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#1
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
I've just updated the page at
http://www.turneraudio.com.au/quad2powerampmods.html If anyone has problems loading the page, let me know. I think old browsers like my Netscape 4.7 won't handle the page content very well. I have re-drawn the original 1955 schematic and included some more notes about Quad ESL57 impedance curves, and the disastrous effects of having an arcing ESL57 midrange-treble panel. Also included is a recent schematic of another Quad 2 amp mod which I would call a minimum upgrade with just power supply caps and biasing made more reliable, and with automatic protection against bias failure or overheating due to KT66 thermal runaway which was caused twice in one monobloc by the crook ESL57 speaker. Quad ESL57 speakers age very ungracefully, and cause amps to fail real badly. The least one could do to save the possible repair costs of replacement power or output trannies for a Quad 2 amp is to provide active bias failure protection because if a KT66 saturates, the mains fuse won't blow. If there is a shorted speaker, or in this case an ESL57 with arcing midrange panel when the speaker input signal went over 2.7vrms, the KT66 will become overheated and begin to turn red, usually one or the other, rather than both, maybe it breaks into RF oscillations, but anyway, the cathode bias doesn't hold down the anode current. The cathode voltage goes high, burning out the cathode bias resistor. The open resistor allows the voltage to rise further, and the bypass cap fails to a short, or perhaps explodes to an open. ( In this case the cap went to a short, twice. ) If the owner doesn't turn off the amp about now, a KT66 sits there getting redder, no fuse blows, and the power tranny gets real hot, and spews its potting tar mixture all over the place and smoke pours out of the amp. The rectifier tube begins to arc due its excessively low load, and finally after the KT66 melts down internally to a short, the mains fuse may blow, but sometimes the OPT primary goes open or creates shorted turns within, or the B+ HT winding goes open. Does ****e happen? as sure as God made 'lil apples, it happens! Do 1955 Morris Oxfords break down in traffic jams? You bet they do, and so do ancient amps and speakers. The protection circuit I designed this time is for the man who doesn't react to problems fast enough, and doesn't notice balance leds being unbalanced, and dididn't quite notice one speaker of two had a fair bit of distortion. The protection circuit this time shuts down the amp completely. Should anyone want to consider an alternative to the SCR which switches on a relay which interupts the mains, they could consider an SCR which simply shunts the B+ output from the silicon rectifiers I have shown in the new PSU to 0V via a 100 ohm resistance to limit peak current discharge to about 3A, but enough to cause the 1A mains fuse to blow. The drive circuit to the SCR could be identical to that shown in the 'improved quad2 schematic jan2007' Notice the ac signal is filtered away to ensure only the dc will trigger the SCR. Patrick Turner. |
#2
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
Nice work, Patrick; I've saved the page in case I ever want to make a
protection circuit, even though I always have my amps in the corner of my eye and switch off tube amps when I leave the room. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review Patrick Turner wrote: I've just updated the page at http://www.turneraudio.com.au.cob-we...erampmods.html If anyone has problems loading the page, let me know. I think old browsers like my Netscape 4.7 won't handle the page content very well. I have re-drawn the original 1955 schematic and included some more notes about Quad ESL57 impedance curves, and the disastrous effects of having an arcing ESL57 midrange-treble panel. Also included is a recent schematic of another Quad 2 amp mod which I would call a minimum upgrade with just power supply caps and biasing made more reliable, and with automatic protection against bias failure or overheating due to KT66 thermal runaway which was caused twice in one monobloc by the crook ESL57 speaker. Quad ESL57 speakers age very ungracefully, and cause amps to fail real badly. The least one could do to save the possible repair costs of replacement power or output trannies for a Quad 2 amp is to provide active bias failure protection because if a KT66 saturates, the mains fuse won't blow. If there is a shorted speaker, or in this case an ESL57 with arcing midrange panel when the speaker input signal went over 2.7vrms, the KT66 will become overheated and begin to turn red, usually one or the other, rather than both, maybe it