http://support.fender.com/schematics/guitar_amplifiers/Custom_Vibrolux_Reverb_schematic.pdf
The link above is the schematic.
Anyway I am having problems with the amp so I took some voltage readings. I got to the Phase inverter and at test point 17 (at the cathode before resistor 39) I am getting 75vdc when it should be 17vdc. I had some resistors laying around and replaced the two plate loaders and that didn't help.
What should be my next step in diagnosis?
The link above is the schematic.
Anyway I am having problems with the amp so I took some voltage readings. I got to the Phase inverter and at test point 17 (at the cathode before resistor 39) I am getting 75vdc when it should be 17vdc. I had some resistors laying around and replaced the two plate loaders and that didn't help.
What should be my next step in diagnosis?
The schematic is wrong, if you calculate the current thru the cathode resistor, using 17VDC, it not even close to the sum of the plate currents. Check your plate resistor voltage drops, calculate the currents, add them together and that will be close to the combined cathode currents, multiply that by your combined cathode resistors and you have the voltage reading you should see, I guessing the 75 volts is close.
What is your initial problem?
Craig
What is your initial problem?
Craig
The initial problem is when playing guitar through the amp there is a crackle/rattle sound that seems to follow certain notes that are sustained. It is present in both channels, I tried different speakers, tubes, and looked for bad connections and bad solder joints ( I basically reflowed all solder connections ). I then started to take voltage readings and stopped on test point 17 because it wasn't close to what fender states.
I will take voltage readings and post ASAP
I will take voltage readings and post ASAP
For whatever reason Fender neglected to put grid stoppers on pin 5 of the output tubes, on the schematic anyway. Try installing some 1.5KOhm 1/2 Watt resistors, exact value is NOT important, right on pin 5 and see what you get. This always seems to fix weird noise problems after you've pulled all of your hair out. Many old Marshalls never had them and with today's tubes they always act up.
Craig
Craig
I think you are on the right track. Every time I get an old amp in and those are missing I add them. It all depends on the output tubes, sometimes they act up sometimes they don't. ALL old Fenders had them and I was actually surprised when I looked at the schematic and with your description of the problem that was the first thing I thought of. Don't know why they would cut corners like that. Try the 2.1Ks, Marshall used 5.6Ks when they finally got around to adding them in the early 70s. Don't go boating yet!!!!!!
Craig
Craig
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Yes, go ahead and try those, it all depends on the output tubes. I did the hair pulling thing years ago, had been building JTM45 clones without grid stoppers just like the originals and then one day on a new amp right off of the bench it did this. After many hours trying to figure out why one amp did and one amp didn't the grid stoppers fixed it.
Craig
Craig
Ok so the noise is in full force with 4.7k resistors. I know that the grid resistors are a good thing so I will leave them in but I am not so positive they are the problem. If I hit an E note on the G string the amp sounds beautiful but as soon as you play a D note on the G string the amp sounds like it is going to fall apart. I have a little time tonight to try some more troubleshooting but my diagnostic skills are not as good with amps as it is with cars. So any ideas would be much appreciated.
I want to thank Craig for taking his time to help me out. I think I will start a new thread titled " Custom Vibrolux Reverb noise issue ".
I want to thank Craig for taking his time to help me out. I think I will start a new thread titled " Custom Vibrolux Reverb noise issue ".
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