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#1
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
Over the last fortnight I rebuilt my benchtop power supply to be able
to test KT120 with a regulated 600Vdc anode supply, at up to 320mA. I placed shunt regulation for the screens on the breadboard for the pair of KT120. Fixed bias was used, with a balance pot to equalise Ia in each tube. Operation in all tests were limited to class AB1 with no grid current. The OPT is a 8Kg monster with negligible winding resistance, and lots of interleaving, so its character does not affect the function of the tubes. The OPT has 20% CFB, so that where there is 200Vrms+ at an anode, there is 50Vrms- at the cathode. Depending on load, applied grid signal is about 60Vrms at middle load values. OPT ZR = 1,233:1, TR = 35.1:1. The drive signal maximum for test1 below were the highest, and up to 115Vrms was needed to be applied to each KT120 grid. This was obtained by using another tube amp and taking out the anode to anode signal via 10uF caps, and applied to quite high value biasing R. The maximum available drive was 276Vrms from each 6CA7 anode from an ultralinear power amp with 17dB global FB, so THD in the drive signal 0.1% at 276Vrms. At the levels of 115Vrms used, THD was considerably lower than for 276Vrms. Using an OPT with 20% CFB meant that the distortion in all the tests remained low, and the Russian KT120 exhibited no nasty surprises when compared to average condition Russian 6550 used to explore the differences between the two tube types. KT120 test1 :- Ea = +600Vdc, Eg2 = +400Vdc and Idle Ia = 50mA, idle Ig2 was 4mA per tube. Fixed Eg1 at -47Vdc. At absolute maximum PO possible, Ig2 rose up to 17mA per tube. Po versus RLa-a as follows, at clipping onset :- 2k0 = 60W, 4k0 = 102W, 4k8 = 115W, 6k0 = 102W, 8k0 = 80W, 12k0 = 56W, 16k0 = 45W. KT120 test2 :- Ea = +500Vdc, Eg2 = +500Vdc, Idle = 50mA, Ig2 slightly higher than in test1. Max Ig2 rose to 22mA per tube at absolute max possible Po. Po versus RLa-a as follows, at clipping onset :- 1k0 = 90W, 2k0 = 127W, 2k5 = 130W, 4k0 = 100W, 4k8 = 85W, 6k0 = 72W, 8k0 = 58W, 12k0 = 40W, 16k0 = 27W. Conclusions. The effect of having the higher screen voltage in test2 raises the knee of the diode curve. The diode curve in fact is the line for Ra when Eg1 = 0V. With Eg2 = +400V, the curve is a straight line from zero to 50V x 350mA, then the line rolls over to the left for 100V x 500mA, and 150V x 550mA. The knee shape could be lower for where class A is intended, so Eg2 could be lower. But for highest class AB Po with the lowest RLa-a load, the Eg2 should be ket as high as possible, and well regulated. With Ea at +600V, I recommend Eg2 not be higher than +450V, which would increase max Po possible to about 125W. With Eg2 = +500V, then the diode line is nearly straight to 100V x 650mA, before rolling over at the knee to 200V x 760mA. The extra +100V of Eg2 gives the tube a much higher max Ia capability with low load values. The curves I plotted for test1 and test2 show that test1 has the higher spread of power, 80W or more is available between 3k2 and 7k9. Test2 shows 80W or more of power is available from 1k0 to 5k3. 6550, Test3 :- The 6550 was not tested wth Ea = +600V and Eg2 = +400V, as it is felt by this technician that reliability can suffer with such high Ea for 6550. But it was tested with Ea = Eg2 = 500Vdc. 1k25 = 90W, 2k5 = 112W, 3k7 = 93W, 4k9 = 77W, 7k3 = 57W, 9k8 = 45W, 12k0 = 32W, 16k0 = 24W Conclusion. The results show that rather a small amount of extra Po is available if the KT120 is used to replace 6550, or KT88 as currently produced. However, the tendency of the KT120 to over heat with abuse is much less than for 6550, so that use with Ea at +600V and Eg2 at +450V would be OK. I fond that a KT120 tended to draw less Ia at idle if plugged into a tube socket which previously had 6550, so it would appear it is safe to plug in the KT120 to replace all other beam tetrodes, ( although please don't assume anything, and don't assume KT12 will be OK where there previously were EL34 or 6CA7. ) The distortion character witnessed in all tests was unremarkable, and what I expected when a substantial amount