How does this work?

[jimi]

Take a look at this schematic of a selmer amp.
It looks a bit complex to me I can't get my head round it. The cathode of the first stage is grounded yet on an amp I have here I measured -0.8volts on the grid. Where does it get this negative voltage from

[jimi]

Link

Here's the link http://britamps.mysite.wanadoo-membe...tic/lgmk1.html

[Alex/Tubewonder]

Look for a continuous DC path from grid to ground i.e through resistive elements only. Grid leak current causes a voltage drop along this path. When you connect your meter between grid and ground it provides additional shunting resistance thus your measured grid voltage is false. Grid leek current is very small. A cleverly wierd or wierdly clever little design, but Selmer had their own thing going.

[Tom Phillips]

There were some very early Fender amps that used grid leak biasing. Some examples are the 5B2 Princeton and the 5B5 & 5C5 Pro. The design was dropped early on.

[Enzo]

Look up grid leak bias, there are good explanations around. Basically, the cathode emits electrons, which have negative charge. They float around in a little cloud called the "space charge", and the closest thing they come to is that grid, so they accumulate there. That gives it the negative charge. There is only the tiniest bit of current available, that is why the grid resistor is a very high resistance. SOmething like 5 Meg being common. Lower resistances would drain away the charge.

So the grid finds itself at -.8 volts or whatever from the accumulated electrons. Since the cathode is grounded, the net bias is .8 volts. As far as the tube is concerned, this is the same as if the grid were at zero volts and the cathode at +.8v. The bias is the difference between the two, not the absolute voltage they are at.

[jimi]

Thanks for the replies looks like a nice simple way to bias a tube. It doesnt seem to be used much so whats the disadvantages with it?

[Ray Ivers]

It doesnt seem to be used much so whats the disadvantages with it?

Generally, high noise and very little headroom (I don't know if that's the case with this specific Selmer circuit, as there's that negative-feedback tone-control thing going on).

Ray