The PS transformer is wound serially so that the two sides of the winding are   
   not the same resistance.   
      
   I did the work on this all of 20 years ago. The results got published in Glass   
   Audio Volume 10, Number 3, 1998 & Electronics World (June 1998). And posted on   
   RAT, several times over several years. I’m surprised you are unaware of it.   
   The resulting    
   problem applies only to SET amplifiers, although PS hum sidebands do show up   
   on the output off PP amps. Measured & posted that too.   
      
   The simple, low cost fix reduces the power frequency (60 Hz)hum component as   
   much as 20 db. A simpler fix reverses the HV leads to the rectifier. That way   
   for a filamentary rectifier such as the common 5U4G one of the HV wdg will be   
   in series aiding    
   while the other is series opposing to the 5V across the cathode surface. That   
   is enough to yield a 15 db reduction in the power frequency component. And   
   costs us nothing. Half of the amps built probably have those leads hooked up   
   wrong way around.   
      
   Better measure some of your power transformer HV windings carefully. That is   
   easy these daze with a digital meter. If you find any where the halves are the   
   secondary are the same resistance, post the p/n & manufacturer here. Any   
   I’ve ever looked at    
   were in all cases different resistance.   
   Most of the schematics show resistance for the halves of the secondary to be   
   the same. But they are not. Quite a few of the old radio schematics out of the   
   30’s show the actual HV resistances. For example, the Philco 37-93 is given   
   as 230R & 250R.   
      
          I'm not sure how having a tube rectifier there, vs solid state, would   
   in    
   any way improve the hum factor.   
      
   But there is a difference. The forward resistance of a SS diode is much less   
   than a Toob. So the charging currents are larger, if all other things are left   
   the same. The there is more hum resulting. Much of the work on that was done   
   in the 30s by Terman &   
    others, all in his textbooks. And copied into the Radiotron Designers   
   Handbook (RDH4), commonly used by many of us old guys. All easy reading.   
      
         As for the PS transformer, they were obviously being cheap if that   
   happens.   
      
   Not cheap at all, simply makes good business sense. Over many years in sales   
   with HP & R&S I had quite a bit of contact with Hammond. One comment I recall   
   had to do with the relative cost of a tap vs. starting another winding. By the   
   time that extra step    
   filters thru to the end user it could be significant. Sales volume depends on   
   reasonable cost as well as perfection.   
      
        one appeal of tube vs solid state rectifiers is the 'punch' effect you    
   get with a guitar amp.  At high load you get an 'attack/decay' amplitude    
   effect because the capacitors discharge for a short time, giving you    
   slightly higher power levels than the power supply can deliver with a    
   sustained signal.  This is due to internal resistance of the rectifier    
   tube.  But if the behavior is non-linear, having a series resistor on a    
   diode won't give you the same effect...    
      
          Anyway, they should be using LC filters after the diode to minimize   
   hum,    
   and a properly balanced power transformer secondary winding.  That's how    
   the old Hi Fi systems were done.  Just boost secondary voltage a bit to    
   compensate...    
      
          /me would consider adding a series voltage regulator using a transistor    
   and zeners and solid state rectifiers and a current limiting circuit, to    
   give you a nice clean primary power supply that doesn't 'surge' the    
   capacitors when it starts up.     
      
   SS circuitry makes life easy in many ways, so SS regulation is a possibility.   
   Not so much when Williamson & the others got going.   
      
        A 25-50V drop would be sufficient, not    
   unlike what you get with a tube rectifier anyway.    
      
         but yeah, that's not very 'purist' of me.  I just think the pre-amp and    
   output tubes are what matters for hifi tube audio, or guitar amp audio    
   for that matter.  The power supply [other than the 'punch' effect] is    
   less important, but could be simulated if needed, using a derivative of    
   a current limiter circuit.    
      
   The guitar pickers & other electronic musicians have much greater problems to   
   solve than PS hum. Ground loops can be a killer. It is somewhat of a surprise   
   that we don’t see a lot more balanced systems as used in broadcast where 600   
   ohms transformers    
   are used. There certainly have the audio bandwidth required. But here again   
   cost is a killer.   
      
   This note is not meant to flame you or anyone else. Just some information   
   where I’ve had a careful look at a potential problem & found a reasonable   
   way out!   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)