not always. With your valves, it's on the sides of the envelope instead.

What's gettering (I hear you cry) ?
When a valve is manufactured, it's essential to get as pure a vacuum as possible inside the glass envelope. They pump out as much air as they can, but even the most powerful vacuum pump can't get rid of every last molecule of the gases that make up our atmosphere (nitrogen, oxygen, CO2 etc.), so inside the valve, they coat an electrode with a small amount of a barium compound in powder form. Once the glass envelope is sealed, they then pass a large current through this electrode and the barium compound explodes like flashbulb, throwing a thin coating of molten barium metal onto the inside of the glass. Barium is a 'hungry' element: it grabs hold of any molecules of gas that might be left inside there and reacts with them to form other harmless compounds. In this way, the gas molecules are 'eaten up' by the gettering, so they can't pollute the vacuum of the valve.[/quote]

Wow, I love your posts Oxblood. Always so informative.

A slight digression from the OP, but are two leads really better than daisy chaining? If a lead fails when daisy chaining you risk losing the sound out of both the cabs, but if a lead fails with two separate outputs doesn't that mean you could be using a single 8 ohm cab with the amp set to 4 ohms? I know using too low a cab impedance with a valve amp can be pretty disastrous, but there was some discussion on one of the older forums that using a valve amp into a higher load than it was set to didn't do it much good either. Or isn't that quite as bad?

It's just that I'd prefer to abandon a song half way through to sort out a lead rather than abandon a set and kiss goodbye to a nice valve amp at the same time. Or am I just misunderstanding what would happen if a lead fails and the dangers of using a valve amp into too high a load?

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[quote name='Musky' post='57270' date='Sep 8 2007, 02:21 AM']Wow, I love your posts Oxblood. Always so informative.

A slight digression from the OP, but are two leads really better than daisy chaining? If a lead fails when daisy chaining you risk losing the sound out of both the cabs, but if a lead fails with two separate outputs doesn't that mean you could be using a single 8 ohm cab with the amp set to 4 ohms? I know using too low a cab impedance with a valve amp can be pretty disastrous, but there was some discussion on one of the older forums that using a valve amp into a higher load than it was set to didn't do it much good either. Or isn't that quite as bad?

It's just that I'd prefer to abandon a song half way through to sort out a lead rather than abandon a set and kiss goodbye to a nice valve amp at the same time. Or am I just misunderstanding what would happen if a lead fails and the dangers of using a valve amp into too high a load?[/quote]

Unless the physics are different for valve amps (jump in valve experts), in the event of a lead failure to one cab (where you have to outputs going to two separate cabs) there would be no chance of damaging the amp running one 8 ohm cab if the amp were selected to run at 4 ohm. You would have a drop in volume but no damage would ensue. However, that isn't going to happen as we all use quality cables and check them periodically [sound of steps disappearing into the distance and the shuffling of gear as the gig case is retrieved].

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[quote name='warwickhunt' post='57285' date='Sep 8 2007, 08:23 AM']Unless the physics are different for valve amps (jump in valve experts), in the event of a lead failure to one cab (where you have to outputs going to two separate cabs) there would be no chance of damaging the amp running one 8 ohm cab if the amp were selected to run at 4 ohm. You would have a drop in volume but no damage would ensue. However, that isn't going to happen as we all use quality cables and check them periodically [sound of steps disappearing into the distance and the shuffling of gear as the gig case is retrieved].[/quote]
The main liability with valve amps is when the speaker blows open or short circuit (or some idiot disconnects the speaker) when it's being played through. Goodbye output transformer and also usually the output valves very quickly! Very expensive too.
In your example above, all should be ok, but not ideal!
The usual problem at a gig is that an old jack plug on a speaker lead goes short or open circuit. Again, bye bye valve output transformer/output valves. If your amp and cabs have XLRs for speaker connections, always, always use them. They lock in place and have proper cable clamps to keep the speaker lead fimly in the plug. I hire out a lot of gear from my studios and they all use XLR or Speakon connectors for the speakers. Result = no problems.
Mr Gig

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[quote name='warwickhunt' post='57285' date='Sep 8 2007, 08:23 AM']Unless the physics are different for valve amps....[/quote]

Alas, they are (they bloody would be, wouldn't they!)

As we all know, with a solid state amp, a higher speaker impedance merely results in less power ouput. But for an amp with a valve output stage, correct impedance matching is important.

Power transistors are marvellously efficient devices. With their very low output impedance, they can deliver current by the bucketload, so they can be connected pretty much directly to the speaker (they only need a capacitor in the way to block the DC). Valves, on the other hand, have very high output impedance, and are awfully inefficient. They suck lots of power from the power supply, but even under ideal conditions only manage to convert maybe 50% of it into power that can be used (via the 'gearbox' of the output transformer) to drive the speakers. The other 50% is thrown away as heat.

Making power valves work into a higher impedance than they're designed for (as when running an 8 Ohm speaker off a 4 Ohm tap) makes them even less efficient. Result: less of the power gets to the speaker, and more is turned into heat.

You can see where this is heading, can't you? Those output valves are now running hotter than ever - hotter than is healthy for them. The results may not be immediately catastrophic: if you're lucky, you might get away with it for quite a while. It will depend on how close to their working limits the amp designer has chosen to run them in the first place (and most designers of high-powered instrument amps like to make them work HARD). At best, though, their lifespan will be shortened.

Now what about the other scenario: where you have your cabs chained,and the speaker cable to the first cab fails. This is the worst case of all. There will now be no load on the amp's output at all (i.e. an infinitely high impedance). If this happens you'd better hope that the amp designer has built in a protection circuit of some sort that cuts off the juice. With no load to dump power into and no protection circuit to save the day, the valves will produce enormous signal voltages which will damage the (very expensive) output transformer, and eventually the anodes of the valves will go into meltdown! :huh: :huh: :huh: :):(:huh:

ADDED LATER:
Re. the opposite scenario - short-circuit of the speaker output: This is fatal for solid state amps, but it does no harm at all to a valve amp.

Edited by Oxblood

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[quote name='Oxblood' post='57345' date='Sep 8 2007, 12:25 PM']Now what about the other scenario: where you have your cabs chained,and the speaker cable to the first cab fails. This is the worst case of all. There will now be no load on the amp's output at all (i.e. an infinitely high impedance). If this happens you'd better hope that the amp designer has built in a protection circuit of some sort that cuts off the juice. With no load to dump power into and no protection circuit to save the day, the valves will produce enormous signal voltages which will damage the (very expensive) output transformer, and eventually the anodes of the valves will go into meltdown! :huh: :huh: :huh: :):(:huh:[/quote]

I stand corrected and have learned something new (for which I am grateful).

In which case there is a 'best' way to connect the speaker leads... that is as was suggested; 2 outputs from the amp each running to a separate cab.

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[quote name='warwickhunt' post='57349' date='Sep 8 2007, 12:44 PM']I stand corrected and have learned something new (for which I am grateful).

In which case there is a 'best' way to connect the speaker leads... that is as was suggested; 2 outputs from the amp each running to a separate cab.[/quote]

Thanx very much everyone. Heavy duty Speakon connectors are the way forward then i take it. Could anyone suggest a log lasting Speakon cable then please.

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