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#1
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I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was not able to get any real detailed information on this. I compared it to several guitar amp schematics which use FOUR 6L6 tubes in push-pull parallel, and this is very similar. It just has 3 tubes on each side of the phase, rather than two. The plates are all wired together and cathode / grid components are duplicates to each tube. They did use as bias adjust potentiometer on each tube, which I have not seen on the guitar amps. Obviously the purpose is to increase wattage output. I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Another question that comes to mind is the impedience of the primary on the audio output transformer. Assuming I could find a transformer that would handle the wattage, would the impedence be the same as those used with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series. Anyhow, looking at this schematic makes me think that any PPP amp with FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is obtainable. |
#2
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Well, yeah, it just scales. More in parallel, more (total) bias and load
current, lower load impedance. By the time you get to 8 or 10 or more, you should probably be asking yourself if a one of those fancy DSP pedals plus a class D amp is cheaper (not to mention more efficient, as we start running into the limitations of a 120V 15A circuit around this level..), or at least another tube type with better economy of scale (i.e. a transmitter tube of some sort), give or take whether we're talking production here (6L6s are still around, if ****ty -- the cheap ones that is) or one-offs from NOS. Heh, funny, come to think of it, that matched tubes are very common, and independent grid bias is reasonably common, but independent grid drive level is not at all common. Really all that matters is balance at the OPT, for magnetic reasons. The more tubes you wire in parallel, the less critical their matching is (assuming independent variables, and assuming any individual does not exceed its plate dissipation rating). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ wrote in message ... I found a schematic of a stereo amplifier containing TWELVE 6L6 audio output tubes. But aside from sharing the power supply, it's two mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was not able to get any real detailed information on this. I compared it to several guitar amp schematics which use FOUR 6L6 tubes in push-pull parallel, and this is very similar. It just has 3 tubes on each side of the phase, rather than two. The plates are all wired together and cathode / grid components are duplicates to each tube. They did use as bias adjust potentiometer on each tube, which I have not seen on the guitar amps. Obviously the purpose is to increase wattage output. I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Another question that comes to mind is the impedience of the primary on the audio output transformer. Assuming I could find a transformer that would handle the wattage, would the impedence be the same as those used with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series. Anyhow, looking at this schematic makes me think that any PPP amp with FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is obtainable. |
#3
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#5
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On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
wrote: On 28.12.18 10:45, wrote: On Thu, 27 Dec 2018 21:11:35 -0600, wrote: I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Tubes may age at different rates, so the characteristics might be quite different after a long time. It might not make sense to replace a single (failed) tube and you may have to replace the whole set of tubes with new tubes preferably from the same manufacturing batch. This can be quite expensive :-) In the 1960's, I made a guitar amplifier with six EL500's. The tubes are not characterized for linear use, and they were not matched in any way. Each tube had an own cathode resistor, and there was no evidence of any overload from unbalanced operation. Using individual cathode resistors to make the whole grid bias helps a lot to equalize the current in each tube. In general one tries to avoid cathode resistor bias in power stages, since quite a lot of the output voltage swing is lost in the cathode resistor(s) and use fixed low power negative grid bias supply instead, but this may require separate bias adjustment for each tube. I am not familiar with 6L6, but the similar EL34 is used in pairs for 30 W for HiFi, using 4 or 6 for "100 W" guitar amplifier with 450 V anode voltage. One bass qui tar amplifier is rated at 100 W using only two EL34 tubes, but runs at 800 V, but only Telefunken "Special quality" EL34s seemed to survive more than a few gigs. The tubes had a tendency of parasitic oscillation, so I added a ferrite bead on the control grids and an inductor-resistor parallel combination on each plate (for constructions, see nearest ham handbook). |
#6
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RCA Theater amps back in the day used between two and six 6L6 tubes depending on the required function. They were all monoblocks, sometimes from a common power-supply. Towards the end of their heyday, when Stereo sound-tracks were becoming more common, the center channel (the speaker directly behind the screen) would run from the optical sound-track, and the left and right from the magnetic sound track.
