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#1
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Which KT88 PP should I go for?
1st post on here, hope you can help me....
History: I have built 10s of SS amps and a couple of valve preamps over the last 20 years or so, but the time has come for me to build a valve/tube jobbie. A friend of mine has a 20 year old Grant Lumley GL50a which has had a new (larger) mains translformer fitted and been re-biased to use KT88s (originally KT77s). The amp always sounded very average with KT77s but sounds superb with KT88s - better than a pair of EAR 509II Anniversarys (~£10K nowadays!!!). I am very tempted to clone the GL50A, the circuit very similar to this (and the Williamson amp): http://www.drtube.com/schematics/grant/g60ams.gif The B+ is 425V, KT88 bias is 40mA and o/p transformer turns ratio is 21.5: 1 (3K7 to 8 Ohms) with 45% taps. If I go ahead with this I will go for 2 X 6SN7 instead of 2 X ECC82 because general opinion is that they sound better and M Jones reckons the distortion is much lower. However there are two other circuits which are appealing: 1) P turner's 5050 2) The GEC 30W: http://www.jacmusic.com/KT88/SCHEMATIC3.gif My speakers are 87dB/W so I guess I need ~ 30W + a bit of headroom. The 5050 seems to only have a gain of about X11 (ignoring V1). Can this be changed to a more usual X 20 to 30? Why is a voltage doubler used? I confess to minimal knowledge about impedance matching.... 3K7:8 Ohms seems like a very low turns ratio compared to the 5050 whcih uses 6K6:4Ohms (with a higher B+). Looking at transformer manufacturers pages they seem to recommend 4K5 or 5K. Please help me understand the tradeoffs and how to choose. I am also very confused about how to select a transformer supplier, here in the UK there seems to be 3 main options: Lundahl - mid priced but seem to be very small cores for the claimed power rating. Hammond - middle of the road? (No drop through style!). Sowter - expensive, never heard a word against them. Custom windings at no extra cost. I am tempted to go with Sowter just to be sure of getting something that will sound "right". Any comments? I guess the main question is: Which circuit should I choose and why? Regards, Dave Last edited by Sliding Bias : January 15th 12 at 05:27 PM |
#2
Posted to rec.audio.tubes
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Which KT88 PP should I go for?
Sowter's transformers are of good quality. But you can probably get
ones from Japan or Taiwan (James Wu) reasonably apart from the shipping, and there may be someone who brings them in with container loads from Japan or Taiwan at lower cost. There may be vendors elsewhere in the EU for these too. Supposedly the company that bought Partridge will still wind Partridge parts for UK customers. You could contact them. They refuse to talk to anyone in North America supposedly due to liability reasons. It's horse**** of course, but that's what I've heard. Anyway, I would look at the Marantz 2 and 5 and the early VTL amps in the Manley book for examples of how to do it simply. This Lumley amp looks something like a VTL at that. Is this where Enid Lumley got her start? |
#3
Posted to rec.audio.tubes
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Which KT88 PP should I go for?
On Jan 15, 11:05*am, Sliding Bias Sliding.Bias.
