I've had numerous L4/L4B amps over my 47 years as a ham and fixed about
just as many. Still have 3 of them here in different stages of "projects".
One thing I have always disliked is the heat from those 50K resistors. I
can fry an egg on the top of my power supplies. That is a lot of wasted
energy and heat. It is simply a poor design. Of course the proper thing to
do is to switch to self bias and then eliminate the 50K resistors
altogether. Then put some simple 100K 3 watt resistors across each cap for
bleeders. Now this idea is just fine, but on occasion these power supplies
get
separated from the respective RF decks and now renders the power useless to
be used with other amps. I don't like the idea of marrying a power supply
to the amp itself. There is a better way. For about $10.00 in parts on
can make a very small power supply that can fit under the chassis of the
power supply. Using a tripler circuit one could send up a bit over 40vdc on
the normal wire carrying the 120vdc from the divider. It is very easy to find
a simple dual primary 12 or 24v mini transformer. I've seen several
candidates on EBAY already today. Simply place the primary leads on the
terminal strip up top and when one changes the amp from 120/240 it will also
change the transformer below. The 40vdc from the tripler is more than ample
to
cut off the tubes in standby. Using this method, it allows the power
supply to be used with any amplifier. I would suggest at the same time to
upgrade the power supply with a Harbach upgrade kit which is a very worthwhile
upgrade. Unfortunately that does not address the heat from the 50K
resistors. Many hams do their own power supply upgrades with new caps and 3A
diodes and use dead bug type construction. Either way a total of the 2
upgrades brings your power supply into the 21st century. If you want more
info
about this mod email me. Lou
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