Help with drawing load lines for 45 output stage
Getting my head around drawing load lines for transformer-loaded output stage, to better understand the beautiful-sounding amp I just built. I've read a number of articles including this
Of Loadlines, Power Output and Distortion.
... and this ...
The Valve Wizard -Single Ended
The valve wizard site states this ...
If you are wondering why it appears that the signal voltage can now swing higher than the HT voltage, it is because this is exactly what happens! Inductances abhor changes in current. When current through the transformer increases it stores energy, which is released when the current falls again, allowing up to twice the HT voltage to be developed. Because of this, the HT in a Class A amp must never be more than half the maximum peak anode voltage rating of the valve, given on the data sheet.
I get that the transformer stores and releases the energy (though I didn't before reading those articles), however I can't find any sheets which reference that value.
The 45 datasheets referenced from TDSL show maximum plate voltage (275V) but I haven't found any which show the maximum PEAK anode voltage. I checked a couple of others too including the Western Electric 300B and couldn't see any reference to the maximum peak anode voltage.
Am I looking in the right place ?
Getting my head around drawing load lines for transformer-loaded output stage, to better understand the beautiful-sounding amp I just built. I've read a number of articles including this
Of Loadlines, Power Output and Distortion.
... and this ...
The Valve Wizard -Single Ended
The valve wizard site states this ...
If you are wondering why it appears that the signal voltage can now swing higher than the HT voltage, it is because this is exactly what happens! Inductances abhor changes in current. When current through the transformer increases it stores energy, which is released when the current falls again, allowing up to twice the HT voltage to be developed. Because of this, the HT in a Class A amp must never be more than half the maximum peak anode voltage rating of the valve, given on the data sheet.
I get that the transformer stores and releases the energy (though I didn't before reading those articles), however I can't find any sheets which reference that value.
The 45 datasheets referenced from TDSL show maximum plate voltage (275V) but I haven't found any which show the maximum PEAK anode voltage. I checked a couple of others too including the Western Electric 300B and couldn't see any reference to the maximum peak anode voltage.
Am I looking in the right place ?
Using a relay to protect the output valve if grid bias supply fails
I'm building a power amp using 45 valves for output, interstage transformer coupled and with grid bias (trimmer surrounded by resistors to make the trimmable range what I need). The grid bias will be fed from a separate rectifier tube and LCLC filtered.
If the grid bias fails there's obviously a risk to the output tube. I'm looking into using a relay so that if the grid bias supply fails the output tube B+ will be cut. In the example circuit that I've seen, from the grid bias supply (approx 120V) a 10k resistor connects in series with a 24v relay. The ground of output tube B+ is connected when the grid coil is energised.
I've never used a relay before - can anyone clue me into how to specify and choose one ? A 10k resistor from 120V will draw around 12mA current, so I'm looking for a relay which will energise at 12mA ? The specs I've looked at on RS or Mouser seem to specify coil voltage so I think I've misunderstood something. Or using wrong terminology.
I'm building a power amp using 45 valves for output, interstage transformer coupled and with grid bias (trimmer surrounded by resistors to make the trimmable range what I need). The grid bias will be fed from a separate rectifier tube and LCLC filtered.
If the grid bias fails there's obviously a risk to the output tube. I'm looking into using a relay so that if the grid bias supply fails the output tube B+ will be cut. In the example circuit that I've seen, from the grid bias supply (approx 120V) a 10k resistor connects in series with a 24v relay. The ground of output tube B+ is connected when the grid coil is energised.
I've never used a relay before - can anyone clue me into how to specify and choose one ? A 10k resistor from 120V will draw around 12mA current, so I'm looking for a relay which will energise at 12mA ? The specs I've looked at on RS or Mouser seem to specify coil voltage so I think I've misunderstood something. Or using wrong terminology.
Higher voltage relay coils generally also have higher resistance and so draw less current. Using a higher voltage coil also reduces the demands on the series resistor.
Relays need something like 60% or more of their rated current to operate and pull in, but once pulled in can continue to remain so down to very low currents. That could be an issue if you are looking for reliable and repeatable operation.