of CFB is used, 20%, in this case. The tube electrode signal relationships with 20% CFB is the same as for UL with 20% taps. I tested output resistance of the output stage as follows :- Set up amp with 10.0Vrms, no load connected. The connect 4 ohms, without changing anything else. The Rout = Vo drop / current in 4 ohms. In this case, with KT120, Rout = 1.0 ohms, which includes maybe 0.1 ohms of Rw at the secondary. With ZR = 1,233, then Ra-a is effectively reduced to 1,233 ohms, or Ra of one tube is around 600 ohms. With the same test performed on 6550 in the same circuit and op conditions, the Rout = 1.5 ohms, giving effective Ra-a = 1,850 ohms, or around 900 ohms per tube. So the KT120 must have a higher figure of merit, ie gm and µ is higher, so Ra must be lower. In conclusion, the KT120 performs rather better than is indicated by the attrocious data sheets currently available from the makers, drawn up by lazy *******s who seem to have forgotten what it takes to really test a tube properly, and to provide curves without lies. The KT120 is a slight betterment on the KT90EH which I like a lot. For those NOT interested in trying to force huge amounts of class AB1 Po from any tubes they lay their hands on, the KT120 allows those interested in pure class A to have Pda at a safe 40W per tube continuously, thus allowing up to 17W of class A1 Po, or 34W from a pair. I have seen ppl try to run KT88 and 6550 at Pda = 40W, and seen how the tubes didn't last long. But with the higher 60W Pda rating, 40W should be OK. I have not tested KT120 for triode connection, but I suspect they should be good. 12Watts in SET may be easily possible from 1 tube, ie, about twice what one gets from a single 6CA7 or EL34. The simplest way to use KT120 for best fidelity and for home hi-fi use is to use the UL connection because CFB OPTs are very rare off the shelf. The KT120 will very much suit use in amps such as my 5050 at http://www.turneraudio.com.au/Integrated5050.htm Everyone knows huge Po is possible from PP KT120, but what audio enthusiasts was is finesse, which means the first 30W is the most important. I think they may find KT120 a fine replacement for all previous octal beam tetrodes. Patrick Turner. |
#2
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
On Feb 5, 6:31*am, Patrick Turner wrote:
Over the last fortnight I rebuilt my benchtop power supply to be able to test KT120 with a regulated 600Vdc anode supply, at up to 320mA. I placed shunt regulation for the screens on the breadboard for the pair of KT120. Fixed bias was used, with a balance pot to equalise Ia in each tube. Operation in all tests were limited to class AB1 with no grid current. The OPT is a 8Kg monster with negligible winding resistance, and lots of interleaving, so its character does not affect the function of the tubes. The OPT has 20% CFB, so that where there is 200Vrms+ at an anode, there is 50Vrms- at the cathode. Depending on load, applied grid signal is about 60Vrms at middle load values. OPT ZR = 1,233:1, TR = 35.1:1. The drive signal maximum for test1 below were the highest, and up to 115Vrms was needed to be applied to each KT120 grid. This was obtained by using another tube amp and taking out the anode to anode signal via 10uF caps, and applied to quite high value biasing R. The maximum available drive was 276Vrms from each 6CA7 anode from an ultralinear power amp with 17dB global FB, so THD in the drive signal 0.1% at 276Vrms. At the levels of 115Vrms used, THD was considerably lower than for 276Vrms. Using an OPT with 20% CFB meant that the distortion in all the tests remained low, and the Russian KT120 exhibited no nasty surprises when compared to average condition Russian 6550 used to explore the differences between the two tube types. KT120 test1 :- Ea = +600Vdc, Eg2 = +400Vdc and Idle Ia = 50mA, idle Ig2 was 4mA per tube. Fixed Eg1 at -47Vdc. At absolute maximum PO possible, Ig2 rose up to 17mA per tube. Po versus RLa-a as follows, at clipping onset :- 2k0 = 60W, 4k0 = 102W, 4k8 = 115W, 6k0 = 102W, 8k0 = 80W, 12k0 = 56W, 16k0 = 45W. KT120 test2 :- Ea = +500Vdc, Eg2 = +500Vdc, Idle = 50mA, Ig2 slightly higher than in test1. Max Ig2 rose to 22mA per tube at absolute max