5d326ada   
   From: info@turneraudio.com.au   
      
   Sorry I have not responded earlier, been busy, work, kids you know the   
   drill.   
     I got my c-cores from ebay, I know it was silly but they dont seem   
   bad. The guy I got them from dissapeared from ebay. He or they seem to   
   be back now as JLW tools. The cores arrived from China in five days to   
   the UK!!. Be carefull though if you want some they seem a bit   
   optimistic with the specs. I went for the next size up from what I   
   needed and Im glad I did. I know you dont need as much iron with c-   
   cores but the ones I have seem to be about the same size/weight as the   
   core of a 150VA torroid so im thinking they should be fine. They are   
   their "core 40" size. They came with bobbin tubes which I will use   
   with added side cheeks. The cast ally (if you can call it that) base   
   they supply is very poor. It could be cleaned up but I think it is too   
   rough to be worth the effort. The cores on the other hand seem OK.   
      I agree about the control tranny. I just have a few lying about as   
   Im an industrial electrician by trade. I might wind my own on torroids   
   after reading a few posts on winding them. Your use of variouse   
   trannys to get the correct voltage/power is a good way to get what you   
   need at a reasonable cost. I have been thinking of building a line   
   stage(eek) to drive some long cables. It will need plus and minus 100V   
   or so and 4 10VA 0-18-0-18V transformers work out perfect.   
                 Mat   
      
   Good luck with your OPT winding. If it all comes out right, you'll deserve a   
   medal because so few ppl are brave enough to discuss their OPT winding   
   activities here.    
      
   C-cores can be deceptive because when you see a pair of O's on the bench the   
   pair may look bigger than an equivalent core using E&I. But on close   
   examination, the Afe of the double C-cores can be much smaller than the Afe on   
   E&I lams. The rules about Afe    
   and turns must be maintained on any sort of core you use. Usually, the ratio   
   of window size to Afe for C-cores favors using more turns and thicker wire   
   than for E&I, thus making it possible to reduce winding resistances. The idea   
   is that most    
   transformers are for mains and the makers want the weight and winding losses   
   to be minimal while driving the iron to Bac = 1.4Tesla. This makes C-cores   
   compete with toroids. But with OPTs, The greater winding traverse width   
   slightly reduces leakage    
   inductance. The higher window height allows thicker P-S insulation which   
   reduces shunt C.    
   Using my OPT design methods should give you an OPT that's better and bigger   
   than most amp company accountants will allow into their products.   
      
   The other good thing about OPT C-cores is that they allow air gapping.   
   Maybe you think this strange because air gaps belong on SE amps only.    
   Well, the permeability, µ, of a pair of C-cores can be up to about 12,000.   
   This would be for fairly good GOSS material used. Coming from China, maybe   
   the  µ is unknown, and you'd have to measure it after assembling the tranny   
   and applying say 100Vac across primary at 50Hz and with 10r0 in series to   
   measure the primary coil    
   current, all with no loads connected.    
   The XLp reactance = Vac across coil / Current flow.    
   From that, the Lp = XLp / ( 6.28 x 50Hz )    
   Then you need to look at the inductance formula at my site and use that to   
   find what µ must be to give you the Lp you have measured. Now µ may anywhere   
   from about 3,000 to 12,000 depending on the C-core material used, which should   
   be cold rolled GOSS,    
   well glued up, and with cuts polished. I have measured 4,500 in one lot of   
   C-cores made in Oz which I bought in 1996. Obviously, the grade of GOSS was   
   lowish, but it did not matter one bit because those C-cores were used in a   
   pair of SE32 amps with 13E1,    
   and then the gap reduces µ to under 500.   
   However, for PP, having very high µ is of little benefit because any Idc   
   imbalance can cause core saturation with the net Idc flow in one direction.   
   The DC magnetization, Bdc, is proportional to turns x µ x Idc.    
   So, its best practice to keep the µ down to around 3,000, and if you have   
   followed all my rules on Fsat < 20Hz, then µ = 3,000 should give you plenty of   
   primary inductance. To adjust the µ downwards you may need a very thin air   
   gap, thinner than an    
   average sheet of paper which is 0.05mm. I have used some shopping bag which is   
   thinner, but just use a small bit for the gap, and leave enough space for   
   varnish to run in around edges of C-core cut surfaces. At F below 20Hz, the   
   core will act as an    
   inductance rather than a saturated reactance and stray VLF signals have less   
   chance of causing saturation, and recovery after overload is better and   
   without the severe effects of possible RF bursts generated in OP tubes when   
   faced with trying to drive a    
   signal into a short circuit, for part of the wave cycle.   
      
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)