[Amps] (no subject)

Rich 2@vc.net
Thu, 26 Dec 2002 20:22:28 -0800


> 
>
>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. 

This usually indicates a push-pull oscillation c. 55MHz.  There is no 
big-bang and no destruction like a push-push oscillation.

> 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.  

Is the 21MHz bandswitch position making contact  with the L ?

>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.  

When a DC blocking cap goes kaput, it shorts.  

>Because when I drive the unit I get
>plenty of plate current and grid current but VERY low output.  
>
If the max P-out occurs at either end of the C scale, the bandswitch 
probably has toasted contacts.
>
>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.

True.  200pF to 10,0000pF makes virtually no difference.

>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).  


-  Rich..., 805.386.3734, www.vcnet.com/measures.