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6CA7 in AB2.
Anyone have any experience in driving 6CA7
in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. What sayest thou, and they of years challenged, and who accompanied history's beginning, and who must have memory of a bygone age? What noble efforts at AB2 were sustained, leading to much music well listened to? Patrick Turner. |
#2
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Patrick Turner wrote:
Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. What sayest thou, and they of years challenged, and who accompanied history's beginning, and who must have memory of a bygone age? What noble efforts at AB2 were sustained, leading to much music well listened to? Patrick Turner. See ABPR for the attachments referenced in the following. Hi Pat- Never ran a 6CA7 / EL34 in AB2 but last year built an experimental 6V6GT rig running AB2. It managed 26 watts in burst mode & 19 watts continuous after the PS sagged to steady state. The 6V6 grids are driven by a PP 6BQ7 CF. That helps to minimize distortion as a result of G1 current. IMO should be OK for a music amp since the grids will be driven +ve only on peaks which are but a small percentage of the time. See the attachment for test results. This all part of an article on Amplifier Burst Testing to appear in the November issue of AudioXpress magazine. Also built an AB2 amp using a PP pair of 33 two volt pentodes. It runs triode or UL. The driver is a triode connected 33, which in turn is transformer coupled to the following grids. That one is published in the "Glass Audio Projects" book from AudioXpress.com. The attached graph is a composite shewing the drive signal to the grids (LHS with X10 Probe) & the resulting G1 current (RHS) measured across a 10R sampling resistor in the grid circuit. This one manages 4.85 W in UL & 2.5 W in Triode mode. Cheers, John Stewart |
#3
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john stewart wrote: Patrick Turner wrote: Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. What sayest thou, and they of years challenged, and who accompanied history's beginning, and who must have memory of a bygone age? What noble efforts at AB2 were sustained, leading to much music well listened to? Patrick Turner. See ABPR for the attachments referenced in the following. Hi Pat- Never ran a 6CA7 / EL34 in AB2 but last year built an experimental 6V6GT rig running AB2. It managed 26 watts in burst mode & 19 watts continuous after the PS sagged to steady state. The 6V6 grids are driven by a PP 6BQ7 CF. That helps to minimize distortion as a result of G1 current. Indeed, I know about the drive requirements needing to be CF, and low impedance, because the grid I reduces R in to the output tubes. IMO should be OK for a music amp since the grids will be driven +ve only on peaks which are but a small percentage of the time. See the attachment for test results. This all part of an article on Amplifier Burst Testing to appear in the November issue of AudioXpress magazine. I wanted to try 6CA7 in a circuit meant for KT88. The limitation, especially in triode mode, is the Vg = 0V line. With say 807, you often see the data curves including for a grid swing up to +15v, allowing 30 watts AB2. Also built an AB2 amp using a PP pair of 33 two volt pentodes. It runs triode or UL. The driver is a triode connected 33, which in turn is transformer coupled to the following grids. That one is published in the "Glass Audio Projects" book from AudioXpress.com. The attached graph is a composite shewing the drive signal to the grids (LHS with X10 Probe) & the resulting G1 current (RHS) measured across a 10R sampling resistor in the grid circuit. This one manages 4.85 W in UL & 2.5 W in Triode mode. Cheers, John Stewart I couldn't see any item under your name at abpr, maybe the currawongs and magpies haven't flown in yet with the info. The data curves for 6CA7 are usually given as EL34/6CA7, and there is a difference, the former is a beam tetrode, the latter is a real pentode, and the couple of 6CA7 I do have in operation ARE different to EL34, with lower Ra and gain, but higher Pd. I like the 6CA7 more than the EL34, and they sound well. If I buy 50 of these now from New Sensor, they will only cost US $280. The driver for the AB2 amp will have a choke with CT grounded, and each end going to the grids of the output tubes. The cathode circuits might be a new AC regulated cathode bias type, which has all the benefits of excellent self regulation of the cathode bias R, but when going into class AB, the cathode cap won't charge up due to the presence of one transistor bypassing the high positive peak currents seen in the