From: themend@gmail.com   
      
   Hi All,   
      
      
   This question pertains to the Sherwood S-5000 restoration I'm doing.  I am   
   interested in getting to the bottom of a power supply resonance I believe may   
   lie in the stock S-5000 power supply.  I've been noticing lately, and have   
   noticed in the past the    
   tendency for this amp to make the speakers flub a bit at an extremely low   
   (1-2hz) frequency when there is a transient with a large attack, and resulting   
   large instantaneous current draw.  I had first thought that it was present   
   only when the amp was    
   driven near its clipping point, but have since noticed it even at normal   
   levels, though it is much harder to notice then, requiring you to put your   
   finger on the cone.  I even recall in my dark memory of other S-5000s I've had   
   that one amp was even self-   
   oscillating at idle at this same approximate frequency when the original power   
   supply caps got really weak.  So it appears to be a bit of a design flaw in   
   general with at least the second version S-5000.  I'm not sure if I've seen   
   the first one do this    
   or not, but I am going to find that out shortly anyways!   
      
   First of all, I am still in need of a new scope since my 535a found a new   
   home, and I am unable at the moment to snoop beyond the resolution of a VTVM.    
   So I have to resort to a more primitive mode of diagnosis at the moment.   
      
   However, there are several reasons why I think it may be a power supply   
   resonance, and please feel free to shoot me down if you think I'm on the wrong   
   track.  First, the oscillation occurs in both channels with the same intensity   
   and always in phase.  I    
   have yet to see this occur in one channel and not the other.  If I set my   
   balance all the way to the right, I still see the oscillation in the (silent)   
   left side.  So all of that tells me that plate voltage is oscillating when   
   transients hit, regardless    
   of channel overload condition.   
      
   Since tube rectifiers have a much higher impedance than silicon diodes, I'm   
   wondering what can be done to eliminate or greatly damp this resonance without   
   throwing in the towel and re-designing the whole power supply for solid   
   state.  It's not like there'   
   s a ton of room in this amp to add a choke and a bunch of big caps, though I'm   
   sure I could fit in a few well chosen components if need be.  It's pretty   
   clear that this amplifier is capable of doing even more than it is now, and it   
   sounds pretty damn    
   good as it is.  But having the HV+ sag enough to actually couple through the   
   output iron at ~1hz is both a testament to this iron and an indictment of tube   
   power supplies, at least this one.   
      
   So, here's the circuit in a nutshell, stock and as it is now.  I should   
   mention that while a couple cap sections have increased in value now, the   
   oscillation hasn't really decreased at all.  No surprise, the resistors and   
   limited value for the first C    
   are probably to blame.   
      
   Stock:   
      
   From the 5AR4, first 20uf, then 33 ohms, 40uf, 4.7K, 40uf, 10K, 30uf, 56K, 20uf   
      
   The plate supply comes from the junction of the 33ohm resistor and the 40uf   
   section.   
      
   Can anyone here suggest some reading material or point me in the direction of   
   how to approach the math here?  I guess I really need to post some current   
   consumption readings, I would guess there's probably in the neighborhood of   
   150ma total, with 88ma or    
   so for the output plate current, but I really ought to measure that...  I'll   
   see if I can come up with idle current readings for each point on the power   
   rail maybe that will start me in the right direction.   
      
   Anyhow, please let me know if you think I'm barking up the wrong tree here or   
   what.  Obviously frequencies that low are damn near DC as far as speakers go,   
   and are certainly not good for the coil, let alone the surrounds which enjoy   
   quite a bit too much    
   excursion for no real reason.  I think this amp is really worth getting this   
   sorted out, it's already the best sounding vintage 7189 amp I've ever come   
   across (Scott 299 not even close).  I once built a block amp using OPTs from a   
   junked S-5000, and even    
   though I didn't bother to really tune the driver circuits, the amp proved that   
   the output iron can really handle low frequency extension even with a paltry   
   10:1 damping factor.   
      
   BTW- It's amazing how little the LF oscillation and extreme speaker excursion   
   is actually audible at all, you only really hear it as you near max output.    
   Still I think a few more clean db might be obtained by fixing this.   
      
      
   Cheers and thanks,   
      
   -forkinthesocket   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)