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#1
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BJT for plate CCS load
Gentlemen,
as it seems suitable BJTs for plate CCS load usage are hard to find at the parts shop at the corner here in Germany. Biggest problem is the needed high maximum emitter-collector voltage of, say, 500 volts (at say, a dozen mA). So far I have used PNP type MJE350 in several circuits, but this only is rated up to 300 Vce max. Any hints on truly obtanium types that will do okay at 500 volts? Tnx2u, Tom -- |
#2
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"Tom Schlangen" wrote
...So far I have used PNP type MJE350 in several circuits, but this only is rated up to 300 Vce max. Any hints on truly obtanium types that will do okay at 500 volts? Looking at http://rswww.com, I see some PNP up to 800V but they are probably too big and clumsy for your kind of current. Some small NPN get up to 400V but higher voltage are again designed for higher power than you need. see eg 2SC3150 Why choose a BJT? Many would use a MOSFET, such as the STD1HNC60 I have used for a mu stage. I know almost nothing about transistors, as perhaps you can tell. cheers, Ian |
#3
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Tom Schlangen wrote: Gentlemen, as it seems suitable BJTs for plate CCS load usage are hard to find at the parts shop at the corner here in Germany. Biggest problem is the needed high maximum emitter-collector voltage of, say, 500 volts (at say, a dozen mA). So far I have used PNP type MJE350 in several circuits, but this only is rated up to 300 Vce max. Any hints on truly obtanium types that will do okay at 500 volts? Tnx2u, Tom I have used many MJE350/340 for CCS in circuits where the B+ was 300v, and never blown one. I suspect at 3 mA, they are very quiet, and never have I noticed noise increases with a bjt CCS. The high hfe of the bjt and their high Gm results in a CCS with a very high finite value, so any noise emanating from the CCS is attenuated by the low anode resistance of the triode, since an effective resistance divider exists. In cathode CCS circuits, they last even though B+ maybe 480V. But in normal triode gain circuits, such as at the input V1 of a power amp, they allow a lower B+, because usually only a few vrms of output is needed, and those few volts are a lot more linear than if you had a resistor to bring DC to the anode, since the distortion of a triode rapidly increases with decreasing resistance values as the loading. The only significant RL on the V1 of an amp with a CCS DC supply is the following cap coupled biasing resistor, and if that is 220k, then its a far better load than say 47k *and* the 220k in parallel. The CCS allows a higher than normal plate current, and a lower Ea, and you still get splendid linearity. I have tried a few mosfets in regs and CCS, and found they were all too easily zapped. Patrick Turner. |
#4
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Hi Ian,
Why choose a BJT? Many would use a MOSFET, such as the STD1HNC60 I have used for a mu stage. I know almost nothing about transistors, as perhaps you can tell. Well, no hard facts from me either (I surely know less about transistors than you do) but the BJTs (mostly MJE... series) I used so far just worked fine in such an app, be it as (PNP) CC plate load or (NPN) CCS for a tubed LTP or diff amp. I am just too stupid, I am afraid, to set up, say, an IRF8xx TO220 type or something for this task (CC plate load) correctly ("biasing" considering depletion mode, gate protection) and so on. If someone would be so kind to give a working example using such a beast as CC plate load, I probably could modify the circuit myself as needed. Tom -- |
#5
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"Tom Schlangen" wrote in message
... Biggest problem is the needed high maximum emitter-collector voltage of, say, 500 volts (at say, a dozen mA). Rip apart a few computer or other switching supplies. Otherwise, they are cheap and plentiful at say, Mouser and Digi-Key. I ordered 10 x MJE18008 (800V 8A) from Digi-key, for $25. That switches 6.4kW! Tim -- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms |
#6
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Hi Patrick,
But in normal triode gain circuits, such as at the input V1 of a power amp they allow a lower B+, because usually only a few vrms of output is needed, Yes, sure. Problem is - unless one uses active voltage regulation instead of the "usual" RC-chain B+ decoupling, the full PSU B+ might be present at the device until tubes start conducting and voltages stabilize. A RC-chain-fed V1 tube might (and will) take this if things are not grossly out of order, but I am not sure a solid state device would be so forgiving ... I have tried a few mosfets in regs and CCS, and found they were all too easily zapped. That is what I fear, too. BJTs seem to be a bit more forgiving. Not as much as tubes, but probably more than FETs, I suppose. -- |
