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

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

Post by RhythMick »

My valve amp uses valve pairs in PP Class A, with a shared cathode resistor for bias (grids are grounded). Works beautifully and sounds excellent.

One problem which caught me out recently was that if one valve stops conducting (eg if a heater fails) then the shared cathode resistor will result in the surviving valve over-conducting dramatically and likely causing significant damage to the valve.

I considered alternative approaches (such as driving the valve heaters in series) but I've settled on providing a warning mechanism if one valve stops conducting. I'd like to minimise the components I introduce into the valve circuit and I plan to use an optocoupler input in the anode, with the output configured to light up a red LED if the valve stops conducting. See basic circuit attached. EDIT: I decided to place the optocoupler in the anode rather than the cathode to avoid amplifying any noise it might introduce.
sketch-1549833676937-1.jpg
Question - the input of the optocoupler is an infrared LED and therefore I need to limit the current. My expectation is that the valve itself will act as the current limiter. In normal operation for example my 45s run at approximately 28mA each, but when the heater of one failed recently the other maxed out at 50mA. It ran like that for quite a while before I realised - it buzzed and I was trying to track down the buzz. I've chosen optocouplers with a maximum input current of more than twice the expected current. https://uk.rs-online.com/web/p/optocouplers/6998177/

Any reason this won't work ? I don't have much SS experience so reaching out for a sense-check. The dropper resistor for the LED may not be right, I need to check the datasheet.
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jack
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#2 Re: Warning LED when valve stops conducting - a little SS help needed

Post by jack »

Personally, I wouldn't go this route.

I'd use one of the galvanically isolated integrated current sensing ICs from Analog Devices, TI or ST etc . - these introduce a non-inductive small resistance to the sense path. They are very accurate...
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RhythMick
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#3 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

jack wrote: Mon Feb 11, 2019 10:24 am Personally, I wouldn't go this route.

I'd use one of the galvanically isolated integrated current sensing ICs from Analog Devices, TI or ST etc . - these introduce a non-inductive small resistance to the sense path. They are very accurate...
Interesting thanks. Could you point me towards an example, ideally a through-hole example as this won't be PCB mounted. DIL sockets perfect.
RhythMick
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#4 Re: Warning LED when valve stops conducting - a little SS help needed

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jack wrote: Mon Feb 11, 2019 10:24 am Personally, I wouldn't go this route.

I'd use one of the galvanically isolated integrated current sensing ICs from Analog Devices, TI or ST etc . - these introduce a non-inductive small resistance to the sense path. They are very accurate...
Those appear designed to measure upwards of 50A. I'm looking at 28mA under normal conditions. Would you still use those?
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#5 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

RhythMick wrote: Mon Feb 11, 2019 6:46 pm
jack wrote: Mon Feb 11, 2019 10:24 am Personally, I wouldn't go this route.

I'd use one of the galvanically isolated integrated current sensing ICs from Analog Devices, TI or ST etc . - these introduce a non-inductive small resistance to the sense path. They are very accurate...
Those appear designed to measure upwards of 50A. I'm looking at 28mA under normal conditions. Would you still use those?
Answered my own question. Hall effect sensors are just not suitable for such low current applications. They can be made to work down as low as 100mA with the help of external magnetic fields, but not a chance at 28mA.
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#6 Re: Warning LED when valve stops conducting - a little SS help needed

Post by IslandPink »

OK< but in your circuit as drawn above, you have a diode in the anode circuit, which seems like a shame ( sonically ) .
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RhythMick
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#7 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

IslandPink wrote: Mon Feb 11, 2019 10:29 pm OK< but in your circuit as drawn above, you have a diode in the anode circuit, which seems like a shame ( sonically ) .
I know. I'm definitely open to alternative suggestions. I figured better in the anode though than the cathode, and hey people use LEDs for bias right ?

So the first question is - would the circuit I laid out above work. Given my lack of experience with SS I just want to make sure I haven't missed something.

If it will work, I'll give it a try. Alternatively I could just accept it may happen and keep an eye on the heaters, or I could change the design to drive the heaters in series.
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#8 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

If you are looking for a failed heater why not use a LED and zener so it lights if the heater voltage does above 2.5v (or whatever the heater in question takes).
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#9 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Mon Feb 11, 2019 11:42 pm If you are looking for a failed heater why not use a LED and zener so it lights if the heater voltage does above 2.5v (or whatever the heater in question takes).
Thanks Nick I'll have a think and play with that suggestion.
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#10 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Nick wrote: Mon Feb 11, 2019 11:42 pm If you are looking for a failed heater why not use a LED and zener so it lights if the heater voltage does above 2.5v (or whatever the heater in question takes).
Actually I should have mentioned the amp uses Andrew's DC boards, which will maintain the voltage at the set value even if the valve heater fails.
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#11 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Nick »

I wonder if its possible to extract a signal from the heater supply when there is no heater current. That might be a useful addition to the circuit.
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#12 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Andrew »

Nick wrote: Tue Feb 12, 2019 8:57 am I wonder if its possible to extract a signal from the heater supply when there is no heater current. That might be a useful addition to the circuit.
You might be able to look and see what the op amps is doing in a "no load scenrio" and use that?
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#13 Re: Warning LED when valve stops conducting - a little SS help needed

Post by RhythMick »

Andrew wrote: Tue Feb 12, 2019 10:24 am
Nick wrote: Tue Feb 12, 2019 8:57 am I wonder if its possible to extract a signal from the heater supply when there is no heater current. That might be a useful addition to the circuit.
You might be able to look and see what the op amps is doing in a "no load scenrio" and use that?
Stupid question Andrew - do you mean the LM358P?
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#14 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Andrew »

RhythMick wrote: Tue Feb 12, 2019 10:47 am
Andrew wrote: Tue Feb 12, 2019 10:24 am
Nick wrote: Tue Feb 12, 2019 8:57 am I wonder if its possible to extract a signal from the heater supply when there is no heater current. That might be a useful addition to the circuit.
You might be able to look and see what the op amps is doing in a "no load scenrio" and use that?
Stupid question Andrew - do you mean the LM358P?
Yes, the output of the op amp or the input to the D44H11 transistor will be close to 0v, or a few 10mV if its off.

But, thinking about it, this is even simpler. Why not place an LED across the cathode/heater with a bias resistor in series with the LED?

If the valve heater stops conducting, no heater current, the DHT circuit will drive the LED to 5v instead of trying to drive the heater. When the heater is conducting the higher impedance of the LED and bias resistor will mean all the current will go to to the DHT heater and the LED will be off, or very very dim, but the LED will light if the cathode heater goes open circuit.

Try an LED in series with 470R or 560R across the heater/cathode.

If you are worried about this introducing noise, which I accept it could, then the next step is to add the LED at the base on the driver transistor, same idea only in this case it will go off if the valve stops conducting. Beware of trying this that you don't steal too much current from the op amp, it might not have the oomph left to drive the D44H11 so the heater goes to full-on, don't try to steal too much and make it too bright.
Last edited by Andrew on Tue Feb 12, 2019 2:06 pm, edited 1 time in total.
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#15 Re: Warning LED when valve stops conducting - a little SS help needed

Post by Andrew »

Thinking about this some more, I don't like the first solution at all now, I have gone off it, I would prefer something like the second, which isn't in the music signal path. Try and LED in series with 820R between the base of the D44H11 and ground and see if the LED lights up and then check if the valve heater makes full voltage. Measure what full is for the valve before and after doing the change.

Then have a listen to make sure I haven't broken anything.

Andrew
Last edited by Andrew on Tue Feb 12, 2019 2:31 pm, edited 1 time in total.
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