Help biasing my Ceriatone Plexi 51 build

[ledvedder]

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

 

[Pete Farrington]

Does removing the valve in V3 affect the voltages at the bias test points?

 

[Jon Snell]

There are no grid leak resistors shown. How does J-Z and K-Z get voltage?

 

[Pete Farrington]

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!
@ledvedder are you sure your meter is working ok, eg try a new battery.

 

[ledvedder]

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.

 

[Pete Farrington]

I also replaced the 4k7 3w at point E with a 10k 3w

That’s just a tweak to the HT supply to the ECC83s, it shouldn’t have affected the bias at all.

 

[ledvedder]

That’s just a tweak to the HT supply to the ECC83s, it shouldn’t have affected the bias at all.

Perhaps is was changing the 180k to 220k?

 

[Pete Farrington]

Perhaps is was changing the 180k to 220k?

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?

 

[ledvedder]

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]

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.