possible Po. Po versus RLa-a as follows, at clipping onset :- 1k0 = 90W, 2k0 = 127W, 2k5 = 130W, 4k0 = 100W, 4k8 = 85W, 6k0 = 72W, 8k0 = 58W, 12k0 = 40W, 16k0 = 27W. Conclusions. The effect of having the higher screen voltage in test2 raises the knee of the diode curve. The diode curve in fact is the line for Ra when Eg1 = 0V. With Eg2 = +400V, the curve is a straight line from zero to 50V x 350mA, then the line rolls over to the left for 100V x 500mA, and 150V x 550mA. The knee shape could be lower for where class A is intended, so Eg2 could be lower. But for highest class AB Po with the lowest RLa-a load, the Eg2 should be ket as high as possible, and well regulated. With Ea at +600V, I recommend Eg2 not be higher than +450V, which would increase max Po possible to about 125W. With Eg2 = +500V, then the diode line is nearly straight to 100V x 650mA, before rolling over at the knee to 200V x 760mA. The extra +100V of Eg2 gives the tube a much higher max Ia capability with low load values. The curves I plotted for test1 and test2 show that test1 has the higher spread of power, 80W or more is available between 3k2 and 7k9. Test2 shows 80W or more of power is available from 1k0 to 5k3. 6550, Test3 :- The 6550 was not tested wth Ea = +600V and Eg2 = +400V, as it is felt by this technician that reliability can suffer with such high Ea for 6550. But it was tested with Ea = Eg2 = 500Vdc. 1k25 = 90W, 2k5 = 112W, 3k7 = 93W, 4k9 = 77W, 7k3 = 57W, 9k8 = 45W, 12k0 = 32W, 16k0 = 24W Conclusion. The results show that rather a small amount of extra Po is available if the KT120 is used to replace 6550, or KT88 as currently produced. However, the tendency of the KT120 to over heat with abuse is much less than for 6550, so that use with Ea at +600V and Eg2 at +450V would be OK. I fond that a KT120 tended to draw less Ia at idle if plugged into a tube socket which previously had 6550, so it would appear it is safe to plug in the KT120 to replace all other beam tetrodes, ( although please don't assume anything, and don't assume KT12 will be OK where there previously were EL34 or 6CA7. ) The distortion character witnessed in all tests was unremarkable, and what I expected when a substantial amount of CFB is used, 20%, in this case. The tube electrode signal relationships with 20% CFB is the same as for UL with 20% taps. I tested output resistance of the *output stage as follows :- Set up amp with 10.0Vrms, no load connected. The connect 4 ohms, without changing anything else. The Rout = Vo drop / current in 4 ohms. In this case, with KT120, Rout = 1.0 ohms, which includes maybe 0.1 ohms of Rw at the secondary. With ZR = 1,233, then Ra-a is effectively reduced to 1,233 ohms, or Ra of one tube is around 600 ohms. With the same test performed on 6550 in the same circuit and op conditions, the Rout = 1.5 ohms, giving effective Ra-a = 1,850 ohms, or around 900 ohms per tube. So the KT120 must have a higher figure of merit, ie gm and µ is higher, so Ra must be lower. In conclusion, the KT120 performs rather better than is indicated by the attrocious data sheets currently available from the makers, drawn up by lazy *******s who seem to have forgotten what it takes to really test a tube properly, and to provide curves without lies. The KT120 is a slight betterment on the KT90EH which I like a lot. For those NOT interested in trying to force huge amounts of class AB1 Po from any tubes they lay their hands on, the KT120 allows those interested in pure class A to have Pda at a safe 40W per tube continuously, thus allowing up to 17W of class A1 Po, or 34W from a pair. I have seen ppl try to run KT88 and 6550 at Pda = 40W, and seen how the tubes didn't last long. But with the higher 60W Pda rating, 40W should be OK. I have not tested KT120 for triode connection, but I suspect they should be good. 12Watts in SET may be easily possible from 1 tube, ie, about twice what one gets from a single 6CA7 or EL34. The simplest way to use KT120 for best fidelity and for home hi-fi use is to use the UL connection because CFB OPTs are very rare off the shelf. The KT120 will very much suit use in amps such as my 5050 athttp://www.turneraudio.com.au/Integrated5050.htm Everyone knows huge Po is possible from PP KT120, but what audio enthusiasts was is finesse, which means the first 30W is the most important. I think they may find KT120 a fine replacement for all previous octal beam tetrodes. Why produce a new beam power tetrode/pentode when the hi-fi market of profitability is triodes? That is where the money is because of pricing. High end audio has access to beam power tubes because of guitar amps, and they don't need any bigger tube than they now have. |
