Ping Sy: LED for Cathode Bias

[SY]

Originally posted by lndm


I like to bias my 6sn7gt's with led's. I don't want to run them at 8-10mA. I do run them at 3-6mA. I'm considering feeding them the few extra mA from the supply, but this is a little ugly. Is it worthwhile?

You can feed the LEDs from the supply via a CCS. This minimizes injection of supply noise. But the 6SN7 really is happier at 8-10mA.

[lndm]

Ok, maybe I'll try 8-10mA.

These are not GTA/B just GT, max Pa 2.5W each with both fired. Will 2W and 10mA allow long life?

At 10mA, a 50k Ra (what I like to use) would drop 500V. Is 20k a reasonable choice for the 6sn7?

[EC8010]

Originally posted by ryssen
I use the one with MJE350 at the anode,wouldnt it be the same to remove R2 and use R3 instead?Better to be able to adjust?

No, use both. Adding R2 limits the range of adjustment anf makes it easier to get the correct setting without any risk of smoke.

[Ryssen]

You can feed the LEDs from the supply via a CCS. This minimizes injection of supply noise.

How is this done,do you mean to make another one just for the Led´s?

No, use both. Adding R2 limits the range of adjustment anf makes it easier to get the correct setting without any risk of smoke.

Ok.

The first schema with 2 Bc549c whats it for?Just for lower current or?

And the one in the middle can I use it for a 6B4G with the "Cathod resitor?"How,Is it explained somewhere?
Nearly identical questions I asked here http://www.diyaudio.com/forums/showt...301#post899301

[EC8010]

The qualities that you want from a CCS are high output resistance and low output capacitance. For a given design, these are optimised by using the most fragile devices you can get away with. Hence, the 2 x BC549C is great when not much voltage is across the upper device and its power dissipation is low. Once the voltage goes up it tends to push the power dissipation up and that means that BC549C is no longer suitable and MJE340 is required. But a CCS using MJE340 as the upper transistor isn't quite as good as one using BC549C.

[Shoog]

"How is this done,do you mean to make another one just for the Led´s? "

That is what SY is seeming to suggest.
My suggested approach has not introduced any noise into my preamp and is considerably simpler than a full CCS.

Shoog

[Ryssen]

In my tweaking progress. I removed the RC filter I had before the CCS,and what I observed is the voltage at the anode
dropped to148v from 170v (6S4) and the B+ got up to 360-365v
So now I have about 300v across the 6B4G little high.Thougt the voltage would be higher at 6S4 with no RC.
But every tweak I have done has bee possitive to the sound,much clearer.So now wich one to test Shoog´s or SY´s,I dont´know.´Then this 300v to the 6B4G could it be lowered with
some CCS at the Cathode"?Apriciate sugestions.

[SY]

How is this done,do you mean to make another one just for the Led´s?

Exactly. It doesn't have to be a super current source, just an OK current source. One end to the B+, the other to the cathode.

Now, the assumption here is that, universally, lower LED impedance is better. But that may not always be the case, as some intriguing results from rdf in another thread suggest.

[Ryssen]

Exactly. It doesn't have to be a super current source

I was locking at MPSA92 it can take 300v if I use 2 of them and 2 red Led´s and a trimpot/channel,cheap and nice,would i be alrigt?Or is it enough with one for both chanels Led`s?

My suggested approach has not introduced any noise into my preamp and is considerably simpler than a full CCS.

Can I do this to and use both metods,fun to tweak.

[lndm]

Originally posted by SY
the 6SN7 really is happier at 8-10mA.

I set my driver at 10mA, 400V supply and a couple of LED's to bias. From what I can tell so far, they seem made for this. Thanks.