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Old 22nd October 2005, 04:16 AM   #1
buliwyf is offline buliwyf  Philippines
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Default help! EL84's dying

the normal glow fades away, making a feedback like sound (coming from speakers), dies and then immediately lights up again, but continously to a bright orange bright glow.

one tube is microphonic.

my amp's got 4 EL84's and this problem only happens to 2 tubes (which are beside each other).

before this, i started to blow fuses.

do you think this is simply tubes going bad?


oh btw, this is a guitar amp. class-a 30w, 3xecc83 preamp, 4xel84 power, combo with 1x12 speaker. i suspect i rattled the tubes to death by playing at really high volumes on-stage? or could it be another problem other than the tubes?

thanks in advance
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Old 22nd October 2005, 07:02 AM   #2
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Sounds more like a problem with the power xfmr. It could have an intermittent short circuit that occurs when it warms up. That would explain the blown fuses, and the loss of heater power, and the feedback when the Vpp goes too low.
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Old 22nd October 2005, 07:06 AM   #3
Enzo is offline Enzo  United States
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What amp, Classic 30? SOmething else?

EL84s are not sturdy tubes and they don't last a long time. When they shake themselves loose inside - and they eventually do - things short to other things. In an EL84 amp, when fuses blow, 98% of the time it is a bad power tube. If a power tube was microphonic that is a bad sign for that tube.
In an amp with four EL84, there are two tubes for each phase of the push/pull. They are wired together. if one shorts out the bias supply, then both tubes of that side lose bias. SO one bad tube can affect the others.

Try a new set of EL84.
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Old 22nd October 2005, 01:26 PM   #4
buliwyf is offline buliwyf  Philippines
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its a Laney VC30, a Vox AC30 clone.

with your explanations, i now see why the other power tube, though still not microphonic and seems to be ok, also glowed like the microphonic/bad tube and became bad too.

im afraid to try a new set of tubes, as the amp might wreck them the minute i power it up ($). is this going to be the scenario (blowing new tubes) if Miles Prower's post is true?

i play it on a stage with lethal volumes (earplug zone). very small stage made up of wood, with a 450w amp driven 4x10 bass cab, and several guitar amps and monitors running full blast. is there any way i can minimize the tubes from getting rattled by the sound (from other sources and from its own speaker)?

currently the tubes installed are 4 Sovtek EL84's for the power section, 2 ECC83's branded as Ruby, and another ecc83 without any brand (i assume its a china tube?)
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Old 23rd October 2005, 01:25 AM   #5
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I would also tend to believe that the most likely problem is a bad tube, which likely led to at least one more bad tube. Another possibility is loss of bias due to a bad cap.

I find that a thick blanket like the kind professional furniture movers use under the amp helps isolate the amp from a vibrating stage.
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Old 23rd October 2005, 06:31 PM   #6
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Quote:
Originally posted by Enzo
What amp, Classic 30? SOmething else?

EL84s are not sturdy tubes and they don't last a long time.

hey that's cruel.....tre's loads on the market used in various play boxes.......I use'em for trumpet practice 6x p--p (45W) and never h'd a problem.

richj
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