wrote: 1st post on here, hope you can help me.... History: I have built 10s of SS amps and a couple of valve preamps over the last 20 years or so, but the time has come for me to build a valve/tube jobbie. A friend of mine has a 20 year old Grant Lumley GL50a which has had a new (larger) mains translformer fitted and been re-biased to use KT88s (originally KT77s). The amp always sounded very average with KT77s but sounds superb with KT88s - better than a pair of EAR 509II Anniversarys (~£10K nowadays!!!). I am very tempted to clone the GL50A, the circuit very similar to this (and the Williamson amp):http://www.drtube.com/schematics/grant/g60ams.gif The B+ is 425V, KT88 bias is 40mA and o/p transformer turns ratio is 21.5: 1 (3K7 to 8 Ohms) with 45% taps. If I go ahead with this I will go for 2 X 6SN7 instead of 2 X ECC82 because general opinion is that they sound better and M Jones reckons the distortion is much lower. However there are two other circuits which are appealing: 1) P turner's 5050 2) The GEC 30W:http://www.jacmusic.com/KT88/SCHEMATIC3.gif My speakers are 87dB/W so I guess I need ~ 30W + a bit of headroom. The 5050 seems to only have a gain of about X11. Can this be changed to a more usual X 20 to 30? I confess to minimal knowledge about impedance matching.... 3K7:8 Ohms seems like a very low turns ratio compared to the 5050 whcih uses 6K6:4Ohms (with a higher B+). Looking at transformer manufacturers pages they seem to recommend 4K5 or 5K. Please help me understand the tradeoffs and how to choose. I am also very confused about how to select a transformer supplier, here in the UK there seems to be 3 main options: Lundahl - mid priced but seem to be very small cores for the claimed power rating. Hammond - middle of the road? (No drop through style!). Sowter - expensive, never heard a word against them. Custom windings at no extra cost. I am tempted to go with Sowter just to be sure of getting something that will sound "right". Any comments? I guess the main question is: Which circuit should I choose and why? Regards, Dave -- Sliding Bias Useable bandwidth depends on quality transformers. Sowter have a strong reputation, lots of continuous history, and are happy to give advice once you've decided what you want to build. They can also supply your power transformer with whatever taps you need. You should read something about valve output transformers, because the basics are too long for here. Menno van der Veen's "Modern valve amplifiers" contains a good mathematical treatment of the subject. The issues you should be aware of a leakage inductance, interwinding capacitance, primary inductance, winding resistance and saturation current. The load seen by the valves comprises the primary winding resistance in series with (leakage inductance in series with the sum of speaker impedance and secondary winding resistance multiplied by the impedance ratio). The primary inductance and the winding capacitance are each connected in shunt with the section in brackets. The impedance ratio is the square of the winding ratio. It's worth sketching that circuit if I've managed to make it clear. You should see that there is a high pass filter resulting from the primary inductance, and a low pass filter arising from the capacitance and leakage. Also, note that the allowable current flowing through the primary inductance is limited by core saturation, and that the current through an inductor depends on frequency and signal amplitude. Hence an output transformer imposes an extra, amplitude dependent limit to bass. That's why Sowter will want to know your target full-power bass frequency limit, in addition to your required small-signal roll-off point. Considering the lowish sensitivity of your speakers and the relatively high output of most modern sources, you may be better looking for more recently conceived circuits than the Williamson. An ultra-linear output stage would give more power than triode. SS current sources, voltage regulators and other odds and ends are cheap and easy, and modern designs can use these to good effect. OTOH, if you know you like the GL50 and you feel confident that you can modify the feedback compensation to suit whatever output transformers you use, that's likely to be a good way to go. It may be worth reviewing your conception of how much power you need for those speakers. Personally, I would want considerably more than 30W. Triode connection will give you a marginally cleaner output for much of the time, but that might not be worthwhile if your short-term signal peaks drive the amp into clipping. I try for a 60:1 ratio of clipping point to mean listening power. Finally, output valves in triode or UL mode are generally loaded with between 2 to 3 times the anode resistance. Linearity is improved as load rises, generally, but output power is reduced. As with mode, if you have no power to spare, then what you choose might depend on which source of distortion you prefer. Ian |
#4
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Thanks for the replies.
I have access to a 2nd hand pair of Sowter transformers. I believe they are 6K6 to 8 and 4 Ohms. Since Patrick Turner seems able to get 50W out of this impedance with ~500V B+ and fixed bias, the 5050 o/p stage and 50mA Iq seems to be a decent starting point for me. Should I go for these or not try to save a few pennies and get some new ones @ 4.5K? I suspect there are some good reasons why he chose the topology he did for the 5050 (i.e. SET, LTP with transistor current source in the tail, PP UL o/p with fixed bias). The alternative is SET, Concertina, PP driver, PP UL o/p with fixed bias (like the GL50A). Any comments? Hopefully I should be OK wrt tweaking the stability margins. I've done this for many SS amps and the GL50 owner is valve savvy, should I need some extra help. Complexity is not too much of an issue (as long as I can fit the whole lot in a normal sized chassis). |
#5
Posted to rec.audio.tubes
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Which KT88 PP should I go for?
