[Amps] GS35b FWB issue

[Jim Thomson]

Date: Mon, 11 Jul 2011 00:10:05 -0400
From: "David H Craig" <davidhcraig at verizon.net>
Subject: [Amps] GS35b  FWB issue


    I lost a diode string on an 8x4 string 1N5408 FWB on my single GS35b 6m HB amp.  To whit, 8 x 1N5408s per leg of a FWB for a total of 32 diodes. 

### with a 3 kv sec  +  245 vac  fed into the 240 primary,  ur  no load B+ =  aprx  3062 x 1.414 =   4330 vdc.     If you want to do this right, you want 12-13 x diodes  PER each of the 4 x legs..and use 6A10's.  If you look at any of these commercially made  FM broadcast  single phase B+  FWB  assy's... they all use  triple the no load B+  



 Presumably a lightening strike / blown lightening arrestor, but now I wonder.  

##  Install a heavy duty lightning arrestor  right at the 240 vac service panel....like the Joslyn series..which is an industry standard.  Consist of 2 x big mother   MOV's.  [ 130-140 vac rated..EACH].     Each MOV is wired from hot to NEUTRAL. The neutral is bonded to the grnd connection inside the main 200A panel.  A fuse/breaker is used  between each MOV and neutral.  That will solve abt 99% of any glitch's /transients/spikes/surges.   Then you are protected  from tranisents/spikes  coming down  EITHER  hot leg..or across both hot legs..which is why they use  2 x 130/150v rated move's  in series.... with the CT  going to neutral. 

## use KM1H's  suggestion , and put a .0047uf @ 10/15 kv disc cap  across each leg [ 4 required].  They are made by Ceramite. Both henry radio + Mouser carry them.   Put 1-2 more in parallel, between B+ on output of lytics and chassis.   Put a couple more at base of plate choke.   [ these are  4700 pf each] 

##  use step start, with a 10 sec timer. If commercial AC power goes off for any reason.. then comes back on   say 5-30 mins later, the B+ supply will simply  go through it's  normal step start sequence again. 

##  Wire a single HV fuse... between  SEC  of plate xfmr ..and input to FWB  assy.   This HV fuse can go in EITHER leg of the plate xfmr sec. 

##  Ur 12 x 450 vdc  @  800 uf caps are not overkill.  12 x 450 vdc =   5400 vdc.    Ur no load B+  [ with 245 vac fed into pri]   is 4330 vdc.    4330/5400 = .8018  or  80% of their max ratings.  This is hardly overkill.  I never run em more than 70-75% of their max rating.     800 uf /12 =  67 uf.    67 uf isn't over kill either.   [ I have tried 450 vdc caps  in 2300/2500/10,000 uf...and discovered the dynamic regulation is beyond superb, provide you have a decent size plate xfmr....and minimal  V drops  to main panel]

##   It appears you have used   6 x 27 K  resistor's  in series [ 162k]  across each HV cap.  That's  just asking for trbl!    You have 6 x 12 = 72  resistor's to mess with, and if any of the 72 ever opened up..then kaboom. 
    Use a SINGLE  100 K @ 3 watt mof   1%  Tol resistor across each cap.   [ VISHAY brand.  Mouser part # 71-CPF3-F-100K-T2
Mouser cat #  CPF-31003FB14      VISHAY  PN CPF3100K00FHB14   .    Use stakon  full circle type crimp connectors, and crimp a lug onto each end of each 100k resistor..then solder em as well.   These 100 K  3 watt mof resistors are superb, and no need  to ever play matchup ever again.  They are well under 1% tol.    Mine are all  just  .2% tol.      You will end up with a precise V  across each HV cap.  The combo stakon crimp tool/wire stripper is standard hardware store stuff. [ home depot/electrical supply, etc].  

## OK, now you require a 6A10  across  EACH  of the  12 x hv caps.     Each  6A10 is  RVS  connected across each cap.   Each diode requires another pair of stakon crimped and soldered connectors.   [ don't try and stuff the 100 k resistor + diode  into just one stakon].      Use an internal tooth lock-washer..and I assume  you are using  10-32 machine screws for each hv cap.    Then crank em down good and tight.   The RVS connected 6A10 diode  across each hv cap is a safety diode.    If any of the 4 x legs in ur  main FWB assy ever shorted out, you don't want to be pumping  raw AC  into the string of hv caps  on 1/2 cycles, and destroying the caps. 

##  Wire some bolt down /block type  250/277 vac MOVS  across the 240  primary of the plate xfmr itself.    Wire a  bolt down block type  130 vac rated MOV  between each 120 vac hot leg..and  neutral. [ 2 x required].  Mouser has loads of these bolt down type movs.   Typ  2" wide at the base x 2.5"  tall x  1/2" thick....with 2 x machine screws  on top for the electrical connections.   The base is bolted to the chassis/metal sidewall...  with 2 x machine screws.  



##  get rid of that  250 ohm  50 watt B-  float resistor.   Wire 6A10's  RVS connected, between  B-  and chassis....again using crimped sta-kon connectors, and solder em.     I use 2-4  diodes in parallel, to make one big diode assy.  2 x  such assy's  are used... so 8 x 6A10 diodes  used in total.   OK,  now ur B-  can never float  more than +/-  .7 vdc  from chassis  potential. 

