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Problems and questions about used SE84C (Read 1012 times)
garnold
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Problems and questions about used SE84C
06/28/13 at 05:55:12
 
Hello, all!

About a month ago I bought a used SE84C, serial #472.  I've been very happy with it -- it plays beautifully on some DIY Betsy open baffles, my first foray into both DIY audio and audioholism. Smiley  So far, so good.

Quick question, here.  I've read various sources saying this amp puts out 2 watts per channel, 2.5 WPC, 3 WPC, and 5 WPC.  What is it, really?

Ok, back to my troubles...
Tonight I sat down and turned on the music, nice and low -- I needed to relax.  After just a few minutes, there was a loud *pop* from the left speaker.  Not the usual blown speaker pop, though.  I looked up at my girlfriend and asked "was that the speaker?"  She also seemed confused, but affirmed.

Knowing the SE84C probably wouldn't like being connected to a dead speaker, I turned it off and started checking the connectors.  Everything seemed fine -- solid connections, no wires shorting.  Hm.  i turned the amp on.  Nothing.  No tube glow, nothing.

Turned it all off again, disconnected everything, let it sit awhile (how big are the caps inside? Smiley ), and then checked the fuse.  Blown.

I quickly hook up a vastly inferior SS amp to sanity-check everything. Strange, that left speaker works just fine.  Maybe some kind of transient did it?

Pop the replacement fuse into the SE84C, hook it all back up, flip the power on.  There's a low, loud "zzzt!" and a blue light coming from somewhere between the base of the 5Y3GT tube and the transformer.  After a half second, it stops.  I turn the power off.  Yikes.

More checking and rechecking and no joy.  Nothing at all happens when I apply power.  The fuse seems fine, the speakers seem fine, just...nothing.

I'm heartbroken!  Since this is my first tube amp, I'm not sure who the likely suspect would be.  Nor am I sure if these are the typical signs of tube death.  What should my next steps be in diagnosis and repair?

I completely love this amp.  While I'm still a toob noob and can't quite discern a deep and wide soundstage from none at all, I can certainly say that this SS amp I've been forced to listen to for the last couple hours is not even in the same league as far as producing enjoyable, beautiful music is concerned.

Help me Obi-Wan Decknobi, you're my only hope!

-g.
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Lon
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Re: Problems and questions about used SE84C
Reply #1 - 06/28/13 at 10:50:49
 
This sounds as if it could be a blown rectifier tube. I'd try replacing it. . ..
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garnold
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Re: Problems and questions about used SE84C
Reply #2 - 06/28/13 at 15:56:42
 
That is exactly what I'm thinking.  While I'm at replacing tubes *rubs hands together* what kind of tube rolling options are available and safe on the SE84C?  It sounds like I should probably stick with the 5Y3GT 6N1P, but can I try replacing the SV83s with EL84s or 6P15Ps?

I'd love to hear of anyone's experiences tube rolling this amp.

Thanks, Lon!
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Lon
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Re: Problems and questions about used SE84C
Reply #3 - 06/28/13 at 16:00:38
 
You should be able to use EL84s on the outputs, and 6922s and 6DJ8s on the inputs. I'd stick with the 5Y3GT tube for a rectifier (it's just my opinion but for me that tube type always sounds most accurate in these amps).

I like the 6N1P tube best myself, but I seem to be in the minority on that one!
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dank
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Re: Problems and questions about used SE84C
Reply #4 - 06/29/13 at 12:09:22
 
I agree with the blown rectifier tube.

Thought I'd also state that 6N1P gets my vote for best input tube and output tube is 6P15P in one of my amps and 6P14P in the other.  They should be identical around the output stage, but one does better with 6P15P and the other with 6P14P, I have no idea why.

Power output is tough to say.  To get the actual power output you need to consider the output tubes (type and age), the output transformer, and the speakers as well as the input AC voltage (120 vac in will give you more output power than 115 vac in).  To get the absolute most power delivered to an 8 ohm load you need to use output transformers that are not stock (they use to be an option -  don't know if they still are).  Then, to get an accurate number you need test equipment to measure with and, in most cases, you will use a sine wave input which means that you are testing at only one frequency which gives you a number that is fairly meaningless.  RMS power is quoted by Steve to mean "real meaningless spec".  Still, I use it to evaluate my equipment, but I'm use a resistor "dummy" load so I'm not really measuring a real system anyway.  2 or 2.5 is probably a good number to go with, its not 5.

Dan
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DirtDawg
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Re: Problems and questions about used SE84C
Reply #5 - 07/01/13 at 13:26:00
 

Interestingly, my first SE84C blew a rectifier tube within a couple of weeks of my purchase and took the fuse with it.

SCARED me! It had been a while since I had used tubes in anything without the Fender name on it.

I agree with Lon about the 5Y3GT along with stock output tubes in that the combination seems to reproduce a sound that is more neutral for most music.

I DO occasionally pleasure myself with various EL84s, however, especially when listening to overly compressed modern music, which usually seems to benefit from a little artificial "warmth" added to the soundstream.
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garnold
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Re: Problems and questions about used SE84C
Reply #6 - 07/06/13 at 21:15:29
 
Thanks for the info, all, but as usual things are more complicated than they seem...

I've purchased a new 5Y3GT and picked up a complement of other tubes for fun.  While I'm spending money, I may as well have some fun, right?

Just as a sanity check, I figured I'd try one more time before I start swapping tubes.  I put in a fresh fuse and powered up.  It works!  Wow!

After a few seconds there's a *pop!* out of the right speaker, and then it's silent.  I wait several seconds, but everything seems stable -- the left channel is working just fine, and nothing seems amiss.

I turn off the amp, and the sound slowly fades away.  I wait for a few seconds after it's gone completely quiet, then turn it on again.

Surprisingly, it comes back on!  Both left and right channels are working!  But then it starts fading away, as if I'd cut power.  Turning it off and on does nothing.  No sound, no glowing tubes, no indication of power.

Figuring maybe it's a bad output tube, I replaced both of them.  Power on...nothing.  I replaced the rectifier tube.  Power on...nothing.

Ok, so this is strange.  Seems it's *not* a tube problem, but something that comes and goes.  A capacitor that's gone bad and something about it holding charge/discharging is messing with everything?

Out of the box thinking would be appreciated, here.  Thanks.

-g.
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« Last Edit: 07/06/13 at 21:28:56 by garnold »  
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garnold
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Re: Problems and questions about used SE84C
Reply #7 - 07/07/13 at 17:09:52
 
Discovered the culprit...output tube(s).  I wasn't thinking about looking at the fuse, duh.  Replacing that and using new output tubes did the trick.  Woohoo!

-g.
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Lon
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Re: Problems and questions about used SE84C
Reply #8 - 07/07/13 at 18:10:17
 
WOOHOO indeed. Good job! Enjoy!!
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