From: info@turneraudio.com.au   
      
   Martin said......   
      
   First of all, Im glad to see some activity here on old Usenet. Most people   
   seem to be in web based forums these days.    
      
   And the web based forums don't work as well as the Usenet forums. BUT, the   
   Usenet forums are non moderated which means all the arsoles can terrorize   
   everyone who does not agree with the arsoles' opinions, so most newbies and   
   ppl who didn't like the    
   slightest challenge to whatever they typed fled in terror to the smaller   
   moderated groups where everyone is forced to be nice to each other. The result   
   is like a lot of benign zombies attempting communication in short almost   
   content free 1 sentence    
   postings, and that's why I've stuck to this place. If people don't like the   
   heat, get out of the kitchen. If a terrorist enters the kitchen, tip a pot of   
   boiling oil on him, kick him out, and set fire to him then close the door.   
   Nobody gets away with    
   terror with me. I'm here to discuss technical issues, not to pose as smarter   
   than anyone else, but to share my experiences regardless of whether I am in   
   error or not.     
      
   In my description of the DC supply, I meant that instead of using the existing   
   16+16uF cap, I placed two separate caps in the circuit - one 33uF before the   
   choke, and a 47uF after. I did not add another choke or anything.   
      
   In Quad-II, the original had C1 16uF + C2 16uF in one box cap, plus 20H choke   
   with Rw about 600r. The B+ for OPT CT was taken from C1 16uF where the Vripple   
   = approx 19Vrms, 100Hz, saw tooth wave. The L + C2 form a filter to the fixed   
   screen supply for    
   KT66, also for input tube B+ and any external tuner or Quad22 preamp. The 20H   
   + 16 uF reduce the Vripple at Eg2 to be about 25mVrms, low enough to keep   
   noise at output low enough.   
   Using GZ34 with 33uF will be OK but only halve the Vripple at OPT CT. The use   
   of 47uF for C2 will further reduce Vripple at Eg2 to about 3mVrms, a   
   considerable betterment.    
   BUT, if you use 8r0 speakers with amp strapped for 9 ohms, the RLa-a anode   
   load is about 3k5, and above what is a low class A threshold, the amp works in   
   class AB where the Vripple at OPT CT is in series with the "ON" half of the   
   OPT load and its tube on    
   every 1/2 wave cycle so the THD and noise increases hugely.   
   To force the Quad-II to work better with low load values, most especially when   
   4ro speakers are used, and, they often are, despite ppl saying oh no they   
   don't,   
   the B+ supply at OPT CT should be far better filtered and be a low impedance   
   to 0V, so I like a modern 470uF cap, rated for 450V, easy and cheap. But 470uF   
   used for C1 in CLC would kill any tube rectifier because the peak charge   
   current rises way above    
   the rating which is often less than 0.2A.    
   SO, I get rid of the tube rectifier. Then I install a pair of 1N5408 in series   
   on each 1/2 primary of HT winding and the B+ at C1 will rise to around +400Vdc   
   if wanted, but better is to use say 150uF for C1, then replace the existing   
   20H choke with a 4H    
   choke with Rw < 50r, and have the 470uF after the L. additional R can be added   
   to between HT winding ends and diodes, say 47r, 68r, 100r to reduce the   
   working B+ to where you want it, or to about +375Vdc, ( +5%, -10%.)   
   The Vripple at OPT CT will be 3mVrms, and then an R&C filter with say 1k0 +   
   220uF for Eg2 will reduce Vripple to 0.021mV, and utterly negligible.   
   With individual R&C cathode biasing networks with 470r + 470uF, the amp will   
   give better bass, and the amount of Vdc change during transients becomes   
   neglible, and the noise injected in class AB is minimized.    
   A lot can be done to make Quad-II amps far better than the original designers   
   intended.   
      
   Phil, thanks for sharing your experiences while I was working on the amps.   
   Since I do not own a dozen of them I could not check what would be the normal   
   behaviour of the circuit.    
      
   Patrick, I have read your interesting site about tube electronics. I   
   understand your angle, and I think it is how one should think about new   
   production amps. But in these Quads I was more concerned with safety and   
   longevity than altering the circuit.    
      
   But the mods I routinely do to old original junk like Quad-II does make them   
   last longer, ie, KT66 life is longer, and their safety is made better. Both   
   issues require circuit changes that exceed what you are doing just by changing   
   the two main electros    
   in PSU.      
      
   I saw a Quad system in a museum recently, together with an early EMT   
   turntable, SPU cartridge, a Revox G36 and so on. Special, quirky but high   
   quality stuff that must to most people seem like steam engines these days.   
   Call me sentimental but I personally    
   dont want to alter such machines too far from how they were made in their   
   time.    
      
   OK, you are in love with the crummy old quality of the past. Maybe you would   
   not mind using hand operated shears to shear a sheep, but I'd prefer the   
   powered wool cutter. I really like old steam engines, and the locals in   
   Canberra have a Garratt    
   articulated loco almost restored, and there are no mods. OK, quite a   
   sight'n'sound to see running. 262 tonnes of power from 1960. Wow. But I'd hate   
   to own the darn thing. Same goes for a Spitfire, or, rather like a Quad amp, a   
   Morris Major. Now I just    
   cannot think of a 1950s amplifier that excites me like say a D Type Jaguar, or   
   a Vincent Rapide. Back in 1950s, amps were useful slaves, seen and not heard,   
   tucked away out of sight, turned on by a switch on a preamp on a console.    
   The amps sold to the public and to BBC were pretty bloody awful, compared to   
   what we can do now because of better diodes and caps, and because decent wire   
   and iron for PTs and OPTs costs far less in real terms than in 1950, when most   
   of the good    
   materials had been sent to the bottom of oceans or blown all because men who   
   ruled human kind are blind, vain, stubborn, vile, ruthless, greedy, and   
   arsolic fuctards.    
      
   I understand the problem with the common cathode resistor, and I wonder if   
   maybe this is one of the reasons many Quads have failed with melted tar all   
   over. In my amps I swapped tubes around and measured voltage differences for a   
   little bit better DC    
   balance. But separated cathodes sounds like a good idea.(I built a Williamson   
   inspired amp once, using huge wirewound pots between the two output tube   
   cathodes and ground. Was easy to balance but maybe not the last word in   
   highend audio :)    
      
      
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