Robinette 6V6 JCM 800 build problem

[JMPGuitars]

Please don't take any offense to the following; my intention is to help, not to offend.

That build looks like a nightmare. It's a rat's nest of wires, that are nearly impossible to follow with any clarity. I also get the impression that you're doing extra things. Do nothing extra. No changes or mods of any kind until you have a working amp.

You shared a layout, not a schematic.

You shared photos of a meter. What you need to share are the voltages- that means a list of target voltages, and actual voltages you recorded. Take a look in the downloads section for an idea of how to present that information.

I think your most important move is to do a highlighter test using the layout AND SCHEMATIC. Verify that every connection and component value are correct, and nothing is shorted anywhere.

I can't look at that build and think the OT is the actual problem. Clean it up! Make it look nicer than Bieworm's and you'll probably fix any problems along the way.

Thanks,
Josh

[bobtait]

OK, I found the schematic. I have not found anything that states target voltages on any of this and I couldn't find the transformer he specified. I can pull the wiring from the HT fuse, the Sag Resistor addition, and the Bias test points. I would have to install a new bias pots as close to the place where they are connected as possible. I would rather not pull the ohm selection switch. I will clean it up and post my progress. I will also chart out every voltage present on every pin and post this as soon as I clean up the amp.

[bobtait]

I haven't gotten my shielded cable for the input, but is this getting better?

[JMPGuitars]

Watch the Soldering Technique videos linked in my signature, then inspect your build.

Thanks,
Josh

[Bieworm]

Set the scope at the grid of a power tube. Play guitar through the input with the amp on on rather low volume. If you read signal at that grid then the OT is the problem. Or a defect speaker.
About the biasing. You mentioned a few times you have 50mA. But that says nothing if you don’t take the accompanying plate voltage into the equation. With fixed bias amps I always put 1ohm 1% resistors between cathodes and ground on the power tubes. That way you can do the math with the robrobinette tube bias calculator.

[bobtait]

Am I using the right kind of solder?
63-37 Tin Lead Rosin Core Solder Wire for Electrical Soldering (0.8mm 100)
Even when I wait til everything is hot enough for the solder to flow everywhere, at first the solder is completely bright, mirror like. Then after a few minutes it looks duller.

[JMPGuitars]

It might be fine as long as it's smooth and concave. Cleaning the solder joint later can improve the shine. You should also look at the mechanical connections, and other choices made regarding soldering and wire mounting. It's not just about shiny.

For that test above, the OT isn't the problem if you haven't verified the wiring first AND the accuracy of the schematic/layout.

[bobtait]

I am getting the guitar signal all the way through the amp to the Screen Grid of both power tubes. The amp seems to be getting really hot so I can't leave it on very long. I started at the input to the preamp tubes all through the circuit to the grid on the power tubes and had signal on the scope.

[Bieworm]

Correct me if I’m wrong, but when the power tubes receive signal on the grids, they should amplify that. When no signal reaches the speaker there is a problem with the OT or its wiring connections.

[bobtait]

You guys are the experts but that is what I was thinking. And I think it might be my fault. i turned the amp on when the OT was grounded to the chassis but I had not grounded the speaker jacks. Also the PT is getting very hot. Maybe because of a problem with the OT? I have reviewed every component and it is all as it should be. What do yall think?

[bobtait]

Also, I was getting wonky voltage readings on the cathode through a 1 ohm resistor. And everything I have read online, for what its worth, has said that a bad OT can cause such things, like voltage piling up and causing excessive heat and such. I know this can be figured out. All I can do is follow the recommendations from you guys and take one step at a time. I like the way you bust my chops over doing it right. Reminds me of my old art school professors.

[JMPGuitars]

Correct me if I’m wrong, but when the power tubes receive signal on the grids, they should amplify that. When no signal reaches the speaker there is a problem with the OT or its wiring connections.

The issue is more likely his wiring than his OT. AFAIK, he still hasn't done a highlighter test or verified the circuit. If he did, he should do it again. Take a closer look at the solder joints and the wiring, and tell me how confident you feel about it.

Testing the OT would make sense if there was confidence in the rest of the build.

[JMPGuitars]

This is another reason I hate eyelet boards. When you try to get so many wires into that eyelet hole, and the builder's soldering is not expert level, you simply don't know what is or isn't connected well.

Then look, and you'll see that at least one of the OT wires is partially melted, and others too.

The rats nest of wires makes it very difficult to follow, but tell me if you see a photo where the SS rec and the OT CT meet together on the cap can as per the layout/schematic.

