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#1
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
Hi, Vcuumlanders,
Perhaps this is just a minor point, or even common knowledge, but it has merit... I had put a pre-set 1 meg log gain pot at the front end of my recently built monobloc (the Williamson near-clone that some here may recall...) It worked, of course, but I was aware that near the mid- point setting the amp had more hum and noise than at either end (with a pre-amp connected.) With no pre-amp connected the hum and noise just went up as the pot was tuned up. The penny dropped! The grid of the first audio tube was picking up "under-chassis" hum and noise when the first stage grid had a high impedance to ground, i.e. 1/2 meg, or so. I had not specifically shielded any under chassis AC wiring and there was a 60 Hz power transformer under the chassis, too, for B+ boost. The solution: use a 47 K log pot for the preset input gain. Now the first grid never sees more than 23 K to ground - very quiet. But now the amplifier now has 47K input impedance. This is OK if the pre-amp has a cathode follower output, i.e. has a Thevenin impedance of, say, 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid state - 47K is easy to drive. I promptly changed my two other monoblocs, a home-brew and a Pilotone commercial unit, to the same pre-set pot input value with some noise improvement - less than the first one, but the idea is a sound one. Cheers, Roger |
#2
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
"Engineer" wrote in message ... Hi, Vcuumlanders, Perhaps this is just a minor point, or even common knowledge, but it has merit... I had put a pre-set 1 meg log gain pot at the front end of my recently built monobloc (the Williamson near-clone that some here may recall...) It worked, of course, but I was aware that near the mid- point setting the amp had more hum and noise than at either end (with a pre-amp connected.) With no pre-amp connected the hum and noise just went up as the pot was tuned up. The penny dropped! The grid of the first audio tube was picking up "under-chassis" hum and noise when the first stage grid had a high impedance to ground, i.e. 1/2 meg, or so. I had not specifically shielded any under chassis AC wiring and there was a 60 Hz power transformer under the chassis, too, for B+ boost. The solution: use a 47 K log pot for the preset input gain. Now the first grid never sees more than 23 K to ground - very quiet. But now the amplifier now has 47K input impedance. This is OK if the pre-amp has a cathode follower output, i.e. has a Thevenin impedance of, say, 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid state - 47K is easy to drive. I promptly changed my two other monoblocs, a home-brew and a Pilotone commercial unit, to the same pre-set pot input value with some noise improvement - less than the first one, but the idea is a sound one. Cheers, Roger Just by the way: If a pot is set to midpoint, and source output impedance is low, then output impedance of the wiper is R/4, not R/2. But... obviously the smaller the pot value -- the less hum and less grid current related noise will be there anyway. |
#3
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
"Alex Pogossov" wrote in message ... "Engineer" wrote in message ... Hi, Vcuumlanders, Perhaps this is just a minor point, or even common knowledge, but it has merit... I had put a pre-set 1 meg log gain pot at the front end of my recently built monobloc (the Williamson near-clone that some here may recall...) It worked, of course, but I was aware that near the mid- point setting the amp had more hum and noise than at either end (with a pre-amp connected.) With no pre-amp connected the hum and noise just went up as the pot was tuned up. The penny dropped! The grid of the first audio tube was picking up "under-chassis" hum and noise when the first stage grid had a high impedance to ground, i.e. 1/2 meg, or so. I had not specifically shielded any under chassis AC wiring and there was a 60 Hz power transformer under the chassis, too, for B+ boost. The solution: use a 47 K log pot for the preset input gain. Now the first grid never sees more than 23 K to ground - very quiet. But now the amplifier now has 47K input impedance. This is OK if the pre-amp has a cathode follower output, i.e. has a Thevenin impedance of, say, 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid state - 47K is easy to drive. I promptly changed my two other monoblocs, a home-brew and a Pilotone commercial unit, to the same pre-set pot input value with some noise improvement - less than the first one, but the idea is a sound one. Cheers, Roger Just by the way: If a pot is set to midpoint, and source output impedance is low, then output impedance of the wiper is R/4, not R/2. Just by the way: R/4 at wiper midpoint only when it's a linear pot, not log. |
#4
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
Is there an input coupling capacitor? Using a lower value pot will increase the high pass rollover frequency.
