Re: A brand new CCS
Konnichiwa,
While no doubt a nice enough idea, I do not think that the result will materially improve on either an optimised "ring of 2" BJT or on either a DN2540 Depletion FET or the IXYS CCS, all of which do not require all the added stuff, this is not even so much a question of cost, but one of complexity, less complexity, less to go wrong.
If you compare Gary Pimm's measurements of CCS's, then you find the simple 3-Pin Ixys part does quite well, both in terms of bandwidth and actual impedance.
Those who have tried it (without additional cascoding) felt it great (that is TubeLab, Pete Millet and some commercial concerns better left unmentioned) often in more demanding applications than what you have in mind.
Sayonara
Konnichiwa,
croccodillo said:
While no doubt a nice enough idea, I do not think that the result will materially improve on either an optimised "ring of 2" BJT or on either a DN2540 Depletion FET or the IXYS CCS, all of which do not require all the added stuff, this is not even so much a question of cost, but one of complexity, less complexity, less to go wrong.
If you compare Gary Pimm's measurements of CCS's, then you find the simple 3-Pin Ixys part does quite well, both in terms of bandwidth and actual impedance.
Those who have tried it (without additional cascoding) felt it great (that is TubeLab, Pete Millet and some commercial concerns better left unmentioned) often in more demanding applications than what you have in mind.
Sayonara
As I explained in another thread, experimentation is the forerunner of innovation, and we need some innovation in this hobby. Giovanni's hybrid CCS may be complex, but it may turn out to work great. No one will know until someone builds it. Any of my attempts to connect opamps to vacuum tubes generally resulted in fried opamps, but who knows until he tries it. If it doesn't work well, he has traded time (and maybe a few parts) for knowledge and experience. If it works great, then we all have one more new circuit to use. Each individual can choose to use it or ignore it.
I (and others) have stated that the IXYS chip works great, but it does have limitations. It handles 450 volts max, and 35 watts max. It also doesn't seem to work well at very low currents. I have applications that require more voltage and more power. IXYS has a new chip that claims 900 volt capabilities, but you can't get them anywhere. When my lab is back up and running, I intend to try a variation of his other circuit with an IXYS chip in the cathode circuit of a pentode to attempt a 100 watt capable CCS.
Giovanni is new to the vacuum tube hobby, but he is an accomplished SMPS designer. Once he gets good with tubes, maybe we can talk him into designing a dynamically modulated plate supply that tracks the peak music voltage. This would feed the CCS just the right voltage at the right time thus eliminating the power dissipation in the CCS. In fact the CCS wouldn't even be needed if the supply was clean enough. I have designed such a power supply for a mobile two way radio, but it worked at 28 volts.
I (and others) have stated that the IXYS chip works great, but it does have limitations. It handles 450 volts max, and 35 watts max. It also doesn't seem to work well at very low currents. I have applications that require more voltage and more power. IXYS has a new chip that claims 900 volt capabilities, but you can't get them anywhere. When my lab is back up and running, I intend to try a variation of his other circuit with an IXYS chip in the cathode circuit of a pentode to attempt a 100 watt capable CCS.
Giovanni is new to the vacuum tube hobby, but he is an accomplished SMPS designer. Once he gets good with tubes, maybe we can talk him into designing a dynamically modulated plate supply that tracks the peak music voltage. This would feed the CCS just the right voltage at the right time thus eliminating the power dissipation in the CCS. In fact the CCS wouldn't even be needed if the supply was clean enough. I have designed such a power supply for a mobile two way radio, but it worked at 28 volts.
Mistake
Sorry,
Just realized that my previous CCS schematic cannot work with tubes, but can work great (IMO) stand-alone.
I will think something different to drive the tube.
George, Agent.5:
Thanks for your support, you exactly centered my kind of thinking: always try to experiment something new, you'll go wrong a lot of time and, sometime, only sometime, will have great success.