#7
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#8
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Tom Schlangen wrote: Hi Patrick, But in normal triode gain circuits, such as at the input V1 of a power amp they allow a lower B+, because usually only a few vrms of output is needed, Yes, sure. Problem is - unless one uses active voltage regulation instead of the "usual" RC-chain B+ decoupling, the full PSU B+ might be present at the device until tubes start conducting and voltages stabilize. I try to have a string of 4 x 75v zeners to regulate the B+ of V1 rail, since it also offers LF stability. A RC-chain-fed V1 tube might (and will) take this if things are not grossly out of order, but I am not sure a solid state device would be so forgiving ... When you use a pnp device, and the turn on voltage goes way above the rating, all 3 electrodes, emitter, base, and collector all are at the high B+ until the triode starts conducting. Then the only way the bjt could be destroyed is if its collector is grounded while the emitter & base is at say 450v. The triode comes on slowly, and pulls the collector voltage down to where it wants it, say +150v, and even if the B+ stays at 450v, the V rating isn't exceeded. Patrick Turner. I have tried a few mosfets in regs and CCS, and found they were all too easily zapped. That is what I fear, too. BJTs seem to be a bit more forgiving. Not as much as tubes, but probably more than FETs, I suppose. -- |
#9
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I have two MJE340K in my amplifier, the HV winding is 400-0-400 so it
comes up very high voltage at no load. Zero failures in twenty years. Jeff Goldsmith Patrick Turner wrote: I have used many MJE350/340 for CCS in circuits where the B+ was 300v, and never blown one. |
#10
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"Patrick Turner" wrote
When you use a pnp device, and the turn on voltage goes way above the rating, all 3 electrodes, emitter, base, and collector all are at the high B+ until the triode starts conducting. Not quite true, Patrick. The valve anode will initially be at 0V. If 500V is supplied instantly to the other end of the current source, then it has 500V across it. The anode capacitance must be charged. The current source may charge this at its designed current, which won't take very long, so you will get an initial short pulse of voltage across the source. Or the source may not be fast enough and you will get a current pulse instead. Then there is the capacitance into the following stage which is probably fed from the anode. That will take much longer to charge at a much lower current, being a larger capacitance and a much higher resistance. Also stray inductance, etc, that will tend to resist current pulses. Also the capacitance of the source device electrodes, and the rest of the source circuit. That is what I fear, too. BJTs seem to be a bit more forgiving. Not as much as tubes, but probably more than FETs, I suppose. Perhaps the mosfets you smoke are more vulnerable to all these start-up transients? Plenty mosfet mu-stages seem to work. It is commonly argued that they make better current sources than BJT (similar to pentode/triode difference?). Problem however with electrode capacitance. cheers, Ian |
#11
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On Tue, 07 Dec 2004 11:28:27 GMT, "Ian Iveson"
wrote: "Patrick Turner" wrote When you use a pnp device, and the turn on voltage goes way above the rating, all 3 electrodes, emitter, base, and collector all are at the high B+ until the triode starts conducting. Not quite true, Patrick. The valve anode will initially be at 0V. If 500V is supplied instantly to the other end of the current source, then it has 500V across it. Hate to disagree with you Ian, but if the valve is not conducting it is not conducting, and therefore the anode will float up to the HT level as no current is flowing through the load resistor/ccs. |
#12
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"bill ramsay" wrote
Hate to disagree with you Ian, You're not, as far as I can see, Bill. but if the valve is not conducting it is not conducting, and therefore the anode will float up to the HT level as no current is flowing through the load resistor/ccs. The anode indeed floats up, we all agree on that. I am saying that it will take time to do so, and it will start from 0V, so the instantaneous initial voltage across the source will be the full HT. The time taken to float up depends on the time constants in the circuit attached to the anode. Just simulating a simple driver and typical output stage, with a perfect CCS set at 10mA between a perfect 500V HT supply and the driver's anode. Coupling is via a 150nF cap into 100k grid leak. The anode voltage takes 1.5ms to reach 100V, 5ms to get to 200V, 10ms to 300V, and nearly 20ms to get to 85% of its final value of 380 volts. So the CCS drops 500V to