cathode circuit, in all AB amps. The choke won't allow the grid bias to change, as it does with a normal RC coupled stage driven into grid current, because of the charge up in the coupling caps. The charging up of cathode biasing and the grid coupling cap both cause a tube amp to become paralysed when pushed a lot. The output stage will be acoustical, so the CFB will INCREASE the input impedance, despite the grid current, in the same way as in a McIntosh. Thus the AB2 amp is possible, and the extra headroom good for dynamics such as loud drumbeats, where the slightly higher distortion after going from class AB1 into AB2 don't matter. The acoustical prevents severe distortion developing in the driver LTP stage, because the voltage demanded still isn't too high for RC coupling. Probably, I will have a direct coupled CF driving the choke, and then the gain LTP can be a 6CG7 which is RC coupled to the CF. Then the usual SE input stage. Two 6CA7 work OK into a 6.6 k RL, without much AB2, with B+ = 450v, and 10k is really nice. But if the load has a dip to 3.3k, then the voltage swing is limited, even in tetrode mode. I was just after a little bit more performance. Patrick Turner. |
#4
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In article , Patrick Turner
wrote: Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. Why is it that the EL34 doesn't like AB2 operation, what happens that is bad? Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#5
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John Byrns wrote: In article , Patrick Turner wrote: Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. Why is it that the EL34 doesn't like AB2 operation, what happens that is bad? Good question. I have never really tried to push an EL34 hard into grid current. But perhaps the 6CA7 is different, since it is a beam tetrode, not a pentode. I was wondering who had experience of this, or whether I will have to try it out myself to see if AB2 works as well as it does say with 807, or 6L6. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#6
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"Patrick Turner" wrote in message ... Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. What sayest thou, and they of years challenged, and who accompanied history's beginning, and who must have memory of a bygone age? What noble efforts at AB2 were sustained, leading to much music well listened to? Patrick Turner. I'm certainly 'of years challenged' Why do you want to run EL34s in AB2 ?? There are easier ways of achieving the same result with 6550/KT88.. The raison d'etre would be helpful regards jim |
#7
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jim wrote: "Patrick Turner" wrote in message ... Anyone have any experience in driving 6CA7 in either tetrode, triode, or UL, into class AB2? The 6CA7 is supposed to be equal to EL34. EL34 don't much like AB2; but since the 6CA7 is a tetrode, not a pentode, perhaps, like the 6L6, AB2 operation is perhaps fine. What sayest thou, and they of years challenged, and who accompanied history's beginning, and who must have memory of a bygone age? What noble efforts at AB2 were sustained, leading to much music well listened to? Patrick Turner. I'm certainly 'of years challenged' Why do you want to run EL34s in AB2 ?? There are easier ways of achieving the same result with 6550/KT88.. The raison d'etre would be helpful regards jim I have a client who has an amp where he'd like the high power ability, but most days he sues low power. This is where class AB2 operation fulfills a niche. Jolida manage 66 watts from their UL amps with 530 for the B+, and that's because the mains inputs are rated for 220v, yet we have 250v most days here, so hence the B+ goes from the design value of 470v to 530v wkg. I know, becuase I tested one. KT88/6550 don't give you much more power. The high screen voltage means the grid bias voltage needs to be more -ve, therefore you get more load swing without running into grid current, with either UL or pentode, ie, all the power is had with AB1 operation. But let us concentrate on pentode operation. If one reduces EG2 to say 300v, then the grid1 bias can be reduced, to have the same idle plate current. But it also means you can't have the same high input voltage to the grid1 because the input voltage will go +ve sooner with respect to the cathode. The lower Eg2 supply relaxes the load on the screen, and gives lower thd at the first 20 watts, because the anode signal voltage spends most of the time at a potential above the screen potential, therefore screen current variations are minimised, and screen current draw during the cycle isn't too linear, which contributes to anode