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#5
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#6
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
On Sep 28, 11:40*am, Engineer wrote:
Hi, Vcuumlanders, Perhaps this is just a minor point, or even common knowledge, but it has merit... I had put a pre-set 1 meg log gain pot at the front end of my recently built monobloc (the Williamson near-clone that some here may recall...) *It worked, of course, but I was aware that near the mid- point setting the amp had more hum and noise than at either end (with a pre-amp connected.) *With no pre-amp connected the hum and noise just went up as the pot was tuned up. *The penny dropped! *The grid of the first audio tube was picking up "under-chassis" hum and noise when the first stage grid had a high impedance to ground, i.e. 1/2 meg, or so. I had not specifically shielded any under chassis AC wiring and there was a 60 Hz power transformer under the chassis, too, for B+ boost. The solution: use a 47 K log pot for the preset input gain. Now the first grid never sees more than 23 K to ground - very quiet. *But now the amplifier now has 47K input impedance. This is OK if the pre-amp has a cathode follower output, i.e. has a Thevenin impedance of, say, 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid state - 47K is easy to drive. I promptly changed my two other monoblocs, a home-brew and a Pilotone commercial unit, to the same pre-set pot input value with some noise improvement - less than the first one, but the idea is a sound one. Cheers, Roger The noise you get with 1M in series between G1 and 0V is the amplified sound of the resistance noise. There is a formula to calculate the noise and it varies with square root of bandwidth, and with square root of resistance value. Thus if you reduce the BW by factor = 1/4, then noise is only halved, and if you reduce resistance by say 1/4, noise is only halved. So that if R is reduced by 1/20, noise is reduced 1/4.4. If BW is reduced 1/100 say from 20kHz to 200Hz, noise is reduced by 1/10. The high Z grid input terminated by 1M also picks up electrostatic effects, higher harmonics from rectifiers especially. Using input resistance = 10k is quite OK for a tube amp with CF, even if it a weak 12AX7 with Ia only 0.7mA. If you get only 0.2mA od peak Ia change, its 2Vpk at the output, and enough to deafen you in most power amps which clip at 1Vrms input. Do the sums. Distortion will be low, hence the rule, at least 600 ohms Rout for line level components, and at least 10k Rin. I've often used a 20k log pot or DACT attenuator switch and the sound remains as good as it can be. The simplest form of preamp is a passive type with source switch feeding a 20k log pot. If its fed with 600 ohms, Rout at -6dB, ie, 1/2 max Vo, then the terminating Rout from the 20k pot = 5k0, and this is 1/50 times less than if you had a 1M pot set at -6dB, so noise is reduced x 1/7, or -17dB. Noise from 5k0 resistance with 20kHz BW might be 4uV, and I'm guessing, so if the power amp gain is 20x, then noise from R = 80uV. Usually, noise from the power amp with input taken to 0V with short lead **should** be 0.5mV if 8 ohm speakers rated for 88dB/W are to be used at 3M distance. The extra 0.08mV from 5k0 won't be heard. But with Rsource = 250k, noise at Vo would be 7 x 4uV x 20 560uV = 0.56mV, so total noise = sq.rt ( 0.5squared + 0.56squared) = 0.75mV. But you have to factor in hum pick up and burst noise and other crap in V1. I suggest you study carefully how to work all this out. You do call yourself Engineer, so be one!!!! When you place a series capacitor ahead of the Rg, you have a HPF filter. So no matter how low the source resistance, at very low F the amp input grid remains terminated by R between grid and ground as determined by the fixed R if used, or by the pot setting from wiper to 0V. Resistors make their noise right down to DC, so with a HPF there can be slight rumble adding itself into a system. If the input is terminated with a coil with low dcr, then LF noise is shunted as XL reduces. But coils are real good at picking up hum, and they always cost, so they are never used at power amp inputs unless someone has a specially designed input transformer to allow a floating balanced input, usually to be driven by a 600 ohm source, which means XL should not be less than 600 ohms at 16Hz, which means L = 6H, quite a lot. Say you have a phono amp with capacitance input to allow base of gate biasing to be more conveniently arranged. Well, if you wish to use a moving coil cart so the amp has to have high gain, then you only need a very small amount of LF noise to get considerable "cone wobbles" at LF. So R needs to be very low, maybe 1k0 max, and C = 10uF, and this means a large sized cap unless perhaps your'e tempted to use a bibolar electrolytic. Electrolytic coupling caps were routinely used for driving base inputs of bjts in bygone days before quiet enough opamps came along. MC carts have very low source Z 20 ohms, which means noise over 20kHz is about 1/16 of say 5k Z of a MM cart. The trouble with electronics is that you must always think of 2 or more interactive factors which often interact non linearly, if not oppositely to common sense. That's why designing and building stuff takes so long to optimise, and bosses and accountants and customers and wives hate all that; they expect immediate and exciting solutions, at -20dB lower cost. Well, I say **** 'em; let em wait until we've got it right. Patrick Turner. |
#7
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Gotta get on the bike now........too much noise here!!
