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V3 Tube Problem? Oh my what did I do???

 
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V3 Tube Problem? Oh my what did I do???
 PostPosted: Mon May 30, 2011 11:01 pm Reply with quote  
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  TheWayTimMoves
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Ok so I have a problem and I decided I'd take it up with you guys before I do something I regret. I have a V3 head and I was playing it through a Carvin 412 with no problem but a few days later when I went to go turn it on, my tubes were blinking blue really fast and the head was making a horrible buzzing sound.

I play my V3 with it on the 50w switch and only the middle 2 power tubes were doing this crazy thing. I was looking over the head and cab when I realized that the head was set to 8 ohm and my cabinet was set to the 4 ohm (stereo) load. I have no idea how my cabinet switched itself but it has me worried now. I was gonna buy a set of new tubes and hopefully that will solve my problem but I hope my head isn't jacked up. I have been searching on the internet and I can't really find anything other than people saying to change the tubes.

Should I first just change the tubes and see what happens or what? Wall
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Mon May 30, 2011 11:19 pm Reply with quote  
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  NCdan
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Change tubes and see if that fixes things.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Sun Jun 05, 2011 3:56 pm Reply with quote  
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  TheWayTimMoves
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So I changed all my tubes and I still have the problem. Does anybody have any advice? I'm going crazy over here hoping my amp isn't totally destroyed.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Mon Jun 06, 2011 12:14 pm Reply with quote  
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  Jack The Ripper
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You might of fried the output transformer!
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Re: re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Mon Jun 06, 2011 2:33 pm Reply with quote  
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  guccieng
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Jack The Ripper wrote:
You might of fried the output transformer!
i'd suspect the OT, too. how long and how hard were you pushing the amp with the impedance mis-match? the OT isn't exactly an easy (or cheap) fix, but i doubt the whole amp is destroyed.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Tue Jun 07, 2011 12:21 pm Reply with quote  
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  TheWayTimMoves
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Thanks guys for all your suggestions and I'm happy to say it's working 100% now. I took it to an amp tech and he seriously just plugged it in and it turned on perfectly. I asked him what he did and he said, "I didn't do anything, it works great." I felt ashamed but he said it's happened before and sometimes the bumpy car ride will put the amp in it's place. He even complimented me on the amp before I left.

So I guess the moral of the story is: Before you go torture yourself over a broken amp, drive over some speed bumps. Mr. Green
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Wed Jun 29, 2011 9:54 am Reply with quote  
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  jcksn1234
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It was your speaker cable.

I use an attenuator with my v3. I had a george l speaker cable going from amp to attenuator, and just an old regular one into the cab. I had the same flickering tubes and the sound was almost like a tremolo. i trouble shot everything from new tubes, asking on the forum, and power cables. just checked from head to cab with the ordinary speaker cable and it's a bingo!

I recut the george l and everything worked fine for a long time. eventually it happened again and i just switched the position of the cables. no problem since and that was a few years ago.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Wed Jun 29, 2011 8:47 pm Reply with quote  
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  whenheavencomesdown
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Funny thing is that I had the exact same thing happen this weekend to my V3m, It seems to be one tube in particular. Anyway, Carvin was really good about sending out a new set of power tubes under warranty really quickly. Haven't got them yet and won't know if my problem is solved until then. I too am hoping that nothing else suffered any damage. I'm glad it worked out for you and good to know that you know why it happened. I don't have an impedance mismatch issue but I've never had a tube go out like that before.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Thu Jun 30, 2011 5:28 pm Reply with quote  
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  MaestroAL
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Perhaps some of you can weigh in as to specific reasons for this, but in carvins FAQ section for tube amps, there's a question about running yellow jackets in a legacy, and which two of the four tubes are going to be used in that case.

Carvin's reply is:

The middle 2 tubes are used in 50W mode.
Also, you should switch the Impedance switch:
If you run 50w mode, set the impedance switch to the next LOWER setting.

For example, if you are running an 8ohm speaker in 50W mode, use the 4ohm setting.


They say "you should", as if it's not crucial, but more healthy for the amp. I guess with only half the tubes, the amp's output imedance setting is halved also (?), so your speaker's impedance should then be doubled. So for a 16ohm cab you'd use the 8 ohm setting on the amp. That means that a 4ohm cab should only be run at 100 watt mode, as there's no 2 ohm setting on the head, to use it in 50w.

Now, running at 8 ohm into a 4 ohm cab is already bad, but it seems that at 50w you would double the detrimental effects, if the above is true.

Of course the question was regarding a legacy, but I think the output topology is the same in the V3 as well.