Kuei Yang Wang:
Thanks for your feedback, I agree with you, but my mind is different from yours.
This should be our hobby, our fun: and, for me, the biggest part of the fun is to create something that nobody has ever seen.
I have no fun to use a ready-to-use solution: I have about 200 schematics of SE, PP, CF schematics collected on my HDD, but no one can be called "mine"...
I'm an R&D engineer, my job is to develop innovative Microwave Histoprocessors (try Google, Milestone is the company for which I work...): I simply cannot follow a solution somebody else has already drawn.
This is a my limitation.
Don't get me wrong if I do not follow your suggestion: I'm a little bit crazy...
Ciao,
Giovanni
Sorry,
Just realized that my previous CCS schematic cannot work with tubes, but can work great (IMO) stand-alone.
I will think something different to drive the tube.
George, Agent.5:
Thanks for your support, you exactly centered my kind of thinking: always try to experiment something new, you'll go wrong a lot of time and, sometime, only sometime, will have great success.
Kuei Yang Wang:
Thanks for your feedback, I agree with you, but my mind is different from yours.
This should be our hobby, our fun: and, for me, the biggest part of the fun is to create something that nobody has ever seen.
I have no fun to use a ready-to-use solution: I have about 200 schematics of SE, PP, CF schematics collected on my HDD, but no one can be called "mine"...
I'm an R&D engineer, my job is to develop innovative Microwave Histoprocessors (try Google, Milestone is the company for which I work...): I simply cannot follow a solution somebody else has already drawn.
This is a my limitation.
Don't get me wrong if I do not follow your suggestion: I'm a little bit crazy...
Ciao,
Giovanni
Re: Mistake
Konnichiwa,
By all means, but your Op-AMp circuit (if made to work) will do nothing else than the on-chip stuff inside the IXYS CCS.
If you want something novel take a 5mA FET CCS and use that instead of the usual resistor to the base of the pass transistor in a ring of two CCS. This cascoded by the KT88 will make a very good CCS that is simple (7 parts including KT88 and it's gridstoppers) and easily build (which will get you a lot of followers).
Sayonara
Konnichiwa,
croccodillo said:
By all means, but your Op-AMp circuit (if made to work) will do nothing else than the on-chip stuff inside the IXYS CCS.
If you want something novel take a 5mA FET CCS and use that instead of the usual resistor to the base of the pass transistor in a ring of two CCS. This cascoded by the KT88 will make a very good CCS that is simple (7 parts including KT88 and it's gridstoppers) and easily build (which will get you a lot of followers).
Sayonara
Kuei Yang Wang:
I cannot find here where I live IXYS chips, and there's nothing else like such a thing (or, better, there's nothing else cheap enough for me); so, I have to build my own CCS.
So, again another one CCS.
This CCS is the simplest I can realise using a high-speed op-amp.
The CCS can handle up to 1000V and up to 1.4Amps.
I've selected the IRFBG20 Mosfet due to its really low input capacitance.
How it works:
Q1 is a CCS with a current of about 4 milliamps, feeding the rest of the circuit; together with the D1 zener forms a stable supply of 6V.
The LM311 low power op-amp drive the Q2 mosfet that is the "real" CCS.
The R2 resistor is used to set the current, that can be regulated from 50 to 200mA.
This circuit should really work fine, and its extremely easy to build.
One can use it without tubes, simply remove D2 and the "TUBE GRID" connector.
Its maximum ratings are: 1000V, 200mA, max. dissipation capacity 54W.
Minimum voltage across connectors: about 10V.
One can parallel more than one mosfets and, thus, multiply the output power.
This is what I will try to build in the next few days.
Ciao,
Giovanni
I cannot find here where I live IXYS chips, and there's nothing else like such a thing (or, better, there's nothing else cheap enough for me); so, I have to build my own CCS.
So, again another one CCS.
This CCS is the simplest I can realise using a high-speed op-amp.
