B+ Precautions

From mgarvi–(at)–anix.com Sun Jan 11 00:56:13 CST 1998
From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Working on amps
Date: 10 Jan 1998 13:46:33 -0500
Xref: geraldo.cc.utexas.edu alt.guitar.amps:79767

>> How >can I drain the electricity?

> strat199–(at)–ol.com (Strat1995) writes:
>You can drain the voltage from the caps by connecting a resistor from one of
>the hot lugs on the cap to ground. I use a 100k.

When power tubes are in place and heated up, they will usually drain
most of the B+ quickly when the amp is powered down. This relates to
output tubes only…preamp stages will not discharge the ps caps
efficiently. If you can get the voltage down to reasonable levels
(< 80v or so), you'll still get a nice jolt, but it won't kill you.
This is still no guarantee.

Unheated output tubes won’t discharge a supply: if you flip an
amp’s power on and off quickly, the supply will stay hot. If
you leave the standby in ‘off’ position, the output tube plates
probably won’t be connected to the first cap in the supply, so
count on that cap being hot. NOTE: cap ‘inrush’ current thru
the standby switch is not good, so the cap is usually before
the switch.

Some amps use two lower voltage caps in series to get the higher
voltage rating for the first supply cap. In this case, there should
be resistors (Fender uses 220k’s) for balancing the voltage across
the two caps. Those resistors will help to discharge the caps, but
the time constant will be relatively long.

Cap voltage decays exponentially thru a resistor, so the first
‘half’ of the B+ will be discharged rel. quickly. This is good.

If I know I will be working on an amp a lot, or if I’m pulling
output tubes to tweak output stages, I often solder a resistor
in permanently. This can be 100k to 200k or so, but figure
out the req’d wattage. B+ squared / ohms = watts. And BE
CONSERVATIVE about the wattage. Measure B+ with the standby in
the ‘off’ position: If you pull the output tubes or don’t flip
the standby, the B+ voltage will increase quite a bit, so
overrate that resistor. 500v across a 250K resistor is 1 watt,
so use a 2 watt resistor, etc. Note that req’d wattage increases
as the SQUARE of the voltage.

Even with the resistor soldered in, you’d best not touch the B+
unless you have to. That’s just a good habit to get into.

Everyone who works on amps gets hit real good eventually. Best make
sure that there’s someone nearby who knows exactly what to do if you
need help. This is one of the ground rules in many electronics labs!
Instruct them to run directly for the breaker box and not to touch you
or the amp. If the breaker box is not nearby, install a master switch
for your shop in a place where it won’t be blocked.

MGarvin

 

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