Leilani amp by Gourley question

[Resonator Guy]

I am in the process of repairing a beauty Leilani amp from 1956, I believe, based on the speaker code.This purple and gold MOT lovely has a 6SJ7, 6V6, 5Y3 line-up, with no on/off or volume control. I have recapped it and although it is still sounding somewhat weak, considering it is going flat out all the time, it is sounding pretty good. The speaker itself is fine.
There is a strange Flash Gordon looking device between the input and the preamp tube. If I bypass this the amp howls and whistles.

Can anyone tell me what this is? I have searched the site for similar questions but can't locate any, although I vaguely recall one last year.
Thanks

[teemuk]

It's a "bias cell".

Just Google search it.

[Resonator Guy]

Thanks teemuk. There seems to always something new to learn about something old. No wonder it howled when I bypassed it.
As it seems to be working fine at the moment I will leave it alone. I will search and see if there is an easy replacement. One site suggested a watch battery.
I am also wondering if my 1956 estimate is wrong, as these bias cells seem to be a 30s and 40s item, but the amp has a permanent magnet speaker, not a field coil, so now I am not sure.