Probably get rained on today. Cheers, John |
#8
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
On Sep 28, 5:06*pm, "GRe" wrote:
"Alex Pogossov" wrote in message ... "Engineer" wrote in message .... Hi, Vcuumlanders, Perhaps this is just a minor point, or even common knowledge, but it has merit... I had put a pre-set 1 meg log gain pot at the front end of my recently built monobloc (the Williamson near-clone that some here may recall...) *It worked, of course, but I was aware that near the mid- point setting the amp had more hum and noise than at either end (with a pre-amp connected.) *With no pre-amp connected the hum and noise just went up as the pot was tuned up. *The penny dropped! *The grid of the first audio tube was picking up "under-chassis" hum and noise when the first stage grid had a high impedance to ground, i.e. 1/2 meg, or so. I had not specifically shielded any under chassis AC wiring and there was a 60 Hz power transformer under the chassis, too, for B+ boost. The solution: use a 47 K log pot for the preset input gain. Now the first grid never sees more than 23 K to ground - very quiet. *But now the amplifier now has 47K input impedance. This is OK if the pre-amp has a cathode follower output, i.e. has a Thevenin impedance of, say, 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid state - 47K is easy to drive. I promptly changed my two other monoblocs, a home-brew and a Pilotone commercial unit, to the same pre-set pot input value with some noise improvement - less than the first one, but the idea is a sound one. Cheers, Roger Just by the way: If a pot is set to midpoint, and source output impedance is low, then output impedance of the wiper is R/4, not R/2. Just by the way: R/4 at wiper midpoint only when it's a linear pot, not log. Alex, right, of course! With a zero Z source, you get 25K in parallel with 25K at mid rotation of a linear pot, a quarter of the original 50K... at some other place further around for the log pot that I used. My fault, in too much of a hurry... Thevenin rules! Cheers, Roger |
#9
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
"John L Stewart" wrote in message ... GRe;939604 Wrote: "Alex Pogossov" wrote in message ...- [...] Just by the way: If a pot is set to midpoint, and source output impedance is low, then output impedance of the wiper is R/4, not R/2.- Just by the way: R/4 at wiper midpoint only when it's a linear pot, not log. Only if driven by a low Z source, which it probably will be. You're in the repeater business? -- John L Stewart |
#10
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
"Engineer" wrote in message ... Just by the way: If a pot is set to midpoint, and source output impedance is low, then output impedance of the wiper is R/4, not R/2. Just by the way: R/4 at wiper midpoint only when it's a linear pot, not log. Alex, right, of course! With a zero Z source, you get 25K in parallel with 25K at mid rotation of a linear pot, a quarter of the original 50K... at some other place further around for the log pot that I used. My fault, in too much of a hurry... Thevenin rules! Cheers, Roger Depending on a pot's taper and quality, for a log pot ccw tap to wiper midpoint is approximately R/10. With a zero Z source and approximately Rx0.9 cw tap to wiper midpoint you'd get around R/11 wiper Z. |
#11
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
On Sep 29, 11:00*pm, John L Stewart John.L.Stewart.
