Tube Types By Manufacturers; Biasing Your Knucklehead Ii - Rivera KNUCKLE HEAD 2 Owner's Manual

POWER STANDBY
®
HANDCRAFTED BY
RIVERA RESEARCH & DEVELOPMENT SYLMAR, CA USA
Tube Types and Manufacturers
12AX7 types. As of 2003, we use a combination of Sovtek types. 12AX7WA tubes are used along with
12AX7LP and 12AX7LPS. By combining these two tube types, we have been able to maximize the amp's
performance. Our earlier amps, starting in 1985, used Chinese 12AX7 tubes. These tubes have higher gain
and more high end than the Sovtek types. However, when the Chinese tubes went out of production in the
mid 1990s, we researched what was available and tried them all. There is the JJ 12AX7, but it seems to have
lower gain and really weird failure modes. Yugoslavia made 12AX7 tubes sound decent, but suffer from
microphonic issues. China has recently begun producing the 12AX7 again, but the number of usable tubes is
low, due to microphonic qualities. Position V4 has a cathode-follower circuit, and we have found that the
12AX7WA/WB types are the best for this position.
As far as NOS (new old-stock) European types go, the Mullard and Philips-Holland 12AX7A, 7025, and
6681 were wonderful, as were those made by Telefunken.. Unfortunately, there are a lot of unscrupulous sell-
ers who demand outrageous prices for tubes of dubious quality or even correct manufacture. Buyer beware!
6L6GC and EL-34. We are using Svetlana 6L6GC and EL-34 tubes, and have found them to be the most
reliable of what is available.
Biasing
Your amplifier was shipped with a matched quartet of power output tubes. These tubes have been rated by
us and are marked on a foil label at the tube base (EL-34) on a scale of 6 to 12, or on 6L6GC with a written
number on the tube base as well as the top of the tube envelope. If you replace the tubes with Rivera tubes of
the same rating, it is not necessary to re-bias the amp. However, if you are using other-value Rivera tubes, or
tubes from another manufacturer, it is wise to have the bias checked and adjusted. We have made it quite
easy to do, but due to hazardous and potentially lethal voltages being in the chassis, we strongly advise that
you have an experienced and qualified technician perform this procedure.
All Rivera amplifiers since 1985 have been equipped with a built-in standby switch in the Speaker Jack #1.
This was for two reasons: (1) It is not uncommon for users to forget to connect a speaker load before turning
on the amp and attempting to play. In most amps this would fry the output transformer and arc the output
tube sockets. (2) This switch lifts the cathodes of the output tubes when a plug is not inserted into the
Speaker Jack #1. By connecting a DC current meter from Pin 8 of the output tubes to the chassis ground,
without a plug in the Speaker Jack #1, the cathode current for all four output tubes will flow through the
meter, giving an accurate indication of the total DC current (the cathodes are connected in parallel, and the
reading is for all four tubes). This is measured at idle with no signal present.
On the motherboard is a trim potentiometer that is intended for setting the bias voltage. At nominal Mains
voltage (115VAC USA, 230VAC Europe/Scandinavia, 250VAC UK and Australia), this is when the bias
current is measured. Use a Variac to adjust the Mains to be at rated nominal voltage as labeled on the back of
your amplifier. If you do not have a Variac to adjust the Mains to the correct voltage, you can compensate in
your calculations and measurement. For example, if your nominal mains voltage should be 115, and you
measure a high Mains condition, like 122VAC, this is almost 10% higher. This means your measured cur-
rent would be around 10% higher than normal and should be adjusted to that. Conversely, if the Mains
voltage measures at 105, you would lessen the idle current so that it was approximately 10% less. Adjust for
165mA DC at rated nominal Mains voltage. Some output tubes, like JJ/Tesla run very hot and may require
less idle current to run reliably.
CLEAN/RHYTHM
PRESENCE FOCUS MASTER 2 BASS
PULL
NINJA
MIDDLE TREBLE GAIN 2 MASTER 1 TREBLE
PULL PULL PULL
NOTCH BRIGHT BOOST SWEET
LEAD/CRUNCH
MIDDLE BASS GAIN 1 INPUTS
LOW
PULL PULL PULL PULL
SCOOP BOOST SELECT
GAIN
HIGH
GAIN