From: info@turneraudio.com.au   
      
    Others mentioned.....   
   > I checked that in Aspen Pitman's ' The Tube Amp Book, 4th Edition'.   
   > About half the amps listed showed some kind of choke in the HV section.   
   > But seldom tell us how many 'Heinrichs'. So difficult to tell how   
   > effective they are.   
      
   Typically 4H / 100R  for a medium powered fenderish or marshallesque PP amp.   
   Up to 15H for some rare amps, 2H or so for cheaper models (ironically most   
   are SE and really need good CLC filtering).   
      
   Indeed a read of the Book will show lots of anode supply chokes. But in   
   'export models' they are often left out so that the majority of guitar amps I   
   have serviced in Oz have no anode choke in a CLC type of B+ supply.     
   As Phil mercilessly points out, 50Hz hum in anode supplies is a largely minor   
   concern not much affected by B+ rectifiers and filtering methods. Usually, the   
   100Hz ripple F + other H are by far the higher quantity of ripple and most   
   prominent source of    
   hum in amplifiers.    
      
   SOME guitar amp makers woke up to the fact that having a well filtered screen   
   B+ supply was more effective at reducing hum than having a clean anode supply.   
      
   I have a Fender Deluxe here with a screen supply filter choke that is 1/5 the   
   weight of an anode choke to achieve the same amount of filtering.   
   Such miniature screen chokes with far less Idc could use very thin wire so   
   lots of turns could be put on a small core. Thus accountants chuckle with   
   delight over reduced costs of production and shipping weight.   
       
   Quad was early to realize the benefit of screen supply CLC filtering so they   
   had their Quad-II screens of KT66 well filtered with CLC but little filtering   
   to anode B+. But with class AB operation, the CMR action ceases to obstruct   
   the hum and the hum is    
   in series with the load on 1/2 the OPT at each wave crest so you'll see lots   
   of noise on wave crests as you come up tp clipping. Some guitar amp players   
   like this because it injects mains related GROWL to a tone being created.    
      
   But boutique guitar amps often have much better CLC anode filtering so GROWL   
   is not possible. In one, with 2 x EL34 output tubes, I recently was able to   
   add a 1/2 an additional  switch to allow B+ to be reduced from +500Vdc with   
   CLC for 50W+ AB to    
   operate as choke input, ie, LC filter, for +320Vdc and have other 1/2 switch   
   to adjust bias for higher Ia. The anode load stays constant but the tubes then   
   produce mainly all class A1 power = 22Watts. The owner was delighted. Is   
   remarkable how many    
   guitar amp users prefer a class A power amp which they say they prefer where   
   huge sound levels are not wanted.    
      
   The guitar amp involved had 100uF - 5H - 100uF, and the switch just moved the   
   C1 to be parallel to C2, so 5H - 200uF. 5H was actually a nice choke, not   
   designed by accountants.    
      
   However, I did have to add a 68r x 10W resistor in series with HT winding CT   
   and 0V to limit peak current from rectifier Si diodes and thus reduce working   
   B+ to +480Vdc. This is necessary because the working B+ in CLC high B+ mode   
   was +510Vdc, and just    
   above the 500Vdc rating for electrolytics. Just when are amp designers ever   
   going to realize it is bad practice to have B+ above capacitor Vdc ratings?   
   Don't hold your breath.   
      
   There was no need to pay any special attention to 50Hz mains hum.   
      
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
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