Problem with 1966 Super reverb (AB763)

[frankeg]

Jezzbo,
I have exactly the same issue with a 70 SF Twin. I have re-soldered connections, changed coupling caps, heated the board and placed some pieces of heat shrink between the boards but it's still there. Looks like I have to separate the boards for inspection (such a pain). Oh well. BTW I can't say exactly why but I have the same voltage readings on the vibrato vol pot and my normal chan is also fine. For your bias circuit, if it's not the famous SF balance system (mine was but is now converted), You would want less DC bias for more current so increase your bias range resistor. Please keep us posted and I'll do the same. BTW, Good call Electron...

[frankeg]

I checked for dc volts on the board and sure enough it's there. My particular problem however, was a bare wire jumper between two eylets following the 250pf treble cap. It was laying tight against the board. On a whim, I pried it up off the board and presto! Noise gone. I will solder a new insulated jumper in when I get some time. I also cleaned the board as recommended with alcohol.

[jezzbo]

frankeg, thanks for your reply.
I will probably have to remove the board too.
And clean it with alcohol (I hope a nice single malt wil do the job )

I'll keep you posted.

[frankeg]

You may not need to remove the board. If yours is like mine, you should be able to lift the tube side of the board with the fewest de-solders. Do a few good cleanings with alcohol and put some heat on it. I'm using a flexible neck desk lamp. Fairly warm but not too hot. Then you can use several single malts while you wait for it to dry out. Advise your friend to keep the amp in a warm dry place when not in use and it may be fine. Replacing the board is a big job and worth avoiding if possible...

[Firestorm]

DC measured at the Volume pot (with the pot dialed in the trouble zone, between 6 and 8:
At the wiper: 3mV
At the right solderlug: 2,7mV.

That's really a tiny amount of voltage; a lot of meters will read a phantom voltage like that out of the air, so to speak.

One thing to consider is that a tube grid will always have DC on it (negative). Electrons from the cathode hit it on the way to the plate and develop a voltage across any resistance to ground (that's how grid leak/contact bias works). A 12AX7 can develop up to 100mV per meg of resistance.

I can't vouch for it, but Gerald Weber claims to cure scratchy volume pot syndrome (though he's talking about voltage leaking from the input jack onto the guitar pot) by using a shielded cable to the grid with the shield connected to the plate instead of to ground (how does that work? I don't know). If you want to see if small negative feedback like this affects your noise, don't do it his way, just put a really small cap (10pF or less) from plate to grid. It wouldn't hurt to shield the grid wires either. BF reverb amps have way long leads on the Reverb channel.

[pdf64]

Definitely try a different tube; as described, some 12AX7s have excessive grid current that results in this symptom. Pete.

[jezzbo]

Definitely try a different tube; as described, some 12AX7s have excessive grid current that results in this symptom. Pete.

Did that allready. No difference.
Thanks for helping.

[ElectronAvalanche]

Hi Y'all,

just a quick follow-up. I had some seriously conductive eyeletboards, so I decided to rip them out and install PCBs.

I made three seperate PCBs, one for the preamp with trem roach, one with the Tremolo oscillator circuit and one for the PI. The PS board was replaced by a PCB as well.

What can I say? NO MORE TICKING. No hum, due to improved grounding scheme (close to what Dumble used in the ODS). The amp sounds great.

Happy camper! Those black eyeletboard should NOT be used for Fender Amp clones unless you find the annoying Trem ticking part of the original sound and not to be missed from your clone!

Happy soldering!

Electron

[Structo]

Wow, nice job on the boards.
Nice thick traces.

I don't think I have ever seen anybody do that to a Fender before.
Usually they just make another eyelet board and if super anal, they make the new one out of fish paper like the old ones.

The problem is the material itself.
It absorbs moisture and becomes conductive to a degree.

Not sure if modern fiber board is the same or is more resistant to moisture.
Either way, cool fix, that amp will never suffer from that again.

[vibratoking]

I have a 65 SR. A few years ago I thought I'd be smart and put OD caps in it. It became a sterile POS. I couldn't get the OD caps out and the blue molded back in fast enough.

[David Root]

+1 on that, and Amen too! Orange drops are not right for BF Fenders IMHO. Especially not polypropylene ODs (715/716).

I'm putting together stuff for a 5E8A build and I bought the fiberboard set from Mojo, largely because Mark Baier uses the fiberboards in his Victoria Amps repro amps. I measured it all over with my Fluke 187, nada conductivity, not even a hint.

Should I alcohol wash it anyway? (I don't drink Scotch any more, gave it up in '77.)

Also, you can ground the shield on a coax run to the plate or to ground, both act as AC grounds. I've done it both ways it does work, but I don't trust the coax's dielectric DC resistance after 20 years or so, so I always chassis ground it at the signal source end.

[husky]

Hi Everyone,

I'm trying to fix a problem on a friends 1966 superreverb (AB763)

This is the problem: On channel 2 (vibrato channel) when I turn the volume pot between 6 and 8, there is this scratchy noise. (like there is some dust in the pot). Before "6" and after "8" the noise is gone.

This is what I allready did to try to solve this problem but with no succes.

1. Changed the volume pot with a new one (did this two times just to make sure the new pot wasn't also broken)
The new pot makes the same noise at approximatly the same location (between 6 and
2. Changed the 3 tonecap (I thought maybe one was leaking DC)
3. changed the treble bleed cap (120pF)
4. changed V2 tube and V4 with a known good one.

thank you for your help.

I know been said before but
Take out the fiber board and soak it in a hot wax tray with the components on it, the vulcanized fiber is hygroscopic.
Especially after rework the original wax stops doing it's job when they remembered to do it. Better to use FR4

JUST CAUSE YOU CANT MEASURE something doesn't mean there wont be a problem !!!! If you use Fiber you should load parts, wires and then wax, trust me !

Fender had to recall thousands of amps to do just this back in the day.

Also be careful with Flux off, it will give you the same symptoms. Only use Anhydrous Isopropyl IF you clean the flux. Sometimes it is better to leave it if you dont get it all off.