Custom Vibrolux Reverb noise issue - diyAudio
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Old 10th May 2011, 10:46 PM   #1
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Default Custom Vibrolux Reverb noise issue

Certain notes make the amp sound like it is going to die For instance an E note on the G string sounds perfectly fine but a D note on the G string sounds absolutely horrible ( Speaker sounds like it is rattling/distorting on the decay of the note ). I have the amp's chassis away from the cabinet, I have tried different speakers and tubes, I installed Grid resistors to the output tubes, I reflowed every solder joint in the amp.

Here is a link to the schematic:

http://support.fender.com/schematics..._schematic.pdf
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Old 11th May 2011, 06:48 PM   #2
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With the vibrato and tremelo off. Try the bright and normal channels one at a time and see if the problem is specific to one or the other. If it is specific to one input channel that isolates it to that channel. If it is common to both, the problem is probally down stream from either pre-amp.
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Old 11th May 2011, 10:01 PM   #3
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It does it in both channels with vibrato and reverb off.
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Old 11th May 2011, 10:34 PM   #4
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Ok that tells a lot. The problem is from V4B on. You might spend some time checking DC voltages. My guess is that part has changed value and voltages can lead to clues. I would be going in with a scope to look carefully at waveforms. Fender has verry kindly supplied AC signal voltage levels and DC voltages. You need a voltmeter to check the DC stuff and a scope to check the AC. Don't install any non standard parts until the problem is fixed. This is a good way to dig the hole deeper. I think you are going to end up getting some help. If you do poke around with it turned on, just bear in mind that its got 437 volts in there. Good Luck.

Last edited by firechief; 11th May 2011 at 10:41 PM.
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Old 12th May 2011, 05:05 PM   #5
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I don't know how much I trust the schematic. There was a typo on test point 17, instead of 71vdc they wrote 17vdc. Anyway it does give values on components to which have been correct so far. The two octal sockets for the 6L6GC's were of poor quality and really loose so I put some Belton's in last night. It didn't cure the problem but gives me that warm fuzzy feeling inside. I do not have access to a scope which would help a lot. I will take more voltage readings and post.
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Old 12th May 2011, 05:28 PM   #6
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on test point 17, instead of 71vdc they wrote 17vdc.

Uh- are you sure that you dident just find the problem? Cathode circut of the phase splitter with 71 volts on it? Check all values of resistors with the power off. In circut is OK. Like R37,38,39,40
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Old 12th May 2011, 06:23 PM   #7
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voltage drops are 77 and 84 across PI plate loaders
77/80000=.0009625
84/100000=.00084
.0009625+.00084=.0018025
.0018025*39820=71.77vdc
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Old 12th May 2011, 07:00 PM   #8
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OK, your right ---.96ma on one half of the tube and .84ma on the other, so the resistor values must be right. Say 77V accross 40K - call it .0019 amps for both tube sections.
Keep poking.
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Old 14th May 2011, 01:19 AM   #9
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I changed the power tubes a few months ago and the plate current of the 6L6GC's were 6ma apart. I now noticed that they have drifted to 11ma apart. Maybe a leaky coupling cap is causing my noise and bias issue? If I measure voltage at pin 5 they should both be the same negative voltage right ( is this a way to see if one is leaky )?
I also noticed that most fenders use a .001uf 400v cap ( polyester film I believe ) as a coupling cap in front of the PI. This amp has a 500pf 1kv ceramic disc in that spot. I have never had to replace any ceramic disc caps before because they hardly ever break.
My question is can a coupling cap create this type of noise issue? And why would they use a ceramic disc for a coupling cap ( probably because of such a low value )?
I think that I will just replace that cap with a .001 600v polyester film and also replace the two .1uf coupling caps to the 6L6GC's.
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Old 14th May 2011, 01:30 AM   #10
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The difference in output tube current is not signifigent. Well within normal range. I believe that there is a faulty part, but I would caution not to deviate from the original values until the problem is solved. You might be at the point of replacing parts until the problem is solved, but stick with like kind and value, until the problem is fixed. You got your teeth in it now!
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