[Amps] [AMPS] Drake L-4 negative grid current reading on meter

[Ian White GM3SEK]

k7fm wrote:
>The Drake L4 uses a small resistor (I recall it is .82 rather than the state
>.082 ohm) in the plate lead of the final tubes.  It is my opinion that such
>a resistor is a bad idea.  Eimac mentions that surge resistor is appropriate
>in the plate - but they want it large enough so that it will handle the
>surge without failure.
>I am thinking something like 20 ohm at 50 watts.
>

The problem with that .82 ohm resistor is the amount of destructive 
energy that it lets through into the tube, while the resistor is taking 
its own sweet time to blow.

>The problem with that particular 2 watt resistor is:
>
>1.  When it "blows" it could easily arc across and therfore not act as a
>fuse.

Quite so. (Look at the construction of a genuine, purpose-designed HV 
fuse. Does it look anything like a 2W resistor?)

But the same problem applies to HV fuses as well. We're accustomed to 
thinking about fuses as protective devices, but we forget that they 
provide NO CURRENT LIMITING during the time they are taking to blow. 
Even a "fast" fuse requires some finite time to blow.  During that time, 
all of the energy that the fuse lets through will be dumped into the 
tube or other components.

Minimizing this "let-through" energy is what Eimac are concerned about 
[1]. They recommend the surge resistor because it acts faster than any 
possible fuse. The resistor limits the current instantaneously, and buys 
some time for a fuse or breaker to follow on.

The surge resistor and the follow-on fuse/breaker are the simplest 
possible strategy to avoid damage... but they work together, so you need 
both of them.

If you do have the HV surge resistor, then I don't believe it's 
necessary to use a HV fuse. A fast fuse/breaker in the HV transformer 
primary will do fine.


>2.  If it does blow, without any other circuit protection, the consequence
>is that the operator will drive the grids with 100 watts and no plate
>voltage, and the grids tubes will be destroyed because of excess power being
>dissipated by the grids.
>
That is a separate problem. At the very least, the amplifier needs a 
simple interlock, to take it off-line if it doesn't have HV.


[1] See pages 65-70 in 'Care and Feeding of Power Grid Tubes':
  http://www.cpii.com/docs/related/22/C&F3Web.pdf

(C&F also refers to Eimac Application Bulletin 17, which seems to have 
disappeared from the CPI/Eimac website but can be found here: 
http://tinyurl.com/2etqak )



>Happy Thanksgiving,

And the same to all of us... whatever we have to be thankful for.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek