[AMPS] L7 Amp Questions

[Tom Rauch]

> 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. 

Grid fuses or resistors designed to fuse should be avoided. 

If there is a HV to grid fault, and the grid path opens, a fuse can 
allow the grid to rise to anode voltage. That allows the anode 
voltage to dump full force into the filament (cathode).

I'd simply ground the grids directly with very short leads. 

If you need arc protection, add a HV rated resistor of 10-20 ohms in series with the anode supply. 

> >advantages/disadvantages of doing it the Drake way? Any harm in just
> >grounding them directly? 

Grounding them directly is safer. Especially if you add a HV 
current limiting resistor.

> >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. 

Don't mix different brands, or new tubes with old tubes, and you'll 
be OK. At one time I did a test of "matched" tubes, and found they 
were not any better matched than random new tubes from the 
same manufacturer.


73, Tom W8JI
w8ji@contesting.com

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