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| Cool amp tools |
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| These are the
amp cradles that I used to set a chassis on so the tubes didn't get damaged.
You just move the right end piece by unscrewing a screw and then sliding the
right end piece in. Rubber bumpers on top keep the chassis from sliding
around. Note the top cradle has markings for all the different Fender
chassis. |
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This device is a circuit
breaker with alligator clips soldered on. When you are dealing with an amp
that is blowing fuses, this tool is the thing to have.
You just press the reset button after the amp
has had it's way with you and you are on your way again. This one is rated
at 3 amps. You can find these devices in old junk gear and at ham fest. |
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This device is a pot that you
only use as a variable resistor. You use one outside leg and the middle lug
of the pot to dial in a specific resistance value. After you find the value
that works for you, you replace the pot with a fixed value resistor. The
value of the pot can vary depending on what you are trying to dial in. A 1
meg pot gives you a huge range but a tiny turn can be a huge resistance
change. |
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Dummy load resistor: This is a large power resistor that you can vary a tap
point on. There is sliding tap point that you can screw down anywhere along
the body of the resistor. I think I have this one set at around 8 ohms. They
get very hot when testing an amp so you need a very high power rating and it
should not be able to touch anything. This one is probably 50 to 100 watts. |
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Ok,
here's the coolest tool of all. This is a stethoscope listening amplifier
that is able to probe a test amplifier and listen in on the signal as it
travels from input jack to speaker jack. The listening amplifier pictured
above is just a cheap solid state bass combo amp that I pulled out of it's
cabinet and put an aluminum cover plate on. You want a clean sounding
listening amplifier because you want to listen to the signal in the test amp
and get an accurate picture of how the signal sounds. You do not want the
listening amplifier to distort the signal. You could probably use an audio
amplifier also. The listening amplifier is hooked up to a speaker cabinet so
you can hear what is going on in the test amp. The test amp is hooked up to
a dummy load. You do not want to hear the test amp.
I have an old multi meter test probe on one end of some mini
shielded cable. The cable shielding stops at the base of the test probe
but the shielding is connected to the 1/4 inch jack on the other end. This
gives you some shielding of the signal as it travels down your test lead to
the amplifier.
The other end of the cable has a 1/4" guitar plug on it and it enters the
listening amplifier input jack.
I have gone inside the listening amplifier and made some mods. The signal
enters the amplifier through the input jack and then goes directly to a
.1/630volt blocking capacitor. This capacitor blocks any DC voltages
from entering the listening amplifier, you only want to listen to AC signal
voltages. Any DC voltages entering the amplifier would blow up the listening
amplifier. You will be able to listen to the AC voltage riding on top of the
DC voltage on the plate of a tube stage and not blow up your listening amp.
:)
After the signal goes through the .1/630 volt blocking capacitor it
enters a 1 meg audio pot. This is your input level trim pot to adjust the
level of the signal that you are listening to. If you are listening to an
input level signal it will only be millivolts but if you are listening to a
signal as it leaves the phase inverter on it's way to the power tubes, the
signal make be 30 volts peak to peak or more. You need to be able to pad
down a 30 volt signal and feed the listening amp a signal that will not
swamp it completely.
You also need to connect both chassis to a common ground or you will not
be able to hear the signal in the test amp. The green alligator clip
in the picture, clips to the common ground of the listening amp chassis and
then you clip it to the common ground chassis on the test amp. This
completes the circuit since the signal probe is only one connector. Make
sure you find a common ground point on your listening amp. If you connect
the green wire to nothing, you will hear nothing.
How to operate this device:
Turn on the listening amplifier and play a guitar through it to make sure it
sounds very clean. Adjust the input level pot for the correct level. Strum
an open E chord and dial in the amps tone controls so that the sound is
squeaky clean.
Hook up a dummy load to your test amp and then turn it on. Clip the green
ground wire to both amplifiers chassis. Plug in a guitar to the test
amplifier. Strum an open E chord and touch the signal probe to the test amps
input jack tip, you should be able to hear the guitar being amplified
through the listening amplifier. Now you are free to probe anywhere you like
in the test amp. But be aware that it is a good idea to turn down the input
level trim pot and bring it up slowly until you hear sound being amplified
in your listening amplifier. You do not want to touch a very high signal
level area with the input level trim pot set for a very low level signal.
It is easier to have a helper strum the guitar very slowly and you
operate the tool. Have them do the same exact thing over and over so you can
hear any changes that may happen as you probe. Tell your helper that this is
not the time to be playing a song, you can hear the song later.
A signal generator does not work for this test. You want to hear a full
guitar chord so you can hear distortions or other problems. A signal
generator is of no help, it does not represent a full spectrum of sound. Put
your oscilloscope away too, it's of no help either. :)
With this device you can listen to the input grids of pre amp tubes and
power tubes. You can listen to the cathodes and the plates also. You can
listen to the plates of output tubes and you can even listen to a speaker
jack.
There is nothing like your own ears for finding where a signal goes bad
in an amp. |
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| This a
visual layout of how the components would be as you go from probe tip to
listening amplifier. I have drawn it this way because you probably could
install the blocking cap and the trim pot in a small metal box and not use a
dedicated listening amp. It's just a theory but it should work. The signal
probe would go to the metal box and then the metal box would go to a
listening amplifier by way of a two conductor shielded cable. You could use
the small aluminum metal enclosure that I sell on my web site. It is about
the size of an old MXR pedal. The pot needs to be grounded to the same
surface that the alligator clip grounds to. The cable leaving the metal box
going to the listening amp needs to be a two conductor shielded cable.
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