breaks into RF oscillations, but anyway, the cathode bias doesn't hold down the anode current. The cathode voltage goes high, burning out the cathode bias resistor. The open resistor allows the voltage to rise further, and the bypass cap fails to a short, or perhaps explodes to an open. ( In this case the cap went to a short, twice. ) If the owner doesn't turn off the amp about now, a KT66 sits there getting redder, no fuse blows, and the power tranny gets real hot, and spews its potting tar mixture all over the place and smoke pours out of the amp. The rectifier tube begins to arc due its excessively low load, and finally after the KT66 melts down internally to a short, the mains fuse may blow, but sometimes the OPT primary goes open or creates shorted turns within, or the B+ HT winding goes open. Does ****e happen? as sure as God made 'lil apples, it happens! Do 1955 Morris Oxfords break down in traffic jams? You bet they do, and so do ancient amps and speakers. The protection circuit I designed this time is for the man who doesn't react to problems fast enough, and doesn't notice balance leds being unbalanced, and dididn't quite notice one speaker of two had a fair bit of distortion. The protection circuit this time shuts down the amp completely. Should anyone want to consider an alternative to the SCR which switches on a relay which interupts the mains, they could consider an SCR which simply shunts the B+ output from the silicon rectifiers I have shown in the new PSU to 0V via a 100 ohm resistance to limit peak current discharge to about 3A, but enough to cause the 1A mains fuse to blow. The drive circuit to the SCR could be identical to that shown in the 'improved quad2 schematic jan2007' Notice the ac signal is filtered away to ensure only the dc will trigger the SCR. Patrick Turner. |
#3
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
"Patrick Turner" I've just updated the page at http://www.turneraudio.com.au/quad2powerampmods.html I have re-drawn the original 1955 schematic and included some more notes about Quad ESL57 impedance curves, and the disastrous effects of having an arcing ESL57 midrange-treble panel. ** Note this ESL57 schematic: http://www.quadesl.com/pdf/quad_schematic.pdf 1. The two bass panels are fed via a series string of 6 x 180 kohms resistors - so even in the unlikely event of a shorted bass panel the secondary impedance is still over 1 Mohm. This equates to 12 ohms on the primary side as the turns ratio is close to 300:1. 2. Similarly the two mid panels are fed via 2 x 150 kohms, with one bypassed by a 560pF cap - so if one of these panels shorts, the impedance is 300 kohms at low frequencies on the secondary side falling to 150 kohms at high frequencies. This translates to 30 ohms on the primary falling to 15 ohms at HF side since the ratio is now 100:1. 3. The treble panel is fed via two 560 pF capacitors in parallel - so a shorted treble panel has virtually no effect on the impedance at low frequencies. The impedance of a 1120 pF cap is 140 kohms at 1000 Hz, falling above that. This translates to 14 ohms on the primary side - or the same load as about a 1 uF cap. So, a shorted panel in a Quad ESL57 will *not* harm the driving amplifier - no matter which one it is. ** However: The very low impedance load situation described in PTs page must therefore have involved an early ESL57 ( pre 1967 ) when treble panels were directly fed from the transformer secondary. In this case, a shorted treble panel will result in the primary impedance becoming about 1 ohm right across the audio band. When fed from a Quad 2 amplifier, this fault would result in a large volume reduction ( circa 20dB) as well as, of course, no mid or treble output at all !!! One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side. BTW: Quad advised owners that all ESL57s be modified with the addition of two 560 pF caps and 270 kohms ( as shown on the schematic) back in 1967 - this coincided with the release of the 303 amplifier. ......... Phil |
#4
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