On Jan 16, 10:52*am, Ian Iveson wrote:
On Jan 15, 11:05*am, Sliding Bias Sliding.Bias. wrote: 1st post on here, hope you can help me.... History: I have built 10s of SS amps and a couple of valve preamps over the last 20 years or so, but the time has come for me to build a valve/tube jobbie. A friend of mine has a 20 year old Grant Lumley GL50a which has had a new (larger) mains translformer fitted and been re-biased to use KT88s (originally KT77s). The amp always sounded very average with KT77s but sounds superb with KT88s - better than a pair of EAR 509II Anniversarys (~£10K nowadays!!!). I am very tempted to clone the GL50A, the circuit very similar to this (and the Williamson amp):http://www.drtube.com/schematics/grant/g60ams.gif The B+ is 425V, KT88 bias is 40mA and o/p transformer turns ratio is 21.5: 1 (3K7 to 8 Ohms) with 45% taps. If I go ahead with this I will go for 2 X 6SN7 instead of 2 X ECC82 because general opinion is that they sound better and M Jones reckons the distortion is much lower. However there are two other circuits which are appealing: 1) P turner's 5050 2) The GEC 30W:http://www.jacmusic.com/KT88/SCHEMATIC3.gif My speakers are 87dB/W so I guess I need ~ 30W + a bit of headroom. The 5050 seems to only have a gain of about X11. Can this be changed to a more usual X 20 to 30? I confess to minimal knowledge about impedance matching.... 3K7:8 Ohms seems like a very low turns ratio compared to the 5050 whcih uses 6K6:4Ohms (with a higher B+). Looking at transformer manufacturers pages they seem to recommend 4K5 or 5K. Please help me understand the tradeoffs and how to choose. I am also very confused about how to select a transformer supplier, here in the UK there seems to be 3 main options: Lundahl - mid priced but seem to be very small cores for the claimed power rating. Hammond - middle of the road? (No drop through style!). Sowter - expensive, never heard a word against them. Custom windings at no extra cost. I am tempted to go with Sowter just to be sure of getting something that will sound "right". Any comments? I guess the main question is: Which circuit should I choose and why? Regards, Dave -- Sliding Bias Useable bandwidth depends on quality transformers. Sowter have a strong reputation, lots of continuous history, and are happy to give advice once you've decided what you want to build. They can also supply your power transformer with whatever taps you need. You should read something about valve output transformers, because the basics are too long for here. Menno van der Veen's "Modern valve amplifiers" contains a good mathematical treatment of the subject. The issues you should be aware of a leakage inductance, interwinding capacitance, primary inductance, winding resistance and saturation current. The load seen by the valves comprises the primary winding resistance in series with (leakage inductance in series with the sum of speaker impedance and secondary winding resistance multiplied by the impedance ratio). The primary inductance and the winding capacitance are each connected in shunt with the section in brackets. The impedance ratio is the square of the winding ratio. It's worth sketching that circuit if I've managed to make it clear. You should see that there is a high pass filter resulting from the primary inductance, and a low pass filter arising from the capacitance and leakage. Also, note that the allowable current flowing through the primary inductance is limited by core saturation, and that the current through an inductor depends on frequency and signal amplitude. Hence an output transformer imposes an extra, amplitude dependent limit to bass. That's why Sowter will want to know your target full-power bass frequency limit, in addition to your required small-signal roll-off point. Considering the lowish sensitivity of your speakers and the relatively high output of most modern sources, you may be better looking for more recently conceived circuits than the Williamson. An ultra-linear output stage would give more power than triode. SS current sources, voltage regulators and other odds and ends are cheap and easy, and modern designs can use these to good effect. OTOH, if you know you like the GL50 and you feel confident that you can modify the feedback compensation to suit whatever output transformers you use, that's likely to be a good way to go. It may be worth reviewing your conception of how much power you need for those speakers. Personally, I would want considerably more than 30W. Triode connection will give you a marginally cleaner output for much of the time, but that might not be worthwhile if your short-term signal peaks drive the amp into clipping. I try for a 60:1 ratio of clipping point to mean listening power. Finally, output valves in triode or UL mode are generally loaded with between 2 to 3 times the anode resistance. Linearity is improved as load rises, generally, but output power is reduced. As with mode, if you have no power to spare, then what you choose might depend on which source of distortion you prefer. Ian The Willy is not too stable at the low end. The circuit he shows IS pretty Wiliamsonish on second look: it has a driving stage after the phase splitter, which is unnecessary on Class AB1 or Class A output stages. I would look at the VTL, which has a long tailed pair phase inverter. Or the Marantzes or even-horrors-the Dyna Mk III. |
