In a message dated 7/23/2012 5:53:55 A.M. Eastern Daylight Time,
jim.thom@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.
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