current distortions. Pentode operation allows for up to 800v for the plate supply, and say 400v on the screens, and a 12k a-a RL, and grid current never occurs. But if we reduced the screen volts to say 300v, then we would maybe run out of grid drive headroom. UL and trioded tubes cannot be run at such high voltages, because the grid 1 would have to be biased at impossibly low -ve bias voltages. Triode operation results in a plate curve line at Vg1 = 0.0v which is a boundary for load voltage swing over which you normally cannot step, because the grid draws so much current itself, that it stalls the driving amp. Some tubes, like the 6L6, 807, and others, don't have so much grid current that they cannot still be driven, although it does take something like a cathode follower to do it. With 6L6,807,KT66, Triode AB2 operation is desirable, if its possible, because we have all the finesse of the triodes during the normal part of the class A, and AB1, that you'd get with any other amp, plus the bonus of the extra 15 watts in AB2. Sure I am a big fan of the EH and Sovtek 6550, and KT88. All four tubes are, afaik, exactly the same electronically, with the same internal electrode structure, but they just have different glass envelopes, and writing on the outside, with the EHKT88 looking the prettiest. These slightly more capable large octals are 3 times the price of the 6CA7. The 6CA7 seems to me to be the best watts/$ value of output tubes at this present time. Sovtek made copies of the Sylvania 6CA7 up to about 1996, and then I never seen any more until EH announced their return, along with a few other tubes which havent been made for 35 years, and including a russian KT90, which will be available later this year. The tube business seems to be growing and diversifying. Anyways, mild reliance on AB2 operation is not evil, and seems a sensible option if done well, it seems to me, so hence my query, how easy is it to drive 6CA7 into AB2? Patrick Turner. |
#8
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Patrick Turner wrote: [...] Pentode operation allows for up to 800v for the plate supply, and say 400v on the screens, and a 12k a-a RL, and grid current never occurs. But if we reduced the screen volts to say 300v, then we would maybe run out of grid drive headroom. UL and trioded tubes cannot be run at such high voltages, because the grid 1 would have to be biased at impossibly low -ve bias voltages. [...] Well, I wouldn't say it's impossibly low. My favourite hi-fi amp uses 6L6GC's at about 620 volts B+ in UL. Grid bias for a reasonable plate + screen dissipation (cathode current 35 mA) is about -78 volts. I don't think that it would be very much different for EL34 or 6CA7, but haven't tried it. (If anyone would care to donate a set, I'd be happy to try them out and report back!) This means, though, that the driver needs lots of headroom; about 160 volts peak-to-peak. The usual concertinas are therefore rather impractical. But what a great application for a long-tail pair with pentode (or transistor) CCS! Lots of output swing -- and gain as a bonus. The net result is lots of power, good clean sound even at full power due to the UL topology, using relatively common-place tubes. I've found AB2 to be a real nuisance for hi-fi use, but it's great for guitar amps because it give a more graduated distortion characteristic, passing from class A to class AB1 to class AB2 to full clipping. It's absolutely *wonderful* for that, the louder you play the "harder" you play, without suddenly crunching into the ceiling like you do with SS amps. But for hi-fi, unless you're a glutton for punishment and want a real challenge to get a low-impedance driver, and then maybe still have to try to iron out the added distortion by using NFB tricks, I wouldn't recommend AB2 using "conventional" valves like the 807, 6L6, EL34, etc. I think you'd be better off with a lower AB1, even approaching B1, than to cope with the difficulty of trying to stay clean whilst drawing grid current. I too am interested in AB2 for hi-fi, but would use a pair of high-voltage TX triodes such as 809 or 811A, in a "darlington" kind of configuration (direct-coupled cathode followers on the control grids). From reports I've seen, and from a very crude mock-up I did years ago, this has the potential of sounding very nice, and have lots of available power. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#9
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"Patrick Turner" wrote in message
... The lower Eg2 supply relaxes the load on the screen, and gives lower thd at the first 20 watts ... - Even though the load impedance is lower, relatively speaking? Tim -- In the immortal words of Ned Flanders: "No foot longs!" Website @ http://webpages.charter.net/dawill/tmoranwms |
