Robinette 6V6 JCM 800 build problem

[bobtait]

Disregard that last post. I forgot that the wires are all on the same coil.

[bobtait]

I hooked up my tone generator and put a signal at the input. On my scope it showed well. At V1 output it was stronger. At the plate on V4 it was really strong. At the speaker output it was there but in was so low I doubt it could drive anything to the speaker

[Bieworm]

Sounds like a bad ground connection on the output or a forgotten coupling cap. Something is nog correctly connected or soldered.
Remove the blue wires from the CC grid load resistors of the PI to the 25k pot and connect them directly to ground. Any better?
What is that pot for anyway?
Edit; forget that. Its a complicated bias circuit. I wonder if it’s all connected correctly

[bobtait]

When I ground the black common secondary wire from the OT at the speaker jacks it shorts to the input of the jack. It is shorting through the OT.

[bobtait]

hmm his layout doesnt show the ground of the OT connected to ground but I assumed because his jack looked like a fender type switchcraft that it was grounding through the chassis.

[Bieworm]

Maybe try this: disconnect the 8 ohm secondary wires and of the OT and connect it directly to a 1/4” jack. Disconnect the black common wire from the OT too and connect it to that sale jack’s negative terminal. Clip in a wire from there to ground too. Now put in a speaker into the jack. Any solace?

[bobtait]

I high lighted through the entire circuit and there are no parts missing. On you last post, I by passed the switch, because I have 5881 tubes in it it is supposed to be on the 16 ohm tap so I used that strait to the jack the black common from the OT straight to the jack ground the jack ground jumped to the chassis. Still shorted out. I had already checked the switch function but I will try anything. The tip of the speaker jack if the jack is grounded it is shorted.

[bobtait]

the 25K pot is the balance for the bias circuit that is controlled by the 50K pot right next it. And the bias is adjusting properly

[bobtait]

I look up stuff and one thing says no continuity and another say yes cause its through the circuit or the speaker. I just checked a 50 year of Fender amp and sure enough pull a speaker cable that is plugged into the amp and even when off there is continuity and that amp has been running for 50 years.

[bobtait]

All I know at this point is that I get signal all the way through the amp that increases in power at each stage to 440V going into the Output Transformer and .2 mV coming out of the OT. I think I need to figure out if another British 18watt OT is ok, or something else. The design only asked for a 7K:8-16, the one in it is 8K:4-8-16. Robinette did say that the OT he specked way beefier than most. The design has to have at least 8 and 16 ohms cause to run 5881 or 6L6 or EL34 you have to plug an 8 ohm load into the 16 ohm out. So you get just a little bit more power and some headroom. So if you can recommend an OT I am all ears.

[JMPGuitars]

The wiring mistake is what likely killed the OT (if it is dead); whether you use the same model or something else is up to you.

Side note: shorting out your OT could draw significant current, and that could cause the PT heat issue. So, it seems likely that you cooked the OT.

An OT will have continuity when not connected/powered on. NEVER test continuity with the amp powered on; you test resistance instead (if needed).

Thanks,
Josh

[bobtait]

Lets just take a break on this one for awhile. I ordered another OT and if it works, we will know. I made that stupid mistake on the speaker out cause I grabbed some Marshall style jacks out of my parts and just put them in. They already had the jacks connected like that and I didn't notice when I was trying to do it like a Tweed Deluxe. One side note: if it all works, I have enough room in the chassis to put the cap can right next to the PT and move the OT and choke down so maybe I can get the wiring closer, tighter and better. But I will cross that bridge after it is up and running. I also think the bias might need to be tamed down and I haven't even adequately checked the new JJ 5881 tubes to see if they were damaged, or even tried to go back to 6V6, so yeah, there is a lot of checking and clean up ahead. Thanks for all the help. I will keep ya'll posted when I get back to it.

[JMPGuitars]

Learning is highly valued, and an extra set of tubes costs less than school.

That said, tubes (that don't say JJ on them) can be pretty resilient. They might be fine.

I had a EL34 in my tube tester, and forgot to check the heater voltage...it was set to 20V from the previous tube. The tube lit up like a light bulb! I tested it later, and it seemed fine. I dunno if I would use it in an amp, but it's fine for testing tube testers.

