readings/adjustments. A digital multimeter can be found cheaply at any hardware store. Here's
the cheapest we've found on the web, which is totally sufficient for the job:
http://www.harborfreight.com/7-function-digital-multimeter-90899.html
With a speaker cabinet plugged in,turn on the amp, let it warm up for about 30 seconds, then
take off Standby into Play mode (The volume can be all the way down for the test). Let the
amp run for a minute or two.
Next, set the multimeter to DC millivolts (mV).
Insert the black test probe into the black jack on the rear panel, and the red probe into either
red jack. The left jack measures the bias of the left output tube (V5), and the right jack
measures the bias of the right output tube (V4).
For the correct bias range you should read between:
•
6L6: 37mV to 43mV
•
EL34: 31mV to 36mV
•
KT88: 31mV to 36mV
Anything lower settings will not harm the tubes, but anything higher than:
•
41mV and up per tube for EL34s
•
50mV and up for 6L6s
•
41mV and up for KT88
will lead to shortened output tube life or imminent failure.
Remove the red probe, and insert it into the other jack to check the reading. Ideally it should
be the same as the first reading, but a couple millivolts difference is no big deal.
Here are some situations where you may need to check the bias:
•
The amp sounds thin, or abnormal in any way.
•
The amp is quieter than normal.
•
The amp hums more than normal.
•
The bias controls are accidentally maladjusted.
If a reading is below the safe range by more than 10-15mV, then it's likely a tube has failed on
that side and needs to be replaced. This will also cause the other side to be higher than
normal.
5
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