Thanks,
Josh

[bobtait]

Look all of your comments could be right. I am not defending my construction. The OT transformer wire burned was only on one side from the soldering iron and then reinsulated with liquid tape. I have gone through the circuit at least 5 times and I checked the values of every component as I was installing them. I went through the amp from input jack to every preamp tube and every component on the eyelet board that could carry a input signal. I got all the way to the output tubes and got the guitar signal there. But i have not been able to validate the cathode signal at the other side of the tube. The power trans is getting probably way too hot. Maybe from the OT trans, i dont know. All I can do is report what I find. You guys are the pros and I will do whatever you tell me to do.

[bobtait]

One moe thing. I disconnected everything i could and shortened every wire i could. I can’t change the fact that the chassis and location of the filter cap and sockets and pots are not optimum for the cleanest layout possible. I had to work with what i had. I am just trying to save something that i was stuck with.

[bobtait]

One more thing. You can criticize my soldering technique from photos and i don’t care. I have been building sound systems and making mic cables and guitar cables and snakes for 55 years. I have seldomly had a cable ever fail from a solder joint. I have mixed major artists on national tv. Working on amps is just something i have always wanted to learn. And i totally defer to your experience in this. But please don’t assume I don’t have at the least some kind of knowledge about electrical stuff.

[JMPGuitars]

One more thing. You can criticize my soldering technique from photos and i don’t care. I have been building sound systems and making mic cables and guitar cables and snakes for 55 years. I have seldomly had a cable ever fail from a solder joint. I have mixed major artists on national tv. Working on amps is just something i have always wanted to learn. And i totally defer to your experience in this. But please don’t assume I don’t have at the least some kind of knowledge about electrical stuff.

Don't take anything personal, or as a comment on anything else. Your solder skills can be improved, that's all you should take from those comments. Believe me, Beiworm and I understand how you feel. I had been soldering electronics for decades before I realized how much improvement my soldering skills needed.

Ask Bieworm about the first builds he showed on this website, and how mean I was. The result of that though is that his skills are excellent now, and he builds reliable amps like the photo he posted above.

If you'd like, I can do what I did for him, and highlight specific details in your amp that show soldering issues. I only do that when requested now because people tend to take it too personally. It's not for any purpose other than to help you improve your skills.

I don't say anything to cause harm. Here's a thought for you, and why I mentioned the things I did:
I pointed out the issues with the SS rec and OT CT (output transformer Center Tap) because those two things need to come together on the first filter cap so that the OT gets the power it needs to function.

Like you said- you have signal up to the power tubes. So what happens from there? Nothing, if the power amp and OT aren't getting what they need. This is also why I asked you to make a voltage chart based on one from the downloads section. Anything with my name on it has a good format. And there are other circuits in there with 6V6 power tubes. They won't be exact, but at least some of the voltages will be similar, and more importantly, we'll get a better picture of what's going on. Making information clear and easy to read/understand makes it easier for us to help you.

Again, we understand your frustration. We also understand how much fun it can be working with less than ideal parts. On the bright side- it's frustrating pains in the butt that help us learn the most.

Thanks,
Josh

[bobtait]

I completely revear and appreciate all the input you guys have given me. I don't take anything personally. And yes there always room for improvement for my skills. And I am just at baffled as anyone at the results of this build, I even tried testing the OT and can find no short circuits. If I had another OT on hand I would just swap it out and see what happened. I am going to go back and check everything. Do a Highlighter review, all of that. I am not giving up. Now if Mojotone contacts me and says they had a batch of bad OT transformers, well what can we do. On the last build you guys helped me with where I thought the OT was bad and bought a new one. I discovered it was the phase inverter. Now that very same OT I purchased then is the suspect now. Eventually the real culpret will be uncovered. And you can be hard on me. It only makes me better. And when a conundrum comes up, well its just problem. And we go back to scratch as many times as it takes. The over heating of the PT is bothering me a bit. I feel like it wouldn't be a bad idea to try another OT. Maybe mojotone will offer to send one. I can afford to buy another one. It is no big deal. In the mean time, I will go back and measure and check everything. It will get there eventually.

[JMPGuitars]

Don't guess; think about what's happening a little deeper. The OT isn't producing what it's supposed to. The PT is probably overheating. The wiring around the can cap and the rectifier are questionable.

Does that sound like things might be related?

Disconnect the OT entirely, and check the cap can to make sure it measures correctly at every step, and is wired correctly. Checking for shorts is a big deal too.

After you verify the cap can has no shorts, and measures the capacitance correctly, wire it up again.

Then power on the amp without any tubes installed, and see how the PT does. Check the voltages at every PT connection.

Thanks,
Josh

[Iain T]

OK, I found the schematic. I have not found anything that states target voltages on any of this and I couldn't find the transformer he specified.

Target B+ voltages are on the schematic. That's a perfect starting point.

I'd second the view on not adding any mods/undoing them to go back to the RR layout and schematic. It's a well tested build to get working.

I'd then add one mod at a time to see how it works.