Red plating and hum after 30 seconds

[What?]

Glad you got it sorted. I had a strong feeling that it must have been red plating. I owned my share of JJ's, and I never saw any of the logos cook like that with healthy amps.

 

[william vogel]

Well, this is pretty clear; as others have suggested, you're missing bias voltage on V4 and V5. That's your problem and why the other two valves had the dye turned to grey. While others are correct that there is a little more stress on two of the four output valves, I was sceptical about the difference being so pronounced. Check the wiring, sockets, contacts. Do you have a schematic (and/or some pics) of the amp as it is right now?

That’s not correct on the screening being heated on the tubes. He added a master volume and when he pulled the chassis to install the master volume he noticed the red paint discolored. The problem is likely with the work around the master volume install/removal.

 

[Phoenix1]

That’s not correct on the screening being heated on the tubes. He added a master volume and when he pulled the chassis to install the master volume he noticed the red paint discolored. The problem is likely with the work around the master volume install/removal.

Yes that is probably when it happened ! Something got pulled or something I actually removed the master volume wires and put it back to stock just to eliminate any extra confusion. Now it is fixed and I am going to rewire the master tomorrow.

 

[PelliX]

That’s not correct on the screening being heated on the tubes. He added a master volume and when he pulled the chassis to install the master volume he noticed the red paint discolored. The problem is likely with the work around the master volume install/removal.

As I suggested, the difference in heat was far more extreme than would typically be witnessed in that scenario. No bias on two output valves explains it. I'm not sure which part of what I outlined is incorrect in your view?

 

[william vogel]

As I suggested, the difference in heat was far more extreme than would typically be witnessed in that scenario. No bias on two output valves explains it. I'm not sure which part of what I outlined is incorrect in your view?

What I was referring was about the two tubes having the paint cooked. That occurred before the bias voltage loss. After the master volume was added, he had a situation where the soldering was compromised coincidentally with the same pair. There’s a good chance that no signal was being delivered to those tubes along with little or no bias voltage. I wasn’t trying to contradict you but I wanted to point out that the amp was operating “normal” before the mod. The effect of discoloring the paint isn’t always normal but I’ve experienced it several times on different Marshall style amps and they were perfectly fine. I attribute it to the style of playing where we drive the power section hard into square wave output and these amps generally always produce nearly double their rated output which would at times cause the tubes to heat but not necessarily be red plating. Yes they are in the upper envelope of dissipation. People always decide bias is the issue and at this level of playing, bias is no longer a factor.

 

[PelliX]

That occurred before the bias voltage loss. After the master volume was added, he had a situation where the soldering was compromised coincidentally with the same pair.

Ah, gotcha. Not quite sure I agree, though - my guess is there was some dodgy connection before then at some point. I see how that's a bit coincidental, but it takes quite a bit of heat to cook the paint like that.

I attribute it to the style of playing where we drive the power section hard into square wave output and these amps generally always produce nearly double their rated output which would at times cause the tubes to heat but not necessarily be red plating.

But wouldn't you agree that it seems like a bit too much of a difference between the two pairs to be from simply running them all hot as heck?

TL;DR: Happy OP got the amp working.

 

[Phoenix1]

TL;DR: Happy OP got the amp working.

Me too ! All those equations were a little over my head by not having someone pointing at what was being referred to.... i am really old LOL ! BUT that being said I was about to just throw in the towel until I just sent a note to the tech (Andy) at Mojotone and he put the problem into easy terms and said I already found the problem. Pete F told me to check the voltages on Pins 3,4& 5 on all tubes and not having any negative voltage on p5 on the first two power tubes was in fact the root of the issue. I have only built two amps and just have basic knowledge, but learning, Andy told me to also Check the 5.1k resistors as well as the 220k and I would find the issue right there. And I did which was the junction of the two 5.1k and the wire tapped off that junction to the 220k. I had slipped a little piece of unsulation on thr long bare resistor wires to where they met and the bigger wire to the board. It looked like some insulation melted into the solder joint and after a few months it finally caused the joint to fail. I guess it had been on and off at first which caused the JJ logo to burn off. Lucky I got the master volume and took the amp apart which exposed the problem. Anyway this was a valuable lesson i wont forget. I do want to thank you all for pitching in to help it really means a lot and says a lot for this Marshall Forum and its value.
THANKS TO ALL !

 

[william vogel]

Ah, gotcha. Not quite sure I agree, though - my guess is there was some dodgy connection before then at some point. I see how that's a bit coincidental, but it takes quite a bit of heat to cook the paint like that.



But wouldn't you agree that it seems like a bit too much of a difference between the two pairs to be from simply running them all hot as heck?

TL;DR: Happy OP got the amp working.

