>Subject: Re: [AMPS] General amp questions
>Sent: 3/25/1997 5:53 AM
>Received: 3/25/1997 6:21 AM
>From: Bill Turner, wrt@eskimo.com
>To: Rich Saik, RICH@mail.azhsv.com
>CC: Amps@contesting.com
>
>The best way to fire up an amplifier (or anything else with unknown
>problems) is to use a Variac and bring up the voltage slowly while
>monitoring the current drawn. Saves a lot of fuses.
>
>73, Bill W7TI
>wrt@eskimo.com
Good advice, Bill --provided the tube or tubes are removed--.......and
for those who don't have a metered variac, a pair of 300w-500w 120v
incandescent lamps will limit the current to a safe amount. In most
amplifiers, you can remove the mains fuses and cliplead the lamps across
the fuseholders. However, check the diagram to see if this is prudent.
In an SB-220, the two 10a circuit-breakers are opened and one lamp is
connected across each open circuit breaker. In a TL-922, pull the two
15a fuses and connect one lamp across each empty fuseholder.
When the power supplies are free from troubles, the HV should come up to
about 90% of normal and the lamps should not glow. Next step: remove the
current-limiting lamps and you should get 100% of the HV. Plug in the
bottles. However, if you are working on an amplifier that uses
voltage-cutoff bias, beware. If a tube has a grid to filament short, and
there is no current limiting resistor between the cutoff bias v supply
and the CT of the filament transformer, the bias v supply will be
operating into a short circuit. A load short can destroy the (unfused)
smaller transformer in a TL-922 or SB-220 in short order. To avoid this
problem, rewire the amplifier for resistor cutoff bias instead of voltage
cutoff bias--or put a 10k-ohm 2w R between the cutoff v supply and the
filament CT.
R. L. Measures, [805]386-3734
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