331e62d6   
   From: mynamespacedbydots@xs4all.nl   
      
   "Patrick Turner"  wrote in message   
   news:6571f445-8098-4834-9880-63a6617c287f@d1g2000pra.googlegroups.com...   
   > Hi all,   
   >   
   > I have a pair of VM220 200W monoblock amps here for a service, and a   
   > stereo amp, VT200, 200W per channel.   
   >   
   > Both sets of amps belonging to two different owners complained about   
   > overheating tubes and burnt resistors and malfunctions.   
   >   
   > My experience is not wonderful with all ARC amps because they end up   
   > in my repair workshop because of their serious design shortcomings. I   
   > have had to totally rewire a VT100 in the past to make it reliable and   
   > easy to adjust bias settings.   
   >   
   > But let me talk about the VM220 monblocs first, so that owners in   
   > Australia might at least be able to live more peacefully with these   
   > amps.   
   >   
   > The VT220 has a sign on the rear panel which says the mains voltage   
   > needed is 220Vac. There are probably 2 x 110Vac windings which would   
   > be paralleled in the USA with 110Vac mains but here in Oz the nominal   
   > mains = 240Vac which may exist of cold nights but otherwise often   
   > measures 250Vrms during the day.   
   >   
   > The result of mains = 250Vac with a mains tranny designed for 220Vac   
   > is that the HT winding voltage rises and the B+ becomes +452Vdc   
   > instead of +400Vdc.   
   > All these ARC amps have screen supply voltage = plate supply voltage   
   > so if the plate supply goes high, then the screen supply goes high   
   > thus raising bias current alarmingly!   
   >   
   > The multiple 470uF x 450V rated electrolytic B+ capacitors work with   
   > +452Vdc across them.   
   >   
   > The working measured heater voltage rises to 6.7Vac instead of the   
   > correct 6.3Vac.   
   >   
   > The amps come from the ARC factory with bias set for the USA   
   > conditions where B+ would normally be just over +400Vdc.   
   > When plugged used in Oz straight out of the box the bias currents in   
   > each 6550 output tube is about 67mAdc, and the Pda per tube averages   
   > about 31W at idle,   
   > I have never seen any publically easily available information for the   
   > biasing of all ARC amps and there is nothing printed on their amps   
   > about biasing, and often the means of biasing is difficult and   
   > extremely dangerous and confusing to many owners. I get so many pissed   
   > off owners of expensive ARC crap I have to say that ARC is overly   
   > optimistic about their amps being able to survive very long, at least   
   > in Oz.   
   >   
   > I'm not sure what the recommended idle current should be in ARC amps   
   > but it seems to me that Pda = 31W is much too high and invites   
   > troubles.   
   >   
   > In the pair of VM220 monos, I mounted a 16 ohm x 60W rated resistance   
   > on a heatsink between the mains and output transformers. This is   
   > connected in series with the 340Vrms HT winding and the diode bridge   
   > charging the multiple electrocaps. I then biased each 6550 to 40mA at   
   > idle and B+ settled at +430Vdc which s unlikely to bother caps rated   
   > for 450Vdc. Pda + Pdg2 is now 17.2W, and the amp runs much cooler and   
   > everything will last a lot better.   
   >   
   > I will gurrantee the sound will NOT be degraded in any way, and I   
   > found 200W was available at clipping with a sine wave, despite the   
   > added series R in the B+ supply.   
   >   
   > The VM220 was only produced between 2004 and 2008, and to me it was a   
   > better design than VT100, VT120, VT200 etc.   
   >   
   > Just why ARC would discontunue a better than before design is a   
   > mystery, indicating their designers have shit for brains.   
   >   
   > The VM220 has separate RC coupling of 6550 output tube grids to the   
   > 6N1P driver tubes. There are 8 pairs of voltmeter jacks allowing easy   
   > monitoring of bias currents while twiddling each one of the 8 bias   
   > pots located near each output tube, via a hole in the top plate.   
   > However, its very easy to not find the pot's screwdriver slot because   
   > their is no guide for a screwdriver, and there should be one along   
   > with a philips headed pot shaft which should of course be metal and   
   > much more rugged. But at least the VM220 has separate pots, one for   
   > each tube, for acuurate bias setting.   
   > The RC coupling is FAR MORE reliable than the stupid VT100 method ARC   
   > has with coupled 6550 grids to driver tube cathodes, with biasing   
   > voltage applied to driver cathode follower grids. When such cathode   
   > follower tubes arc over inside because the driver tubes have too much   
   > Ea betwen anode and cathode, the Vdc applied to output grids can go   
   > very positive, and saturate output tubes which try to conduct 10 times   
   > their idle current thus perhaps causing fuses to blow, but maybe not,   
   > maybe burning out cathode resistors and or screen resistors.   
   >   
   > There is no active protection circuitry in ARC amplifiers - its a   
   > hostile form of negligence in this day and age where owners just wanna   
   > plug and play without smoke ever happinging!!!!   
   >   
   > Anyway, I suggest my little mod to the VM220 monos will allow them to   
   > last a whole lot better than they otherwise might.   
   >   
   > Now the owner of the VT200 just wants me to repair but not modify   
   > because he says there isn't enough power.   
   >   
   > The first dreadful and fucking awful thing about the VT200 is the   
   > weight. What on earth goes on in ARC designer's heads when they fail   
   > dismally to to produce goods which cause serious spine problems every   
   > time some unfit audiophile tries to move such a thing??????   
   >   
   > The VT200 is like having 2 x VM220 inside the one box, but instead of   
   > 2 power trannies, there is just one big one.   
   > But lifting VT200 is a two person job. The VM220 monos are too heavy,   
   > but trying to deal with something twice this weight is a challenge,   
   > and damned dangerous to health. The sooner someone sues the arse off   
   > ARC, the better.   
   >   
   > Anyway, The VT200 had caused at least two clouds of smoke to appear   
   > while being used in Jindabyne, not far south of here.   
   >   
   > I remove the 6mm al plate side covers to get access to the underside   
   > of the 2 pcbs where there are 8 x 1 ohm resistors per channel, one for   
   > each output tube, between each cathode and common connection to the   
   > OPT secondary windings which are used for local cathode FB, rather   
   > like Quad-II amps. These cathode R allow the monitoring of bias   
   > current in each OP tube, but its a terrible shame one has to remove a   
   > cover to check the bias current for each tube. Where are the plug in   
   > test points??   
   >   
   > I find two of these 2W cathode resistors have burnt to a crisp before   
   > going open, hence there must have been two clouds of smoke.   
   > I replaced one, and turned on the amp.   
   > The tube with its open cathode 1r has no bias current, and I have yet   
   > to see if the tube is stuffed, or the screen series R has also gone   
   > open. But a tube nearby has a red glowing anode, although its current   
   > is only 70mA, so probably its screen has overheated in the past and   
   > screen wires have become warped deformed in the heat and the tube   
   > function is fucked.   
      
   [continued in next message]   
      
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