#10
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Fred Nachbaur wrote: Patrick Turner wrote: [...] Pentode operation allows for up to 800v for the plate supply, and say 400v on the screens, and a 12k a-a RL, and grid current never occurs. But if we reduced the screen volts to say 300v, then we would maybe run out of grid drive headroom. UL and trioded tubes cannot be run at such high voltages, because the grid 1 would have to be biased at impossibly low -ve bias voltages. [...] Well, I wouldn't say it's impossibly low. My favourite hi-fi amp uses 6L6GC's at about 620 volts B+ in UL. Grid bias for a reasonable plate + screen dissipation (cathode current 35 mA) is about -78 volts. I don't think that it would be very much different for EL34 or 6CA7, but haven't tried it. (If anyone would care to donate a set, I'd be happy to try them out and report back!) True, some folks do use really high voltages for UL or triode. Its sudden dearth territory, and many older tubes just can't be biased down to be stable, 6CM5 is a classic. I don't like such high voltages for trioded or UL amps, because it means the idle current has to be low, to get the tubes to be biased a LOT below the rated max. This means the tubes are not working in the most linear region for the first few watts, which are the most important, and the amp mainly class B. +500v is as high as I like to go. Acoustical goes really well at all voltages, and is a better way to do UL, and the screens are given a reprieve from high screen currents, since the screen voltage can be a lot lower than the anode supply. This means, though, that the driver needs lots of headroom; about 160 volts peak-to-peak. That's only around 55 vrms, per each input, and that's easy to do, and a heck of a lot easier than a McIntosh, where the drive to each output tube is maybe 140 vrms. Same for EAR509. The usual concertinas are therefore rather impractical. But what a great application for a long-tail pair with pentode (or transistor) CCS! Lots of output swing -- and gain as a bonus. Well exactly. The net result is lots of power, good clean sound even at full power due to the UL topology, using relatively common-place tubes. I've found AB2 to be a real nuisance for hi-fi use, but it's great for guitar amps because it give a more graduated distortion characteristic, passing from class A to class AB1 to class AB2 to full clipping. It's absolutely *wonderful* for that, the louder you play the "harder" you play, without suddenly crunching into the ceiling like you do with SS amps. One might have to have some limiting R between a CF driver and the output grids, say 1k, maybe 2k, to give a nice squishy overdrive, ie, a natural compressor effect. Also, grid current needs to be limited somewhat in amps subjected to continual overdrive all night. But for hi-fi, unless you're a glutton for punishment and want a real challenge to get a low-impedance driver, and then maybe still have to try to iron out the added distortion by using NFB tricks, I wouldn't recommend AB2 using "conventional" valves like the 807, 6L6, EL34, etc. I think you'd be better off with a lower AB1, even approaching B1, than to cope with the difficulty of trying to stay clean whilst drawing grid current. I would still have the same highish value load I normally use, so the first 25 watts are still the same low thd as the AB1 amp. If the load is halved, the power is lower, due to grid voltage excursions needing to exceed the bias voltage, to get the preferable extra load voltage swing. The extra thd above 25 watts isn't going to be huge, and is for the occasional drumbeats. I too am interested in AB2 for hi-fi, but would use a pair of high-voltage TX triodes such as 809 or 811A, in a "darlington" kind of configuration (direct-coupled cathode followers on the control grids). From reports I've seen, and from a very crude mock-up I did years ago, this has the potential of sounding very nice, and have lots of available power. !/2 a 6SN7 is a fine CF driver for 6L6 in AB2, and I have a Brimar circuit in an old tube manual of theirs with this set up, using 807, with +600v for B+, and +300v for EG2, and it makes 80 watts, and that's what folks used for PA, back in 1953, complete with hardly any FB, and 10% thd max. At 10 watts, they weren't too bad. The 807 meant you didn't get arcing on the tube socket, and the Eg2 voltage meant the screens didn't fry every time some galoot ran a shorted speaker lead, and turned up the volume. Patrick Turner. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#11