One option might be to use a higher voltage relay coil and a series zener diode and resistor. That way you could get a more defined turn off as the supply falls a little.
Another real problem is that a contact breaking DC current tends to draw an arc and so it is common to see a relay coil specified as perhaps 250 vac/10 amps and maybe only 30 volts at 1 amp (or whatever). It can be as dramatic difference as that.
Relays need something like 60% or more of their rated current to operate and pull in, but once pulled in can continue to remain so down to very low currents. That could be an issue if you are looking for reliable and repeatable operation.
One option might be to use a higher voltage relay coil and a series zener diode and resistor. That way you could get a more defined turn off as the supply falls a little.
Another real problem is that a contact breaking DC current tends to draw an arc and so it is common to see a relay coil specified as perhaps 250 vac/10 amps and maybe only 30 volts at 1 amp (or whatever). It can be as dramatic difference as that.
Would a simpler solution be a very small fuse in the cathode of the 45 tube ? Normally 30 to 35mA of current flowing. I'll have a 10R resistor there for measuring current, I could put a slow-blow fuse under that to blow at a low value. With 200V on the plate, the tube would have to draw more than 50mA to exceed it's 10W max dissipation.
The fuse would be directly in the signal path though.
The fuse would be directly in the signal path though.
The old-fashioned way to provide this protection is via a clamp valve. This is a power pentode (but somewhat smaller than the output valve) which is normally held cutoff by the grid bias supply. If the supply fails the clamp valve turns on and drags the output valve screen supply low enough to reduce the current. It was commonly used in Class C transmitters, which rely on grid drive to provide grid bias.
Here's an idea I want to use. You don't want to play with the B+ directly so it's better to break the line voltage. Use a relay that can be powered by part of the bias voltage, 6-12v, and make/break the primary line. To do that you will need a Power On/Off switch and Start button. A simple N/O momentary contact switch on the primary after the main switch that energizes the primary and the bias voltage to the relay coil which will open if bias supply fails, power fails or you flip the main to off. Nothing will go through the primary of the PT until you push the start button. But you have to know if the bias supply can handle the coil current.
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The "contacts arc" part of the problem can be avoided by the MOSFET choke, where you turn the Gate on by the photo-coupled SS-relay whose LED is fed from Bias supply rail. This has reliably worked in my Luxman for almost a year. Plus, same Gate control is actually used for the HV delay.
Further, to make it easier for MOSFET you could shunt it with 24-30K resistor, makes the rectifier feel better with big filter caps + a solution for stand-by mode with HV about 100-150V, where the output lubes practically would not have any idle current.
Or, if you do not want a choke - use a triac.
Further, to make it easier for MOSFET you could shunt it with 24-30K resistor, makes the rectifier feel better with big filter caps + a solution for stand-by mode with HV about 100-150V, where the output lubes practically would not have any idle current.
Or, if you do not want a choke - use a triac.
I think the ideal fixed bias supply should have very low ripple and fast settling time. Usually the load of this psu is determined by the trim pot and the surrounding resistors to a few mA. This makes it easy for a simple low impedance Pi filter to meet the above mentioned requirements. Inserting a relay in the circuit will increase ripple and ask for heavier filtration resulting in a "slower" psu. Adjusting operating point of output tubes may be a pain especially with tube/LCLC psu...
You really should be monitoring cathode current. This way you can also protect the output transformer if a valve starts to run away. Just use a simple comparitor to monitor the voltage across your 10R resistor in the cathode. You probably will need to put a filter in front of it to stop false trips on bass peaks. From the comparitor use a buffer transistor to drive a relay switching either the mains in or BOTH HT lines from the power transformer. Don't try switching HV DC with a relay. As previously said it will arc and continue conducting whilst melting and generally making a mess without protecting anything.
Cheers
Matt
Cheers
Matt
Page 15 of this KT88 datasheet discribes a circuit for this.
https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf
https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf
ok thanks for replies. I'm trying to avoid any silicon in this amp, at least for now, patly for purity and partly because I have zero experience with it. Something I'll come back to later.
