Orange OR50 40th Anniversary Bias?

[misterc57]

Anyone know if there is a bias adjustment on this amp? Also trying to find a schematic.

Is this it in the second pic?

Thank you, Mark

[Delta362]

Looks like it would be the bias pot. I see the grid wires from the octal sockets going to that part of the board. Just looking at the nearby components, I'd say yes. To be safe, you can try tweaking it with the power tubes removed and monitoring pin 5 for change.

[g1]

Other people have reported them having adjustable bias. Are there any other trim-pots in there? Do you find negative DC on TP3 or 4?

[Mars Amp Repair]

Just from the layout of the board, it looks like Vr1 would be a bias control. Glen

[misterc57]

Yes so that is the only trim pot I can find on the board and it does control bias.

EL34 reading bias v -29 current 49mA

[g1]

Yes so that is the only trim pot I can find on the board and it does control bias.

EL34 reading bias v -29 current 49mA

That seems a lot of current if for a single tube. But -29V isn't much bias, something like -38V may be more appropriate, depending on other power tube voltages.

[misterc57]

That seems a lot of current if for a single tube. But -29V isn't much bias, something like -38V may be more appropriate, depending on other power tube voltages.

I suspected that was high. These reading are on the existing 6 year old Mullard EL34. Amp is sounding fine with no issues.

I am going to install new pair of Black Sable EL34. Will dial up the bias and report back with voltages and current.

Thank you

[misterc57]

Black Sable readings

bias -31 V, 38mA
plate 372 V

[g1]

That seems a lot better.