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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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14th Oct 2009, 11:19 pm | #21 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
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Re: Output transformer essential?
LEDs could be useful for cathode bias at lowish (<20mA) current since their forward voltage drop can be several volts (depending on colour). The low slope resistance should mean that a decoupling capacitor would not be needed.
I can't see them helping with an output stage, though. Chris |
15th Oct 2009, 10:13 am | #22 |
Heptode
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
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Re: Output transformer essential?
Using LEDs to set the output stage bias is possible and has been done but you need a lot of them in series/parallel combination to set the correct voltage and to have the current capability.
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15th Oct 2009, 10:27 am | #23 |
Retired Dormant Member
Join Date: Sep 2009
Posts: 487
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Re: Output transformer essential?
yeah, I figured you would need a whole load of them too, but reading some of the posts in different places, you`re left with the distinct impression that you only need a single One to do the job.
even to suggesting that you can mount this LED (singular) on the front panel as an idicator! now I`v popped an LED with a watch battery in the past, I can imagine a live valve setup blowing through a whole stack of them with the same ease if not extremely careful. I think this topic may be worthy of a thread on it`s own? |
16th Oct 2009, 8:42 pm | #24 |
Heptode
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
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Re: Output transformer essential?
If you google "red light district 15w push-pull" you will find discussion pages and a finished article on how to build a push-pull EL84 amplifier with red LEDs used to bias the output stage. You need 36 LEDs in a six-by-six array for this circuit. I use single red LEDs to bias the signal triodes in my phono preamp.
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16th Oct 2009, 9:45 pm | #25 |
Retired Dormant Member
Join Date: May 2006
Location: London, UK.
Posts: 129
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Re: Output transformer essential?
Rather than dozens of LEDs, wouldn't it be easier just to use a single Zener diode?
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21st Oct 2009, 11:43 am | #26 |
Heptode
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
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Re: Output transformer essential?
Probably because Zener diodes over 5.6V have a positive temperature coefficient and the higher the regulating voltage the higher the ptc, which means that they could self-destruct in a hot environment which the area near an output valve's cathode tends to be.
You would also need to bypass it with a capacitor to minimize noise injection, which you don't need to do with an LED. There are probably other reasons that I don't know about, but nobody seems to do it. Another solution is to use a suitably biased LM317 voltage regulator. LEDs are a lot prettier, though. |
21st Oct 2009, 12:56 pm | #27 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
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Re: Output transformer essential?
Barretter is right about the PTC of zener diodes over 5.6V. But, that'll actually give a small stabilising effect.
Suppose the temperature rises slightly, then the zener voltage increases (PTC). This then gives increased bias to the valve, which thus drops its anode current. So the zener power dissipation reduces slightly, negating the initial temperature rise. Zener diodes less than 5V or so have a negative temperature coefficient, which would thus have a destabilising mechanism. However, you'd need a serious amount of current coupled with a small diode, to be in danger of runaway. There's also the matter of bypassing with a capacitor. In an output valve, I'd be really surprised if the noise from a Zener diode is enough to be significant, so the capacitor could well be the icing on the cake. Of course, whether anyone wants to contaminate their valve circuits with nasty PN junctions, holes, etc is another matter! |