The Red Light District - another PP EL84 amp

[TheGimp]

I just measured 40 LEDs (T-10) I have from the 80s and they were all between 1.63 and 1.65V at 10mA. If I crank them up to 30mA they get up to 1.69-1.72V.

Next I guess I need to measure the dynamic impedance.

I'm looking for less than 5 Ohms, right?

Otherwise, why not configure the LM317 as a voltage source and bias the cathode at 10.5V? Eliminate the current source for a voltage source which is what the LEDs are doing any way.

I'll try all three and see for myself.

[GK]

Originally Posted by TheGimp

Otherwise, why not configure the LM317 as a voltage source and bias the cathode at 10.5V? Eliminate the current source for a voltage source which is what the LEDs are doing any way.

Won't work. An LM317 has a single NPN series pass element, so, as a voltage regulator, it can only source current, not sink it. In other words, the cathode current of the tube will reverse bias the pass device and the voltage will shoot up to whatever.

It is fine to use an LM317 here as a current regulator/sink as it will need to be bypassed with big electro anyway. This makes the HF performance of the current regulator/sink irrelevant.

But anyway, all this still looks quite pointless to me. I'd just connect the cathodes to ground and use (adjustable) fixed bias on the grids.

[ttan98]

Originally Posted by GK

But anyway, all this still looks quite pointless to me. I'd just connect the cathodes to ground and use (adjustable) fixed bias on the grids.

With fixed bias, cathode current tends to drift esp. when the tube ages.

[GK]

Originally Posted by ttan98

With fixed bias, cathode current tends to drift esp. when the tube ages.

That is why I said adjustable. In any case, LED or voltage regulator bias at the cathode basically is fixed bias with the same issue WRT tube aging and drift / balance. You're just raising the cathode to a fixed potential above ground instead of lowering the grid to a fixed potential below ground. The former is the hard way to go about it.

[SY]

That's correct, Glen, the bias from LEDs is fixed, but again, the issue is recovery after overload (blocking). Using conventional fixed bias, a clipping event becomes a clllliiiiiipppppiiiiinnnnngggg eeeeevvvvvveeeeennnnntttttt.

[GK]

Originally Posted by SY

That's correct, Glen, the bias from LEDs is fixed, but again, the issue is recovery after overload (blocking). Using conventional fixed bias, a clipping event becomes a clllliiiiiipppppiiiiinnnnngggg eeeeevvvvvveeeeennnnntttttt.

You've raised the cathode potential to avoid the need to lower the grid potential (conventional fixed bias, as I understand it). How does that improve the revovery from blocking?

[SY]

Read the references. Especially the treatment in Crowhurst and Jones. And, of course, as I point out in the text, this was experimentally verified.

[GK]

I don't have Crowhurst and Jones handy, but, I just skimmed through your blog. You don't actually explain why fixed bias applied to the cathodes should improve the overload recovery there either, but I'm keeping an open mind.

The benefit of the large grid stopper resistors used is valid, but that goes for their inclusion in any conventional fixed bias circuit (cathodes grounded) as well as your LED bias circuit.

When you say "conventional fixed bias", do you mean "conventional fixed bias without the large grid stoppers"?

[SY]

Large grid stoppers definitely help conventional fixed bias as well (one more argument in favor of pentodes). If memory serves, this was a Good Trick used by ElectroVoice back in the '60s.

[Yvesm]

IMHO, clipping on a sudent "Needle in the red on the Vumeter" is not a problem by itself and the ear accomodates easily for this brief overload/distortion.
Worst is what happen just after if the peak somewhat altered the bias and until it recovers its correct value.
A cathode bypass cap may retain a wrong bias voltage for more than half a second.
Of course the link cap may possibly do the same, it shouldn't be over sized . . . il not removable !
BTW, biasing using a positive voltage source at the cathode or a negative one at the grid makes no difference for me.

Yves.