The mains transformer is protected by secondary fuses at very low values (160mA at the moment).
I'll fit a 50mA fuse in the cathode leg while I get it working. I can then bypass it later to hear what difference it makes to the sound.
The mains transformer is protected by secondary fuses at very low values (160mA at the moment).
I'll fit a 50mA fuse in the cathode leg while I get it working. I can then bypass it later to hear what difference it makes to the sound.
Assuming the OP is reluctant to deal with micros and ss comparators this appears to be the best possible solution - a tube comparator doing exactly what is required.
As much as i personally love tackling little issues like this with micros, experience has taught me that keeping the generated RF away from the audio circuits is far from trivial. Having made the effort for tube bias one is justifiably picky about stuff that can ruin the sound.
Thanks. That site doesn't seem to be accessible for me now but I'll try later. It must be working because I took a quick look on the tablet last night - the piece is very brief and I confess I didn't "grok" it in that brief glance.
I’m wondering if music can still bring joy to some listeners. Probably they listen only to find a disturbance, a little hiss or crack that, according to so called “HiFi purists”, is caused by the quality of a mains cable or even the direction the wires are used or so many more problematic use of components that disturb the sound in one way or the other.
As a technician I hear these stories over and over again. I’ve been asked what I think about it and my response is a very clear one:
“If your amplifier and all the rest is in good condition, listen to how the sound make you feel. Is it OK, then it’s OK. If it’s not, you probably have a bad day. Come back later. Don’t make a problem out of it…”
Most listeners and even musicians think to clean. They are no longer interested in “the soul of the music” but only in the purity. And a song or whatever music piece that has no soul, that’s something you can “feel”.
Everybody may have his opinion on the technical “Quality” of music and how it “Sounds” but chasing ghosts is a very expensive hobby. A hobby that will end one day, not because these ghosts are “gone”, due to the many “variations of snake oil” bought or used, but reason that will tell “in fact it dosn’t matter”.
Let the music flow through your brain and enjoy. You will feel good (and your wallet as well)
As a technician I hear these stories over and over again. I’ve been asked what I think about it and my response is a very clear one:
“If your amplifier and all the rest is in good condition, listen to how the sound make you feel. Is it OK, then it’s OK. If it’s not, you probably have a bad day. Come back later. Don’t make a problem out of it…”
Most listeners and even musicians think to clean. They are no longer interested in “the soul of the music” but only in the purity. And a song or whatever music piece that has no soul, that’s something you can “feel”.
Everybody may have his opinion on the technical “Quality” of music and how it “Sounds” but chasing ghosts is a very expensive hobby. A hobby that will end one day, not because these ghosts are “gone”, due to the many “variations of snake oil” bought or used, but reason that will tell “in fact it dosn’t matter”.
Let the music flow through your brain and enjoy. You will feel good (and your wallet as well)
It's easy to misinterpret written word without non-verbal cues, so I'll assume your response is meant to be constructive and not condescending (which is how I first read it).
Thank you for your advice, however you don't know me at all and shouldn't try to judge my aspirations or objectives. The soul of the music is everything. I have often heard that "soul" or "warmth" and "clarity" or "purity" are opposite ends of the spectrum. I fundamentally disagree and the improvements I've made to my hifi system over the years have proven that to me.
If you're saying "stop trying to improve it and just play some music", well I have a little sympathy but I truly enjoy increasing my understanding of how it works, what makes it sound better and love to be able to DIY. It's a hobby I greatly enjoy. Please don't think to tell me to stop doing it.
Thank you for your advice, however you don't know me at all and shouldn't try to judge my aspirations or objectives. The soul of the music is everything. I have often heard that "soul" or "warmth" and "clarity" or "purity" are opposite ends of the spectrum. I fundamentally disagree and the improvements I've made to my hifi system over the years have proven that to me.
If you're saying "stop trying to improve it and just play some music", well I have a little sympathy but I truly enjoy increasing my understanding of how it works, what makes it sound better and love to be able to DIY. It's a hobby I greatly enjoy. Please don't think to tell me to stop doing it.
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