Ceriatone Plexi 50 Lead build kit Red plating (SOLVED)

sdn25

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SOLUTION: I blew my OT due to having my bias set at 70mA as a result of a measurement error, had to get that replaced.

So basically, I had built a plexi 50 lead kit from Ceriatone, a 68' circuit that is for the exception being that I have the 47k NFB going off the 4ohm tap. I was recently playing the amp and all of a sudden the volume just tanked. I looked over and the tubes were red plating. I immediately put it on standby and decided to check the bias once the tubes cooled down a bit. I always run the amp between 7 and 10 on the volume, so full blast.

It has only been 3 days since the amp was built, I had a pair of brand new JJ EL34s installed from TAD.

I used a multimeter to check idle bias. At idle the amp did 33mA from the bias probes on the back of the amp. But at idle the pins on the tube sockets read 55mV, which is too high right?

So, I lowered the bias using the bias pot to where it read 35mV on the pins at idle, but it then read 18mA at the bias probe points on the back of the amp.

The tubes were ONLY redplating when a signal was passed through. I’m aware the plates inside the tube started to glow red and not the gas glow. All tests were done with a load.

While checking the amp with the lights off and I passed a signal through again, the tubes made a crackling sound and they just died. I should mention that throughout all this the volume output from the amp was severely diminished as that’s why I checked it in the first place.

The tubes redplated even when the volume was very low.

Now, I checked all the pins on the socket and compared it to the voltage chart that Ceriatone have on their website for the plexi 50 and they were within spec. This was tested before the tubes died. Could this still suggest that I have wired something wrong?

I then decided to test the amp again once more after the tubes died so I pulled the power tubes and tried to test since I've read these amps can be tested without power tubes, I still had a load connected. When I powered it on the HT fuse blew.

I was hoping if any amp wizards here have any suggestions as to what may cause this. Thanks in advance!

SIDENOTE: I use a Suhr reactive load as my load. Impedance selector at 8ohm setting.

Here is the layout of the amp from their website.
 

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william vogel

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Multiple chassis pictures and a voltage chart would be bare minimum needed to begin troubleshooting.
 

sdn25

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Multiple chassis pictures and a voltage chart would be bare minimum needed to begin troubleshooting.
I will get you the chassis pictures soon im at work rn. By voltage chart do you mean the readings I get? or the chart published by Ceriatone?
 

Dblgun

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You can check things such as unloaded plate voltage at p3 and negative voltage at p5 but you cannot check bias current without tubes in the amp. Being that it has bias checking posts at the rear panel what pin are you measuring the bias current at where you are getting different readings? As @william vogel said post good detailed photos of your amp and you may get some help figuring it out. My guess initially is that you want to make sure you wired the bias checking posts correctly and with the correct resistor value which should be 1 ohm.
 
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sdn25

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You can check things such as unloaded plate voltage at p3 and negative voltage at p5 but you cannot check bias current without tubes in the amp. Being that it has bias checking posts at the rear panel what pin are you measuring the bias current at where you are getting different readings? As @william vogel said post good detailed photos of your amp and you may get some help figuring it out. My guess initially is that you want to make sure you wired the bias checking posts correctly and with the correct resistor value which should be 1 ohm.
Yes I have 2 1R resistors on the bias posts, they are wired how the layout I had attached in my initial post shows it. And yes I will send detailed chassis pictures when I get back from work. Thanks!
 

Pete Farrington

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I used a multimeter to check idle bias. At idle the amp did 33mA from the bias probes on the back of the amp. But at idle the pins on the tube sockets read 55mV
I’m confused by the above; which socket pins were you measuring 55mV at?”
 

sdn25

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Your actual voltages. The Ceriatone information is a guide. We need the information from your amp to help.
I am waiting for new fuses to arrive since my HT fuse blew when I turned the amp on after removing the power tubes. I also don't have power tubes to test since the amp blew em, have some on order though coming in a day or two. Im however afraid to put new tubes in since I don't want another pair to blow up is there any way to check the integrity of the amp without installing the power tubes?
 

sdn25

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Multiple chassis pictures and a voltage chart would be bare minimum needed to begin troubleshooting
Also just a quick question, What exact part of the chassis do you need a picture from? I ask since if its the tube sockets I have to remove the power mode switch as well as the PPIMV to expose the socket lugs as otherwise its covered by those.
 

william vogel

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Also just a quick question, What exact part of the chassis do you need a picture from? I ask since if its the tube sockets I have to remove the power mode switch as well as the PPIMV to expose the socket lugs as otherwise its covered by those.
The whole interior of the chassis. Multiple pictures because trying to investigate is difficult when you only include a small piece of the puzzle.
 