#3
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
On Feb 6, 11:49*am, wrote:
snip for brevity... *Why produce a new beam power tetrode/pentode when the hi-fi market of profitability is triodes? That is where the money is because of pricing. *High end audio has access to beam power tubes because of guitar amps, and they don't need any bigger tube than they now have. Well, I'll probably have to let you consider a couple of things. The reason for much of Russian tube production is that there are at least 100 tube guitar amps for every hi-fi tube amp in the world. Musos like as much bang for buck as can be cheaply afforded, so if someone comes up with a way of making a better beam tet than has existed before now, then its a winner, even if there were no takers for triodes at all. I have not tried KT90 strapped as a triode, but methinks it'd have to be awfully good compared to using a 300B. Any NEW beam tet usally means another good triode if that is wanted. Having a *bigger* KT120 with rated Pda of 60W means that there *IS NO* problem using just a pair where before you'd have used a quad of EL34, 6CA7, 6L6, KT66, etc. Audiophiles will, I think, embrace the KT120 favourably because the higher Pda allows for higher class A operation. ARC are using them already. For all I know, maybe one could add yet another 12mm to the anode height and make a tube with Pda = 72W, but probably it won't be done because it may not fit under covers over tubes in existing amps fitted with 6550/KT88, etc. So, in conclusion, I think Mike Mathews deserves a gong for contributing what appears to be another very fine tube to the list of those already being produced. In about 2005, I fitted KT90 to a pair of Quad-II amps I re-engineered and all operating measurements improved. They were set up to draw slightly less Ia than the original KT66 which laboured hard in those amps. Ea was kept the same. So, KT90 used like that last a heck of a long time, and give better sound. I've also used KT88 in Quad-II. Maybe KT120 would be *over the top*. One reason is that heater current for KT120 is 10% more than KT88, and this frightens ppl into not using them. But KT88 work OK in Quad-II because the amps are made to power old fashioned AM and FM tuners etc and a No 22 preamp unit. I fitted KT88 to a Quad-II amp in 1998, and its never missed a beat since, despite the hot weather in Cooma summers. But then I also reduced Ia, junked the GZ32 rectifier entirely, used fixed bias, and drilled lotsa holes in the bottom plate and re-set the sub-plate holding KT88 sockets to allow an air flow up through the amp, so, it deals with its waste heat better because there is less of it and improved ventilation that Peter Walker never considered. In about 2002, I emailed the EI factory in Yugoslavia, with a request that they make a PT40, ie, a Patrick Triode 40Watt tube with an OCTAL base, and 6.3V indirectly heated cathode, so that its a drop in replacement for most octal beam and pentode tubes, except it'd be a REAL triode without a screen. They already had been making their own version of KT90, and making a triode with an anode with same internal dia as the screen helix so that KT90 bits and pieces could be used. Well, their response came weeks later, and it seemed the guy was drunk. It was also after the Kosovo war, and the EI factory became moribund aftewards because many staff were needed to rebuild th country, or were generally so ****ed off they took to the bottle. They also said how they'd been trying to develop exactly what I said audiphiles wanted. There were a couple more emails, then just bull**** from them, and I guess there just wasn't any money to develop a new tube, and no real incentive, and probably much mental depression, even though they had been making some good tubes. Companies should never sit on their laurels, they need to get off their arses and inovate, and improve, and not stagnate, let alone go out under a fog of vodka and negativity. So the *OLD* experts have to stay with a spring in their step and retain the freshness of youth but without being youthful fools. Jus' my 3.14 cents whirth. Patrick Turner. |
