Page 48 - Triwatt Custom Lead Amp Builder's Guide
P. 48
KEEP IN MIND that every time you power up from now on that B+ will be high. In all the
following, allowing B+ to bleed at each power down is implied.
If all is well, check that B+ is at the OT on all the primary taps. Without the power tubes installed,
the OT primary should be at B+. If not, something is wrong at the OT. Power down immediately
and check for shorts of the OT primary. This should not be the case, however. An OT short
should have been caught by now by checking B+ levels in the previous steps. This is really just a
final sanity check to really make sure the trannies aren't going to be killed by any mistakes. Leave
the amp on for a few minutes and make sure neither tranny is getting warm. The OT should stay
dead cold and the PT should get just a little warm supplying the pilot lamp and heaters. Make sure
the negative voltage is being generated and check that it gets to the power tube grids and is
adjustable with the bias pot.
Now we’re ready to put some tubes in. Power down and install all the signal tubes. You also need
to hook up a speaker or dummy load for the OT. Use an old or less valuable speaker to get started.
Turn volume and gain pots all the way down and tone controls to center. Turn the 10K bias
control pot all the way down (fully anti-clockwise) to start (most negative grid voltage possible).
Power up and, again, watch for the following signs:
pilot lamp comes on brightly;
all tube filaments light up right away;
tube plates do NOT glow red (overheat) paying attention to power tubes.
Bias Setting: Check power tube current and bias appropriately soon after power up to make sure
all is well.
Set the tube selector toggle switch to select your type of power tubes - 6V6 or KT66.
Set your meter to the mV scale. With the 10K bias control pot all the way down (fully anti-
clockwise), insert the black (negative) test probe into the Black bias test point jack. Put the other
into the Red bias test point jack.
Turn on the amp. You should have a small reading in mv. Since you are measuring across a 1
ohm resistor, then this is the equivalent ma bias. i.e. 10mv on your meter is reading 10 ma bias.
Slowly move the pot with a small screwdriver until the readings are as follows:
6V6 Bias 15 ma min 23 ma max
KT-66 bias 30 ma min 40 ma max
You should be able to hear a little hiss or hum from the speaker. Of course, hopefully this is at a
low level, requiring your ear up next to the speaker to tell (make sure the Overdrive control is
pushed in -ie Off). If hiss and hum is loud at this point, there are problems. If there is dead silence,
something is likely wrong, too.
TrinityAmps TRIWATT Builders Guide Ver. 2.31.docx- 48 -