wrote: Gotta get on the bike now........too much noise here!! Probably get rained on today. Cheers, John -- John L Stewart I have got to ride after too very wet deighs hear. I spent tyme rewiring a truly frightfully made stero amp made by Earl Weston. Looks very pretty, goes like ****. Just one little drop and you get lotsa splinters and broken glass.. Plus more work rwirin' Quad-II Forties. Nuttin's sacred 'round 'ere ! Singing lessons are compulsory curiculum for all amps entering the shed door.Gently encouarged with hot soldering iron, long nose pliers, side currers and a very stern chior leader. Probably one of THE WORST things being inflicted by Chinese indistries have been tube sockets. In Quad-II-Forty, they have white ceramic sockets meant for PCB because the contacts are just extended pins meant to poke into drilled holes for soldering to tracks. The trouble is the tube pin grippers. These are made zinc or cadmium plated metal, not sure what metal, stamped to a Y shape with top of the Y made to engage the tube pins. But the metal has very little springiness, so after pushing a tube is and out 10 times, the tube becomes quite loose as the distance between the Y increases. The Y grippers have had two cuts in the metal bottom leg so that when inserted into the slots in the ceramic, the bottom Y leg is given twist so the leg section below the cuts turns out to prevent the pin coming back up out of the slot its in. But you only have to bend the bottom section of the pin gripper side ways say 45 degrees, and many will just break off because the metal is so KUNTY and FUCTATIOUS. It has been said they "don't make thigs like they used to" and this is very true of many things relating to tube amps, and especially to such things made in China where the ruling elite who run Chinese industries to provide western customers with Quad amps costing many thousands of dollars while paying the chinese workers only an average of 64c a day, and while using **** METALURGY in critical items like tube sockets. The Chinese tube sockets fitted into hi-end Chinese amps or sockets bought by wannabe makers like Earl Weston are often utter garbage, and I don't know if the situation is improving because the Chinese hate being told to do better about product reliability. They think they are superior, but I know their **** stinks just like anyone elses. The Chinese have a long way to go before making tube sockets which are as good as the McMurdo and other weston made sockets which last well against many plug ins/outs, and having tubes pushed over while in sockets which is particularly bad for the Chinese sockets. Good tube sockets allow grippers to move and follow tube sways, without the metal deforming, bending, or yielding. While gutting the Quads in preparation for re-wiring my way, a contact broke so very easily on a tube socket. I was able to repair it by using some tinned wire 0.5mm dia, in a loop over the Y grippers, then flattened with pliers and soldered. The two wire ends are taken down through the slots in ceramic as the slots are wide enough. The grippers can be bent with an awl carefully used to make the gap smaller to grip tube pins tighter, but after I did this the fix didn't seem to last long, and the tube still felt like not much was holding it in place. Audiophiles change tubes like underpants. They are always trying to prevent ****ty sound by changing tubes, and they do themselves no good because sooner or later the grippers will become intermittent and cause smoke and silence. The S&S brought the Earl Weston amp to me for a fix. Fulla chinese tube sockets which fail so easily. Owner had tried a big 5R4 because some idiot ****wick@somewhere had said bass would improve. He'd also tried 5AS4. But 5AR4 is the best. A heavy tube like 5R4 nearly can fall out of the socket if the amp is turned upside down. It is in- evitable that amps like this get moved around as audiophiles lend them to mates and sell them to one another. With a big 5R4 with only 4 4 grippers left in the socket, there isn't much to hold the tube, and they easly get bent over during moves as there is no cover screwed down over tubes. The rectifier heat cycles also tend to fatigue the metal. The Earl Weston had KT66 with 43% UL taps on OPT, in class A, for 14 watts with Ea at 350V. But just 1/2 6SN7 input powering one OP grid with a 1/2 6SL7 driving the other grid, with this single 1/2 6SL7 a floating self balancing phase inverter circuit. So the other 1/2 6SL7 remained unused. NO GNFB applied, so Rout measured 10.6 ohms, noise& distortion all too high, but who cares anyway after they spend $5,000. The wooden chassis base using 18mm thick sides