LEDs used as Cathode Bias

[NickG]

Can I also suggest that if you use battery grid bias going to 0v you need to have a blocking cap between the input and the grid. If you use the battery in line as it were, you don't need that, assuming whatevers driving that doesn't mind the voltage on its output.

[Richard]

Originally Posted by NealG

Sure you can Richard, I did it here and also did it the same way on my WAD 300B's

Nice one Neal, so it can still be done both ways. Sound would be different with Kel84/Kit6550 though as the bypass network ends up being different.

Rich

[Richard]

Originally Posted by NickG

Can I also suggest that if you use battery grid bias going to 0v you need to have a blocking cap between the input and the grid. If you use the battery in line as it were, you don't need that, assuming whatevers driving that doesn't mind the voltage on its output.

Not sure I follow the "battery grid bias going to 0v" bit Nick but agree about a cap between input and bias feed.

Rich

[NickG]

Not very clearly described, what I meant was there are two ways of doing this,

1, Connect a battery with its +ve to 0v, and its -ve to a resistor, and teh end of the resistor to the grid of the valve, the input signal is connected to the grid, and a blocking cap is needed.

2. connect the grid of the valve to the -ve of the battery, the +ve of the battery to the input. Use the low resistance of the source to reference the battery to ground.

[Richard]

Ah, thanks Nick,

1) I understood and

2) I've not seen before, don't think it would it work with a pot between battery and source as in Kel84. Does the signal like going through a battery, never thought what the impedance or capacitance might be do you have any typical figures, can you get BG batteries

Rich

[NickG]

It should work with the pot, as the pot will give the +ve a ground reference.

As to how it sounds, that I don't know, just pointing them out as ways it "could" be done. It would work best I guess with a transformer coupled stage. What I wonder is would it sound any different with the battery between the TX and the grid or between the TX and ground. I keep trying to convince myself the signal isn't "passing through" the battery in the second case, but it is rearly. Don't know, as there is only the current to charge the miller passing through it, I can imagine it may not have much effect. Then again, is it much different with the amplified signal passing through the battery if its in the cathode circuit?

I do know that Paul and myself did find in at least one case, that cathode bias sounded far better than battery bias, but I can't remember details.

In effect its the same idea as the odd biasing method in Romy's 6c33c amp that someone posted a link to, except that generated the voltage across a resistor.

[Richard]

Yes you're right it should work it's only potential no current.

My overview is that it will sound different in the case of Kel84 if only for the reason that I don't think the original bypass network can be kept (but Neal's probably working on it ).

I think also that John's right in that you can't have (complete) battery bias and feedback. Neal and my suggestions both rely on a 100R divider leg which will introduce some resistor cathode bias.

In all cases the battery is being used as an AC path and we don't know the impedance/capacitance/reactance/inductance; if it's linear etc.

Rich

[IslandPink]

Have a look at :

http://www.izzy-wizzy.com/audio/preampnew.html

( Steve Robinson's site - scroll down to final circuit )

http://artemislabs.com/docs/PL1_Manual_1.pdf

( John Atwood's Artemis Labs design )
Can vouch for the excellent perfomance of the latter ,
despite fairly standard valves .

Mark

[Richard]

Yes both TX coupled inputs though Mark. Kel84 could be grid fed with a cap blocking the input or cathode biased as in this neat design, anyone tried it, for records I think ,



feedback for Kel84 could be applied with a divider in both cases.

Rich

[NealG]

My overview is that it will sound different in the case of Kel84 if only for the reason that I don't think the original bypass network can be kept (but Neal's probably working on it ).

Hehe, OK anybody have a Kel84 they can lend me!