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positive red probe lead in the red tip jack of BIAS V1. Referring to the
recommended bias settings below, slowly adjust the corresponding bias pot while
paying attention to its sensitivity. Make a note of your bias setting if you are using
2 or 4 of the same tubes.
3. Repeat step 2 for BIAS V2, BIAS V3, & BIAS V4. Then recheck and make
further adjustments as necessary.
4. Once the bias is set, tighten down the lock nuts on the bias trim pots, but not too
tight. Now you are ready to play.
Recommended Bias Settings:
EL34/6CA7/KT77
25mV to 35mV
6L6 /5881/KT66
30mV to 42mV
6550
35mV to 51mV
KT88
44mV to 61mV
Note:
The upper range of the recommended bias settings should be considered a
maximum setting. Feel free to experiment within the given ranges to suit your taste. It is
normal for tubes to drift a little, especially when they are new, and will stabilize after a
few hours of use. Also, bias will drift with variations in AC line voltage. This is not a big
deal so don’t get obsessed by constantly checking the bias to keep it at your chosen
settings. To balance out the currents of the output tube when using 4 of the same type of
output tubes, place one meter probe (black or red) into BIAS V1 red tip jack then the
other meter probe into the BIAS V4 red tip jack. Then adjust either one of the
corresponding bias pots to get a 0 reading on your meter. Repeat the above for BIAS V2
and BIAS V3. If you are using 2 different output tube types, (e.g. 2-KT88 and 2-EL34),
place the 2-KT88s in either V1 and V4 or V2 and V3 positions, then the 2-EL34s in the
remaining positions and follow the above procedure to balance out the currents.
Special
note for the adventurous souls
: When using a different tube type in each position, please
keep the imbalance to a maximum of + or - 15mV DC. Using this simple formula of
(BIAS V1 + BIAS V2) – (BIAS V3 + BIAS V4) = maximum plus or minus 15mV DC
will ensure optimum performance. Nothing will happen to the amp if it exceeds this value
but you may experience an audible hum in the output and loss of low end response at
extreme volume settings. Mixing different tube types is an easy and fun way to combine
characteristics of your favorite tubes for a “best of both worlds” approach. Although tone
is subjective, all tubes have their own distinct sonic character and “feel” that you can use
to your advantage in further fine tuning your BONES amplifier. Taking this approach one
step further, you can also experiment with different preamp tubes.