Pete Farrington

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pin 1 read 55mV.
So the voltage between the red and black pin jack test points was 36mV, but 55mV is measured between pin1 and the chassis?

If so, there may be a bad connection between the black pin jack’s solder lug and the chassis.

I suggest to use a light bulb limiter on the amp, until you get this resolved.
 
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sdn25

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So the voltage between the red and black pin jack test points was 36mV, but 55mV is measured between pin1 and the chassis?

If so, there may be a bad connection between the black pin jack’s solder lug and the chassis.

I suggest to use a light bulb limiter on the amp, until you get this resolved.
No, I read 33mA at the bias test points, and 55mV from pin 1 to ground. I think I realised my mistake here. I measured current rather than voltage at the bias test points. However, I'd like to know whether I am able to test the integrity of the amp without installing power tubes. I will also re do my solder joints and try to clean up the amp. Sorry for my naïveté as this is my first amp build.
 

william vogel

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No, I read 33mA at the bias test points, and 55mV from pin 1 to ground. I think I realised my mistake here. I measured current rather than voltage at the bias test points. However, I'd like to know whether I am able to test the integrity of the amp without installing power tubes. I will also re do my solder joints and try to clean up the amp. Sorry for my naïveté as this is my first amp build.
So you’re suggesting that you had your meter set to the wrong measurement? If yes, the meter would have definitely given a false reading of current because the parallel path through the meter and across the resistor would be a false reading. I suggest to never attempt to measure current with a meter because you have to break the circuit to insert the meter in series and creates a lot more work to perform the measurement. Voltage drops across known resistances are accurate enough to derive current in nearly all instances. You need to remember though that probing grid feeds generally create false readings because the meter’s impedance loads circuit.
 

sdn25

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u
So you’re suggesting that you had your meter set to the wrong measurement? If yes, the meter would have definitely given a false reading of current because the parallel path through the meter and across the resistor would be a false reading. I suggest to never attempt to measure current with a meter because you have to break the circuit to insert the meter in series and creates a lot more work to perform the measurement. Voltage drops across known resistances are accurate enough to derive current in nearly all instances. You need to remember though that probing grid feeds generally create false readings because the meter’s impedance loads circuit.
understood. I will make sure to only measure the voltage. I will send you a picture once before I turn the amp on after cleaning up my soldering just for you guys to have a look to spot any issues.
 

Pete Farrington

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Voltage drops across known resistances are accurate enough to derive current in nearly all instances
It’s worth bearing in mind that’s all a DMM is doing when set to measure current.

my HT fuse blew when I turned the amp on after removing the power tubes
I'd like to know whether I am able to test the integrity of the amp without installing power tubes
I suggested that you use a light bulb limiter to protect your amp from fault current because it’s suspicious that the HT fuse blew with no output valves fitted. Hence something other than the output valves must be drawing fault current.
 

sdn25

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I suggested that you use a light bulb limiter to protect your amp from fault current because it’s suspicious that the HT fuse blew with no output valves fitted. Hence something other than the output valves must be drawing fault current.
Okay so after some exploration, I found out that one of the 325V wires from the PT Into the rectifiers was sort of hanging on by a few strands. Would that have caused the excess current ?
 

sdn25

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Here are some pictures from the chassis. Hope they can help.
 

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sdn25

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okay good news. I was able to power up the amp after my cleanup without power tubes. No fuse blown yet. what voltages do you guys need ? Id like to report what I've checked so far though.

Heaters read 6.7VAC
VDC at diodes are 480V
VAC at diodes are 352V
Pin 5 on power tube socket measure -28.6VDC
pin 4 and 6 on power tube socket read 477VDC
HT reads 480VDC
 

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