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  #46  
Old 01-24-2015, 11:35 PM
moon moon is offline
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Make sure you're checking voltages relative to ground rather than across the individual components (I think that's right). Sorry if I keep telling you stuff you already know

You'd expect a mistake or a fault in the circuit to show up as a bad voltage somewhere. Could it possibly be the newly-wired cab? If you can test the cab with another amp that might at least rule it out.

Something at or near the reverb pot is probably the prime candidate though.

Nearly there
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  #47  
Old 01-25-2015, 01:00 AM
clintj clintj is offline
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Bad speaker? Hmm. Could plug it into my Tweed for a test.

I'm taking a day off from this project. It's kind of starting to annoy me, and the next step is to hand trace every last signal path with my multimeter. I know that both power tubes have voltage, and are idling properly. Either something is bleeding signal to ground, or a tube is just not developing gain like it should, or one half the phase inverter is not playing nice. We'll see.
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  #48  
Old 01-25-2015, 10:16 AM
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It's a 4x10 cab isn't it? That increases the chances of a bad solder joint or wiring problem.

It probably is unlikely but it's the philosophy that's important really. When you're debugging you have to check everything, even your own assumptions because that's where the fault may be hiding. It can be difficult even to realise the assumptions you're unconsciously making. There's an old joke:

An economist, a mathematician and a philosopher travel to Australia. On their first day they encounter a black swan.

"Look!" Says the economist: "in Australia all swans are black".

"Perhaps." Says the mathematician. "But all we can say for sure is that in Australia there is at least one swan which is black."

"Gentlemen," says the philosopher, "you are both wrong. All that we can say for certain is that in Australia there is at least one swan one side of which is black."

Real-life doesn't usually require that level of detail but debugging faulty systems needs your inner Sherlock Holmes.

Simplifying the system can help although that requires that it can break down in a modular way. I've done a bit of programming and debugging complex systems can be an absolute nightmare so you deliberately design them as lots of discrete, co-operating, modular parts which can be tested independently.

Gathering as much information as possible helps too, such as voltage readings. Maybe different pot settings could reveal something: a fully open pot might be sending current straight towards a fault but maybe a fully closed one would cut off the part of the circuit with the fault.

Don't have much specific help to offer I'm afraid. Try it out with tubes from your other amps, if you haven't already.
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  #49  
Old 01-25-2015, 11:13 AM
Paultergeist Paultergeist is offline
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I built an AB 763 circuit some years ago......took me a long time.....I think it was my 5th amp build. Indeed, there is a lot of complexity in that amp with two channels, reverb, and vibrato. I don't know if I have be a lot of help at this point in time, but I will try.

Do you have access to an oscilloscope?

Can you post tube pin voltages (referenced to ground)?

Lastly (and most importantly) are you confident that you can safely operate inside a powered-up tube amp?
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  #50  
Old 01-25-2015, 12:07 PM
clintj clintj is offline
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Quote:
Originally Posted by Paultergeist View Post
I built an AB 763 circuit some years ago......took me a long time.....I think it was my 5th amp build. Indeed, there is a lot of complexity in that amp with two channels, reverb, and vibrato. I don't know if I have be a lot of help at this point in time, but I will try.

Do you have access to an oscilloscope?

Can you post tube pin voltages (referenced to ground)?

Lastly (and most importantly) are you confident that you can safely operate inside a powered-up tube amp?
I can compile a list of pin voltages, but it will take me a little time. I checked as many points as I could using the schematic listed voltages and didn't find anything odd. I installed my bias probe on each power tube and got 35ma and 38ma on each tube, or about 65% dissipation at idle. At this point I'm starting over and using my meter to verify each and every circuit has correct connections and continuity. I looked up the Tube Amp Troubleshooting page on Geofex for some more possible problems too.

As far as an oscilloscope, not yet. I started looking around online last night for one as it would really come in handy for these problems.