The CCS can handle up to 1000V and up to 1.4Amps.
I've selected the IRFBG20 Mosfet due to its really low input capacitance.
How it works:
Q1 is a CCS with a current of about 4 milliamps, feeding the rest of the circuit; together with the D1 zener forms a stable supply of 6V.
The LM311 low power op-amp drive the Q2 mosfet that is the "real" CCS.
The R2 resistor is used to set the current, that can be regulated from 50 to 200mA.
This circuit should really work fine, and its extremely easy to build.
One can use it without tubes, simply remove D2 and the "TUBE GRID" connector.
Its maximum ratings are: 1000V, 200mA, max. dissipation capacity 54W.
Minimum voltage across connectors: about 10V.
One can parallel more than one mosfets and, thus, multiply the output power.
This is what I will try to build in the next few days.
Ciao,
Giovanni
Konnichiwa,
You can buy the "a'la carte" from Mouser and other such places, with a credit card or in extreme case money transfer and they do ship to the EU.
Past that, you can buy a little PCB with DN2540 (depletion MosFet) from K&K Audio:
http://www.kandkaudio.com/accessories.html
And if you e-mail Brian Cherry from DIYHIFiSupply you may be able to get a set of parts and little PCB's from him including the IXYS Parts, i know he has a bunch he had made which need a slight mod to work reliably (I have some) otherwise very neat:
http://www.diyhifisupply.com/index.htm
Does it HAVE TO HAVE an Op-Amp?
BTW, this looks exactly like the inside of an IXYS CCS ;-)
From: http://www.ixys.com/98703.pdf
BTW, are you sure that Q1 will work as CCS? I suspect the IRF part you have is an enhancement FET, not depletation (eg it wants it's grid...uuuhm...gate positive to conduct).
A KT88 readily dissipates 35W static on the anode. 35W Static dissipation in a solid state device need a lo of heatsinking, which is why I have been toying with the "Tube Cascoded CCS" idea, it makes it easier to dissipate the heat.
Sayonara
croccodillo said:
You can buy the "a'la carte" from Mouser and other such places, with a credit card or in extreme case money transfer and they do ship to the EU.
Past that, you can buy a little PCB with DN2540 (depletion MosFet) from K&K Audio:
http://www.kandkaudio.com/accessories.html
And if you e-mail Brian Cherry from DIYHIFiSupply you may be able to get a set of parts and little PCB's from him including the IXYS Parts, i know he has a bunch he had made which need a slight mod to work reliably (I have some) otherwise very neat:
http://www.diyhifisupply.com/index.htm
croccodillo said:
Does it HAVE TO HAVE an Op-Amp?
BTW, this looks exactly like the inside of an IXYS CCS ;-)
An externally hosted image should be here but it was not working when we last tested it.
From: http://www.ixys.com/98703.pdf
BTW, are you sure that Q1 will work as CCS? I suspect the IRF part you have is an enhancement FET, not depletation (eg it wants it's grid...uuuhm...gate positive to conduct).
croccodillo said:
A KT88 readily dissipates 35W static on the anode. 35W Static dissipation in a solid state device need a lo of heatsinking, which is why I have been toying with the "Tube Cascoded CCS" idea, it makes it easier to dissipate the heat.
Sayonara
Some questions:
1. If I use a KT88 as a triode CCS in the upper side of a para-feed output stage, do I need separate filament supplies for it and the low side and output drivers?
This is because I can have about 400-500V between the cathode of the upper triode and the cathode of the lower one.
I know the filament is insulated from the rest, but the maximum voltage between it and the cathode is reported as 250V.
2. Is it better to power the filament with AC or DC? I mean, does it worth to use DC in manner to avoid "hum"?
3. If I use separate filament supplies for "high side" in para-feed output stage (floating supply), and I use DC filament voltage, can I electrically connect the filament to the tube cathode?
