From: analogdino@rogers.com   
      
   On Saturday, 29 September 2012 18:32:57 UTC-4, Analogdino  wrote:   
   > Hi, Vacuumlanders,   
   >    
   > I'm seeking advice and opinion on a vintage Heathkit audio amplifier – the   
   W7-M.   
    (snip)    
   > Cheers,>    
   > Roger   
      
   Hi, Flipper, Doug John and Patrick,   
   Many thanks for the most useful replies. I've made some progress to date but   
   first some replies to the comments made:   
      
   From Flipper: "The UL taps should be enough to tell you it isn't a 'P.A.' amp.   
   It's the most powerful of their 'W', so called, 'Williamson' amp line except   
   the W7-M is obviously not a Williamson circuit while the others, W3-M. W3A-M,   
   and W5-M, are."   
   Me: Agree.  I identified it loosely as a PA amp due to the 250 ohm o/p option   
   but the UL format implies it was meant to be Hi-Fi .  Not "Williamson", of   
   course, as no driver stage.  Odd that they used the "W" designation...  IMO,   
   the W4 and W5 are    
   clearly higher quality amplifiers (I don't know the W3 but did refurbish a   
   W4-AM.)  Your points on the 6L6 sub are taken... but see below! Also, 3,500   
   ohms P-P is a given... I'm not about to but a new OPT!   
       
   From John: (re. my 6L6 substitution.)   
   Me: Right!  I put in two "somewhat tired" EL34's from my Leak TL/25 and, with   
   a bit of bias twiddling, got 49 watts RMS at clipping, but still with my   
   kluged transformer drive to EL34 grids.  Getting there!  Now looking for two   
   "good+" EL34's.  I sense    
   that this amplifier runs them hard. I plan to try a pair of 5881's that I have   
   (but they're not matched)... not done yet.   
       
   From Doug: "Your 6L6s have approximately half the gm of the EL34s and will   
   consequently require about twice the driving voltage for the same plate   
   current... It would be a good idea to check for problems in the 6AN8 stage...    
   You should check the primary    
   inductance of the OPT.     
   Me: This suggests I need to add a driver stage (6SN7 or 12AU7) and redesign   
   the whole front end to have a true Williamson, i.e. my option 4 in the   
   original post.  I'd expect to end up with  a very good 25 watt amplifier to   
   rotate into my vinyl tube    
   stereo system... have to reliable it "Ex W7-M".  OPT is a given but I'll check   
   for shorts (I'm pretty sure it's OK.)   
      
   From Patrick: "You should find that at some low value RLa-a you will get 55   
   Watts with fixed bias and class AB1. The PO amounts you quote would be high %   
   class A1 but do the load line analysis and all would be revealed... The   
   Heathkit input driver stage    
   looks like an RCA or Dynaco schematic with bean counter minimalist use of two   
   tubes, pentode input, triode CPI. "   
   Me: Thanks for the excellent analysis and advice.  I'm stuck with the 3500   
   ohms P-P OPT, but I'll likely still give around 25 watts RMS from "average"   
   6L6's (have several!) and, see above, close to 50 watts RMS with good EL34's   
   (none to hand!) Re. 6AN8:    
   IMO, likely not worth fiddling with... redesign stages.   
       
   So, briefly, to date, I plan to:   
   1. Find some good EL34's, or settle for a decent 25 watter with 6L6's     
   2. Rebuild front-end to Williamson all-triode architecture. Sadly, the W7 is   
   not a "valuable collectible" like the W5, or even my refurbished W4, so it's   
   not a big loss to the world! Input sensitivity then not a problem and VLF   
   stability is easily fixed (   
   see the thread on this from a while back.)     
   3. It will run with a WA-P2 pre-amp (as does my home-brew "near Williamson"   
   and my W4.)   
      
   Again, thanks to all for the thoughtful replies... we are not alone in our   
   challenges!   
   Cheers,   
   Roger   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)