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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 4:35 am
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+1 Stratele, That was my question, as far as BJ is hoooot. May be out of spec grill voltage that has to be checked otherwise in a couple of week: new valves even worse ... ! May be a bad set of valves ??? (I've 1 new JJ EL84 that makes poping noise at start on my vox)

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 5:01 am
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Right Tis-san,

Whe I talk about biasing power tube, I prefered to talk about Kathode current, not grill voltage.

The same grill voltage could be good for one set of tube ( the right Kathode current) and could be too hot or cold for another set of tubes.

I'm shure you know that the grill voltage is the way to obtain the Kathode current. But the goal IS the kathode current, not the grill voltage.

All schematics write only grill voltage, but their use is for electronics technician who know how to use this information. Not as only AND final adjustments.

Unjoy the sun of Paris, in Québec province ; rain,rain.....

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 5:27 am
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tele_head wrote:
An update...

Picked up a set of new Groove Tubes EL84-S matched tubes. Got them in, and the amp sounds good as news. I just have a sneaking suspicion that this might happen again...

Thanks all for your info so far!


Blues Junior is a fixed bias amp. You need to check the bias when swapping tubes. Unless you have the tools and knowledge to do this, let a tech do this as amps have very high voltages inside.

Do Fender tubes come with ratings? If so, then maybe try tubes of the same rating as the original or even go down a notch or two cooler. But really, you need to know the hard numbers (plate voltage and cathode current or plate current).


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 6:33 am
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I still recommend checking the bias every time you change power tubes in any fixed bias amp. It doesn't hurt anything especially if you keep having them fail on you. Different tube manufacturers vary in build quality and can handle voltages and abuse better than others.

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 7:04 am
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Definitely get the bias checked.
Just wondering if it could also be a tube socket needs re-tensioning, maybe the grid is losing bias supply due to poor a pin contact and the current has totally run away on a couple of occasions and cooked the tubes with a pop.

Snowy


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 8:22 am
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Wow, That is smoking hot! Mine was idling at around 13 watts per tube, only about 10% higher than the max Plate dissipation rating of the EL84, but anything over the max rating is beyond ridiculous. I hope that they fix it for you, and I'm especially interested to hear how they deal with the hot bias problem (if at all). Let us know.

BTW, I also chose the GT EL84 S, great tubes. :)

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 10:17 am
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shimmilou wrote:
Wow, That is smoking hot! Mine was idling at around 13 watts per tube, only about 10% higher than the max Plate dissipation rating of the EL84, but anything over the max rating is beyond ridiculous. I hope that they fix it for you, and I'm especially interested to hear how they deal with the hot bias problem (if at all). Let us know.

BTW, I also chose the GT EL84 S, great tubes. :)


What are the tonal/behavioral differences between the GT EL84-R and GT EL84-S?


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 11:55 am
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Fully right Stratele, P(plate) = I(plate) * V(Plate) and I(plate) = f(VGK)

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 12:31 pm
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Rossfloss wrote:
...What are the tonal/behavioral differences between the GT EL84-R and GT EL84-S?


Hmmm, I would describe the S as a much more strident, crisper, clearer, richer and louder sounding tube than the R. The S still retains some warmth but doesn't breakup as easily as the R. I like my grit from the preamp anyway (or pre-preamp), so they were great for my application. Time will tell if the S is as rugged as the R, but so far, so good. The R is obviously Russian made (Sovtek?), and the S is made in the JJ factory although it is different from the JJ tubes. Unfortunately, they haven't released the specs for the S, so who knows the ratings.

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 1:20 pm
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Are the Groove Tubes still color coded? And if so can't you just get the same color and not have to wring your hands over the bias?


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 1:34 pm
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In my opinion, you have to check the bias even if you use the same Groove Tube color code. No garantee the production is the same spec/ quality /material.


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 1:55 pm
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candycoke09 wrote:
Are the Groove Tubes still color coded? And if so can't you just get the same color and not have to wring your hands over the bias?


That's the idea, and is true especially with GTs, no problems, although they will vary slightly. For example, the red label can be an 8, 9, or 10, and if you replace a red 10 with a red 8 the bias will be just a little different but should still be reasonably close. But, as stratele52 and others have pointed out, the best way to be sure is to check it. The new style GT labels now are different, a paper label instead of the older style "printed-on" label. The new label just has the color written on it along with the number instead of the label actually being the color.

Image

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Wed May 25, 2011 11:13 am
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so I'm going to definitely need to get my amp rebiased if I switch from GT-EL84-R's to GT-EL84-S's?


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Wed May 25, 2011 12:03 pm
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Like the others have pointed out, the only way to be sure about the bias is to check (before and after tube change for comparison). But the BJr doesn't have an adjustment pot and would require some modification to alter the bias circuit in order to change the setting (involving pulling the main PCB). It seems that most people just change the tubes and go, without checking the bias. Personally, I would put in a pot for adjustment, but it's up to the user I suppose, as stock the BJr is likely running a very hot bias no matter which tubes are used. :idea:

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Wed May 25, 2011 12:31 pm
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shimmilou wrote:
Like the others have pointed out, the only way to be sure about the bias is to check (before and after tube change for comparison). But the BJr doesn't have an adjustment pot and would require some modification to alter the bias circuit in order to change the setting (involving pulling the main PCB). It seems that most people just change the tubes and go, without checking the bias. Personally, I would put in a pot for adjustment, but it's up to the user I suppose, as stock the BJr is likely running a very hot bias no matter which tubes are used. :idea:


Well the R's I have are rated 7 and I was thinking about just going cleaner since I like adding dirt earlier like yourself, so I might just get S's rated at 10.
My question is since they would be cleaner power tubes would that mean that holding the bias constant they wouldn't get as hot as the R's I have in now?


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