Thanks,
Giovanni
1. If I use a KT88 as a triode CCS in the upper side of a para-feed output stage, do I need separate filament supplies for it and the low side and output drivers?
This is because I can have about 400-500V between the cathode of the upper triode and the cathode of the lower one.
I know the filament is insulated from the rest, but the maximum voltage between it and the cathode is reported as 250V.
2. Is it better to power the filament with AC or DC? I mean, does it worth to use DC in manner to avoid "hum"?
3. If I use separate filament supplies for "high side" in para-feed output stage (floating supply), and I use DC filament voltage, can I electrically connect the filament to the tube cathode?
Thanks,
Giovanni
Konnichiwa,
Yes. One of the problems in Indirectly heated valves is that the Cathode sleeve and the heater wire form a parasitic and inefficient but nevertheless present diode and if the heater is negative to the cathode current will flow in this diode.
Yes, sure. Directly Heated Triodes have no cathodes, just the heater wire, so this works just fine, though it needs to be done with care to avoid hum loops etc....
Sayonara
croccodillo said:
Yes. One of the problems in Indirectly heated valves is that the Cathode sleeve and the heater wire form a parasitic and inefficient but nevertheless present diode and if the heater is negative to the cathode current will flow in this diode.
croccodillo said:
Yes, sure. Directly Heated Triodes have no cathodes, just the heater wire, so this works just fine, though it needs to be done with care to avoid hum loops etc....
Sayonara
In the case of an output stage using an indirectly heated tube. Hum is usually not an issue, so the standard practice is to use AC filaments. There is no reason why you can't use DC on the filaments, and I have done so many times.
"Referenced to the cathode" implies that the filament is at the same voltage potential as the cathode. In most applications the filament winding on the transformer is connected to a voltage divider on the power supply such that it operates at a voltage within the H-K ratings of the tube.
In your case the two can be connected together, but keep in mind that the cathode of the upper tube carries the entire output voltage swing of the amplifier. You want to avoid connecting it to anything that has a large capacitance to ground, or anything that would couple signal into low level circuits, or the circuits of the other channel.
"Referenced to the cathode" implies that the filament is at the same voltage potential as the cathode. In most applications the filament winding on the transformer is connected to a voltage divider on the power supply such that it operates at a voltage within the H-K ratings of the tube.
In your case the two can be connected together, but keep in mind that the cathode of the upper tube carries the entire output voltage swing of the amplifier. You want to avoid connecting it to anything that has a large capacitance to ground, or anything that would couple signal into low level circuits, or the circuits of the other channel.
Thanks for your answers...
So my last idea could work.
I'm still thinking about the CCS, and it's a challenge to do it on my own...
So, if I can use the filament voltage to power the CCS it should be easy to build up a very stable thing.
Look at this .
It is a discrete op-amp (the two inputs are the emitter of Q1 and the anode of D3).
When there's no current flowing trhough R2/R4/R5 te voltage across them is 0V, so the voltage at Q1 base is fixed by D3, and it is equal to the one at its eitter, fixed by D4: in this condition the transistor Q1 is off, and the Q2 mosfet will be turned ON.
When the voltage across R2 reaches 0.7 Q1 begin to conduct (the voltage at its base is 0.7V lower than the one at its emitter), regulating Q2.
In other words, the voltage across R2 will always the same as forward voltage across D4 (op-amp).
The temperature drift of Q1 will be compensated by D3 and D4, making it very stable.
With the R2/R4/R5 value in schematic one can regulate the output current from 70mA to 100mA.
Of course I have to find a way to fix the whole circuit far away from all the rest of the circuit, but it can be done.
Your opinion please?
So my last idea could work.
I'm still thinking about the CCS, and it's a challenge to do it on my own...
So, if I can use the filament voltage to power the CCS it should be easy to build up a very stable thing.
Look at this .
It is a discrete op-amp (the two inputs are the emitter of Q1 and the anode of D3).
