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Post subject: Super Reverb Issues
Posted: Sun Oct 06, 2013 7:37 am
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Hi everyone! Hoping somebody can help me troubleahoot this thing.
The amp in question is a 65 blackface ab763 clone bought from Marsh, in kit form, and made by mojo. The only things that veer away from the original circuit are a master volume and a 2/4/8ohm output tranny. I built this amp myself about 5 years ago and it has been runnin strong since, until recently....
Now, I can follow a schematic and layout, and build with the best of them, but as you know, troubleshooting is a whole other animal. So here is the problem (finally) Im getting a horrible crackling sound thats almost pattern like, on the pick attack and the sustain. Ive narrowed it down to something in the reverb circuit (I think). Here are the things I've tried...
Unplugged the tank, sound is still there. Changed out v3 and v4 with known good tubes, still there. Plugged into both channels, still there. Removed v3 entirely, AHA! That stops it! So, ive searched all over and come up with 3 culprits so far. Reverb tranny, 25/25 cap, or blocking distortion.
Here is a vid of the problem:
http://youtu.be/MDZFZiWpFa8
If anyone can think of anything off the top of their heads please chime in. Sorry for the long winded post and thanks in advance!


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Post subject: Re: Super Reverb Issues
Posted: Sun Oct 06, 2013 8:00 am
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I'd opt for replacing the 25/25 cap tying V3's cathodes to ground. Leakage there may be the source of the noise. As well, if you've used a carbon-comp resistor in that RC circuit it may have picked up some moisture during the intervening years. If you have a fresh 2.2KΩ laying around, you might throw that in there too.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Super Reverb Issues
Posted: Sun Oct 06, 2013 9:59 am
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Defective solder ?


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Post subject: Re: Super Reverb Issues
Posted: Sun Oct 06, 2013 11:34 am
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RedtheRiffer, welcome to the Forum! :mrgreen:

http://www.webphix.com/schematic%20heav ... _schem.pdf


First thing is to try tapping ALL solder points with a WOODEN CHOPSTICK . Tap all wires, solder points, and components attached to offending socket. Power is going to be on --- so be careful!!!! If no luck...

V3 is the 12AT7 reverb driver. It has a jumper running from pin 2 to pin 7. Often, the socket tangs are folded over and soldered. If the solder point becomes cold you can get a micro-crack formed. That leads to buzzing when a note is hit. Anyhow, try resoldering this point.

Also, looks at the solder on pins 4 & 5 and 9. These are the heater lines. Cold solder here can lead to static from V3, yet no problems from other tubes, as the heater line is in parallel to all the tubes.

If that doesn't work, try resoldering all points on V3 socket. Now, check the solder points from the reverb tranny. That's the small tranny next to the 12AT7. Usually canted at an angle between sockets ---- in the AB763 circuit. Try tapping the tranny to see if that induces the static. May need a new one.

If that doesn't work, flip the amp over to the filter cap's "doghouse." Check the first 20mfd/500VDC cap (next to the two large 70mfd/350VDC--- or whatever value of the first two filter caps). This cap feeds power to the anode plates of the 12AT7. Check the leads off this cap for poor soldering.

Tap the resistors under the doghouse. Replace any that induces static.

Good luck! Keep us informed. :D


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Post subject: Re: Super Reverb Issues
Posted: Mon Oct 07, 2013 2:55 pm
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Ok I lifted the leg on that 25/25 cap and it appears to have taken care of it. Would you say that a replacement of this cap is all that is in order? Thanks


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Post subject: Re: Super Reverb Issues
Posted: Mon Oct 07, 2013 3:44 pm
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Possibly, but you won't be able to confirm that until you try it.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Super Reverb Issues
Posted: Mon Oct 07, 2013 6:13 pm
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Nope, didnt work :(


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Post subject: Re: Super Reverb Issues
Posted: Tue Oct 08, 2013 4:28 am
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Sounds like you may have a coupling cap leaking DC. Or perhaps too high voltages applied to the anodes of that 12AT7.

Prolly best to do a fairly comprehensive test of all components off that tube.

Good luck!


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