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Section 12
Testing
OK! Your amplifier is built – take a deep breath and well done!
The first thing to do is to check – and check – and check again your work!
No need to insert any tubes at this point!
Let's go through a series of Ohm checks to make sure the amp is more or less constructed
correctly! Go through the Interwiring chart and visually inspect everything. Also if you have
patience you can Ohm out the connections to make sure all the connections are good.
12.1 Power On and Voltage Checks
There are a couple of voltage basics that can really help the cause. There are two types of
voltage: DC and AC. DC is a constant voltage, for example the High Voltage in a 300B amplifier
is typically 425V DC, so when you measure from the high voltage point to any ground in the
circuit (even the chassis), you will measure 425V DC. The other type of voltage is AC voltage,
this is basically a voltage that is constantly changing in the form of a SIN wave. The filament
voltages on an EL34 are 6.3V AC or alternating voltage. With your voltmeter, you can measure
DC or AC and makes troubleshooting straightforward. The other key tool on your meter is the
Ohm meter, you can take a resistor and measure the resistance in Ohms. For example, you pick
up a resistor, put a probe on each end and set your meter to Ohms and measure 1000 Ohms for
example, this is then a 1K resistor. Just as important as resistance, being able to measure
continuity is one of the most powerful debug tools, in other words, is there zero Ohms or a
straight connection between this point in the amp and another point?
12.1.1 Mains Fuse Installation
Install the fuse in the small plastic fuse holder and insert it into the IEC. (If you are operating in
120V you can disregard the lettering that says 240V only – there is only one type of fuse holder,
no matter whether it's for 120V or 240V.) Note that the kit is supplied with several 2A slow blow
fuses. After the amplifier has been "up and running" for a while, if you have any issues with
the Mains fuse blowing occasionally you may increase the fuse rating up to 6A slow blow. If
you have any issues or concerns regarding this please contact
When you turn on the amplifier for the first time you can turn it on without installing any
tubes and do some voltage checks as follows:
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