Hi Mike,
> I have inherited an old homebrew 4 x 811A amp which I am
> trying to get back into working shape (my friend has dubbed
> it "The Widowmaker" as it had +400V on the case of the power
> supply when I first turned it on - high impedance thank God).
The case and chassis must not be grounded to the safety wire of
the outlet. Bad news!
Some amplifiers also use the chassis to return the 120 v common.
Bad news.
> I was wondering what the "preferred" method for biasing would
> be?
Ground the grids, open the filament center tap. It will float up to cut-
off bias all on its own. Add a 100 k cutoff bias resistor if you have
no other leakage path from center tap to chassis.
> grids are RF bypassed to ground via ceramic disc capacitors (220pf
> I think). All four grids are tied together via four 47 ohm resistors
> (one to each grid). This common point is fed to the antenna
> relay thru an RF choke. During standby, the RF choke is
> switched to a 170 volt supply which presumeably cuts off
> the tubes from conduction. When the antenna relay switches
> from standby to operate, this node is disconnected from the
> cutoff bias, and shunted to ground thru the grid current
> metering circuit. This scheme presumeably provides a negative
> "self bias" proportional to the envelope of the drive signal.
I don't think that was the intention, but that circuit sounds like
trouble!
> My question is, what class of operation does this sort of
> biasing scheme produce with 811A's? Is it linear enough
> for SSB service? Is there a better alternative?
Calculate the reactance of the grid capacitors on 1.8 MHz.
Calculate it on 30 MHz. Now consider the grid to filament
impedance through the RF cycle, and look at grid voltage
considering the capacitor in parallel with any resistance.
You'll see the grids not only move around a lot, they move around
not-in-phase on all bands. You have somewhere between in-phase
and out-of-phase feedback that varies with power level and
frequency.
Not the best idea anyone ever came up with!
Ground the grids, or at least bypass them thoroughly and get rid of
that nonsense. Use a conventional metering system.
73, Tom W8JI
w8ji@contesting.com
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