Thanks,
Josh

[bobtait]

I was trying to remember how I made such a blunder on those speaker jacks. And I realized that this build was so delayed by getting the wrong parts and having to get them replaced and waiting for this and that, I lost track of what was going on. I wanted marshall style jacks on the speakers, no problem, I have a bunch of them so I grabbed a pair. A pair that had been wired like many speaker jacks with the wires going through all the posts. Then a week or 2 went by and I thought, hey, I guess I could wire the jacks like a tweed deluxe so it shorts if nothing is plugged in. All cool but, I left all the contacts hard wired on the outside and added the grounding connection for the main jack along with it. oops. The lesson in this is, to me, amp builds sack up and if you are not extra careful, time delays can make you sloppy. So, you gotta slow down. If you have a delay, like a week, you gotta go back to square one and check everything again. Cause you forget where you were. The other thing I have learned from this build. It is component placement in the chassis is critical to having a tight, clean build. I still may have to move some stuff like the cap can to clean up the build. I pass this stuff along maybe to help others in the same place. It is all a learning experience.

[Bieworm]

the resistance between all the secondary impedance wires is all .5 or .6 ohms either between each other or each one and the ground wire from the secondary

I'm confused. Here you state to read resistance and further on you state there is a short?
I'd suggest disconnecting all OT secondary wires so nothing touches nothing. Now measure the resistance between the 16 ohm tap and the black common wire. If it isn't shorted or infinite then, then there is no reason to assume the problem resides within the OT.

[Bieworm]

I was trying to remember how I made such a blunder on those speaker jacks. And I realized that this build was so delayed by getting the wrong parts and having to get them replaced and waiting for this and that, I lost track of what was going on. I wanted marshall style jacks on the speakers, no problem, I have a bunch of them so I grabbed a pair. A pair that had been wired like many speaker jacks with the wires going through all the posts. Then a week or 2 went by and I thought, hey, I guess I could wire the jacks like a tweed deluxe so it shorts if nothing is plugged in. All cool but, I left all the contacts hard wired on the outside and added the grounding connection for the main jack along with it. oops. The lesson in this is, to me, amp builds sack up and if you are not extra careful, time delays can make you sloppy. So, you gotta slow down. If you have a delay, like a week, you gotta go back to square one and check everything again. Cause you forget where you were. The other thing I have learned from this build. It is component placement in the chassis is critical to having a tight, clean build. I still may have to move some stuff like the cap can to clean up the build. I pass this stuff along maybe to help others in the same place. It is all a learning experience.

A multi cap can is not an ideal solution for various reasons.
1. it inherently implies that there will be multiple long wire runs, carrying high voltage, that will automatically be close to other wires or components. This increases the chance of crosstalk, noise, etc
2. a filter cap should be positioned as close to the part of the circuit it's supplying as possible. Again for a number of reasons.. the positive and negative sides are strategically better separated from other parts of the circuit, the negative side is closer to the components for shorter wire runs and senseful grounding of some very important or sensitive parts (reverb, tremolo,...)
3. a multi cap can that loses one of the separate caps means you'd have to replace the entire can and - apart from the work involved - is pretty expensive.
4. in case of most applications there is a mutual ground terminal on these things, meaning there is no way to separate ground references for preamp and power amp. This separation alone is a pretty crucial element to obtain a quieter amp

[bobtait]

I have plenty of room to move the cap can right next to the rectifier and move the OT and choke over some. Just an extra hole in the chassis. That build requires at the least an 8 ohm and 16 ohm OT for the way he sets it up to run bigger tubes you plug a 8 ohm load into the 16 ohm tap. You really only pick up 4 watts doing that but I wanted to try it. He said you get a little more headroom. I will keep you posted how it goes.

[bobtait]

I wired in another OT with no change. So, I started tracing through the circuit with a sine wave. Everything was good with pre and master volumes working. Had a good sine wave all the way to the phase inverter when it went from a sine wave to something totally wonky. I found a wire to the phase inverter pins 8 and 3 that had broken off the eyelet board and slipped underneath it. I reconnected the wire and got sound with the tone generator to the speaker but very weak unless you put up to 8 volts on the input. A guitar would not even drive it. I am still not sure now if the original OT was damaged and just weak. Should I try the new OT again or keep looking elsewhere

[JMPGuitars]

Put the original OT back in, and see if the behavior is the same. Assuming it is, do a highlighter test properly. That includes looking for shorts that shouldn't be there. Follow your schematic, and find any other faults. Also try a different tube in the PI after confirming it's wired correctly.

[bobtait]

The original is the one that is in it now. I only patched in the new one for a few minutes. But I am going to keep going through the circuit too. I checked the values of every part I put in as I was doing it but I will check them again. It is pretty easy to just patch in the other OT too. I'm just glad it's making any kind of noise at this moment. And thanks!