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Speaker Outputs
The GA-30 models have three 1/4" jack sockets for connection to 8Ω or 16Ω loads. The
two 8Ω sockets are in parallel. The GA-60 has an additional 4Ω output.
On the combo models the two 8Ω outputs are connected to the two internal 16Ω
speakers, however other cabinets may be used for different sounds/occasions as desired.
When using other extension speaker cabinets the overall effective impedance should be
correctly worked out and the appropriate socket(s) used.
Impedance Formula
For the two cabinets in parallel this can be done as follows:-
ZT (total impedance) = Z1 x Z2
Therefore, as an example, if two 16Ω cabinets are used,
16 x 16
16 + 16
For more advice on this please contact the Service Dept. or distributor in your territory.
Output Biasing (GA-60RV)
Do not even attempt to make any adjustments to this before reading and understanding
this section thoroughly. As with any power amplifier, incorrect biasing can result in the
complete electronic destruction of the output devices, in this case expensive valves. As
stated on the WARRANTY FORM, valves are not covered by the standard two year
warranty except under certain conditions at the discretion of GIBSON.
If you are in any doubt as to what you are doing refer the unit to an appropriate
electronics service centre.
The holes in the chassis marked BIAS MEASUREMENT and BIAS ADJUST allow the
biasing of the output valves to be checked or reset easily and safely if necessary. It also
allows the use of several different types of output valve including EL34, KT88, 6L6 and
6550.
On a new unit the biasing will be factory set for the particular set of valves supplied with
the amplifier. Although this should not need to be adjusted unless a new set of output
valves is fitted, on all amplifiers as valves age their bias requirements may change.
10
Z1 + Z2
256
=
= 8
32
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