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Tim Williams wrote: "Patrick Turner" wrote in message ... The lower Eg2 supply relaxes the load on the screen, and gives lower thd at the first 20 watts ... - Even though the load impedance is lower, relatively speaking? Well if you read the operating specs for 6550 as tetrodes, 440v for Ea, and lower V fpr Eg2, they get thd down to 0.6% at 40 watts, I have forgot the exact figures, but the secret is the low Eg2 relative to the anode supply, once that supply is over +300v. There simply isn't any need to have Eg2 so high as they so commonly do these days, its just that makers are lazy to make a dropped value of Eg2, which costs them an R&C. Most AB1 tetrode/pentode amps DON'T need to have a grid voltage which goes anywhere near making grid current; the peak input grid swing is usually a lot less than the bias voltage value, AND this bias voltage could be reduced, IF the screen voltage was lowered, and maximum power output would stay the same, or only be marginally lower. Patrick Turner. Tim -- In the immortal words of Ned Flanders: "No foot longs!" Website @ http://webpages.charter.net/dawill/tmoranwms |
#12
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jim wrote: "Fred Nachbaur" wrote in message news:NwC_a.8048$zE1.6670@edtnps84... Patrick Turner wrote: [...] Pentode operation allows for up to 800v for the plate supply, and say 400v on the screens, and a 12k a-a RL, and grid current never occurs. But if we reduced the screen volts to say 300v, then we would maybe run out of grid drive headroom. UL and trioded tubes cannot be run at such high voltages, because the grid 1 would have to be biased at impossibly low -ve bias voltages. [...] Well, I wouldn't say it's impossibly low. My favourite hi-fi amp uses 6L6GC's at about 620 volts B+ in UL. Grid bias for a reasonable plate + screen dissipation (cathode current 35 mA) is about -78 volts. I don't think that it would be very much different for EL34 or 6CA7, but haven't tried it. Likewise. The EL34/6CA7 is an unknown quantity to me. We're not really answering Patrick's question here, but with a variable PSU. I've run UL Reflektor 6L6GC pairs up to 550v with idle dissippation of 24W into loads between 4 and 6k and like the way they sound. If you had a 4 k load, and used the amp at elevated levels, perhaps the Pd might exceed the Pd limit. Load line analysis would show that, but one can get away with a lot with tubes, and music. (If anyone would care to donate a set, I'd be happy to try them out and report back!) This means, though, that the driver needs lots of headroom; about 160 volts peak-to-peak. The usual concertinas are therefore rather impractical. But what a great application for a long-tail pair with pentode (or transistor) CCS! Lots of output swing -- and gain as a bonus. The net result is lots of power, good clean sound even at full power due to the UL topology, using relatively common-place tubes. I've found AB2 to be a real nuisance for hi-fi use, but it's great for guitar amps because it give a more graduated distortion characteristic, passing from class A to class AB1 to class AB2 to full clipping. It's absolutely *wonderful* for that, the louder you play the "harder" you play, without suddenly crunching into the ceiling like you do with SS amps. But for hi-fi, unless you're a glutton for punishment and want a real challenge to get a low-impedance driver, and then maybe still have to try to iron out the added distortion by using NFB tricks, I wouldn't recommend AB2 using "conventional" valves like the 807, 6L6, EL34, etc. I think you'd be better off with a lower AB1, even approaching B1, than to cope with the difficulty of trying to stay clean whilst drawing grid current. I too am interested in AB2 for hi-fi, but would use a pair of high-voltage TX triodes such as 809 or 811A, in a "darlington" kind of configuration (direct-coupled cathode followers on the control grids). This looks kind of elegant if you draw it out. It looks so simple, I put one together a couple of years ago. As I remember ...... 550v B+... a pair of FB UL KT88s..... 2 x 6SN7 CFs off the same 550v supply, direct coupled to the KT88 grids ..... 