> current tube is GU84B, but problem existed with 4cx1500b
> 2300 vdc on plate under load
> screen 400
> grid -60
> It does it with cathode directly tied to ground
>
> ::Then what was the "cathode circuit" referred to in the earlier post??
>
>
> I can measure grid E and I, screen E and I with built in amp metering
>
>
> ::How is the E and I measured? The comment in the earlier post about
> "grid microamps" made no sense, as grid microamps is too small a quantity
> to affect the tube, whose grid current would normally be in the milliamps.
>
>
> input circuit rf in to a .01 cap tied to 50ohm resistor, and bypassed
> on the
> cold end.
>
> Grid bias connected to cold end of 50 ohm resistor. 50 ohm
> resistor and
>
> ::So, only 60 Ohms total is isolating the bias supply from the grid of the
> tube? That's it? This is unconventional, for sure. For a grid-driven
> tetrode, I normally use 100-200 Ohm carbon R, then an RF choke or string
> of ferrite beads, then a wideband bypass cap (like a button mica) to keep
> RF out of the bias supply. If all you have between the bias supply and
> the tube grid is 60 Ohms of R, I'm surprised the bias supply isn't going
> bonkers. In fact, I'm kind of surprised you don't have a 1500W
> oscillator, since the bias wiring is carrying RF around the chassis...
>
>
> .01 cap tied to 10 ohm resistor to control grid pin on socket. Short lead
> length.
>
> ::I don't get the ".01 cap tied to 10 Ohm resistor..." part. Where's that
> cap? Are you sure you haven't created a(n undesired) grid tank circuit
> with all these .01 uF caps?
>
> ::-WB2WIK/6
>
> yes, I am feeding the grid
>
> Lynn
>
>
> Steve Katz wrote:
>
> > > I am working on an older homebrew amp with a tetrode.
> > >
> > >
> > > ::What's the tetrode? What are the operating parameters (voltages
> applied
> > > to all tube elements)?
> > >
> > >
> > >
> > >
> > >
> > > The grid current is normal on 160, 80, and 40. As you begin to
> > > approach escessive
> > > drive, the grid current rises to 20 or 30 microamps. The grid current
> > > meter measures from 0 to 100 ua.
> > >
> > >
> > > ::Microamps? Microamps? Regardless of the meter scale, what's this
> thing
> > > really indicating?
> > >
> > > Screen current does not flow until the tube output increases -
> > > voltage is steady.
> > >
> > > ::What is the screen voltage? How do you know it doesn't change
> (under
> > > load)?
> > >
> > > Grid voltage does not change.
> > >
> > > ::What is the grid voltage? How do you know it doesn't change (under
> > > load)?
> > >
> > > I have tried several configurations of input circuits and they all
> > > act the same.
> > > Things that have been replaced or substituted.
> > > tube socket
> > > tube
> > > input circuit (but not the 50 ohm load resistor)
> > >
> > > ::What's the circuit? Where's the 50 Ohm load resistor?
> > >
> > > cathode circuit components
> > >
> > > ::What are they?
> > >
> > > I am somewhat suspicious of the tank circuit but unable to
> verify.
> > >
> > > I think fixing the grid current issue will help get to the root
> of
> > > the problem.
> > >
> > > ::Sounds like you're driving the grid, but then you talk about cathode
> > > circuit components. What is this circuit? Sounds like something
> > > "charging up" on the input side of the amplifier.
> > >
> > > ::WB2WIK/6
> > >
> > > Any ideas?
> > >
> > > Lynn, NO9Z
> > >
> > >
> > >
> > > _______________________________________________
> > > Amps mailing list
> > > Amps@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
|