>
>I recently got an L7 Amp with a bad 3-500Z. The tube plate had shifted and
>shorted to the grid. This caused the RF choke that Drake uses in the grid
>to ground circuit to go up in smoke.
>
>Does anyone know the value of the RF choke used and where they can be
>obtained?
As you know, the choke value is not shown on the diagram. My guess is
roughly 1mH. I would replace the chokes with fuses or fusing resistors.
Add about 1000pF of grid to ground bypass C to protect the fusing on 10m.
>I would also like to know where I might obtain a parts list for
>the amp since the manual that I got with the amp does not have one.
>
>Another question is on Drake's method of grounding the grids using RF
>chokes and shunt capacitors to ground. The ampss that I've seen in the
>Radio Handbook ground the grids directly. What's the
>advantages/disadvantages of doing it the Drake way? Any harm in just
>grounding them directly?
>
grounding the grids directly eliminates the possibility of grid fusing.
>And yet another question. I believe that one of the tubes is OK so I really
>only need one tube for the amp. The write ups I've seen on using two of
>these tubes says they must be matched.
The main thing is don't mix a Mu=200 tube with a Mu=130 tube. RF Parts
sells the latter.
>How closely do they need to be
>matched?
>Does one just check for idling current and verifies that each tube
>draws the same current +/- some percentage, does the grid current have to
>match within some amount, or what?
>
If they show similar amounts of redness, they are matched close enough.
later, Harry
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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