Ceriatone Plexi 50 Lead build kit Red plating (SOLVED)

sdn25

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I fear it may be the OT :( . How do I test the OT to ensure its functionality?
 

neikeel

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The OT will not create runaway bias.
You can do static tests for resistance to determine shorts, open circuit first. Then you need to put a voltage through the primary and measure secondary voltages progressively up to working voltage. Need a variac and/or o’scope to do that.
 

sdn25

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the amp was working fine until this whole volume loss/tube failure issue. The bias seems to work fine. All the other voltages are fine. I mean what else is there to check at this point.

its getting quite tiring and de motivating ngl. The distortion from the amp sounds fine but its the volume output that's the issue. Would you guys know what exactly are the components that are responsible for the volume output? so that I can test each component individually? Apart from that I will remove the PPIMV and see if that is causing any issues.
 

Spanngitter

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You might have a leaking coupling cap (0.022uF connecting to the 5k6 Grid Stopper) which effects your bias voltage at Pin 5, another possibility could be a resistor drifting away under load due to heat. For checkng the OT the only thing you could do is measure (with Amp unplugged, caps discharged and power tubes removed) resistance between OT CT connection (red cable) and Pin 3 of each power tube socket (blue/brown). the result should normally nearly identical (it can't due to the principle of winding OTs) and in a range of approx. 42 Ohms (typical value for a 50Ohm Marshall OT).
 

sdn25

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You might have a leaking coupling cap (0.022uF connecting to the 5k6 Grid Stopper) which effects your bias voltage at Pin 5, another possibility could be a resistor drifting away under load due to heat. For checkng the OT the only thing you could do is measure (with Amp unplugged, caps discharged and power tubes removed) resistance between OT CT connection (red cable) and Pin 3 of each power tube socket (blue/brown). the result should normally nearly identical (it can't due to the principle of winding OTs) and in a range of approx. 42 Ohms (typical value for a 50Ohm Marshall OT).
Oh, so despite the amp being only a couple days old you think Its possible to have leaky caps and faulty resistors? And about the OT, I read 15ohms from CT to blue primary and 63ohms from CT to brown primary. Does this mean my OT is shot? I sure hope not.
 

sdn25

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how might I check for leaky coupling caps?
 

Pete Farrington

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I read 15ohms from CT to blue primary and 63ohms from CT to brown primary. Does this mean my OT is shot?
That’s not good, sorry.
What’s the blue to brown resistance?

It’s worth contacting Ceriatone, they might feel that their OT should have coped, bearing in mind the HT fuse didn’t blow.
 
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william vogel

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Okay so at the setting with the most -ve voltage on the bias pot, the meter reads 3mV!! I think this is progress. Now what must I
I’m sorry you’re having troubles. I would have thought the transformer would live through a 500mA cycle. I hope the ht fuse is a fast blow. A slow blow takes about 1amp and a decent period to open. A fast blow takes more than 500mA but it opens quickly.
 
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XTRXTR

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I feel sad for you🙁You built this amp, you were rockin' and now sadness. I know that sucks. Don't beat yourself up you had it, just perhaps one mistake and everyone makes mistakes.

You will only ever measure voltages and resistance in an amp so the misinterpretation of your measurements using your DMM for mA instead of mV may have been the mistake. This likely led to bias voltage set too high and a high plate dissipation which could have led to the OT being current overloaded. This is all speculation on my part but seems a likely possibility.

It is odd that the HT fuse didn't blow immediately. Perhaps you shut it down just before it was about to blow and then the OT still being hot the insulation of the OT windings burned through and shorted together. Then fuses blew after pulling tubes and powering back on due to the short in the OT.

There are some startup procedures you can follow that may help you on your next build: https://robrobinette.com/Tube_Amp_Startup.htm

And some troubleshooting info: https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

After you replace your OT you should follow that startup procedure and test each thing as you go to verify you have a working circuit before adding tubes. Build a lightbulb limiter, its only $20 US and can really save your amp from disaster.

As Pete said it is possible Ceriatone may cover the OT.
 

sdn25

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Alright thank you guys for the help! I will contact ceriatone about the OT. And about the HT fuse, it was a slo blow that was installed previously but I replaced it with a fast blow one. I’ll try and update you guys about what ceriatone says.
 

sdn25

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The slo blo was the fuse that was provided by ceriatone.
 

neikeel

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As above really. Usually expect 40-45 ohms on a Marshall type 50w ot.
 
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william vogel

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You really need to re measure the resistance of the output transformer. It should be around 40 ohms per side to center tap. I don’t know what specs are for the Ceriatone transformer. They could be as much as 65-70 ohms per side but as I said I don’t know. Even if one side of the transformer was semi shorted the amp would output sound or it would blow the ht fuse from the short. I’m not looking at the amp and can’t determine the problem.
 

Seanxk

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Re-measure and remember you're looking at Ohms not K Ohms, clean your leads and their contacts in the DMM, clean the amp contacts, clip solid the CT contact and keep measuring each side until you get a change, If you don't then talk to Ceriatone.

Blue to Brown is not that far off, but your CT measurements are.
 

sdn25

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You really need to re measure the resistance of the output transformer. It should be around 40 ohms per side to center tap. I don’t know what specs are for the Ceriatone transformer. They could be as much as 65-70 ohms per side but as I said I don’t know. Even if one side of the transformer was semi shorted the amp would output sound or it would blow the ht fuse from the short. I’m not looking at the amp and can’t determine the problem.
I will re measure. There is faint output as I said before when switched to play mode. HT fuse does not blow. All solder joints have been re done.
 

sdn25

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You really need to re measure the resistance of the output transformer. It should be around 40 ohms per side to center tap. I don’t know what specs are for the Ceriatone transformer. They could be as much as 65-70 ohms per side but as I said I don’t know. Even if one side of the transformer was semi shorted the amp would output sound or it would blow the ht fuse from the short. I’m not looking at the amp and can’t determine the problem.
im getting the same 15ohm from CT to blue and 62.5 ohms from CT to brown. Blue to Brown is 77.5 ohms. Meter leads are clean they tested 0.1 ohms when shorted.
 

Pete Farrington

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I think the audio symptoms align with the primary resistance measurements, leading me to put a shorted section of the primary as the chief suspect.
As any shorted turns will cause the primary inductance to collapse, which will hobble the amp’s power output.
The amp will still be able to put out sound, but at a small fraction of the intended power output. And it will put the output valves under stress, as their load is now pretty much just the primary resistance.
 

sdn25

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In case the OT is dead, is the hammond 1750N any good?

 


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