No, literally a LED inside a bridge rectifier. Rectifier with ac terminal one to each resistor cathode, LED with dropper resistor between the bridge + and - terminals.
Warning LED when valve stops conducting - a little SS help needed
#91 Re: Warning LED when valve stops conducting - a little SS help needed
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#92 Re: Warning LED when valve stops conducting - a little SS help needed
haha, you're right of course
I think I have the following options...
1) Live with it, keep an eye on the heaters especially if it starts buzzing
2) Rewire the heaters in series, so if one fails they both stop
3) Warning LED per valve in heater circuit (comparator, powered from 8v on DC board)
4) Warning LED per valve pair comparing current through valves
Both 3 and 4 are interesting learning exercises for me, even if in the end I go with 1.
I haven't seen any recent opinions on 2.
#93 Re: Warning LED when valve stops conducting - a little SS help needed
#94 Re: Warning LED when valve stops conducting - a little SS help needed
Forgive the hasty sketch, but you mean like this ?
No external power, so the led lights IF the difference between the voltages at the ends of the resistors differs by more than 3 diode drops plus the drop over the current limiting resistor.
Interesting, but wouldn't that be directly in the music path ?
#96 Re: Warning LED when valve stops conducting - a little SS help needed
Yep, thats it, throw a cap across it of you like, to stop it lighting on audio. Yes, it would be in the signal path, but if its not conducting would it be there?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#97 Re: Warning LED when valve stops conducting - a little SS help needed
Fascinating approach and good learning. Simple to experiment with. I would expect the bridge to impact the sound, but should be easy to prove
#98 Re: Warning LED when valve stops conducting - a little SS help needed
Bet it wont be easy to "prove" given that you expect it willI would expect the bridge to impact the sound, but should be easy to prove
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#100 Re: Warning LED when valve stops conducting - a little SS help needed
OK, yeah! Filament goes t!ts up, other valve is saved. Hoorah! All gorn quiet, = alerts user that summat's up. Nowt ambiguous about that.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#101 Re: Warning LED when valve stops conducting - a little SS help needed
Ha indeed.
It would mean losing the virtual cathodes and of course if there are minor differences in the filaments in the two valves then the voltages may not be identical.
What I'm most interested in is views as to the impact on the music.
#102 Re: Warning LED when valve stops conducting - a little SS help needed
Actually the 10R resistors wouldn't drop enough voltage for the 45s let alone the 26s. When the heater failed I measured 50mA across one resistor and obviously 0 across the other, so across the 2 would give 500mV ish. Not enough to exceed the forward voltage of the 2 diodes and the LED. I could try it if I changed resistors.
#103 Re: Warning LED when valve stops conducting - a little SS help needed
And used Schottky diodes.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#104 Re: Warning LED when valve stops conducting - a little SS help needed
Can we please revisit the virtual cathodes?
I see the sense and purpose of these in a AC heated circuit, turns single ended noise into common mode noise etc, but I'm not seeing the value here.
The DHT heaters contribute very liitle to the noise in an amp, so there's no noise rejection benefit; there must be another reason you have them, Mick?
I could very well be missing something, however? I'd need to be reminded of the full circuit....
I see the sense and purpose of these in a AC heated circuit, turns single ended noise into common mode noise etc, but I'm not seeing the value here.
The DHT heaters contribute very liitle to the noise in an amp, so there's no noise rejection benefit; there must be another reason you have them, Mick?
I could very well be missing something, however? I'd need to be reminded of the full circuit....
Analogue, the lost world that lies between 0 and 1.
#105 Re: Warning LED when valve stops conducting - a little SS help needed
Being brutally honest I didn't understand enough to go against the advice given when I raised the question, also that from Jac Van Der Walle. However it's entirely possible that I misunderstood the advice. I agree it's worth reopening the discussion ...Andrew wrote: ↑Sat Feb 16, 2019 10:23 am Can we please revisit the virtual cathodes?
I see the sense and purpose of these in a AC heated circuit, turns single ended noise into common mode noise etc, but I'm not seeing the value here.
The DHT heaters contribute very liitle to the noise in an amp, so there's no noise rejection benefit; there must be another reason you have them, Mick?
I could very well be missing something, however? I'd need to be reminded of the full circuit....
1) with a DC heated DHT using Andrews DC board, is there a benefit to using virtual cathode resistors
Once that's resolved, the 2nd question...
2) with a pair of DHT with a shared cathode, is there any reason not to wire the heaters in series