GB Battery +/- polarity confusion

[pwdrive]

Nah...Positive  

Lawrence.

[Terry]

At least, that's my attitude - your's might differ ...

Please, let's keep this civil  

No intention to offend, Mark. I wasn't happy with it when I wrote it but couldn't come up with a suitable alternative at the time.

The point I was really trying to make is that not everybody thinks the same way - and there is no reason why they should - but in this case I thought it was of paramount importance to get the facts absolutely right to avoid confusion.

It really dates me to recall that GB Batteries were still being sold by the likes of Woolworths into my early adulthood!

That makes two of us then (at least!)

They were incredibly useful batteries for early experiments with bulbs, electromagnets, solenoids, home made bells and buzzers, etc, always being able to provide the correct voltage for each device. 

The real beauty, I suppose, was having this variable voltage supply in one compact package complete with wander plug sockets.

It would be so much harder these days to juggle with multiple AA cells in a variety of multicell holders ...

[Terry]

Negative.

Nah...Positive  

Lawrence.

Confusing, isn't it!

Still, you got it right the second time! 

[pwdrive]

Just Testin'

Lawrence.

[Nowhere-Man]

I'm just making a replica Drydex H.1001 Grid Bias battery and have got myself into a state of confusion  

It would seem there is still a bit of juice in the original upon which the replica is based, but the polarity is the opposite of what I expected. As you can see from the attached picture I have the positive DMM probe in the + terminal socket and the negative probe in the 1.5 negative socket, the voltage reads (pos) +1.15. Surely, it should read (neg) -1.15?
Wouldn't this be putting a positive voltage on the valve grid?

Can someone help this dim-wit please?

Nick.

From what I gather from my very old sets, the Grid Bias battery would be connected + to chassis and - to grid. Its purpose, of course, is to stop the grid becoming too positive. It can be a bit confusing at first as both HT and LT are shown as + - connections with - to chassis but the GB battery is shown differently.

One thing that had me thinking was also there was a need to use a grid leak as well. And now it starts to get a bit more involved.

Starting off with the bias on the grid, most modern mains sets will do this a large resistor in series with the cathode and leading to chassis. This prepares to set the bias. The end of the resistor close to cathode will be more positive and the chassis end will be negative and can be tapped into. Add an RF bypass capacitor and that eliminates any R.F. there So, now you can add another resistor - the Grid Leak. In the very early sets this would go between the grid and the + filament. It seems any correct positive connection will do the job at the correct voltage to allow electrons to escape from the grid if it becomes too negative. So, in a way, it's a bit like another bias on an established bias - all to make sure the grid doesn't go out of wack

Then it starts to tie

[Terry]

... From what I gather from my very old sets, the Grid Bias battery would be connected + to chassis and - to grid. Its purpose, of course, is to stop the grid becoming too positive.

This has already been covered further back in this thread ...

One thing that had me thinking was also there was a need to use a grid leak as well. And now it starts to get a bit more involved.

The grid bias battery (or other supply) will be a short circuit to audio or RF signals appearing at the grid so the resistor is needed  to feed the negative grid bias to the grid whilst maintaining a reasonable isolation between the signal and the bias supply.

So, now you can add another resistor - the Grid Leak. In the very early sets this would go between the grid and the + filament. It seems any correct positive connection will do the job at the correct voltage to allow electrons to escape from the grid if it becomes too negative. So, in a way, it's a bit like another bias on an established bias - all to make sure the grid doesn't go out of wack

I've re-read this several times and still can't get my head round it!

Are you seriously proposing two grid leak resistors? There will only ever be one, returned to the bias supply or, if there is no separate bias supply as in, say, a one-valve TRF, to the earth rail.

[pwdrive]

Maybe he means means two resistors, a grid resistor and the cathode bias resistor.

Then there's grid resistors and contact potential....maybe...possibly...interesting, some stuff here on that:

http://frank.pocnet.net/other/AWV_Radiot..._20_11.pdf

Lawrence.

[Terry]

I don't think he's referring to cathode bias here and the potential created by the space charge surrounding the filament/cathode is a bit beyond the average experimenter - I doubt the average set designer ever paid much attention to it either, but interesting all the same.

[Nowhere-Man]

Maybe he means means two resistors, a grid resistor and the cathode bias resistor.

Then there's grid resistors and contact potential....maybe...possibly...interesting, some stuff here on that:

http://frank.pocnet.net/other/AWV_Radiot..._20_11.pdf

Lawrence.

Yes, I meant more or less that. The Hartley Oscillator circuit is also worth getting your head around with respect to bias.

[pwdrive]

He does mention cathode bias and also grid leak, I think he's getting there with the understanding but maybe has difficulty cramming everything into his post(s) in a clear fully explanatory way due to his limited internet time access arrangements.

EDIT: Whoops, posts crossed.

Lawrence.