KBD - Kick Back Diode

[hego]

In some schemas for ultra-lineal pentode I can see a diode between the shield grid and the limiter resistence of this tap. It seems avoid the energy generated from de voice-coil.I dont know.

¿Any experience about this?

Cheers

 

[trobbins]

I think you mean a configuration that Dennis Grimwood likes:
Tubes

 

[hego]

Thanks !!!!

 

[jazbo8]

So are they really "kick back diodes"? In any case, I haven't seen too many schematics with them TBH.

 

[trobbins]

I don't believe Dennis has them functioning as a form of kick back diode.

I can't see any benefit in them for a typical pentode mode screen that is always going to force the diode to conduct.

 

[hego]

May be only make something with the AC component.

 

[trobbins]

As long as DC + AC current is still above zero, then diode is conducting. Anode current can fly around (ie. not just sit on a straight load-line), but as far as I can appreciate it doesn't go negative. I can't yet rationalise when or how the screen current could go negative (and hence be influenced by a diode).

 

[DF96]

Putting a diode here is complete nonsense. If the screen current stays positive (as it nearly always will) the diode does nothing apart from dropping 0.7V, so a trival change in bias. If the screen current tries to go negative (which is possible with tetrodes, not sure about pentodes) then the diode will cause a massive change in bias and so huge distortion. So it either does nothing much, or it does massive harm.

You may sometimes see 'explanations' written by people who seem to believe that a diode passes DC but blocks AC (or is it the opposite?). Pure nonsense.

 

[jazbo8]

So it is no surprise that other than Grimwood that no one else is advocating for such "design feature". OTOH, real kick back diodes are often installed to protect the output tubes from damage due to excessive kick back voltage from the OPT.

 

[hego]

An others KBD-supporters from single-ended.com

http://www.single-ended.com/images4/KT90-EL156-KT88SE-standards.png

http://www.single-ended.com/images4/kt88-s.png

http://www.single-ended.com/images4/stopping-diode.png

I can´t find the KBD-sense.

 

[12E1]

There is no sense in this. If the diode ever did become reverse biased, then the connected grid would effectively float, and that could result in any amount of strange behaviour. This seems to be a "fashion" item with no merit.

 

[hego]

I take a chinoise EL34 SE kit and apply the controversial diodes and.... I think the sound is diferent: clear , flat, dinamic, powerfull....better.

I understand nothing.It have no sense.

¿Suggestion?

The netx probe: take off the diabolic diodes.

 

[artosalo]

...I think the sound is diferent....better..

I was sure that someone would soon say something like that

 

[DF96]

I can´t find the KBD-sense.

That is because it is nonsense.

 

[hego]

Because the suggestion is very strong.
Stronger as the technical reasons and the real world.

Uffff....I go to sleep.

 

[hego]

I take a chinoise EL34 SE kit and apply the controversial diodes and.... I think the sound is diferent: clear , flat, dinamic, powerfull....better.

I understand nothing.It hat no sense.

¿Suggestion?

The netx probe: take off the diabolic diodes.

This is stupid. I know.

 

[smoking-amp]

TV Sweep tubes (using lower Vg2) can sometimes get operated too near to the grid 2 heating limits in designs that are pushing the limits. If grid2 wires get red hot, they could become an electron emitter toward the plate when the OT voltage swings up to 2X B+. So the diodes might prevent flashover and destruction in that case (no back conduction allowed).

Maybe those Chinese EL34 are running near grid2 meltdown. Might change the sound with some diodes then.

 

[smoking-amp]

Could temporarily connect an LED across the diode in the reverse conduction direction (versus the diode). If the LED lights up at all with continuous audio signal applied, then some reverse current is flowing. (and possibly an alarm should sound!) There could be a -little- capacitive current that would flow from the wildly swinging plate V coupling to g2. (could show up with high frequency audio signals)

 

[jazbo8]

That is because it is nonsense.

The so-called KBD from the Japanese site is a misnomer, I don't think Grimwood ever use such term. The "real" KBD are commonly connected from the primary windings/anodes to ground (as shown in the example below), and you can see them in some guitar amplifiers.

 

[DF96]

Yes of course. Sometimes people try to slip nonsense past other people by giving it a sensible-sounding name.

'Hego' needs to understand that asking a question, then answering it himself, and then arguing with people who offer a quite different answer is classic trolling behaviour.