Hi Byron, the 7mA is not grid current, even though it shows on the
grid current meter. I believe it rather is the bias current for the
zener diode that sets the 8877 resting cathode (zero drive) current.
Because the grid of the 8877 is grounded to the chassis, the grid
current flows from the cathode to chassis and from the chassis back
into the grid pin. Normally there is a low value resistor (10 ohms or
so) between the cathode and chassis, and the grid current flows
through that resistor. The voltage drop across that resistor is
proportional to the actual grid current and is what is displayed on
the grid current meter. The 7mA is a useful at-a-glance indication
that the bias circuit is working normally.
A couple of caveats about the circuit. Be sure you have no other
resistances between the cathode and chassis, except the one that
provides the path to the chassis for the grid current. If you have a
separate HV supply for the 8877, be careful you don’t have a low value
resistor in the power supply between B- and the chassis. Any such
resistor will be in parallel with the grid current measuring resistor
and will contribute to inaccurate meter readings and potential
amplifier instability. Second, be sure you don’t have back-to-back
safety diodes between the chassis and B-. Back-to-back diodes
sometimes crept into early designs of grounded grid amplifiers, but
that is a mistake. You should have one safety diode (e.g. a 2n5408)
with its anode (unbranded end)tied to the chassis and its other end to
B-, and that’s all. This diode comes into play if there is ever an HV
arc to the chassis, but otherwise does nothing.
73,
Jim w8zr
Sent from my iPhone
> On Jan 5, 2022, at 7:19 PM, Byron Tatum <bjtatum1@att.net> wrote:
>
> Hello- Possibly someone can shed some light on this for me. Previous years
> I built the K1AGB 144 Mhz 8877 stripline amplifier. I duplicated the QST
> article very closely and used the exact same metering and bias circuitry. The
> amplifier works really great with no complaints and is reliable. However,
> when the amplifier is keyed it always shows 7 mA of "resting" grid current,
> without any drive whatsoever. The idling cathode current is 110 mA. I used
> the amplifier to test quite a few 8877 type tubes over the years and this 7
> mA. of grid current showed up every time no matter which tube. I cannot
> recall if I keyed the amplifier without a tube to check grid current though.
> I am curious about how the metering resistor string, as shown on schematic
> of K1AGB amplifier, for the HV returns to chassis ground instead of the B
> minus. Whenever you float the B minus above chassis ground if you return
> anything (such as metering resistor string) to chassis ground will it show up
> as "grid current" due to grid metering scheme? I had two resistors going from
> B minus to chassis, the second one was for safety. I used double the
> resistance on each of them as compared to the single resistor shown, so that
> I had the same resistance total.Thanks,Byron W5FH
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