As far as live circuits, I have a set of Class 0 rated insulated gloves good to 1KV, and never have both hands in contact with the chassis. One hand is either behind my back or somewhere else like the tabletop.
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  #51  
Old 01-25-2015, 05:54 PM
clintj clintj is offline
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Eureka! Found and fixed a cold solder joint on one of the input jacks, and got a squeal after a little motorboating this time. Got a suggestion to swap the PT primary leads as the negative feedback might be reversed. Man, Channel 1 sounds good now. Loud, too!
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  #52  
Old 01-25-2015, 06:20 PM
clintj clintj is offline
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Oh, sweet tube sounds! The reverb hums a little when turned up, but that's nothing compared to the earlier mysteries. We have a warm, chimey, very loud amp now.

Time to go help cook dinner now.
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  #53  
Old 01-25-2015, 06:52 PM
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  #54  
Old 01-25-2015, 07:12 PM
Dan Lampton Dan Lampton is offline
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Great job! Enjoy making music with your new amp.
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  #55  
Old 01-25-2015, 10:42 PM
clintj clintj is offline
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Quote:
Originally Posted by moon View Post
Thanks! I feel a real sense of accomplishment after this one. Next one will be, um, not nearly as complex.
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  #56  
Old 01-26-2015, 07:08 AM
jonfields45 jonfields45 is offline
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Maybe I missed it in an earlier post, but who manufactured the kit? Thanks, Jon
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  #57  
Old 01-26-2015, 07:32 AM
clintj clintj is offline
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Quote:
Originally Posted by jonfields45 View Post
Maybe I missed it in an earlier post, but who manufactured the kit? Thanks, Jon
It was a Mojotone kit. You get some nicely built parts like the chassis, cabinet, and faceplate, very well organized parts containers with everything labeled as to what it is, and a schematic and layout on two very large sheets of paper. What you don't get are written directions on this kit, or a pic of suggested underboard jumpers. They definitely market these for people with some experience in both building and schematic reading, and the difficulty is rated as the highest for the blackface Fenders. The Tweed Deluxe of theirs I built came with more illustrations and more complete directions.

In all fairness to them, there are two ways to install the output transformer and if not for the negative feedback loop many Fenders use, it wouldn't matter which primary wire went to which power tube. Either way would work fine. I just got it backwards the first try and didn't catch it on first power up. I also wrongly assumed that if it were feeding back, it would be audible. Not necessarily.
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  #58  
Old 01-26-2015, 09:38 AM
Dru Edwards Dru Edwards is offline
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Quote:
Originally Posted by clintj View Post
Eureka! Found and fixed a cold solder joint on one of the input jacks, and got a squeal after a little motorboating this time. Got a suggestion to swap the PT primary leads as the negative feedback might be reversed. Man, Channel 1 sounds good now. Loud, too!
Fantastic!!!! Congrats Clint. I was going to recommend that you check the solder joint on the input jack. Ok, I wasn't.

How did you know it was the solder? The look of the joint (i.e. that non-shiny color)? Did you just resolder hoping it was it?
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  #59  
Old 01-26-2015, 09:44 AM
clintj clintj is offline
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Quote:
Originally Posted by Dru Edwards View Post
Fantastic!!!! Congrats Clint. I was going to recommend that you check the solder joint on the input jack. Ok, I wasn't.

How did you know it was the solder? The look of the joint (i.e. that non-shiny color)? Did you just resolder hoping it was it?
Actually, I pressed the meter probe against it to check that there was a good connection to ground and it popped loose from the jack. Explains why I was getting an erratic reading from the cable tip to ground (1M, open, 1M, etc.)

Still didn't solve the feedback problem, but it did make it audible finally. That "aha!" moment.

Once I get the reverb hum fixed, I'll set up a couple of microphones and record some sound clips.
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  #60  
Old 01-26-2015, 08:49 PM
clintj clintj is offline
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I'm starting to feel like the codebreakers working on the Enigma machine. Try A, see what comes out the other side. Tried adjusting lead dress and grounding to kill the hum. Oddly enough, if I turn on the Bright switch for that channel, it almost completely kills the hum on the reverb.
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