UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 14th Oct 2009, 11:19 pm   #21
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
Default 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
Herald1360 is offline  
Old 15th Oct 2009, 10:13 am   #22
barretter
Heptode
 
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
Default 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.
barretter is offline  
Old 15th Oct 2009, 10:27 am   #23
YT2095UK
Retired Dormant Member
 
Join Date: Sep 2009
Posts: 487
Default 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?
YT2095UK is offline  
Old 16th Oct 2009, 8:42 pm   #24
barretter
Heptode
 
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
Default 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.
barretter is offline  
Old 16th Oct 2009, 9:45 pm   #25
Barry Lloyd
Retired Dormant Member
 
Join Date: May 2006
Location: London, UK.
Posts: 129
Default Re: Output transformer essential?

Rather than dozens of LEDs, wouldn't it be easier just to use a single Zener diode?
Barry Lloyd is offline  
Old 21st Oct 2009, 11:43 am   #26
barretter
Heptode
 
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
Default 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.
barretter is offline  
Old 21st Oct 2009, 12:56 pm   #27
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default 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!
kalee20 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:10 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.