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
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Here is a link to the schematic:
http://support.fender.com/schematics/guitar_amplifiers/Custom_Vibrolux_Reverb_schematic.pdf
.Here is a link to the schematic:
http://support.fender.com/schematics/guitar_amplifiers/Custom_Vibrolux_Reverb_schematic.pdf
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.
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.
Good Luck.
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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.
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.
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.
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!
The negative bias voltage was both the same ( -42vdc ) on both tubes at pin 5. I am trying to narrow down the parts to replace by starting with the more likely culprits. Are the coupling caps a good place to start. Voltages for V4B are 286 before plate resistor, and 190 after, I am getting 1.4vdc before cathode resistor ( looks good to me ). PI voltages look good too.
Intresting - for some reason they have labled the fixed bias adjustmet (I know it's an oximoron) as the "intensity" control. The control puts a variable amount of negative voltage on the grids of the output tubes. With the amp turned on and plugged in, try adjusting the negative bias and see it the problem goes away.
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The "intensity" is for the vibrato control. The amp did have a fixed bias adjustment R59 18k. I swapped it out a while ago for a 10k pot and 5k resistor totaling 15k.
My question now is on the primary side of the reverb transformer is 1160 ohms. Is that kind of high? The secondary side has 1.5 ohms. Do these numbers look right? That ratio doesn't seem right to me.
Ok Im with you. The "intensity control" sets the relationship between the grids being controled by the vibrato and a fixed DC. Try lifting C29 so the the vibrato is not in play.
As far as the reverb input transformer ratio. Uh- you must have looked that up - because the schematic dosent say anything about it but a part number. Good enough, but I think that the approach should be to disconnect all the reverb and tremelo and see if you can get a working amp without the frills. I hope Im helping rather than clouding things up. You obviously have some chops your-self. Im thinking output section. See what youve done, you made me print the schematic so I could get a better look at it.
As far as the reverb input transformer ratio. Uh- you must have looked that up - because the schematic dosent say anything about it but a part number. Good enough, but I think that the approach should be to disconnect all the reverb and tremelo and see if you can get a working amp without the frills. I hope Im helping rather than clouding things up. You obviously have some chops your-self. Im thinking output section. See what youve done, you made me print the schematic so I could get a better look at it.
Can they get noisy and still ohm out to specified resistance? It does it in both channels so I did replace the PI plate loaders ( 1/2 watt ). I could try V4B plate load resistor, it ohms out but I think that is a 1/4 watt.
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