22K loads off a fixed 150v low imp line and variable -ve bias on the 6SN7 grids to set the KT88 idle current. Up front was a 6SL7 LTP. I could swing 170V pk-pk on the KT88 grids before it cut the tops off a sine wave input. The PSU would start a broken down truck. From reports I've seen, and from a very crude mock-up I did years ago, this has the potential of sounding very nice, and have lots of available power. Up one paragraph. Did it sound nice ?? It didn't actually sound any better or worse than the same KT88s, without the CFs, with either a LTP or Williamson front end up to the sound level where you had to hide behind something. The "sound" is mainly determined by the output stage operation, since it usually makes far more thd than the input stages, with or without CF drivers to the OPV grids.. Such a layout may be useful for getting 100W out of a pair of EL84s whilst staying within the stated max anode dissippation by driving them hard enough to cut them off for over 50% of the input cycle, but this is not really hi-fi. I have seen EL34 used with B+ at 900v, and Eg2 at 450v, and Po = 125 watts. It makes for a very fragile amp, if ever the load becomes lower than rated for 125 watts. I just realized that big noise and low distortion starts with a low component count, big volts, big toobs and big OPTs. Patrick has a man who insists on AB2 This is hard work regards jim In pentode/tetrode, using 2 x EL34, or 6CA7, with RL = 8ka-a, with Ea at 500v, and Eg2 at say 400v, you get 636 vrms into 8k, ie,51 watts AB1, with about 8 watts of class A. Pd is OK, and biasing at 25 watts is possible. Using KT88, you get no more than this, with the same RL. trying to drive AB2 would be pointless, since the Emin swing is already close to the vertical axis on a loadline graph. Say you reduce RL to 4ka-a. 6CA7 will have Emin swing cut off by the Eg = 0.0v line, so you get 462vrms into 4 k, ie, 53 watts, class AB1, and prolonged use at such Po will turn the plates red. Class A power is only 5 watts. With KT88, the AB1 swing will be 560vrms into 4k, ie, 78 watts, but the extra Pd ability allow continuous duty cycle. But if we could push the EL34, or 6CA7 into some grid current, then we may be able to force the voltswing on the load to very nearly that of the KT88. We have to accept we won't be asking the amp to make say 70 watts into 4k for 100% of the time, not in a hi-fi amp at least, but we do garnish our construction with three things, low cost output tubes, nice high peak power ability, and greater load tolerance. Waht if we had six output tubes? we could thus get 210 watts reliably from 6 x 6CA7, although aiming for 150 would be better. Who uses more than 150 watts for hi-fi? Not many, although I know folks who like the utterly effortless sound of a huge but agile tube amp, working well away from areas of high thd, except for the occasional huge crescendo, or cannon shots. I might add that one could get a pair of 6L6 or 807 to make 80 watts with a 600v B+ supply, using simple AB2 techniques. Power isn't everything, and a plain old trioded williamson with 16 watts does sound a treat, and can offer superlative listening, but I like to explore the world beyond 16 watts, and a land where speakers of 80 dB efficiency have to be driven with tube amps. We should be able to make the best of whatever mode we chose, A1, A2, AB1, AB2. Musical Reference get 36 watts from a pair of EL84, AB1, acoustical. There is a 200 mA fuse in the cathode circuit, which is large enough to prevent nuisance fuse blows, but small enough to prevent tubes from bias failures. A bloke I know has been using the same tubes since he got the amp in 1996. Patrick Turner. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
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"Fred Nachbaur" wrote in message news:NwC_a.8048$zE1.6670@edtnps84... Patrick Turner wrote: [...] Pentode operation allows for up to 800v for the plate supply, and say 400v on the screens, and a 12k a-a RL, and grid current never occurs. But if we reduced the screen volts to say 300v, then we would maybe run out of grid drive headroom. UL and trioded tubes cannot be run at such high voltages, because the grid 1 would have to be biased at impossibly low -ve bias voltages. [...] Well, I wouldn't say it's impossibly low. My favourite hi-fi amp uses 6L6GC's at about 620 volts B+ in UL. Grid bias for a reasonable plate + screen dissipation (cathode current 35 mA) is about -78 volts. I don't think that it would be very much different for EL34 or 6CA7, but haven't tried it. Likewise. The EL34/6CA7 is an unknown quantity to me. We're not really answering Patrick's question here, but with a variable PSU. I've run UL Reflektor 6L6GC pairs up to 550v with idle dissippation of 24W into loads between 4 and 6k and like the way they sound. (If anyone would care to donate a set, I'd be happy to try them out and report back!) This means, though, that the driver needs lots of headroom; about 160 volts peak-to-peak. The usual concertinas are therefore rather impractical. But what a great application for a long-tail pair with pentode (or transistor) CCS! Lots of output swing -- and gain as a bonus. The net result is lots of power, good clean sound even at full power due to the UL topology, using