#4
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
On Feb 5, 7:31*am, Patrick Turner wrote:
(SNIP) The drive signal maximum for test1 below were the highest, and up to 115Vrms was needed to be applied to each KT120 grid. This was obtained by using another tube amp and taking out the anode to anode signal via 10uF caps, and applied to quite high value biasing R. The maximum available drive was 276Vrms from each 6CA7 anode from an ultralinear power amp with 17dB global FB, so THD in the drive signal 0.1% at 276Vrms. (SNIP). Patrick Turner. Patrick, Just one comment on the above driver test rig... I have been bread- boarding a pair of 7534 tubes with a view to making a more than "30 watt amplifier" (not the subject of this post.) To get a low enough impedance drive at a high enough voltage, I used a smallish P-P tube OPT, reversed, driven by a separate 10 watt audio amplifier from its 8 ohm tap to the small OPT's speaker side; amplifier terminated by 15 ohms across 15 ohm tap to keep it happy. A lot of drive volts were available on the OPT "plate side"... I did not measure them but I eventually had the pair of 7534 tubes giving 60 watts RMS (via a big Hammond OPT) at 400 Hz at clipping, with the grid drive side undistorted on the 'scope (class AB1 7534's cathode biased on 500 VDC B +, but that's another story.) This way you get to keep your fingers out of the driver amplifier's HV section! Of course, you can even use a spare s/s receiver amplifier as a driver unit. Cheers, Roger |
#5
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
On Feb 6, 4:04*am, Patrick Turner wrote:
On Feb 6, 11:49*am, wrote: snip for brevity... *Why produce a new beam power tetrode/pentode when the hi-fi market of profitability is triodes? That is where the money is because of pricing. *High end audio has access to beam power tubes because of guitar amps, and they don't need any bigger tube than they now have. Well, I'll probably have to let you consider a couple of things. The reason for much of Russian tube production is that there are at least 100 tube guitar amps for every hi-fi tube amp in the world. Musos like as much bang for buck as can be cheaply afforded, so if someone comes up with a way of making a better beam tet than has existed before now, then its a winner, even if there were no takers for triodes at all. I have not tried KT90 strapped as a triode, but methinks it'd have to be awfully good compared to using a 300B. Any NEW beam tet usally means another good triode if that is wanted. Having a *bigger* KT120 with rated Pda of 60W means that there *IS NO* problem using just a pair where before you'd have used a quad of EL34, 6CA7, 6L6, KT66, etc. Audiophiles will, I think, embrace the KT120 favourably because the higher Pda allows for higher class A operation. ARC are using them already. For all I know, maybe one could add yet another 12mm to the anode height and make a tube with Pda = 72W, but probably it won't be done because it may not fit under covers over tubes in existing amps fitted with 6550/KT88, etc. So, in conclusion, I think Mike Mathews deserves a gong for contributing what appears to be another very fine tube to the list of those already being produced. I am not saying this is a bad tube, just that production resources could be more profitably aimed at making triodes rather than a beam power tube that guitar amps probably won't much use. The trend is smaller tubes. Even at that I have long advocated a BASS amp with a single pair of transmitting tubes, the Svet SV811-x lo mu single ended 811 mutants would work great. Then again so would some pentodes or tetrodes, the 4-65A is much underused as is the 4E27. The 813 is too physically big. |
#6
Posted to rec.audio.tubes
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KT120 tests, compared to 6550.
