ASK AMP MAN
Choke Troubleshooting
Hey Jeff,
I just finished reading your column in Premier
Guitar. Great column by the way! But I was
wondering if you could finally be the person
to help me?
My question is, I just finished installing
and wiring up a choke in place of the 400
ohm power resistor in my Peavey 6505+
head, and as far as I know it is located in
the correct spot (between the screen and
plate) from everyone I’ve talked to. But I
must be missing something when I did this
swap because when I put the head back on
standby after playing it for awhile I get a
“pop” when I throw the
standby switch on.
Could you please
advise me on just
what I may be missing
in this conversion to
the choke? The choke
is from Mercury by
the way.
Thanks for your valu-
able time.
Scott
Hi Scott,
Thanks for your question
and thanks for reading
Premier Guitar.
If you installed the choke
across the two terminals on the output board
where the 400 ohm resistor was removed,
then it is installed in the correct location in the
circuit. And you are correct; this is between
the main B+ supply to the output transformer
and the screen grid supply. I don’t believe
you “missed” anything here, but installing the
choke may have changed the operation of
the power supply slightly. Let me attempt to
explain what I believe is happening and then
pose a potential solution.
The configuration of the power supply in the
6505+, as well as in some other Peavey amplifiers, is slightly different than the standard configuration in most other manufacturers’ guitar
amps. In most standard power supply configurations, when the standby switch is engaged,
each section of high voltage supply powers
JEFF BOBER
up simultaneously. In the power supply of the
Peavey, the high voltage is constantly supplied
to the plates of the output tubes regardless of
the standby switch setting. The standby switch
simply energizes the remaining sections of the
power supply. By installing the choke in place
of the resistor, you have either changed the
rate at which the later sections of the power
supply respond to being energized or de-ener-gized by the standby switch, or the choke is
causing a spike in the power supply due to a
collapsing or expanding field within the choke.
You may not fully understand that explanation, but here’s the bottom line. Something is
now causing a spike in the power supply and
because the output tubes in the 6505+ are
more “at the ready” than most other amps,
they are able to reproduce this spike more
quickly then an amp with a more standard
power supply design. If this theory is correct and that is what’s happening, I may
have a solution. It requires a bit of rewiring, but I believe it will solve the problem.
Before you attempt this rewiring, please
read the following disclaimer:
If you are not familiar with working around
potentially lethal voltages and do not
know the proper safety measures that you
should take, or do not know how to employ
proper and safe wiring techniques, DO NOT
attempt this modification yourself. Find a
knowledgeable and experienced technician.
First, trace back the two wires from the
standby switch to the points on the circuit
board where they’re connected. Remove the
connections to the standby switch and connect
those two points on the circuit board together,
removing the standby switch from this point in
the circuit. Next, on the side of the main circuit
board where all the fuses are located, find fuse
F1. Immediately to the side of F1 you’ll see
two high voltage leads coming from the mains
transformer. Disconnect one of these leads
from the circuit board and insert the standby
switch here. Either of the two leads is fine,
but if you’re unsure, disconnect the lead con-
nected to F1. Connect one lead of the switch
to the transformer wire
and the other end to the
connection on the circuit
board where the wire was
removed. This will locate
the standby switch at the
AC input of the power
supply, enabling all of
the voltages to rise and
fall simultaneously when
you operate the standby
switch. While the power
supply “spike” may still
happen electrically, it
shouldn’t result in an audible “pop,” since the output tubes won’t be able to
respond as quickly.
With the choke installation
you may also notice that
the amp seems a bit louder or has more gain.
This is not solely due to the characteristics of
a choke, as opposed to a resistor in the power
supply, but it is due to the fact that the voltages in the amplifier may have risen slightly. The
DC resistance of the choke is probably lower
than the 400 ohm resistor it has replaced, causing voltages subsequent to the choke to rise.
There you have it. Hopefully, this will
give you noise free operation of your
standby switch, and we can save the Pop
Phenomenon for the media!
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