relatively common-place tubes. I've found AB2 to be a real nuisance for hi-fi use, but it's great for guitar amps because it give a more graduated distortion characteristic, passing from class A to class AB1 to class AB2 to full clipping. It's absolutely *wonderful* for that, the louder you play the "harder" you play, without suddenly crunching into the ceiling like you do with SS amps. But for hi-fi, unless you're a glutton for punishment and want a real challenge to get a low-impedance driver, and then maybe still have to try to iron out the added distortion by using NFB tricks, I wouldn't recommend AB2 using "conventional" valves like the 807, 6L6, EL34, etc. I think you'd be better off with a lower AB1, even approaching B1, than to cope with the difficulty of trying to stay clean whilst drawing grid current. I too am interested in AB2 for hi-fi, but would use a pair of high-voltage TX triodes such as 809 or 811A, in a "darlington" kind of configuration (direct-coupled cathode followers on the control grids). This looks kind of elegant if you draw it out. It looks so simple, I put one together a couple of years ago. As I remember ...... 550v B+... a pair of FB UL KT88s..... 2 x 6SN7 CFs off the same 550v supply, direct coupled to the KT88 grids ..... 22K loads off a fixed 150v low imp line and variable -ve bias on the 6SN7 grids to set the KT88 idle current. Up front was a 6SL7 LTP. I could swing 170V pk-pk on the KT88 grids before it cut the tops off a sine wave input. The PSU would start a broken down truck. From reports I've seen, and from a very crude mock-up I did years ago, this has the potential of sounding very nice, and have lots of available power. Up one paragraph. Did it sound nice ?? It didn't actually sound any better or worse than the same KT88s, without the CFs, with either a LTP or Williamson front end up to the sound level where you had to hide behind something. Such a layout may be useful for getting 100W out of a pair of EL84s whilst staying within the stated max anode dissippation by driving them hard enough to cut them off for over 50% of the input cycle, but this is not really hi-fi. I just realized that big noise and low distortion starts with a low component count, big volts, big toobs and big OPTs. Patrick has a man who insists on AB2 This is hard work regards jim Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
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"Patrick Turner" wrote in message
... - Even though the load impedance is lower, relatively speaking? Well if you read the operating specs for 6550 as tetrodes, 440v for Ea, and lower V fpr Eg2, they get thd down to 0.6% at 40 watts, I have forgot the exact figures, but the secret is the low Eg2 relative to the anode supply, once that supply is over +300v. I meant, you use say a 5k load for class AB1, it needs to be lower, maybe 3 or 4k, to accomidate the higher peak current and power in AB2. But what happens to the distortion level when it's still within A or AB1? Tim -- In the immortal words of Ned Flanders: "No foot longs!" Website @ http://webpages.charter.net/dawill/tmoranwms |
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Tim Williams wrote: "Patrick Turner" wrote in message ... - Even though the load impedance is lower, relatively speaking? Well if you read the operating specs for 6550 as tetrodes, 440v for Ea, and lower V fpr Eg2, they get thd down to 0.6% at 40 watts, I have forgot the exact figures, but the secret is the low Eg2 relative to the anode supply, once that supply is over +300v. I meant, you use say a 5k load for class AB1, it needs to be lower, maybe 3 or 4k, to accomidate the higher peak current and power in AB2. But what happens to the distortion level when it's still within A or AB1? Tim The lower the RL, the higher the thd, right across the power range, for all amps. For the first few watts in a FB amp with 4k a-a, the thd is only marginally more than with 8ka-a. Maybe 0.1% at 3 watts, rather than 0.07%, with similar spectra; this has been my experience. But for pentode/tetrode, using NFB, the higher the RL, the higher the class A1 %, and at B+ = 500v, and RL = 8k, there is no capability to use AB2, just AB1, and thd is not bad for 50 watts. With RL = 12ka-a, power has gone down, but is all class A, but so has thd, because gain is higher, and applied FB is effectively a higher amount of dB, if you check your gain/feedback equations. This is another common thing about most FB amps, the thd goes down with the higher RL. Patrick Turner. -- In the immortal words of Ned Flanders: "No foot longs!" Website @ http://webpages.charter.net/dawill/tmoranwms |
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