When there's no current flowing trhough R2/R4/R5 te voltage across them is 0V, so the voltage at Q1 base is fixed by D3, and it is equal to the one at its eitter, fixed by D4: in this condition the transistor Q1 is off, and the Q2 mosfet will be turned ON.
When the voltage across R2 reaches 0.7 Q1 begin to conduct (the voltage at its base is 0.7V lower than the one at its emitter), regulating Q2.
In other words, the voltage across R2 will always the same as forward voltage across D4 (op-amp).
The temperature drift of Q1 will be compensated by D3 and D4, making it very stable.
With the R2/R4/R5 value in schematic one can regulate the output current from 70mA to 100mA.
Of course I have to find a way to fix the whole circuit far away from all the rest of the circuit, but it can be done.
Your opinion please?
Yes, of course I have a scope.
But I do not think this little CCS will oscillate, if you build it carefully.
If you add too much capacitors and resistor you'll reduve speed and bandwidth... but I understand what you mean.
I was thinking a different approach now: the same circuit, with a little separate supply, that will substitute completely the upper tube.
It should work, the only think is I have to find an high voltage mosfet.
In such a case the minimum CCS voltage can go down to, let's say, 5 volts, and I can use a lower supply voltage: I have a transformer with an output, once rectified, of about 600V.
Using a SS CCS I can operate the lower tube at, let's say, 400V and still have an (theoretical) ouput swing up to 600V and down to, for example, 200V (400V of swing).
If instead I use a tube CCS I must provide an higher supply voltage (say 700V), and up to now I do not have a transformer with such an output.
Ciao,
Giovanni
But I do not think this little CCS will oscillate, if you build it carefully.
If you add too much capacitors and resistor you'll reduve speed and bandwidth... but I understand what you mean.
I was thinking a different approach now: the same circuit, with a little separate supply, that will substitute completely the upper tube.
It should work, the only think is I have to find an high voltage mosfet.
In such a case the minimum CCS voltage can go down to, let's say, 5 volts, and I can use a lower supply voltage: I have a transformer with an output, once rectified, of about 600V.
Using a SS CCS I can operate the lower tube at, let's say, 400V and still have an (theoretical) ouput swing up to 600V and down to, for example, 200V (400V of swing).
If instead I use a tube CCS I must provide an higher supply voltage (say 700V), and up to now I do not have a transformer with such an output.
Ciao,
Giovanni
I've got it!
OK, at the end I ended up building one CCS...
Here it is:
This little board is working now on my workbench (unfortunately I do not have an high voltage power supply now, so the CCS is working at 30V):
THIS is the schematic, and
THIS is the PCB, seen from the bottom side.
It is working fine, but of course I cannot be sure until I will connect it to a real amplifier.
The BU508A transistor can handle a continuous voltage of about 700V, and a current up to 1.8A. It can dissipate 125W.
In its final form T1 will be mounted on a large heatsink, THIS (dimensions expressed in millimeters).
I want to use it as stand-alone, not connected to a tube.
The PS (Power supply) pin will be connected to the B+ rail, while the OUT (output) pin will be connected to the anode of the output tubes (two paralleled KT88, connected in para-feed configuration).
I changed the value of R2 and R6 to 27 and 100 Ohm, respectively.
This, togheter with R7, permits you to change the output current from 120 to 170 mA.
The bad thing is that you need an external power supply for the CCS, but this ensure you have a perfectly stable output current.
Q1 and D1 form a differential amplifier, making a well regulated controller.
D1 serves also to fix the temperature drifting of Q1: having the same behaviour, it will compensate Q1 almost perfectly.
Next step is to build up a raw high voltage power supply (about 550V) and build the following circuit to test it:
Test circuit
I will drive it with my portable CD player, just to try it.
Your comments? Can this schematic work? What about the capacitor on the input?
And what about the one on the output stage, is the value correct?
