[Amps] Drake L4B 50K Resistors HOT!!

[Jim Thomson]

From: Gudguyham at aol.com 
Sent: Monday, July 23, 2012 5:00 AM
To: jim.thom at telus.net ; amps at contesting.com 
Subject: Re: [Amps] Drake L4B 50K Resistors HOT!!



In a message dated 7/23/2012 5:53:55 A.M. Eastern Daylight Time, jim.thom at telus.net writes:
  was just repairing a Drake L4B that I picked up from my want listing on QTH.com. I just happened to measure the temperature of the 50K 50 watt resistors in the power supply with my infrared thermometer and WOW!! 

  The middle of the resistors run 510 degrees f !! I could not believe it!

  Needless to say there outta here! I won?t need them anyway. I?m using the amp to prototype my QSK board for the Drake L4-L7 amplifiers that will use resistor cutoff bias instead of the +120 volts that the resistors provide.

  73
  Jim W7RY

OH SO TRUE!!!!!!!  it cracks me up.  Swan built the Mark I and then the Mark II.  Then they revised the
  Mark II.   Heathkit built the Warrior and then the SB series came in.  When Collins decided to raise the grids above ground with caps and use masses amounts of cut off bias voltage the other companies followed suit with raised grids above ground and high cut off voltage.  The Swan Mark II revision (second edition) had the raised grids and higher voltage cut off . They completely forgot or second guessed what they learned from their previous amps.  Heathkit followed suit with the SB-200 and 220 with raised grids and masses amounts of cut off bias.  Drake came on the scene at the time with their L4 and later L4B  they started right off the bat with that nonsense.  Now poor Jim (and the rest of us should) go in their and do what Jim has done.  The problem is when you do that it marries the power supply to that amp!  It seems as though Collins had a stability problems with the 30L1 without enough cut off bias the 4 811A tubes could take off so once you plaster 120vdc on them they aint goin' anywhere in cutoff.  The Yeasu FL-2100B has same problem so in the Z model they put in a V tripler so add more V for cut off.  Even when Viewstar caught wind of this they did it also.  It seemed to be the status quo  to follow what Collins did because he was trying to stabilize the 30L1 which the 3-500 tube amps didn't need.  Oh, not to Mention the Kenwood TL-922 did the same thing as well.  The only one out there that kept the faith was Denny.  he didn't fall for that nonsense he just had a different set of problems.

###  I forgot to mention that I also tossed ALL the grid to chassis caps  and ALL the grid to chassis chokes.  Replaced with wide CU strap.   Drive requirements dropped  20-25 watts in all cases, and all bands. 
Then the xcvr po is reduced by 20-25 watts pep.   Then the xcvr’s   IMD is a lot less .   Cleaner signal into the amp means a cleaner, lower imd output from the amp. 
If u want a grid fuse,  simply wire a 3agc fuse holder on the rear apron..and wire the FAST grid fuse between  chassis  and lower end of grid shunt.  If grid fuse pops open, no path  for dc grid current, and amp shuts down.
No PO... and  input ser rises to infinity.   Xcvr then shuts itself off.   Use the vacated spaces  of the pair of oem  50 k 50 w  ww..and install a 50 ohm @ 50 watt  WW..and that is ur new glitch R.  install a HV fuse in series with the
glitch R... and now its fully protected.   Whoa.... the drake PS already uses a .82 ohm  tiny resistor as a HV fuse in the B+ lead..and it will work. 

later... Jim   VE7RF