begin with, falling to 400V after 1.5ms, 300V after 5ms, etc. The valve uses a cathode resistor for bias, so the valve is hard on to begin with (but with a high resistance until the anode voltage rises). A simulated piece of wood instead of the valve takes pretty much the same time. Most of the delay is due to the coupling caps to the next stage it seems. But the real picture would be much more complicated due to reactance in the CCS circuit, etc, etc. cheers, Ian in message ... On Tue, 07 Dec 2004 11:28:27 GMT, "Ian Iveson" wrote: "Patrick Turner" wrote When you use a pnp device, and the turn on voltage goes way above the rating, all 3 electrodes, emitter, base, and collector all are at the high B+ until the triode starts conducting. Not quite true, Patrick. The valve anode will initially be at 0V. If 500V is supplied instantly to the other end of the current source, then it has 500V across it. |
#13
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Tom Schlangen wrote:
Gentlemen, as it seems suitable BJTs for plate CCS load usage are hard to find at the parts shop at the corner here in Germany. Biggest problem is the needed high maximum emitter-collector voltage of, say, 500 volts (at say, a dozen mA). Transistors intended for horizontal output duty in TVs and computer monitors can handle up in the thousands of volts. Unless you are designing for production quantities, try salvaging one out of a discarded monitor. Or visit repair shops that will sell small quantities to people. |
#14
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Toshiba makes a bunch of high voltage high power transistors. Here's a link: http://www.semicon.toshiba.co.jp/ eng/prd/pdf_presen/tr_hv_e.pdf There's also another link whose author is messing around with high voltage switching transistors that gives some part numbers and specs: http://www.hills2.u-net.com/electron/ignit.htm -- Fred Gilham In 1981, an OSHA epidemiological study of 2,500 workers, half of whom had been exposed to PCBs for over 17 years, found that the number of deaths from cancer was 10 percent lower than what would be expected for a group with the same profile in the general population. |
#15
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"robert casey" a écrit dans le message de news: et... Tom Schlangen wrote: Gentlemen, as it seems suitable BJTs for plate CCS load usage are hard to find at the parts shop at the corner here in Germany. Biggest problem is the needed high maximum emitter-collector voltage of, say, 500 volts (at say, a dozen mA). Transistors intended for horizontal output duty in TVs and computer monitors can handle up in the thousands of volts. Unless you are designing for production quantities, try salvaging one out of a discarded monitor. Or visit repair shops that will sell small quantities to people. Be aware they usually exhibit very lo beta, down to 5 ! |
#16
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Ian Iveson wrote: "Patrick Turner" wrote When you use a pnp device, and the turn on voltage goes way above the rating, all 3 electrodes, emitter, base, and collector all are at the high B+ until the triode starts conducting. Not quite true, Patrick. The valve anode will initially be at 0V. If 500V is supplied instantly to the other end of the current source, then it has 500V across it. But no current flows, and the anode voltage whizzes up to whatever voltage the CCS collector wants it to be. The anode capacitance must be charged. Its only a few pF. The current source may charge this at its designed current, which won't take very long, so you will get an initial short pulse of voltage across the source. Or the source may not be fast enough and you will get a current pulse instead. It depends on how the CCS is set up. I have an R and C between the base and B+, then an R from base to 0V, so the CCS B+ to base voltage takes some time to establish, the anode capacitance is low, no problems so far.... Then there is the capacitance into the following stage which is probably fed from the anode. That will take much longer to charge at a much lower current, being a larger capacitance and a much higher resistance. Still only pF quantities.... Also stray inductance, etc, that will tend to resist current pulses. Also the capacitance of the source device electrodes, and the rest of the source circuit. That is what I fear, too. BJTs seem to be a bit more forgiving. Not as much as tubes, but probably more than FETs, I suppose. Perhaps the mosfets you smoke are more vulnerable to all these start-up transients? I'm not the one to smoke mosfets, because I don't use then for CCS when there is a cheaper easier locally available bjt. Plenty mosfet mu-stages seem to work. It is commonly argued that they make better current sources than BJT (similar to pentode/triode difference?). Problem however with electrode capacitance. I am not sure. Patrick Turner. cheers, Ian |
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