Help biasing my Ceriatone Plexi 51 build

ledvedder

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Hi guys, I decided to remove the effects loop out of my Plexi 51 build, since I don't use it. It's completely pulled out of the amp. I connected the treble wiper to point Q on the layout below. I also replaced the 4k7 3w at point E with a 10k 3w, based on someone's suggestion. Now, the highest I can get the bias is 32mV. Any thoughts?

My voltages are as follows:
Pin 3 on power tubes: 399v
Pin 5 on power tubes: -32v
Pin 1 on PI: 200v
 

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Pete Farrington

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There are no grid leak resistors shown. How does J-Z and K-Z get voltage?
From the Z terminal on the board, between the J and K terminals for the V3 LTP outputs. The Z terminal being linked to the bias supply output.
Pesky Ceriatone layouts! :mad2:
@ledvedder are you sure your meter is working ok, eg try a new battery.
 
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ledvedder

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I put the 3w 4k7 back in and changed the 180k to 220k. I can now bias to 35-36mA. I'm getting 400v on the plates, and 329v at the junction of the 100k/82k resistors. I'm also getting -31 at point Z.
 

ledvedder

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At A4? Yes, that will move the bias range hotter.
So it was probably running in the low 30s before you took the fx loop out?
Yep A4. Before I took out the fx loop, I could bias up to over 40, but I was setting it at 35. When I removed the fx loop, I also changed the 4k7 at E to 10k. That's why I was thinking that caused a biasing issue.
 

Pete Farrington

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Possible explanations -

the mains voltage was high before, now it’s low. Always check mains voltage when checking voltages inside an amp.

the EL34 have become a fair bit weaker.

different master volume etc settings may be allowing hiss etc to add to the EL34 idle current.
 

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