From: info@turneraudio.com.au   
      
   On Monday, 17 February 2014 08:53:17 UTC+11, John L Stewart  wrote:   
   > This note describes a simple, low cost power supply improvement. It   
   >    
   > allows reduction of the power frequency ripple component (60 Hz in North   
   >    
   > America & 50 Hz in Europe) by up too 20 db. The problem this   
   >    
   > modification resolves occurs only in center tapped, full wave rectifier   
   "John L Stewart"   
   >   
   >  ( snip )   
   >   
   > Depending on the loading conditions of a triode power stage, from 2/3 to   
   > 4/5 of this ripple voltage will be applied to the load. Unfortunately,   
   > most loudspeaker systems have one or two   
   > resonance's below 100 HZ. One of these could be at or near the power   
   > frequency. Presto!! We have objectionable hum in the listening room.   
   >   
   > Figure One illustrates an ordinary center tapped full wave power supply.   
   > Two sides of the HV winding measured 38 & 40 ohms.   
      
      
   ** FFS  -  just add 2 ohms to the 38 ohm winding.   
      
    Problem gone.   
      
    Dodgy 5U4s etc not withstanding.   
      
    SET amp loonies sure are a bunch of neurotic old women.   
      
   ....  Phil     
      
   I'm not so sure an added 2r to a winding with 38r would fix the "problem".   
   What if there is slight difference in turns of the CT winding? What if there    
   is Idc imbalance in the two output tubes?   
      
   Now, consider Quad-II amps which have about the worst PSU of any amplifier   
   ever made and which looks exactly like an "accountant's delight"   
      
   There is a GZ32 charging 16uF and the B+ at approx 140mA Iadc is applied to   
   the anode winding CT on OPT. The Vr at 16uF is approx 18Vrms! However, CMR   
   reduces this to negligible levels and any differential Vr across OPT anode   
   winding appears at cathode    
   windings where the local CFB reduces it. The KT66 screen supply has a 20H   
   choke followed by a second 16uF which then reduces the 18Vrms   
   by a factor 0.008 to approximately 0.15Vrms. Having a very well filtered   
   screen supply is more important than a well filtered anode supply.     
   The GNFB then reduces any residual hum further to be what Peter Walker   
   considered low enough, and was less than the THD measured. This assumes that   
   the rectifier has equal Ra anodes, and tubes are new and have equal Idc.    
      
   Where the tubes are old and Idc imbalanced, then THD and the hum can rise to   
   be the same as if there was no NFB in the amp.   
       
   My solution to the "Quad problem" has been to completely revise the Quad PSU   
   using Si diodes feeding say 150uF then 150ohms to 470uF mounted where tube   
   rectifier once was, then use individual R&C biasing on each KT66, and then the   
   amp works fine with    
   less Idc. The screen choke can be retained, but bypass cap is 100uF. Thus   
   makes the amp dead quiet, even if output tubes are badly matched and a fault   
   is present.    
      
   Where there is a CLC filtered supply, the Vr can usually be much reduced by   
   adding a C+R series network across the choke, arranged so that C is resonant   
   with the *working* choke inductance at twice the mains F. R should be about   
   22r. The damped parallel    
   resonance usually reduces the main harmonic of 2H    
   in ripple ( 100Hz or 120Hz ) by 12dB. Other H are not much reduced but then   
   the    
   following C2 of the CLC filter has an increasingly lower XC thus shunting the   
   higher ripple H.   
        
   Many Guitar amps have a hum nulling pot so hum from heaters can be nulled but   
   often is has no effect. Some allow balancing of the Idc in each output tube.   
      
   Few guitar amps have CLC anode filtering. Hum seems only objectionable if the   
   volume is turned up. Switching noise of diodes can often be heard as pulses at   
   100Hz or 120Hz. But for hi-fi, owners expect dead quiet when volume is up and   
   methinks one has to    
   go to more trouble to get quietness than either Phil or John suggest. I have   
   also repaired and re-engineered many amps where the earth path has had to   
   entirely re-wired, including position on chassis for green-yellow Earth wire   
   from mains input wiring. a    
   connection lug should be well away from say a bolt holding the PT to chassis.    
      
   Noise is not always easily reduced.   
      
   Most amps with CT HT windings don't make much more Vr than amps with single HT   
   windings with 4-diode bridges or voltage doublers with single HT winding and   
   two diodes - all things being equal with current to capacitance ratios.   
   Keeping ancient electrolytic caps and tube rectifiers in ancient amps is not   
   good practice and it is always an up-grade to change to Si diodes and healthy   
   value filter caps. Especially with SE amps, where noise is not reduced be CMRR.   
   Many SE amps have triode output tubes which means the primary load is in   
   series with lower tube Ra and the B+ rail so that 2/3 to 4/5 of Vr is across   
   the OPT as JS mentions above. So if Vr across OPT primary = 0.5V, and OPT TR =   
   25:1, then Vr hum at sec =   
    20mV and 12dB GNFB might reduce it to 5mV but its far too much hum for hi-fi,   
   so the Vr at OPT MUST be much less, so there is no cheap solution and the   
   dopey cost avoiding hobbyist MUST let moths from his wallet.   
      
   RDH4 has other solutions of using twin T R&C networks for B+ hum reduction    
   but none apply to output stages with high Idc.    
   The other benefits of having a LARGE value C between OPT B+ connection and 0V   
   is to give a very low impedance anchor to the connection and this minimizes   
   IMD generation in tubes caused by having high amounts of LF appearing in   
   series with the higher F.    
   It applies especially to SE amps and to those working in Class AB - including   
   Quad-II, which works in AB with RLa-a only 4k0 when you have a load of 16r   
   connected to the OPT when strapped for 16r.    
   There is less hum when PP OP tubes work in class A. Nearly all PP amps made now   
   work in in class AB and owners insist on great bass at high levels and the use   
   of 470uF caps between OPT CT and 0V is very common. 35uF = 91r at 50Hz, 470uF   
   = 6.7r at 50Hz.   
   Many amps now proudly display the very large C value fancy brand electrolytics   
   above the chassis. But so often that alone does little to change their other   
   dreadful shortcomings with noise and other issues.    
         
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)