#6
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The Willy is not too stable at the low end. The circuit he shows IS
pretty Wiliamsonish on second look: it has a driving stage after the phase splitter, which is unnecessary on Class AB1 or Class A output stages. I would look at the VTL, which has a long tailed pair phase inverter. Or the Marantzes or even-horrors-the Dyna Mk III.[/quote] Referring to the Grant G60AMS,the nervous LF of a typical Willy is partially fixed by a 15 db shelf in this cct. But we don't know which OPT is used. A successful shelf depends on the OPT characteristics. Otherwise very ordinary. The lower RH corner shows a cct with no labels, perhaps a low or hi bias indicator? And what does the mysterious 3.9 V off the heater supply do? Cheers, John |
#7
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Quote:
The lower RH corner shows a cct with no labels, perhaps a low or hi bias indicator? And what does the mysterious 3.9 V off the heater supply do? Cheers, John[/quote] I'm not sure about the 3.9V. The G50 is a more basic and earler version of the G60 it seems, and it does not use the opamps etc. The Marantz model 5 looks interesting - similar topology to the 5050 except for the dc levels/coupling. It is hard to figure out what's goiong on with the feedback though - there seems to be a resistor shown in the "com" o/p conection that could be related to the same symbol in series with the extra o/p winding. There seems to be an updated schematic for the Dyna MkIII which also seems to use (more or less ) this topology: http://www.diytube.com/unidriver/poseidon.pdf Not sure why they don't use a resistor for the o/p tube screens, but otherwise this looks decent. I would use tag strip to build though, not pcbs. I cannot find any schematics for VTL. Maybe I need to buy the Manley book, where can I find this book?? |
#8
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Quote:
The Marantz model 5 looks interesting - similar topology to the 5050 except for the dc levels/coupling. It is hard to figure out what's goiong on with the feedback though - there seems to be a resistor shown in the "com" o/p conection that could be related to the same symbol in series with the extra o/p winding. There seems to be an updated schematic for the Dyna MkIII which also seems to use (more or less ) this topology: http://www.diytube.com/unidriver/poseidon.pdf Not sure why they don't use a resistor for the o/p tube screens, but otherwise this looks decent. I would use tag strip to build though, not pcbs. I cannot find any schematics for VTL. Maybe I need to buy the Manley book, where can I find this book??[/quote] ---- I think I got my copy of Vacuum Tube Logic by David Manley about 10 years ago from Antique Electronic Supply of Arizona- http://www.tubesandmore.com/ Seems to me AudioXpress Magazine also had the book- www.audioxpress.com Try a used book site like Alibris- www.alibris.com I got some nice copies of Fred Terman's books at Alibris at reasonable cost. Perhaps VTL it is out of print. All the grids in any ordinary audio amp should have grid stopper resistors to prevent RF interference & parasitics. I've seen parasitic oscillation to 150 MHz while using hi G tubes. This can be avoided by using 1/2 Watt resistors of one to 10K on all the control grids, mounted as close as possible to the socket. The screens with 100R, again of 1/2 Watt. With the new front end the Dyna is no longer a Dyna in my opinion. In its original form the Dyna got its stability by using one less stage than the other guys. That was possible because they used high G tubes such as the EL34, not available when earlier amps such as the Wiliamson were introduced. But the more recent driver should work well anyway. My 2€ antway, Cheers John |
#9
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Quote:
The Marantz model 5 looks interesting - similar topology to the 5050 except for the dc levels/coupling. It is hard to figure out what's goiong on with the feedback though - there seems to be a resistor shown in the "com" o/p conection that could be related to the same symbol in series with the extra o/p winding. There seems to be an updated schematic for the Dyna MkIII which also seems to use (more or less ) this topology: http://www.diytube.com/unidriver/poseidon.pdf Not sure why they don't use a resistor for the o/p tube screens, but otherwise this looks decent. I would use tag strip to build though, not pcbs. I cannot find any schematics for VTL. Maybe I need to buy the Manley book, where can I find this book??[/quote] Here is a link to quite a good paper on taming the Williamson. Many of the points covered would be useful in other amplifiers. http://www.frihu.com/content/diy/rc-...erstaerker.pdf Cheers, John |
#10
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Thanks John. I had previously found that article in the "Williamson Resources" thread. Very helpfull!!!