Phil Allison wrote: "Patrick Turner" I've just updated the page at http://www.turneraudio.com.au/quad2powerampmods.html I have re-drawn the original 1955 schematic and included some more notes about Quad ESL57 impedance curves, and the disastrous effects of having an arcing ESL57 midrange-treble panel. ** Note this ESL57 schematic: http://www.quadesl.com/pdf/quad_schematic.pdf 1. The two bass panels are fed via a series string of 6 x 180 kohms resistors - so even in the unlikely event of a shorted bass panel the secondary impedance is still over 1 Mohm. This equates to 12 ohms on the primary side as the turns ratio is close to 300:1. 2. Similarly the two mid panels are fed via 2 x 150 kohms, with one bypassed by a 560pF cap - so if one of these panels shorts, the impedance is 300 kohms at low frequencies on the secondary side falling to 150 kohms at high frequencies. This translates to 30 ohms on the primary falling to 15 ohms at HF side since the ratio is now 100:1. 3. The treble panel is fed via two 560 pF capacitors in parallel - so a shorted treble panel has virtually no effect on the impedance at low frequencies. The impedance of a 1120 pF cap is 140 kohms at 1000 Hz, falling above that. This translates to 14 ohms on the primary side - or the same load as about a 1 uF cap. So, a shorted panel in a Quad ESL57 will *not* harm the driving amplifier - no matter which one it is. That ESL57 schematic is informative thankyou. I have it already, and there are other "versions" of the same speaker with varied R & C used between the panels and HV step up tranny. ** However: The very low impedance load situation described in PTs page must therefore have involved an early ESL57 ( pre 1967 ) when treble panels were directly fed from the transformer secondary. The schematic I traced of the wire up of the HV tranny has only one cap, and is different to what must be later speaker versions; indeed the two ESL57 I have here must be "earlies", and probably quite old, and which may have suffered in unwise hands prior to their recent purchase, ie, the guy bought them with serious faults from someone else who was happy to part with the bothers... In this case, a shorted treble panel will result in the primary impedance becoming about 1 ohm right across the audio band. This is what I noticed, except the short is at a voltage input threshold above 2.7Vrms, and at a volt input the Z of the crookie is very similar to its mate which appears healthy. When fed from a Quad 2 amplifier, this fault would result in a large volume reduction ( circa 20dB) as well as, of course, no mid or treble output at all !!! But the short is a dead short at low voltages.... One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side. But the amps cope at low voltages. The owner MUST have had the amps cranked well up to excite the crookie to make the amp see the bad load; I noticed the amp behaved like an over loaded amp with clipping behaviour after 3Vrms, and raised cathode dc bias voltages, as it vainly struggled with the low load, and suffered Pda which became excessive, one tube thermalled out, 470 ohm Rk fuses open, Ek shoots up, exceeding 63V Ck rating, Ck shorts, owner notices something wrong then! Damage had probably been done previously to the other amp which may have been used with the crook speaker. It seemed so because one KT66 grid was +9Vdc after being on for an hour, even with separate Rk resistors to better regulate the bias. It was on the verge of failing, and left any longer, may have caused real damage. Did Peter Walker have shares in the MOV GEC company making KT66? He would have done well due to all the unecessary early failures of all those KT66.... But I suspect the owner who just bought all this kit was saddled with ancient KT66 on their last legs, just about to blow; buying old tube amps is often like buying a used car, the tyres are crap, the radiator is full of sawdust to stop water leaking out, the diff is full of banana skins to keep the noise down, and so on. Invariably, you buy another dude's problem he hasn't the money to fix. Even with one output tube pulled out of a QuadII amp, many peole wouldn't notice anything wrong since that single remaining tube beavers away as an SE class A amp producing up to about 4.5Vrms output into a normal 8 ohm load but with much extra 2H. BTW: Quad advised owners that all ESL57s be modified with the addition of two 560 pF caps and 270 kohms ( as shown on the schematic) back in 1967 - this coincided with the release of the 303 amplifier. No such 270k R exist on these speakers. I know what you mean and may have to alter the speakers to suit the later precautionery approach. I tested the response of the good ESL57 against my own which have a flatter response than the '57. This was done using speakers side by side, 3.5M away from the mic set at ear height. Same amp, levels, and pink noise source and mic were used. My own speakers are very slightly less sensitive, ie, less SPL /W/M, and this took some calculation because of Z differences. But for the same voltage input, '57 was 4dB down on the Seas drive units in mine. The '57 has a brightness peak between 2k and 7k of +5dB max, and explains the slightly more detailed sound whch could be awful if the music source was toppy. The '57 bass had a big peak at 85Hz which rolled off more than 18dB/octave so that bass was -3dB below average overall level at 44Hz, wheras mine go down to at least 22Hz even using my limited mic, so