Ciao,
Giovanni
OK, at the end I ended up building one CCS...
Here it is:
An externally hosted image should be here but it was not working when we last tested it.
This little board is working now on my workbench (unfortunately I do not have an high voltage power supply now, so the CCS is working at 30V):
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
THIS is the schematic, and
THIS is the PCB, seen from the bottom side.
It is working fine, but of course I cannot be sure until I will connect it to a real amplifier.
The BU508A transistor can handle a continuous voltage of about 700V, and a current up to 1.8A. It can dissipate 125W.
In its final form T1 will be mounted on a large heatsink, THIS (dimensions expressed in millimeters).
I want to use it as stand-alone, not connected to a tube.
The PS (Power supply) pin will be connected to the B+ rail, while the OUT (output) pin will be connected to the anode of the output tubes (two paralleled KT88, connected in para-feed configuration).
I changed the value of R2 and R6 to 27 and 100 Ohm, respectively.
This, togheter with R7, permits you to change the output current from 120 to 170 mA.
The bad thing is that you need an external power supply for the CCS, but this ensure you have a perfectly stable output current.
Q1 and D1 form a differential amplifier, making a well regulated controller.
D1 serves also to fix the temperature drifting of Q1: having the same behaviour, it will compensate Q1 almost perfectly.
Next step is to build up a raw high voltage power supply (about 550V) and build the following circuit to test it:
Test circuit
I will drive it with my portable CD player, just to try it.
Your comments? Can this schematic work? What about the capacitor on the input?
And what about the one on the output stage, is the value correct?
Ciao,
Giovanni
Hi Giovanni:
I have been down the bipolar CCS road before. There is a serious pitfall to look out for. Look at the ST data sheet for the BU508A. On the second page there are "safe operating area curves". These curves show that there is an evil destruction mechanism that exists in Bipolar Transistors called secondary breakdown. Even though the transistor is rated for 1.8 A and 700V at 125W you can't pull high currents with a large drop across the transistor. It may work for a while, but they have a habit of failing at the least oportune moment. I was using the MJW21195 from ON Semi, because it claimed a large SOA for audio applications. They all eventually blew up.
Since you have already built this good looking circuit, I would try it out, but use a variac and test it in voltage increments of avout 50 volts starting at 300 volts or so.
I think that your test circuit will work OK. You might want to add some small grid stopper resistors at the grid pins of the KT-88. The 1uF input cap is probably overkill, but it shouldn't hurt anything. The 4uF output cap is probably OK. Some have suggested much larger. You can calculate its impedance at your lowest frequency. It should be below 10% of your OPT impedance. I use a 6uF because I have one.
You may want to start with 1 KT-88 at half the total current to avoid stressing the CCS.
I have been down the bipolar CCS road before. There is a serious pitfall to look out for. Look at the ST data sheet for the BU508A. On the second page there are "safe operating area curves". These curves show that there is an evil destruction mechanism that exists in Bipolar Transistors called secondary breakdown. Even though the transistor is rated for 1.8 A and 700V at 125W you can't pull high currents with a large drop across the transistor. It may work for a while, but they have a habit of failing at the least oportune moment. I was using the MJW21195 from ON Semi, because it claimed a large SOA for audio applications. They all eventually blew up.
Since you have already built this good looking circuit, I would try it out, but use a variac and test it in voltage increments of avout 50 volts starting at 300 volts or so.
I think that your test circuit will work OK. You might want to add some small grid stopper resistors at the grid pins of the KT-88. The 1uF input cap is probably overkill, but it shouldn't hurt anything. The 4uF output cap is probably OK. Some have suggested much larger. You can calculate its impedance at your lowest frequency. It should be below 10% of your OPT impedance. I use a 6uF because I have one.
You may want to start with 1 KT-88 at half the total current to avoid stressing the CCS.