Peter Wieck Melrose Park, PA |
#7
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On 12/28/18 3:45 AM, wrote:
On Thu, 27 Dec 2018 21:11:35 -0600, wrote: I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Tubes may age at different rates, so the characteristics might be quite different after a long time. It might not make sense to replace a single (failed) tube and you may have to replace the whole set of tubes with new tubes preferably from the same manufacturing batch. This can be quite expensive :-) Unless the tubes are arranged truly symmetrically, e.g. in a circle with vertical air flow, they won't all run at the same envelope temperature, which means they won't have the same cathode temperature either. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com |
#8
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On 12/28/2018 03:45 AM, wrote:
On Thu, 27 Dec 2018 21:11:35 -0600, wrote: I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Tubes may age at different rates, so the characteristics might be quite different after a long time. It might not make sense to replace a single (failed) tube and you may have to replace the whole set of tubes with new tubes preferably from the same manufacturing batch. This can be quite expensive :-) All amps of that scale should have active protection circuitry/monitoring of currents, biases, and temperatures via microprocessor to avoid potentially catastrophic faults. Not putting that into an amp with the kind of non-inconsiderable expense one will sink into building it on the grounds of "purism" or whatever is mad, the kind of irresponsible behavior that the less well-heeled of the world get called out on. Work hard. Save money. Don't burn up 2 grand of parts/risk your life cuz one thinks silicon is too new-fangled. |
#9
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#10
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On 12/28/2018 06:32 PM, Chris wrote:
On 12/28/18 03:11, wrote: I found a schematic of a stereo amplifier containing TWELVE 6L6 audio output tubes. But aside from sharing the power supply, it's two mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was not able to get any real detailed information on this. I compared it to several guitar amp schematics which use FOUR 6L6 tubes in push-pull parallel, and this is very similar. It just has 3 tubes on each side of the phase, rather than two. The plates are all wired together and cathode / grid components are duplicates to each tube. They did use as bias adjust potentiometer on each tube, which I have not seen on the guitar amps. Obviously the purpose is to increase wattage output. I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Another question that comes to mind is the impedience of the primary on the audio output transformer. Assuming I could find a transformer that would handle the wattage, would the impedence be the same as those used with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series. Anyhow, looking at this schematic makes me think that any PPP amp with FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is obtainable. If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that which uses 6 or 8 6080 double triodes to build a direct coupled amplifier, no output transformer and dual power rails, positive and negative. Never built built it here, but a very original design. Could probably update that to use power mosfets... the two big issues with OTL tube amps is the intrinsic impedance mismatch of a tube cathode vs a tube plate in a totem pole arrangement looking into the load, and related lack of any intrinsic power supply noise rejection in the output stage the way a push-pull transformer-coupled output stage has when both halves have similar output impedances working into the same reflected load. Global negative feedback can't do anything about the second and there isn't usually enough open-loop gain available to do a good job of correcting for distortion caused by the first across the audio band. often leading to a kinda poor-performing amp. |
#11
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On 12/28/2018 08:18 PM, bitrex wrote:
If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that which uses 6 or 8 6080 double triodes to build a direct coupled amplifier, no output transformer and dual power rails, positive and negative. Never built built it here, but a very original design. Could probably update that to use power mosfets... the two big issues with OTL tube amps is the intrinsic impedance mismatch of a tube cathode vs a tube plate in a totem pole arrangement looking into the load, and related lack of any intrinsic power supply noise rejection in the output stage the way a push-pull transformer-coupled output stage has when both halves have similar output impedances working into the same reflected load. Global negative feedback can't do anything about the second and there isn't usually enough open-loop gain available to do a good job of correcting for distortion caused by the first across the audio band. often leading to a kinda poor-performing amp. These issues can be addressed but it requires more work than I'm guessing they put in in the mid 60s (can't immediately find the article in question online) |