utterly no need for a sub-woofer. Subjectively, the '57 are very nice, with probably enough bass for anyone without a sub unless you need to hear modern crap with deliberate bass factored in. My own speakers have zero detectable box artifacts due to good woodwork, and sounded smoother, simply because the treble response is flatter. To good old blokes whose hearing is down -6dB at 7kHz, not uncommon, ( mine isn't though ) and whose tinnitus obstructs anything over 10kHz then ESL57 would be just fine, with their wives none the wiser since they seem to have more tolerance of response differences, and sometimes put up with attrocious systems. This test of ESL57 is of only ONE ancient sample. I don't have another 20 speakers to compare to get an average response which could be said to be The Response of The Quad '57. The fact is, speakers vary with age, and one would expect differences after 50 years. But that a 50 year old speaker picked at random can sound so good compared to state of the art domes and cones is a credit to Peter Walker. I am told that upgraded or repaired panels with better diaphragm materials will give better sound. Patrick Turner. ........ Phil |
#5
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
"Patrick Turner" The very low impedance load situation described in PTs page must therefore have involved an early ESL57 ( pre 1967 ) when treble panels were directly fed from the transformer secondary. In this case, a shorted treble panel will result in the primary impedance becoming about 1 ohm right across the audio band. When fed from a Quad 2 amplifier, this fault would result in a large volume reduction ( circa 20dB) as well as, of course, no mid or treble output at all !!! One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side. But the amps cope at low voltages. ** Then there is simply no fault at low volumes ( under 500mW). When turned up to higher power levels, exactly what I said above must happen. My comment still stands: " One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side." ......... Phil |
#6
Posted to rec.audio.tubes
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Quad 2 monobloc amp mods
Phil Allison wrote: "Patrick Turner" The very low impedance load situation described in PTs page must therefore have involved an early ESL57 ( pre 1967 ) when treble panels were directly fed from the transformer secondary. In this case, a shorted treble panel will result in the primary impedance becoming about 1 ohm right across the audio band. When fed from a Quad 2 amplifier, this fault would result in a large volume reduction ( circa 20dB) as well as, of course, no mid or treble output at all !!! One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side. But the amps cope at low voltages. ** Then there is simply no fault at low volumes ( under 500mW). When turned up to higher power levels, exactly what I said above must happen. My comment still stands: " One would have to be * deaf or semi-unconscious * not to notice something was VERY wrong and discontinue usage of the system on that side." ........ Phil Your comment does stand indeed, although a little wobbly. I tried music and the effect on the crook speaker was if I had a 1 watt amp. 500mW is enough to give average listening levels. ESL57 are not all that insensitive. With two channels playing at low level many ppl wouldn't notice the occasional clipping of peaks in one channel. The owner said there was a little hum from one channel, but that was after the tube in the amp affected went beserko, because when the amp has grossly unbalanced Ia in the OPT, its acts in Class F1, ( and if drawing grid current, Class F2.) The hum at the CT becomes large, and in series between anode and PS, with no CMR, so lots of hum. In the affected amp the owner did eventually notice something wrong and turn it off, and the KT66 which had fused the 470 ohm Rk and shorted the bypass cap survived OK and remains quite usable, but I don't trust any of the tubes in that amp, nor do i trust the ESL57 speakers to not give trouble in future, so i suggested active protection be installed and my offer was accepted. He won't blow up a PT or OPT in future, which would be 20dB more hassle and expense. 2.8Vrms into 16 ohms is 500mW, the level at which the speaker went to a low Z. I should test the crook speaker at night and I will probably see where the arc is occuring. It arced with the EHT turned off, so its most likely a stator to stator short, and perhaps it just needs dismantling, cleaning, and reassembly, and maybe with nylon bolts instead of metal rivetts, and with the later modified RC networks. I can see why so few people know anything much about these speakers. Patrick Turner. |
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