Warning LED when valve stops conducting - a little SS help needed

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RhythMick
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#121 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

26s back to individual DC boards, no virtual cathodes, bias resistor connected to the +ve filament pins.

Valves are in balance to within 0.25mA of each other.

Warning up with TV then a quick listen. Unfortunately I'm at work early... :(

... But hey it all works... :D
RhythMick
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#122 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Well I feel very confident in saying that those virtual cathode resistors were damaging the sound.

I now have them out on both 26 and 71A stages. 45 still have them as it's a more awkward access.

After a week of listening I can confirm first impressions that the music is so much better all round. It feels more real and in the room, more musical, more foot tapping and dancing from everyone who hears it (no prompting).

Bass is tighter, more controlled and more musical for it. Voices are so much more emotional. Live performances more convincing. There's also less top end hardness with those tracks that could become shrieky.

No question for me. I connected cathode resistors to the +ve end of the heaters and arranged the bias so the midpoint hit my operating points. Everything bang on. There's a small buzz still, very low. I'll chase that down after I get rid of the VC on the 45s.

Happy happy boy. Now I'll look at warning leds
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jack
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#123 Re: Warning LED when valve stops conducting - a little SS help needed

Post by jack »

RhythMick wrote: Sat Feb 23, 2019 10:16 am ... Now I'll look at warning leds
It's taken 9 pages to get back to where we started 😎
Vivitur ingenio, caetera mortis erunt
RhythMick
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#124 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

jack wrote: Sat Feb 23, 2019 3:46 pm
RhythMick wrote: Sat Feb 23, 2019 10:16 am ... Now I'll look at warning leds
It's taken 9 pages to get back to where we started 😎
Fair point
RhythMick
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#125 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

However - that was a very productive diversion from the original question. I've tried and dismissed the idea of series filaments. Having lost the virtual cathode resistors that now opens up a number of options.

1. Transistor switch powered from D44 pin 2
2. Sense resistor in heater -ve with comparator powered by D44 pin 2
3. Optocoupler in cathode or anode
4. 10R sense resistor in cathode with comparator powered by independent source (either battery or DC-DC converter)


1 and 2 would work, as long as the DC board itself was powered. However if that's the fault condition (eg wire works loose from the screw terminals) then the warning circuit also fails.

3 would work but I'm reluctant to put the diode in the anode.

4 would work and catch any fault condition which causes the valve to stop conducting. Battery would require the outputs to be XOR'd to avoid them lighting up when the amp is powered off. DC-DC converter to power the comparator is attractive. Single LM293P would monitor both valves. The music signal could cause the LED to flicker, but hysteresis could be used to overcome that and only warn when the current drops and stays low.

I'll finish getting the VCs out of the 45 circuit then look at a test circuit for the comparator, battery to begin with.
RhythMick
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#126 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Further update. I decided to try separate cathode resistors (with decent bypass caps). I'd shied away from that before, believing that the shared cathode gave differential action and that removing it would make the amp "not balanced". Clearly I had very much to learn when I started this - slightly better now.

Well the separate cathodes turned out to improve the sound, as well as giving better DC balance between the valves. Perhaps the former can be attributed to the latter. The 26s have been done and the others will shortly follow.

Anyway, now they are on separate cathodes the original risk has gone. I will however follow through on the designs we've discussed for interest and personal learning and will report back.
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Mike H
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#127 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Mike H »

Image Watching.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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