OK, I couldn't just read this without building anything. My work room is still a mess, but I cleaned off a spot, and "built" a basic amp. It is an unsafe mess, so as they say on TV, "Don't try this at home."
Kuei Yang Wang made the following statement, it convinced me that I had to try it.
"Here a little trick for you.
Combine a KT-88 and the IXYS CCS Tubelab mentions (replace the cathode R of the upper SRPP device with the CCS). This way dissipation in the Solid State CCS is limited, most is dissipated in the KT88. You can take the output from the lower Valve Anode (basically traditional SE parallelfeed) or from the upper KT-88 Cathode for basically a Power Mu-Follower."
So, I wired an IXYS chip up to an octal socket, and built a CCS. I tested it with a box full of different tubes. I took the best 10 tubes, and tested them at 3 different currents. What I found is that there is a big difference in the minimum voltage required for CCS operation.
A (Chinese) KT-88 needs 100 volts to create a 30 mA CCS, and it needs 145 volts to provide 80 mA. Overall, sweep tubes work the best. My favorite, the 6LW6, requires only 36 volts for a 30 mA CCS, and 50 volts for 80 mA. I will post the entire list later.
Since the 6LW6 required the lowest voltage, and my power supply only goes to 400 V, I used a 6LW6 for the top tube. I wired up a KT-88 in a manner similar to your latest test circuit, except that I grounded the cathode and used fixed bias.
The first thing that I found is that my CD player does not have near enough drive, neither does my audio generator. I used a filament transformer to step up the output of the generator. Now I could drive the "amp" to clipping.
Output is only 3 watts, this is about right for a 400 volt supply. Frequency response, measured at low signal (without the step up transformer) using a resistive 3 K ohm load (no OPT) was amazing. 18 Hz to 800 KHz (3 db points). The limiting factor will be the OPT.
Here is a picture of the experiment.
Next, build a 600 to 700 volt supply, depending on whatever transformer I can dig up. Then I want to experiment with screen drive. Then depending on the outcome, I will prototype the driver, then build the amp. The results will be posted here, and on the web site.
Kuei Yang Wang made the following statement, it convinced me that I had to try it.
"Here a little trick for you.
Combine a KT-88 and the IXYS CCS Tubelab mentions (replace the cathode R of the upper SRPP device with the CCS). This way dissipation in the Solid State CCS is limited, most is dissipated in the KT88. You can take the output from the lower Valve Anode (basically traditional SE parallelfeed) or from the upper KT-88 Cathode for basically a Power Mu-Follower."
So, I wired an IXYS chip up to an octal socket, and built a CCS. I tested it with a box full of different tubes. I took the best 10 tubes, and tested them at 3 different currents. What I found is that there is a big difference in the minimum voltage required for CCS operation.
A (Chinese) KT-88 needs 100 volts to create a 30 mA CCS, and it needs 145 volts to provide 80 mA. Overall, sweep tubes work the best. My favorite, the 6LW6, requires only 36 volts for a 30 mA CCS, and 50 volts for 80 mA. I will post the entire list later.
Since the 6LW6 required the lowest voltage, and my power supply only goes to 400 V, I used a 6LW6 for the top tube. I wired up a KT-88 in a manner similar to your latest test circuit, except that I grounded the cathode and used fixed bias.
The first thing that I found is that my CD player does not have near enough drive, neither does my audio generator. I used a filament transformer to step up the output of the generator. Now I could drive the "amp" to clipping.
Output is only 3 watts, this is about right for a 400 volt supply. Frequency response, measured at low signal (without the step up transformer) using a resistive 3 K ohm load (no OPT) was amazing. 18 Hz to 800 KHz (3 db points). The limiting factor will be the OPT.
Here is a picture of the experiment.
Next, build a 600 to 700 volt supply, depending on whatever transformer I can dig up. Then I want to experiment with screen drive. Then depending on the outcome, I will prototype the driver, then build the amp. The results will be posted here, and on the web site.
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