#12
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On 29.12.18 01:32, Chris wrote:
On 12/28/18 03:11, wrote: I found a schematic of a stereo amplifier containing TWELVE 6L6 audio output tubes. But aside from sharing the power supply, it's two mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was not able to get any real detailed information on this. I compared it to several guitar amp schematics which use FOUR 6L6 tubes in push-pull parallel, and this is very similar. It just has 3 tubes on each side of the phase, rather than two. The plates are all wired together and cathode / grid components are duplicates to each tube. They did use as bias adjust potentiometer on each tube, which I have not seen on the guitar amps. Obviously the purpose is to increase wattage output. I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Another question that comes to mind is the impedience of the primary on the audio output transformer. Assuming I could find a transformer that would handle the wattage, would the impedence be the same as those used with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series. Anyhow, looking at this schematic makes me think that any PPP amp with FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is obtainable. If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that which uses 6 or 8 6080 double triodes to build a direct coupled amplifier, no output transformer and dual power rails, positive and negative. Never built built it here, but a very original design. Could probably update that to use power mosfets... I found it in my copy, 16th edition, 1963. The amplifier is not direct coupled, there are several stages with coupling capacitors. The final is series-connected with 3 series pairs of 6082 triodes in parallel and direct feed to a 16 ohm speaker. The power supplies are + and - 140 V. -- -TV |
#13
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#14
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On 12/29/18 10:27, Tauno Voipio wrote:
On 29.12.18 01:32, Chris wrote: On 12/28/18 03:11, wrote: I found a schematic of a stereo amplifier containing TWELVE 6L6 audio output tubes. But aside from sharing the power supply, it's two mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was not able to get any real detailed information on this. I compared it to several guitar amp schematics which use FOUR 6L6 tubes in push-pull parallel, and this is very similar. It just has 3 tubes on each side of the phase, rather than two. The plates are all wired together and cathode / grid components are duplicates to each tube. They did use as bias adjust potentiometer on each tube, which I have not seen on the guitar amps. Obviously the purpose is to increase wattage output. I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Another question that comes to mind is the impedience of the primary on the audio output transformer. Assuming I could find a transformer that would handle the wattage, would the impedence be the same as those used with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series. Anyhow, looking at this schematic makes me think that any PPP amp with FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is obtainable. If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that which uses 6 or 8 6080 double triodes to build a direct coupled amplifier, no output transformer and dual power rails, positive and negative. Never built built it here, but a very original design. Could probably update that to use power mosfets... I found it in my copy, 16th edition, 1963. The amplifier is not direct coupled, there are several stages with coupling capacitors. The final is series-connected with 3 series pairs of 6082 triodes in parallel and direct feed to a 16 ohm speaker. The power supplies are + and - 140 V. sounds like a gimmick. Tube plate/cathode currents are just way too small to properly drive a speaker without an impedence matching transformer. Had they thought of it "back then" they could've used toroidal transformers with PWM push-pull amplifiers modulated up to 70 or 80 percent as a class 'G' amplifier. By the time that was invented, it was all transistors, though. [and no benfit to using tubes]. I suspect that the push-pull configuration's advantages [and disadvantages] disappear when you use a transformerless configuration, and new ones take their place. If you want high power, you should consider using KT88's and a matching transformer from Hammond. A single pair should get you at least 100W RMS. I saw a stereo amplifier built using these at a state fair once, in the 70's, basically a clone of the GE tube manual's reference design. It was on an oversized chassis, though. I would've preferred it as 2 "monoblock" amplifiers that could fit together on a shelf... even the high heater current requirements of a KT88 would be smaller than 6 or 8 6L6's [and is physically smaller]. But I suppose it looks 'cooler' to have all of those bottles grouped together. -- (aka 'Bombastic Bob' in case you wondered) 'Feeling with my fingers, and thinking with my brain' - me 'your story is so touching, but it sounds just like a lie' "Straighten up and fly right" |
#15
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On 12/28/18 09:16, bitrex wrote:
All amps of that scale should have active protection circuitry/monitoring of currents, biases, and temperatures via microprocessor to avoid potentially catastrophic faults. I think it works ok without "all that". fuses where needed of course, to avoid things catching on fire. But tubes are amazingly strong at absorbing conditions that transistors would melt under. Sure the plates turn pink [like when half of a push-pull transformer melts away] but if you don't push them to the edge of physics they generally take it ok and survive getting a new output transformer. Or if a single tube fails in multi-pair configuration, the others will take up most of the slack without too much bad behavior. You'll probably hear the bad quality sound at high volumes, but without extended operation "that way" the other tubes should survive. the more you have paralleled, the less impact a single tube failure will have on the other tubes. Even briefly overvolt or overcurrent on a typical transistor, and you'll be replacing it VERY soon. I made the mistake of designing a circuit that operated close to the maximum Vceo (these 60V transistors should be able to handle 45-50V right?), and the transistors never lasted long under load (replaced 3 times, and 3 blown fuses that were supposed to protect them). Replaced with transistors that had twice the Vceo and no problem. -- (aka 'Bombastic Bob' in case you wondered) 'Feeling with my fingers, and thinking with my brain' - me 'your story is so touching, but it sounds just like a lie' "Straighten up and fly right" |
#16
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"Big Bad Bob" wrote in message
... I think it works ok without "all that". fuses where needed of course, to avoid things catching on fire. But tubes are amazingly strong at absorbing conditions that transistors would melt under. Sure the plates turn pink [like when half of a push-pull transformer melts away] but if you don't push them to the edge of physics they generally take it ok and survive getting a new output transformer. A lot of sweep tube datasheets contain the rating, "short term overload duration: 220W for 60s" or something like that. Transistors can only dream of such abuse, gone in milliseconds! But the purpose of that rating must be understood. These were TV tubes, and the sweep tube in particular often took a beating as other tubes heated up and other signals stabilized: horizontal oscillator and sync, and the damper diode especially -- its high cathode voltage isolation takes a long time to warm up. There was definitely no money in adding a protection circuit! Burn a tube, pop it out, take it down to the corner drugstore and buy a new one for a buck or a few. Nowadays, with both tubes and transformers being rather pricey, let alone the repairman -- the balance changes, and especially with how little hardware is involved in adding a protection circuit (if one does not mind that it contains silicon), it's well worth it. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ |
#17
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"Big Bad Bob" wrote in message
... sounds like a gimmick. Tube plate/cathode currents are just way too small to properly drive a speaker without an impedence matching transformer. So what, nothing comes out?... They were definitely below the maximum power point (at clipping), if that's more accurately what you meant. 6AS7/6080, 6S33S and the other regulator tubes have peak cathode current in the ballpark of an ampere, so a modest number of tubes offers a modest power output, say 20 or 40W, preferably into a higher load like 16 ohms. The efficiency is poor, with more heat dissipated in the heaters alone, than delivered to the output. If you put dozens in parallel, the efficiency keeps going up as you get closer to matching, but now your whole system consumes multiple kilowatts... There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms voice coil at good efficiency, but those speakers are so rare that this is practically a unique case. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ |
#18
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On 12/31/2018 11:32 PM, Tim Williams wrote:
"Big Bad Bob" wrote in message ... I think it works ok without "all that".* fuses where needed of course, to avoid things catching on fire.* But tubes are amazingly strong at absorbing conditions that transistors would melt under.* Sure the plates turn pink [like when half of a push-pull transformer melts away] but if you don't push them to the edge of physics they generally take it ok and survive getting a new output transformer. A lot of sweep tube datasheets contain the rating, "short term overload duration: 220W for 60s" or something like that.* Transistors can only dream of such abuse, gone in milliseconds! But the purpose of that rating must be understood.* These were TV tubes, and the sweep tube in particular often took a beating as other tubes heated up and other signals stabilized: horizontal oscillator and sync, and the damper diode especially -- its high cathode voltage isolation takes a long time to warm up. There was definitely no money in adding a protection circuit!* Burn a tube, pop it out, take it down to the corner drugstore and buy a new one for a buck or a few. Nowadays, with both tubes and transformers being rather pricey, let alone the repairman -- the balance changes, and especially with how little hardware is involved in adding a protection circuit (if one does not mind that it contains silicon), it's well worth it. Tim Yep, the OP is talking about monoblock amp with 6, 8, ten power tubes. The power supply iron and output transformer will be large, possibly custom, and not cheap. $5-10 worth of microcontroller or ICs and relays to at the least monitor tube cathode currents and grid voltages and cut the HT if things start going tits-up compared to a melted half of an output transformer that might cost $3-500. Big BUMMER! |
#19
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On Mon, 31 Dec 2018 11:14:33 -0800, Big Bad Bob
wrote: On 12/29/18 10:27, Tauno Voipio wrote: On 29.12.18 01:32, Chris wrote: clip If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that which uses 6 or 8 6080 double triodes to build a direct coupled amplifier, no output transformer and dual power rails, positive and negative. Never built built it here, but a very original design. Could probably update that to use power mosfets... I found it in my copy, 16th edition, 1963. The amplifier is not direct coupled, there are several stages with coupling capacitors. The final is series-connected with 3 series pairs of 6082 triodes in parallel and direct feed to a 16 ohm speaker. The power supplies are + and - 140 V. sounds like a gimmick. Tube plate/cathode currents are just way too small to properly drive a speaker without an impedence matching transformer. Search for 6082 "50 ohm" and you find quite a lot hits with single pair driving 50 ohms. With tree pairs, the load could be 16 ohms. |
#20
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On Mon, 31 Dec 2018 22:39:02 -0600, "Tim Williams"
wrote: There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms voice coil at good efficiency, but those speakers are so rare that this is practically a unique case. Usually closer to 1000 ohms. These speakers were sold for a few years, but later on, they went on sale. Adding an 8:1000 ohm transformer and you could cheaply build a good quality speaker box for an ordinary 8 ohm tube or transistor amplifier :-) |
#21
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"Big Bad Bob" wrote in message
... On 12/28/18 09:16, bitrex wrote: All amps of that scale should have active protection circuitry/monitoring of currents, biases, and temperatures via microprocessor to avoid potentially catastrophic faults. Even briefly overvolt or overcurrent on a typical transistor, and you'll be replacing it VERY soon. I made the mistake of designing a circuit that operated close to the maximum Vceo (these 60V transistors should be able to handle 45-50V right?), and the transistors never lasted long under load (replaced 3 times, and 3 blown fuses that were supposed to protect them). Replaced with transistors that had twice the Vceo and no problem. Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their rating at high voltage is much lower than their power ratings would imply. I designed mosfet amp in the early 80s. I tested it by putting in a full level signal with an output s/c. The only protection was a zener across the gates to limit the current to the device ratings. I left it cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems. -- Kevin Aylward http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html |
#22
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On 1.1.19 06:39, Tim Williams wrote:
"Big Bad Bob" wrote in message ... sounds like a gimmick.* Tube plate/cathode currents are just way too small to properly drive a speaker without an impedence matching transformer. So what, nothing comes out?... They were definitely below the maximum power point (at clipping), if that's more accurately what you meant. 6AS7/6080, 6S33S and the other regulator tubes have peak cathode current in the ballpark of an ampere, so a modest number of tubes offers a modest power output, say 20 or 40W, preferably into a higher load like 16 ohms.* The efficiency is poor, with more heat dissipated in the heaters alone, than delivered to the output. If you put dozens in parallel, the efficiency keeps going up as you get closer to matching, but now your whole system consumes multiple kilowatts... There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms voice coil at good efficiency, but those speakers are so rare that this is practically a unique case. Tim The final triodes (6080) are special high-current tubes. The amplifier designer rated the thing at 25 W into a 16 ohm voice-coil speaker. -- -TV |
#23
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On 01/01/2019 10:27 AM, Kevin Aylward wrote:
"Big Bad Bob"Â* wrote in message ... On 12/28/18 09:16, bitrex wrote: All amps of that scale should have active protection circuitry/monitoring of currents, biases, and temperatures via microprocessor to avoid potentially catastrophic faults. Even briefly overvolt or overcurrent on a typical transistor, and you'll be replacing it VERY soon.Â* I made the mistake of designing a circuit that operated close to the maximum Vceo (these 60V transistors should be able to handle 45-50V right?), and the transistors never lasted long under load (replaced 3 times, and 3 blown fuses that were supposed to protect them). Replaced with transistors that had twice the Vceo and no problem. Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their rating at high voltage is much lower than their power ratings would imply. Ya, BJT amps tend to have oversized output devices as compared to what their rated maximum "RMS power" output would imply. 75 watt-rated devices in 25 watt amps. The problem is risk of second breakdown when working into reaactive loads, not de-rating BJTs appropriately when they're handing significant powers into reactive loads common newbie mistake I designedÂ* mosfet amp in the early 80s. I tested it by putting in a full level signal with an output s/c. The only protection was a zener across the gates to limit the current to the device ratings. I left it cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems. -- Kevin Aylward http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html |
#24
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"Kevin Aylward" wrote in message
... Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their rating at high voltage is much lower than their power ratings would imply. Used to be true -- modern MOSFETs are more than current-dense enough to exhibit 2nd breakdown. I shouldn't actually say modern, because apparently SuperJunction process has... PTC source connections or something? I haven't seen one without a square SOA yet I don't think. So by now, it's actually previous generation that you have to watch out for. I forget if lower voltage (SJ goes away under ~400V I think it was?) processes are still prone. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/ |
#25
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"Tim Williams" wrote in message ...
"Kevin Aylward" wrote in message ... Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their rating at high voltage is much lower than their power ratings would imply. Used to be true -- modern MOSFETs are more than current-dense enough to exhibit 2nd breakdown. I shouldn't actually say modern, because apparently SuperJunction process has... PTC source connections or something? I haven't seen one without a square SOA yet I don't think. So by now, it's actually previous generation that you have to watch out for. I forget if lower voltage (SJ goes away under ~400V I think it was?) processes are still prone. Exicon are the audio mosfets of choice today, apparently. They are laterals. http://www.exicon.info/ They have the usual power limited SOA. I don't know who actually makes them, but my guess is someone like XFAB. Standard fab vendors will make any asic for any fabless company, even if the asic is just the one big transistor! -- Kevin Aylward http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html |
#26
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Posted to sci.electronics.design,rec.audio.tubes
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On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
wrote: On 28.12.18 10:45, wrote: On Thu, 27 Dec 2018 21:11:35 -0600, wrote: I'm not intending on building this, but it's interesting and makes me wonder if someone could use EIGHT 6L6 tubes, or TEN? (Of course adding more tubes mean bigger audio output transformers and heftier power supplies. Tubes may age at different rates, so the characteristics might be quite different after a long time. It might not make sense to replace a single (failed) tube and you may have to replace the whole set of tubes with new tubes preferably from the same manufacturing batch. This can be quite expensive :-) In the 1960's, I made a guitar amplifier with six EL500's. The tubes are not characterized for linear use, and they were not matched in any way. Each tube had an own cathode resistor, and there was no evidence of any overload from unbalanced operation. The tubes had a tendency of parasitic oscillation, so I added a ferrite bead on the control grids and an inductor-resistor parallel combination on each plate (for constructions, see nearest ham handbook). Back in the late 60s early 70s, I had several (self refurbished) mono-block power amps that used four 6L6 output tubes. I did not even know about matching those tubes. I just put in any tube marked as a 6L6. I recall having both the glass (GC) types mixed with the black metal ones. I always had good sound and lots of power. I do recall that replacing the metal cased ones with 6L6GC did increase my power though. But until I could afford new tubes, I used what I had. And even with the new tubes, I never matched them. |
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