##  we are not done yet.   Between  B+  and ur output  HV connector.... u need the 2nd HV fuse + glitch R .    From B+ on output  of string of caps, wire to HV fuse 1st.    Then  wire from output of   HV  fuse  to input of  a 50 ohm / 50-225watt wire wound glitch R.   You always want this 2nd HV fuse to precede the 50 ohm  glitch R.     50 ohm @  225 W  CCS  wire wounds are dirt cheap @ Mouser.   I have also used 2 x 100 ohm resistor's in parallel....and also  4 x 200 ohms in parallel. [ all in 225 W  CCS variety]    I have also used  4 x 200 ohm  globars  in parallel.  [ 1" diam x 12" long] .    If ur sucking 2-3 A of plate current, key down...even for just 10 x secs  to either tune an amp, or just to get some steady state plate / grid current readings.. u need the big stuff.    I also fan cool some of the glitch R assy's.  

##  I use a magnetic-hydraulic breaker in the 240 vac line.... where the 240 vac  1st enter's the HV supply box.    Use  a super fast one. The fast ones have NO oil in the internal resovoir  and are ... 'instant trip'.  

##  Wire a B+  meter  directly between  B+  and  B-  of your string of lytics...[ inside HV supply].         Wire a 2nd  B+ meter inside the RF deck, between B+   and  B-      NEVER wire any HV meter between  B+ and chassis.  You also have to bond the RF deck chassis and  HV supply chassis  directly together.... and not via a star grnd either.  Use  6-10 ga wire with big lugs on each end....and terminate each end  with   1/4-20  SS bolts  on both chassis's.   use  SS flat washer's / split ring lock washers  etc. 

##  OK, now you are done.  Everything is now FULLY protected. 

##  IF  you blow the HV AC  fuse between plate xfmr sec + FWB...  BOTH HV meters  will drop to zero.      IF you blow just the HV DC  fuse [ just b4 the 50 ohm glitch]... the HV meter in the RF deck will read zero.... but the HV meter inside the HV supply will STILL read    4330 vdc.     A  simple 0-500 ua dc meter +  10 x series  1 meg @ 2-3 watt mof's  in series  will suffice for a  0-5 kvdc meter.  A 0-1 ma with the same 10 x resistor's will  read 0-10 kvdc.  

##  IF any individual HV cap  shorted, nothing will happen..except ur max cap rating went down by 1 x cap.   IF you have a short across the sec of the plate xfmr, the  240 vac breaker on input to plate xfmr will trip open.   IF you have a short, or grnded B+   in the string of caps, the AC HV fuse [ between sec of plate xfmr +  FWB diode board]  will  blow open.  

##  IF  u get a short  from B+  inside  RF deck to chassis/metal... the  50 ohm glitch R will LIMIT the  fault current to just  4330/50 =  87 A.      The DC  HV  fuse just prior to the glitch R  will  blow open < 2 msecs.  You have now INTERRUPTED the fault current completely.    In that case, the fault current path is  from B+ to chassis..then from chassis  up through the 6A10 safety diodes, and back to B-     If 2-4 x 6A10 diodes are used in parallel, their combined surge rating is an easy 800-1600A.  They won't open up..since the  glitch R  limited the fault current to just 87 Amps.   

##  I'd use more than 2 seconds  for the step start delay.   I use  10-15 secs..so as to get the b+  up to it's  highest level, to minimize any sec surge.   Now this all depends on what size step start resistor u used, and also what  size hv caps. [ total uf]  etc.  

##  Now u are protected  from anything and everything.   If you or anybody else requires HV lytics, I have caseloads of em.  All  450 vdc  @  2300uf [2.5" OD  x 4"]....... and also 2500 uf  [2" OD  x 4"]   CD and also  united chemi con brands, all new, complete with 10-32 machine screws,etc.  I can also toss in the stakons and 6A10 diodes and also the 100 k  @ 3 W @ 1% tol resistor's if u require them.  

## The above scheme is  100%  bomb proof.    It's  what's  I  used on 5-15 kw out  HF  RF decks.    It's used on a 2 x GS35B  6m amp as well. [ 5.2 kw out]

later......... Jim   VE7RF



    I replaced the diodes on the blown string & tested again- worked fine with just HV trans running to the bridge.  Then, put back together w/ cap bank- blown fuse again & no solid HV.  Checked again, another leg of the FWB blown.  Replaced those 8 5408s & tried again with the cap bank inline. Ergo, another string was blown.  Replaced them, and then tested with just HV & the FWB.  No problem.  Then connected the cap bacn (overkill, 12 x 450V in series, nominal 800 MFD Mallories w/ 6 x 27k ohm Rs across each cap) WITHOUT the RF deck connected, and another audible glitch.  Tested all the 5408s in the bank, and 3 of the (just) replaced diodes show x Meg ohm in the WRONG direction (yes, I tested all before & after installation before poweup).  All the rest were fine.  Then, tested each cap in the 12 x 800 MFD bank, and discovered nothing (none shorted), but on a hunch, tried to tighten each screw, and low and behold, after 2+ years in service, EVERY screw needed 1/8 to 1/4 turn 
 to get tight.  Question:  Could the combined losening of the screws create enough resistance to draw enough current to blow a diode string in the FWB?  I have checked everything else in the PS, and the common denominator seems to be the cap bank.  Am I missing something?

    Without load at least, the high current relay works 100% & the HV soft-start relay is working 100% as is the 24 volt ps that works the relay.  The 2 second TDR that controls the soft-start is similarly fine.

    The 250ohm 50w B- float resistor is also fine (tested).

    Having isolated the problem to the HV supply, I am baffled.  I plan to replace the now-bad diodes, again, but am not sanguine it will work, and I am running out of 5408s...  I don't want to waste bigger diode blocks on this if they'll short too.

    Any advice or suggestions would be appreciated.

    73 Dave N3DB