is very poor carpentry, and stainless steel top-plate and bottom plate held to wood with only 4 screws, so one drop and the whole damn thing flies apart into splinters. There was a signed note on the inside of the woodwork that the amp was supplied as a replacement for another that had been dmaged in transit, and that a warranty was not offered. So along with the poor workmanship, no good will either. The world is fulla ppl making lousy quality garbage because they won't put the effort in that real quality demands, and its because more effort means less earnings. I'm replacing all circuitry with equivalent of my 5050 schematic which works so much better than Earl the Charlatan could ever manage. What gives good bass is adequate PSU capacitance and low Rout, ie, good damping factor, by means of class A triodes OR else adequate GNFB for the UL connection. So folks, there is a huge amount of difference in tube sockets, and there are many gullible people who would not recognise a decent tube socket even if it bit them on their arse !. The problem of poor sockets isn't monopolised by the Chinese though. I completely re-wired an ARC VT100 some years ago, and its tube sockets were so bad tubes would creep out of sockets. ARC had 8 horizontally mounted tubes in a close box, so covers had to be unscrewed to push tubes in. I soldered 0.8mm wire to bases of 6550 and lashed the tubes in by tying the wire down around the socket stand-offs, so no more problems with tube creep. The ARC also gave lots of Smoke&Silence, and the complete rewire changed all that. Noys hear isn't two bad though, cood be wirse, three or four bad. But when I ryde, I nough I am pe-sewd by Uncle Fate and peneenless customers close behynde. Patrick Turner. |
#12
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Again, in a real cct more random noise is generated by the input characteristic of whatever gain device is used, be it SS or vacuum. For vacuum it turns out triodes are best while running lots of plate current. The cascode arrangement is quite good. I've used that in the past for input from reluctance pickups, where only a few millivolts signal are available. I used RF triodes such as the 6BK7/6BQ7/6BZ7 family with much success. 6DJ8 & 6AQ8 should work very well also. But for the input of a power amp a little more noise is not a problem. Hum is. Here I would use an EF86 or Z729 if it was critical. But I've found rather ordinary things like the 6AU6 do well in most applications. Others may differ. But I'm sure that all this is well known here!! Biked another 50 km today. Looking for a 600 mile Septmber by this time Friday afternoon. Weather don't look good. Cold & wet predicted. Cheers, John |
#13
Posted to rec.audio.tubes
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Monobloc pre-set gain pot value
John mentioned...
The noise formula referenced predicts only the noise voltage or current resulting from an ideal resistor. In a real resister noise would be more. In my tests of tubes and noise I have found the resistance value dominates the noise production and that the formulas are about right. My tests for noise in tubes and any added grid resistance were done using a sample triode with grounded or ungrounded Rg, then amplifying the anode signal using a quiet amp with gain of at least 1,000 x or +60dB. If the tube under test has gain = 16, then you know the noise seen at the output of the 60dB amp is about 16,000 times higher than any grid signal. So if grid noise = 1uV, there will be 16mV at the 60dB amp. If a resistance is added then a calculation can be made to find its noise. N total = sq root ( NRg squared + other N squared ). Perhaps cheap old carbon composition resistors are more noisy than metal film types. Again, in a real cct more random noise is generated by the input characteristic of whatever gain device is used, be it SS or vacuum. For vacuum it turns out triodes are best while running lots of plate current. The cascode arrangement is quite good. I've used that in the past for input from reluctance pickups, where only a few millivolts signal are available. I used RF triodes such as the 6BK7/6BQ7/6BZ7 family with much success. 