Can anyone confirm/correct me on this?
(btw, I found this bit of info only in the FAQ, not in any of v3 or legacy manuals)
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Thu Jun 30, 2011 7:45 pm Reply with quote  
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  guccieng
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it is crucial. it's kinda like saying 'you should look both ways before crossing the street'. sure you cross the street without looking, but it sure ain't a good idea!
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Thu Jun 30, 2011 8:48 pm Reply with quote  
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  MaestroAL
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guccieng,

so, do you know the electrical reasons behind that suggestion?
And if this is the same on a v3 as well as the legacy?

Seems that if it was so important, Carvin would state so in the manuals. But they do not. They just tell you that for earlier tube clipping, you can run it in 50 watts, while the volume will only reduce about 3db. There's not a single word about changing impedance when using 50w.
Surely, it cannot be assumed as "common knowledge". Moreover, I'm sure that such details are very much design-specific.
Sure, it might apply across the board to any push-pull design using 4 or more tubes, but most players are not electronics specialists, and obviously cannot be expected to arrive at such conclusions on their own. At best, you'd hope they'll read the manual (though half of us probably won't even do that - yeah, like you buy a screaming new head, and first sit down and read the papers). But if the manual doesn't include that to begin with, one assumes there's nothing to worry about.

Now, I'm not yet an expert on either guitars or amps, but out of these five years that I began playing, I've just now "stumbled" on this half-power/impedance bit.
I need to find out more.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Thu Jun 30, 2011 9:11 pm Reply with quote  
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  guccieng
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if you are using a switch on the amp to go from 100/50/25 or something like that, then you do not change impedance settings. if you remove tubes, you will have to change impedance settings. 4 tubes push harder than 2. if you remove 2 of them, you have to 'trick' the output transformer into making up for it by numerically halving the impedance. for example, 4 ohms is easy to push and 16 ohms is harder to push. if you have the amp set at 8 ohms trying to push 16 ohms, you create heat in the output tranny, and it will eventually fail (the harder you push it, the quicker it will fail). by removing 2 of the 4 tubes, you just took away half of the amps ability to push. the amp's impedance setting needs to be adjusted to 'easier' by going from 16 to 8 or 8 to 4 (and you're right: don't use a 4 ohm speaker with pulled power tubes).
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Thu Jun 30, 2011 9:12 pm Reply with quote  
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  Delta362
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It really depends on how the power reduction is designed. The X100B reissue changes the phase inverter circuit for the 100/50/25W design so no power tubes should be pulled with that amp when changing the output power switch.

For the others, you can move the switch to 50W and not worry about it as Carvin amps have beefy trannies to handle that situation. However, to be impedance correct, you should do both. Not doing so will not harm your Carvin amp.
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Fri Jul 01, 2011 5:10 am Reply with quote  
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  MaestroAL
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ah, thank you.
I think that's the clincher:

Physically pulling the outside tubes to create a 50w situation is different than just flipping the switch to the 50w mode, correct?

So no need to half the impedance setting on the amp if all 4 tubes are in, even though only two provide the push?
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Fri Jul 01, 2011 12:01 pm Reply with quote  
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  guccieng
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exactly!
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re: V3 Tube Problem? Oh my what did I do???
 PostPosted: Fri Jul 01, 2011 8:41 pm Reply with quote  
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  Barnacle Bob
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It makes sense to adjust the output impedance, because it will yield maximum power. An impedance mismatch causes the saturation voltage of the tubes to rise, or increases the max plate voltage when the tubes are off (because the other tubes in the push-pull pairs are driving). I've got a post here somewhere that shows the effects of 2:1 mismatches, and the effects aren't severe.

Load 1/2 optimal:



Load twice optimal:



You get into much more trouble with no load or a load short circuit, but even those pale in comparison to an intermittent speaker connection, where a substantial current is set up in the transformer, the speaker is momentarily disconnected, the current has no place to go, and the tubes encounter a big back-drive voltage spike.

Open load:



Shorted load:




Based on another old thread here, Carvin "officially" doesn't require a change on the impedance switch on the amps with 2 tube/4 tube switching (Legacy, ValveMaster, etc.). Flipping the impedance switch with the amp running is much worse than running with a 2:1 mismatch.

Power stages are designed with the speaker modelled as a resistor. It isn't a resistor:



In fact, the speaker impedance, at least in an open-backed cab, varies by at least a factor of x8.

So it's best to set the impedance switch to a lower impedance (with the tubes at least on standby) when you remove half the tubes, if you don't have a reason not to, or if you're not sure that your tubes are well matched and haven't drifted, or if you've cranked the bias way up.

London power labels their impedance switch as a tone switch.
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