I think it’s strange that it only burned paint on a pair on one side of the output transformer. When I burned the paint off tubes it was all four on a 100 watt or the pair on the 50 watters. Maybe it was happening all along the way but he described it as only after the mod and we’ll never know for sure. The paint they use is surely the same formula for all their tube offerings and I’m sure the KT88’s and 6550’s probably put more heat in the glass than EL34’s but I can’t say for sure because I’m not a frequent user of JJ tubes. I’ve got a 1971 Super Lead I bought recently and it has their EL34II’s in it. I haven’t looked around back to see if they’re still red paint or not but I don’t think anything about it if it’s burnt. I set my bias on it at about 68% and that’s where the waveform stays crossover distortion free until clipping.

 

[Phoenix1]

I think it’s strange that it only burned paint on a pair on one side of the output transformer. When I burned the paint off tubes it was all four on a 100 watt or the pair on the 50 watters. Maybe it was happening all along the way but he described it as only after the mod and we’ll never know for sure. The paint they use is surely the same formula for all their tube offerings and I’m sure the KT88’s and 6550’s probably put more heat in the glass than EL34’s but I can’t say for sure because I’m not a frequent user of JJ tubes. I’ve got a 1971 Super Lead I bought recently and it has their EL34II’s in it. I haven’t looked around back to see if they’re still red paint or not but I don’t think anything about it if it’s burnt. I set my bias on it at about 68% and that’s where the waveform stays crossover distortion free until clipping.

I did no mods it was just a finished Mojotone kit that was working fine. It just happened to be a bad solder joint. These amps work in pairs and the first pair after the preamp stage works harder from the info gathered and explained several times in this thread than the final two. It just burned the paint some on the first two. The pairs of 5.1k joined together circled in green and red are the weak spot ,if any, but the one on the right next to the preamp section was the culpret. There happened to be some plastic insulation the leached its way into the solder joint that caused an intermittent problem that caused now and then a red plate situation which in the process of adding a master volume control found the paint burned off the tubes in time to save the amp.
In my opinion there should have been a solder lug which I might add on both sides to eliminate this problem from happening again

 

[william vogel]

I did no mods it was just a finished Mojotone kit that was working fine. It just happened to be a bad solder joint. These amps work in pairs and the first pair after the preamp stage works harder from the info gathered and explained several times in this thread than the final two. It just burned the paint some on the first two. The pairs of 5.1k joined together circled in green and red are the weak spot ,if any, but the one on the right next to the preamp section was the culpret. There happened to be some plastic insulation the leached its way into the solder joint that caused an intermittent problem that caused now and then a red plate situation which in the process of adding a master volume control found the paint burned off the tubes in time to save the amp.
In my opinion there should have been a solder lug which I might add on both sides to eliminate this problem from happening again
View attachment 155490

The master volume is a mod. I’ve built and sold 50+ Plexi clones over the past five years and not a one of them has master volume because it’s not part of the circuit as Marshall built it. I’m not degrading anything here but adding a component is considered a modification and from what I thought I read was that you built and played the amp and then at a later date you’ve decided to add the master volume. You noticed the discolored tubes while pulling the chassis to install the master volume. I could have this all wrong but I’ve followed the threads.

 

[dommarsh]

Once power tubes red plate, I consider them damaged. The tubes also seemed to have gone thru several cycles of red plating and for an extended period of time judging by the scorched print! Red plating can compromise the structure of the metal in the tube. Well done on the repair, now you owe yourself a new set of power tubes!

 

[Phoenix1]

The master volume is a mod. I’ve built and sold 50+ Plexi clones over the past five years and not a one of them has master volume because it’s not part of the circuit as Marshall built it. I’m not degrading anything here but adding a component is considered a modification and from what I thought I read was that you built and played the amp and then at a later date you’ve decided to add the master volume. You noticed the discolored tubes while pulling the chassis to install the master volume. I could have this all wrong but I’ve followed the threads.

What ever you want ! This was finished and fixed and I seriously said thank you to everyone that offered any constructive help through this thread yesterday and as far as I am concerned I see what the problem was and the amp is done

 

[Blacksabbath1987]

I’m getting one to build in 2- weeks . I’m looking forward to it glad you got it figured out nothing worse getting everything done and then have to chase a problem good work . How do you like the build and the components from Mojotone kit

 

[Phoenix1]

Very good ! I just followed the basics froM another JTM 45 build sort of order of assembly thing and it went great ! Worked first time LOL ! Just this little glitch of a contaminated solder joint.

 

[Phoenix1]

I was finishing up and starting to put the chassis back in the cabinet and noticed all this "maybe" debris on the Spacer/ retainers in the power tubes. Since they got over heated I did swap the two rights to the left position so both took a hit while diagnosing and fixing the problem. Any opinions on the damage ?