Vampower bias circuit

[Stratfordade]

Could anyone please explain to me how this bias supply works on a Vampower 100W amp? I don’t have the actual amp but am considering one so just educating myself.

The bias supply (it’s fed to presets subsequently and so adjustable for each EL34) just looks like a chain of resistors from the HT secondary, with no voltage dividing going on. How does it take the HT down to a reasonable level (say -50V) for fine tuning after that?

[Jon Snell]

That is a simple capacitor fed DC supply.
The capacitor (2 x 4uF 2uF in this case supply a low current to the clamp diode. The clamp diode allows the voltage to charge in the 64uF only the one way, (possitive voltage being blocked by the diode) providing a very simple -ve DC supply.
Not my choice of doing it. I would use one PMP capacitor and two diodes.

[Stratfordade]

Oh that’s very clear now thank you Jon. I’d no idea supplies were ever arranged in this fashion.

[J M Fahey]

Very infficient but bias takes microscopic power so it´s acceptable.

[Mick Bailey]

with no voltage dividing going on. How does it take the HT down to a reasonable level (say -50V) for fine tuning after that?

It's the reactance of the capacitor(s) that gives the effective voltage reduction. Similar circuits are used to reduce mains voltage in LED bulbs. I just made an LED night light built into a UK mains plug that uses the same idea. AC capacitor voltage reduction is dependent on having a fixed frequency and reasonably fixed load, so a bias supply is a good candidate. Marshall used this quite a bit and it eliminates the need for a dedicated bias winding.

[Helmholtz]

This bias supply is actually more tricky than it seems.
It's a kind of charge pump and operation involves 4 diodes and the B+ load.