Fender 1965 Blackface Vibrolux Heat Related Distortion Issue

[Paul98038]

I have a vintage 1965 Blackface Vibrolux with the AA864 circuitry. It plays fine for a short while but after 15 or so minutes of playing all channels get terrible distortion and crackling. Tapping on the amp causes the same. I assume this is heat related. Any tips for what to look for what is causing the issue? I have looked for loose soder joints and found and fixed one but that did not solve the problem. The amp appears to have all original parts except for the speakers.

[xtian]

Hi, Paul, welcome to TAG.

If your amp malfunctions reliably, that makes it so much easier than chasing an intermittent issue.

While the amp is misbehaving, use a wooden chopstick to probe every component and connection. Press, wiggle, flex. Hopefully, you can tighten the net of possible culprits.

[TUBEDUDE]

How old are the filter caps? Make sure there are no cracked resistors

[Charlie Wilson]

Hello, one of the first things I always check for with this sort of problem is the 100 ohm resistor at the PI tail. It is the only resistor on a blackface board that runs in the direction of the length of the circuit board and when the fiberboard warps that resistor always wants to pop out of its eyelet.
CW

[Stevem]

The thing to do is play the amp until it acts up and the to narrow down where the issue is remove the small preamp tube next to your 2 big output tubes on the left rear side of the amp.

If the issue goes away when you turn the amp back on then the problem is likely to be bad preamp tube plate load resistors , or taping each preamp tube with your finger nail may reveal a bad preamp tube or loose tube socket.

Any of these means a trip to a repair shop!

[Paul98038]

Thanks. As far as I know, all the parts are original parts. I bought it around 1988 and it has not been repaired/serviced since. I did resoder a resistor that ran the lengthwise of the circuit board. It was popped up from one its joints like you mentioned. I'll try the tube pull.

[Paul98038]

Finally tracked down what my problem was. I found a couple more cold solder joints on the board and I also found a ground wire that came off the board that went to the metal frame that had a loose solder joint. I think that the circuit board was flexing up after heating up and making the one ground wire lose contact. I also retensioned all the tube sockets and cleaned the reverb RCA jacks. Good to have the amp working like it used to again. Can't complain that it needed a little work. It is 54 years old.

[Stevem]

With all my Fender amps of your amps age and older it really pays to replace the output tubes sockets with the Marshall style sockets with there pin grippers and also in the process get both those resistors up graded to larger wattage and out of the direct heat area of the output tubes that changes there values drastically even well before they crumble from the heat!

Sorry, but at this point in my life I am really tired of retentioning Fender Octal sockets as part of a repair. lol!

Go to hear you found the amps issue by the way!

Also if it's been recapped make sure the bias filter has been up rated to 75 volts as the stocker is 50 volts and is being hit with more voltage then it's rating.