help please

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gurusimran
Posts: 7
(1/26/04 10:33 pm)
  help please
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 Hello, I just finished a plexi 50, and then i switched on but in standby, it was ok, a small orange light were in all tubes, then I switch standby of but I heard no sound, and a blue light appears in the power tubes and then an orange light of the same color of the heaters appears but in much more quantity, I think it was too much, so I turned it off, then I tried again and it was the same, but I left it in a little more, and suddenly the strong orange light turned off, so I turned it off, and I checked the fuses, de 3 Amp was ok but the 0.5 Amp was blow.
Never hear anything from the speakers. I didn´t ckeck the bias voltage I left the trimpot with no resistance to make sure that the tubes had a -b and no zero -b.
Thanks.
Kristian.
 
GroundhogKen
Forum Moderator
Posts: 2233
(1/26/04 10:42 pm)
  Re: help please
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 Turn the amp back on. Leave it in standby mode. Measure the voltage at pin 5 of the EL34 tubes. Make sure you have at least -38 to -42 volts with respect to ground..

You can do this without replacing the .5A fuse in case you don't have a spare.

Ken
 
gurusimran
Posts: 8
(1/26/04 10:53 pm)
  help please
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 ok Ken, but I left the bias circuit after the standby switch, like the original schematic, because I wasn´t shure about that. I think I will leave it Like hoffman´s layout, before the standby switch. I will check that voltage right now.
 
gurusimran
Posts: 9
(1/26/04 11:52 pm)
  help please
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 I got around zero Volt on pin 5 in the el34, I can´t get more but after the rectifier betwen the 15K resistor and the trimpot I got around 1 Volt, i´m using the 340-0-340 power source from the`power transformer, Ithink that I will use the 60V source from the power transformer that I didn´t use, because I wasn´t shure About if I can have the -B out of the standby switch
 
Rotting27
Junior tube assistant
Posts: 94
(1/27/04 12:40 am)
  Re:
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 The REALLY bright orange is your tube experiencing "plate burn". Your're biasing it WAY too hot. Check that bias supply circuitry over and over and find out what the problem is. You should be getting negative voltage referenced to earth. Make sure your diode is "backwards" with the band being toward the AC supply side. Then make sure you've got the right resistors and all the solder joints are good and all the right connections. Tackle that first, then, if you haven't done so already, trace the entire board. Make sure you've got everything wired 100% correctly. Everyone, to a man, makes at least one little wiring mistake on their first amp.

Hope that helps.
 
gurusimran
Posts: 10
(1/28/04 12:12 am)
  help please
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 I corrected the problem, the board was underwired wrong. I blow the second 12ax7 preamp tube so I changed it. Now the pin 5 on both el34 reads -40V. So I turned the amp on.
It makes a sound, but it is like a loud low frecuency feedbak, it is like a vibration, inmediatly when I remove the standby. All the tubes heats ok. The power tubes has a blue light at the same time when the sound is produced.
the guitar sound a little in the middle of that loud sound.
Can be that I blow the power tubes in the past try?
I try to read the voltage between pin 8 and ground with a 1 ohm resistor and I got zero Volt in both power tubes.
 
HStraub
Senior tube assistant
Posts: 210
(1/28/04 12:53 am)
  Help
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 Swap the output transformer leads that are on pin 3 of the power tubes.
That is what is causing the 'loud' feedback sound(it is oscillating).
You are getting closer.....keep us posted.
Harry
 
Rotting27
Junior tube assistant
Posts: 97
(1/28/04 1:45 am)
  Re:
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 The blue light is not something to get worked up over, unless it's super-prevalent. It's just some gas in your not-so-vacuum tubes.

You're not gonna read much of anything useful across that 1 ohm resistor while it's on standby, as long as your standby lifts to plate voltage (which most do). As HStraub suggested, swap the Pin3s on your power tubes and fire it up, it should sound normal. Then, with the standby in the "play" position, you should get a voltage reading across that resistor. Again, make sure your meter is in mV, if it's not autoranging.
 
gurusimran
Posts: 11
(1/28/04 3:00 am)
  Re:
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 The amp is working, I´m very happy.
Thanks to all
 
gurusimran
Posts: 12
(2/25/04 2:29 pm)
  I modified this amp but...
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 This Plexi was working, the normal chanel was ok only because if you turned up the treble control more than 3/4 the sound gets terrible, and the bright channel sound terrible with high frecuency noises .I made a jcm 800 mod, looking for tube distortion but it sound terrible
the sound its like a high frecuency noise mixe with the guitar sound , like a squeak??? I don´t know it this word describes the sound correctly anything up 1/4 of gain makes that terrible ultra low quality sound, forget about turning up the trble control, it gets worst, the mid control too. Only the bass control works well, and the presence control doesn´t feed back I think.
I shortened the grids wires on all tubes and shortened the lenght of the resistors on the power tubes and I put shielded cable on the input, but it sounds terrible. I think it is a high frecuency parasit oscillation, can it be fixed?

 
Lucid Alice
Senior tube assistant
Posts: 166
(2/25/04 8:30 pm)
  Re: I modified this amp but...
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 Do you have a drawing of what you built, with the mods? If so, find some place to post it and paste a link here.

If it's not too much trouble to undo the mods, it might be best to get the amp working stock first... then put the mods back and trouble shoot them separately. Any mod that adds gain can be susceptable to oscillation problems. In this case you may also have an oscillation problem from the build.


 
gurusimran
Posts: 13
(2/26/04 3:23 pm)
  I modified this amp but...
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 I made the plexi 50 thunder layout, then I modified it to the top fuel jcm800 thunder layout
 
 
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