On Feb 7, 2:19*pm, wrote:
On Feb 6, 4:04*am, Patrick Turner wrote: On Feb 6, 11:49*am, wrote: snip for brevity... *Why produce a new beam power tetrode/pentode when the hi-fi market of profitability is triodes? That is where the money is because of pricing. *High end audio has access to beam power tubes because of guitar amps, and they don't need any bigger tube than they now have. Well, I'll probably have to let you consider a couple of things. The reason for much of Russian tube production is that there are at least 100 tube guitar amps for every hi-fi tube amp in the world. Musos like as much bang for buck as can be cheaply afforded, so if someone comes up with a way of making a better beam tet than has existed before now, then its a winner, even if there were no takers for triodes at all. I have not tried KT90 strapped as a triode, but methinks it'd have to be awfully good compared to using a 300B. Any NEW beam tet usally means another good triode if that is wanted. Having a *bigger* KT120 with rated Pda of 60W means that there *IS NO* problem using just a pair where before you'd have used a quad of EL34, 6CA7, 6L6, KT66, etc. Audiophiles will, I think, embrace the KT120 favourably because the higher Pda allows for higher class A operation. ARC are using them already. For all I know, maybe one could add yet another 12mm to the anode height and make a tube with Pda = 72W, but probably it won't be done because it may not fit under covers over tubes in existing amps fitted with 6550/KT88, etc. So, in conclusion, I think Mike Mathews deserves a gong for contributing what appears to be another very fine tube to the list of those already being produced. *I am not saying this is a bad tube, just that production resources could be more profitably aimed at making triodes rather than a beam power tube that guitar amps probably won't much use. The trend is smaller tubes. The world mainly does not share your views about what is/isn't a waste of resources or what constitutes profitability. My forecast is that many people will vote with their wallet with KT120. And if you want a nice fat triode, connect the screen to anode. *Even at that I have long advocated a BASS amp with a single pair of transmitting tubes, the Svet SV811-x lo mu single ended 811 mutants Yeah, ppl everywhere COULD have used different tubes than what you might dream about but most just won't, and if they want high power for a bass they'd head towards getting an Ampeg capable of 300 W using 6 x 6550. I guess ppl already with an ampeg could plug in KT120,and maybe they'd get 330Watts with the same load, and maybe more with a slightly lower load but SUSTAINED power with a sine wave measured at clipping might show the PSU limitations. Probably, the KT120 will last longer in an amp which has just the one shared grid bias voltage supply, or two, to the 1, 2, or 3 tubes each side of the PP circuit, and this is because one would use the same bias current in KT120 as in 6550, in a bass amp. But tube matching drifts well apart as tubes age, and Pda for each tube of a six pack might vary between say 40 Watts and 15 Watts, and soon, at least one or two 6550 will overheat and run away thermally and die, maybe causing collateral damage. But KT120 have 60W Pda limit, so large variations in Pda with ageing will e better tolerated. Of course the ONLY correct way to apply fixed bias is to have a separate adjustment for EACH output tube no matter what that tube is, which means having 6 pots and 6 LEDs to tell you if the bias of any one tube is too high. But all such measures of intelligent management of errant tube behaviour in the music instrument amp setting have been universally condemned by all amp makers - too fukkin difficult, fidly, and too fukkin expensive to put in during construction, and if we did we'd be rooned. The major brands of amps treat the buying public as dumb jerks who are NOT intelligent, and who exist to be relieved of their cash in the easiest legal way possible. Probably, one could build a nice powerful amp using compact sized forced air cooled transmitting triodes or tetrodes normally used in high power RF transmitters. May as well use mosfets, eh? would work great. Then again so would some pentodes or tetrodes, the 4-65A is much underused as is the 4E27. The 813 is too physically big.- There is no end to what's possible, but there are severe limitations on what is profitable, and what will sell like hot cakes to the gullible and fickle public. Marketing is a ******* of the thing; caused a few suicides..... Patrick Turner. |
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