I need some opinions from you experts. I have been modifying a Drake
L7. I have replaced the HV caps, removed old diodes/xistors and
installed meter protection diodes per Mr. Measures's instructions.
Before I went any further I decided to put some hours on the mods. I
also ordered and received new 3-500s. I wanted to test it out before
putting the new tubes in place. I had removed the parasitic suppressors
and then removed and measured the resistors to see if there had been any
change, there had not been any change. I put old tubes in place and put
the parasitic suppressors on, sans xistors. I just wanted to light it
up for a moment and watch for smoke. No smoke but holly runaway
oscillator Batman. I had my finger on the AC line safety switch and
released it before the plate meter movement was destroyed. Seriously,
the plate meter began to climb rapidly so I removed power. I repeated
this 2 more times, at different plate and tune positions as well as
different band selection(thinking the plate choke and found a freq it
liked), switching the power off at about 800 mills. I replaced the
xistors (in the parasitic suppressors) and all looked OK, no more
runaway. I never detected anything smoking or overheating. It looked
stable so I decided to install the new tubes. I did so and then let
them burn in/stabilize what ever, for 4 hrs. I removed the new tubes
and installed the old tubes for a test. Everything appeared to work as
before, in other words OK. I tried the unit on different frequencies so
confirm proper operation. When I went to 21 mc the plate refused to
dip. So I then tried it on frequencies it worked on previously and now
there was no dip there either. I didn't hear or see anything to
indicate a part had faulted. I have ordered some caps to replace the
original DC plate blocking caps. Because when I drive the unit I get
plenty of plate current and grid current but VERY low output.
2 questions
1. Am I on the right track here????
2. As I remember from past experiences building amps (20+yrs ago)
the value of the dc blocking caps did not influence the tank circuit.
Read here: Can't find the same value caps so I am substituting a
reasonable value (according to me anyway and I am omnipotent I hope you
realize).
Mike k8ze
K8ze@comcast.net
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