Hi Charlie,
Let me point out one very serious flaw in the 220, along with some history.
Since that is a common amp, and similar circuits are used in the TL-922 and
other amps, and are also suggested in articles related to "improving"
amplifiers, it may be a problem of general interest.
I was a contract RF engineer for Heath, so I speak with direct experience on
the 220.
In a message dated 97-02-12 11:44:06 EST, you write:
>
>A thorough visual inspection will be performed next. Actually, I started to
>do that last night, but then noticed that my TS-930 had decided to stop
working.
Woops, the 930 may have taken a hit from the 220. Here's how it could easily
happen.
The grids of the 220 are not grounded directly to the chassis, they connect
to the chassis through RF chokes and bypass caps. This is just plain stupid.
Besides the degradation in RF performance this causes, it also sets you up
for exciter disaster if you have a gassy tube, or a anode to grid fault.
When the grid floats, through any impedance above a direct short, if the tube
flases over or shorts to the grid the grid quickly elevates to the full
amount of the high voltage. This either turns the tube on hard, or causes the
grid to arc to the filament. The cathode (filament) then becomes elevated to
full high voltage.
This generally blows the zener, the cathode meter, and couples high voltage
out through the input circuit path to the relay where it flashes over the
contacts and much to the users delight smacks the expensive rig with a few kV
of pulse for a few milliseconds.
The grids in any GG PA belong grounded, don't let ANYONE tell you otherwise.
Unless the grids are solidly bonded to the chassis through low impedance
direct connections, you have little hope of protecting your rig from damage
due to a sharp pulse if the tubes ever arc.
73 Tom
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