On balance (no pun intended) I think I will go for the 5050 type topology i.e. using an LTP phase splitter. General opinion seems to be that this sounds better than the concertina phase splitter + drivers topology and it's also easier to keep stable. I think I'll also go for the 2nd hand 6K6 o/p transformers as these should allow a decent power o/p with good distortion figures. Thanks for everyone's help. Dave PS I was rather hoping Patrick Turner would be along to explain why he chose a voltage doubler and if the low gian of the power amp part of the 5050 can be tweaked. But heyho. Last edited by Sliding Bias : January 21st 12 at 05:58 PM |
#11
Posted to rec.audio.tubes
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Which KT88 PP should I go for?
On Jan 22, 4:45*am, Sliding Bias Sliding.Bias.
wrote: Thanks John. I had previously found that article in the "Williamson Resources" thread. Very helpfull!!! On balance (no pun intended) I think I will go for the 5050 type topology i.e. using an LTP phase splitter. General opinion seems to be that this sounds better than the concertina phase splitter + drivers topology and it's also easier to keep stable. I think I'll also go for the 2nd hand 6K6 o/p transformers as these should allow a decent power o/p with good distortion figures. Thanks for everyone's help. Dave PS *I was rather hoping Patrick Turner would be along to explain why he chose a voltage doubler and if the low gian of the power amp part of the 5050 can be tweaked. But heyho. Remarkably, as if by some obscene Quirk of Fait, Patrick Turner has materialised right in front of you in your down town local cyber hide- out. The 5050 design is based on the historical development of PP amps. Before Mullard's 520 with EF86, 2 x 1/2 12AX7 in LTP, and UL OP stage 43% taps, maybe not many ppl used the Mullard topology, and perhaps most used the Williamson config. I say that knowing that by 1955, nearly everyone insisted on at least 12dB GNFB even in an amp with triode output stage. Before GNFB, people could leave out a whole gain stage in a given amp. Anyway, the GOLD STANDARD of hi-fi performance by 1955 was the PP triode amplifier with its disgustingly low gain OP stage where tubes with µ of not more than 6 were considered best. 300B gave A 4, 6L6 or 807, also very low gain, but who cared? As long as the sound was free of noise, had wide bandwidth, low THD, low IMD, low Rout, and enough PO for typical speakers of 1955 which often had sensitivity of 95dB. People tried using beam tetrodes and pentodes with hideous results unless you applied a lot of NFB. OPTs needed to be extremely good to allow say 26dB GNFB around a pure pentode/tetrode output stage, and then the copious mix of ghastly upper harmonic products remained intact, but just reduced in level by about 20dB. With a triode OP stage, the local NFB operating within the triode minimised the higher H products by a huge amount, so you'd need only 12dB GNFB which was easier to apply, and the H mix just wasn't as complex as the pentode/ tetrode. The high gain of pentode and tetrode is due to the screen preventing internal electrostatic FB from operating. Strapping the screen to anode conveys the anode voltage field effect to the electron stream, and voila, magic, and a shirt and trouser load of grunge is nicely cleaned away. In the 1950s, if not officially in the late 1950s and 1940s, people were experimenting with any possible arrangement of tubes to get a marketting edge or better sound, or to get some darn thing, as ppl do - given 26 letters in the alphabet, ppl are still adding words listed in dictionaries. That's our nature. Anyway, the UL stage was invented which gave the world OP stages with many benefits. THD / IMD for the same PO as triode connected pentore/tetrode tubes was found to be as