6DJ8 & 6AQ8 should work very well also. The noise in tubes varies much more than it varies for the same value of resistance. A good 1/2 12AX7 sample will have 1uV of grid noise, and if you have dc on the heater and amplify the noise, its like a sputtery barbecue, with considerable LF burst noise, ie, the tube noise has more LF content than HF content. This prohibits 12AX7 being good enough for MC carts. Gm = approx 1.2mA/V, and noise reduces with square root of increase in Gm, so that if you had 5 x 12AX7 with all 10 triodes in parallel, Gm = 12mA/V, 10 times the single 1/2 triode. One would expect noise to be 0.316 times less, but damned if I've ever see such a reduction. With a 6DJ8 with Ia = 5mA, Gm = approx 6.6mA/V, and both halves give Gm = 13mA/V, and yet noise rarely measures any better than the single 1/2 12AX7. Using a cascode does NOT improve anything because the grid noise is still amplifed by the cascode gain. What the cascode does do is reduced the triode Miller effect because the bottom tube has very low gain drinving the top tubes whose gain = anode RL / tube gain. The total cascode gain works out at gm x RLa, so for cascoded 6DJ8, with Gm = 6mA/V, and RLa = 20k, expect gain = 120, and if the top tube's gain = 18, then bottom tube's gain = 6.6, as the cathode input resitance = 1.48k, and gain for load = 1.48k = 6.6. ( Ra = approx 5k ). Some ppl have used MC input to the cathode of a grounded grid tube. This can be tricky and biasing the grid negatively will reduce Ek to 0.0V and allow the cart to be between cathode and 0V without any DC flow, or very little. Noise rules still apply. But the quietest cascode comes when a high Gm J-fet is used to drive a triode or another J-fet. Typical noise figures for 2SK147 driving say 1/2 6dJ8 are -20dB lower than tube grid inputs. If RLa = say 16k, and Ia = 5mA, gain = Gm x RL = 0.04A/V x 16k0 = 640. Usually an unbypassed Rsource is used to reduce this gai to about 200. The j-fet noise does not have high LF sputter noise of the tube, so when used in an MC amp or mircrophone amp the fet is a very effective device to minimise noise. But for the input of a power amp a little more noise is not a problem. Indeed this is the case, and most good tubed power amps exhibit quite adequate input noise figures. If amp gain = 20, and input noise = 5uV from a not so wonderful input tube, then output noise = 100uV, plus whatever else is generated within the PO, and yes, its often mainly hum. If there is dc to all heaters, and the PSU is remote, then the hum will be largle absent and noise is very low. I've made 55Watt amps using a pair of 845, and had noise 0.25mV with input grounded. 55W into 5 ohms = 16.5Vrms, and if the noise does not increase for mas PO, the unweigted SNR -100dB. Hum is. Here I would use an EF86 or Z729 if it was critical. But I've found rather ordinary things like the 6AU6 do well in most applications. Apart from good B+ rail filters, DC on the heaters of all input/driver tubes in power amps is the best way to reduce hum noise. Others may differ. But I'm sure that all this is well known here!! Biked another 50 km today. Looking for a 600 mile Septmber by this time Friday afternoon. Weather don't look good. Cold & wet predicted. It rained and blew hard yesterday here and the 60km ride was cold and miserable. My ankle has decided to be a real pest - it was badly smashed up in a motorcycle accident 45 years ago, and maybe it tips me off the bike. I might have to buy a kayak to get exercise if I can't use the legs. But in a month it will be quite warm here. We don't export our spring sunshine. But I'd gladly export a magpie which attacked me yesterday. Between August and October its the breeding season, and they become very teritorial, and its usual to get attacked by 2 or 3 during a ride, so over the years I've probaby been attacked hundreds of times. But yesterday's bird was the worst because it pecked me 20mm from my eye, and would have got my eye were it not for wrap around sun-glasses. A child here was recently blinded this way. If I manage to grab this bird in my next ride when he attacks, I'll gladly thrash it to death quickly, as its genes are way too agressive. Magpies do very well in suburbia and city landscapes where they benefit from so much junk food scraps littering the land scape. The road kill of animals on country roads is a smorgasboard of food for maggies in spring, so they like to nest in trees near roads, and from there they terrorise passing cyclists. Putting a few cable ties in the helmet helps keep most away but some come in from the side, then upwards to get eyes. I'll have to apply to the Pentagon to get one or two spare drone aircraft to accompany me on rides to fire warning missiles at the maggies. But I reckon the maggies would attack the drones and win. Maybe there's a real good market to export magpies to Ghan, where they could be used against the Taliban - have 'em on their knees in the dust begging for a peace deal and handing in all their AK47s and IEDs. Patrick Turner. Cheers, John -- John L Stewart- Hide quoted text - - Show quoted text - |
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