Ceriatone Plexi 50 Lead build kit Red plating (SOLVED)

sdn25

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Okay guys so nik got back to me just now. He basically confirmed that my OT is dead based on the imbalance in the resistance readings. He mentioned how tubes can cause this.
 

Dblgun

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What did Nik say, is he going to help you out with the transformer?
 

sdn25

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What did Nik say, is he going to help you out with the transformer?
I don’t think he will be covering for the OT because I think at the end of The day it was my mistake In setting the bias too high. This was his exact response :


Hi

If that’s the case, indeed the OT is shot.

It should be about balance.

A tube, for example, can kill the OT like this.

At the same time, your screen resistors might be toast, as well.

Thanks!

nik “.
 

Dblgun

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I don’t think he will be covering for the OT because I think at the end of The day it was my mistake In setting the bias too high. This was his exact response :


Hi

If that’s the case, indeed the OT is shot.

It should be about balance.

A tube, for example, can kill the OT like this.

At the same time, your screen resistors might be toast, as well.

Thanks!

nik “.
Yes, that would be my take on his response as well. I can certainly understand his position but it was worth contacting him to confirm. I think you will be very happy with the Heyboer. Have you finished your work on the amp in preparation of the new transformer arriving?
 

sdn25

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Yes, that would be my take on his response as well. I can certainly understand his position but it was worth contacting him to confirm. I think you will be very happy with the Heyboer. Have you finished your work on the amp in preparation of the new transformer arriving?
Yep, just took the old one out and did some other cleanliness work.
 

sdn25

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GOOD NEWS EVERYONE: Just got the new OT today and installed it, and it works!! The struggle is over I believe, checked all voltages, bias etc and they're all stable. Is there anything else I should check?
 

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Dblgun

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Great news! Sorry if I missed it but did you determine the cause for the demise of the original O/T?
 

sdn25

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Great news! Sorry if I missed it but did you determine the cause for the demise of the original O/T?
Yes, My wrong method of reading bias current led me to have the bias set at about 70mA. This caused the power tubes to meltdown and the overcurrent shorted the OT windings.
 

sdn25

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Update: Im getting 422V on the screens of the el34 biased to 30mA is that normal?
 
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Pete Farrington

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the power tubes to meltdown and the overcurrent shorted the OT windings
What’s strange / concerning is that a fuse didn’t blow in response to the output valve/s shorting. That’s kinda the point of them, the OT isn’t supposed to fail before a fuse.
Hopefully the faster responding fuse type you’ve got in the HT now will prevent that recurring.
Im getting 422V on the screens of the el34 biased to 30mA is that normal?
If the HT is around 430V (eg at the rectifier output, reservoir cap or HT fuse), I think so.
The 1k screen grid resistors might typically drop 3 to 6V at idle (varies according to the individual valve and its anode current).
I suggest to always check the mains voltage when checking voltages inside an amp. All the power supply voltages will largely be a ratio of the particular mains at that time. So if the mains is lower or higher than you’re normally used to seeing, all the internal voltages will follow suit.
 

sdn25

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What’s strange / concerning is that a fuse didn’t blow in response to the output valve/s shorting. That’s kinda the point of them, the OT isn’t supposed to fail before a fuse.
Hopefully the faster responding fuse type you’ve got in the HT now will prevent that recurring.
I asked Nik about that, he said the fuse not blowing depends on how the failure occured, I don't know what he meant by that.
If the HT is around 430V (eg at the rectifier output, reservoir cap or HT fuse), I think so.
The 1k screen grid resistors might typically drop 3 to 6V at idle (varies according to the individual valve and its anode current).
I suggest to always check the mains voltage when checking voltages inside an amp. All the power supply voltages will largely be a ratio of the particular mains at that time. So if the mains is lower or higher than you’re normally used to seeing, all the internal voltages will follow suit.
Mains is 241V. maybe now that I have it setup properly its reading lower I presume? its in fact within the voltage chart published by Ceriatone. This is because before I was reading 455-460 on the screens when I had the blown OT installed.

I also had another question, whenever I power up the amp, or when after I turn it off, the tubes make a sort of glassy crinkling sound, not from the speakers, but from the tubes themselves, is that normal? Is it due to the thermal expansion and contraction?
 

Pete Farrington

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whenever I power up the amp, or when after I turn it off, the tubes make a sort of glassy crinkling sound, not from the speakers, but from the tubes themselves, is that normal? Is it due to the thermal expansion and contraction?
I think that’s likely, I don’t worry about it.
 

sdn25

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Is this normal? When a signal is passed the inside of the power tubes glow, v4 glow is more prominent than v5. Its not red plating. One picture shows the tube without signal passed, the other with signal passed.
 

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Pete Farrington

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When a signal is passed the inside of the power tubes glow
That’ll probably be the screen grids. It’s fairly typical with Marshalls for first push pull side to take more of a beating that the second.
From that, it’s perhaps a good idea to swap the valves around every once in a while.
 

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