low or lower, and Ra-a went down from say 60k for a pair of OP tubes to 6k0, and about equal to the reflected load, so DF went drom say 0.1 to 1.0, so not much GNFB was needed to increase DF to say 5, which many ppl find OK. Gain was about twice triode gain. PO was near that of tetrode and pentode, so after 1955, UL became king, because for the price of 30 minutes of factory labour the amps could be made to work far better. Peter Walker marketed his Acoustical connection in his Quad-II amps. This acts to reduce gain in an OP stage to be about equal to triode, and give the same Ra-a as triode, further reducing the need for high amounts of GNFB. In fact, the KT66 act as though they were in an amp with 10% UL taps, but which also have 10% local series voltage FB. The equations fully describing what is happening are far to complex to mention here, but IMHO, the Acoustical Connection with local CFB is the best way to cajole wayward larikinesque pentodes and tetrodes into cultivated clever dicks who can be relied upon to put music to our ears without many artifacts. I prefer using 20% of CFB. Some miht go further, and sure, have 20% CFB, then have a CT choke feeding the two screens at slightly less Eg2 than Ea, while bypassing each screen to the cathodes of the opposite tube. Some would otherwise just have UL taps so Ea = EG2 where both are not very high, then have the CFB, and OP stage gain does go LOWER than the triode - but - the sound is magnificanto! Even with only 12.5% CFB, and with UL taps, and with 4 x 6550, one can get 0.7% THD at 50W in class A, without any GNFB. I couldn't find any way of achieving this while keeping the OP stage gain about the same as a triode OP stage so that it is easily driven by any one of several driver stage topologies found at my website and others and all of which work better than the old fashioned crap of 1955. One does not have to build a McIntosh or EAR 509 to get good results. Voltage doublers with sililcon diodes offer better regulation of B+ than any PSU scheme with tube diodes. Modern cap values of 470uF offer a decently LOW IMPEDANCE anchor to run any tube amp from, SE or PP. For examples of pure crud in a PSU, see Quad-II, and ST70, and much bean counter compromised 1955 crapolgy. Garbage. And winding the HT secs on a given PT for a doubler for a given B+ involves 1/4 of the turns used for a CT full wave jobby. The doubler wire is then much thicker and easier to wind, without wasting so much space on enamel and interlayer insulation, so for the same allowed current per sq.mm of Cu section, more I out is available; ie, the doubler tranny can be more efficient. Efficiency is not an obvious sonic artifact. But more of it in PTs helps the sound. Always assume you can do better, and never ever let me see anyone assuming junk from the past was always fabulous stuff; it just wasn't, when compared to what is possible now. Utter Crud is still being pumped out as it always has and always will be because that's how markets work. BTW, 6k6 OPTs might be a compromise for the 5050 unless you have 4, 8 16 ohm outlets. The 5050 has Ea = 500V and the OPT is better if if has well over 6K6. 50W is on the high side of the power. For best performance, connect 8 ohm speakers to a "4 ohm" labelled outlet, thus giving say 13k2 RLa-a and the tubes then are working in mainly class A and it sounds best. Maybe you get less than 50W, but so what, the first 10 watts are what count the most; you want your amp to give finesse, and not just be a noisy brute. The 16 ohm labelled outlet is about as useful as tits on a bull. If you connect 8 ohms, sure, max PO is high, but all other parameters are craperized. Patrick Turner. Sliding Bias |
#12
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Wow, thanks for such a detailed answer Patrick!!!
Looks like I should be OK with the 6K6 to 4 and 8 Ohm transformers (Sowters usually have separate secondary windings, but I need to check in this case). The voltage doubler bit is VERY interesting. I thought you were going to say "I had one lying around" It seems that I can do quite a bit better than my friend's Grant which in turn sounds better than another friend's EAR 509IIs IMO. I still have one question about the 5050 though. Have I understood correctly that the gain is ~11 without the SET stage V1 i.e a bit less than 1 + R33/R10? To increase the gain to ~22 I could reduce R10 to 470R. This would of course 1/2 the feedback. Is this OK? Thanks and regards, Dave |
#13
Posted to rec.audio.tubes
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Which KT88 PP should I go for?
On Jan 23, 2:47*am, Sliding Bias Sliding.Bias.
wrote: Wow, thanks for such a detailed answer Patrick!!! Looks like I should be OK with the 6K6 to 4 and 8 Ohm transformers (Sowters usually have separate secondary windings, but I need to check in this case). The voltage doubler bit is VERY interesting. I thought you were going to say "I had one lying around" It seems that I can do quite a bit better than my friend's Grant which in turn sounds better than another friend's EAR 509IIs IMO. I still have one question about the 5050 though. Have I understood correctly that the gain is ~11 without the SET stage V1 i.e a bit less than 1 + R33/R10? To increase the gain to ~22 I could reduce R10 to 470R. This would of course 1/2 the feedback. Is this OK? I think you have got mixed up. R10 is 100r beneath V2 in its cathode circuit. NFB is established at top R10 with feed by R33, 1k0, from Vo terminal. The GNFB can be reduced roughly 6dB by reducing R10 to 47r. But why? What's so wrong with gains as the amp is now? If you want twice the input sensitivity, then you could use 6DJ8 / 6922 at V2 instead of 6CG7 and this doubles V2 gain. This doubles the open loop gain of the whole power amp includingV2,3,4,5,6. Closed loop gain will hardly change, so amount of GNFB applied will rise from 17dB to 23dB, and the amp might become unstable. There is no need for 23dB FB, so R10 could then be reduced to 47r to reduce NFB with V2 = 6DJ8 back down to 17 dB. If the the stability was OK with V2 = 6CG7, it should remain so if when 6DJ8 is used if the amount of NFB is made the same. Compensation values for shelving open loop HF and LF and Zobel network on output and compo cap across R33 are what I used, but any OPT made by some other dude will have different values or else there **will be** oscillations. So C7, R11, C6, R12, C12, R32, C11, and R33 itself all have to be carefully chosen after building the amp. A number of ppl have built the 5050, and most have had success. I've answered the inevitable questions when they email to say they wanna use say 5814A insead of 6CG7, or use some different OPT, different OP tubes, operating voltages etc. So they end up building a 4747, or a 5353, but not a 5050. That does not matter. What matters is that you know what you are doing while you are doing it, and that you can addapt the basic design to suit what you have to get the best performance. But sadly, for every 5 ppl who start to buy parts for a tube power amp project in their spare time, maybe only 1 gets a good working amp after 12 months. I hear about their attempts, but there must be many who don't ever contact me and don't ever finish. There are far more Gunner Dooz than Igotit Dunns. And along the way they learn SFA, and try to make changes but can't analyse anything, because they never bother learning 101 basics. For 5 sets of transformers I've sold to ppl only one build an amp after 12 months. One started 8 years ago, he has a lovely box of bits. Spends time playing PC games, getting divorced, working, and General Living. When he retires in 35 years and maybe finds time, I will not be here. Nobody will even remember me. Maybe the Green Police will arrest him for being in possession of Earth damaging goods, ie, vacuum tubes etc, when what he must purchase, by law, is the latest 2047 twigital amp, that produces sound from the smell of compost. Patrick Turner. Thanks and regards, Dave -- Sliding Bias |
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