There are also some compatible substitutes that you can try which include 5Y3GT and
5AR4. The 5AR4 is an indirectly heated rectifier with a soft start up so it may be a bit
more expensive than the 5Y3GT and 5U4. A popular rectifier tube that many people try
to substitute is the 274B. There are different manufactures and different types of 274B
rectifier tubes. Of the ones we’ve examined, they all required no more than a 10uf
capacitor in the first section of the power supply. Very few tubes amps today use that
small of a value. The SE34I.3 uses a 47uf capacitor in this location which can cause
the 274B to arc on start-up and thus reduce it’s life.
BIASING and METERS
The SE34I.3 is a self biasing, self balancing design meaning there is never a need to
adjust the bias of the output tubes. The amp does it for you. It also insures that you
get matched output power from each channel even if the tubes are not matched. And
most importantly, when compared to the common “fixed bias” approach where you
adjust the bias with a bias control, this way sounds better. The reason everyone
doesn’t do it this way is because it sacrifices power. So it’s an issue of better sound or
more power. We choose better sound.
If you have perfectly matched tubes,
your bias meters of which there is
one for each of the two output tubes,
will read the same. This is typically
around 38 millivolts per side. Of
course different brands of tubes may
read as much as 10 millivolts higher
or lower than that figure.
These bias meters will tell you at a
glance if your output tubes are
healthy and if they’re matched. As a
completely analog device, they can
tell you other things as well. For
example, you can watch how long it
takes the tubes to warm up and come to full bias by watching the meters when you first
turn the amplifier on. If one tube takes a lot longer to arrive at it’s bias point than the
other tube it can be an early warning that there are differences developing between
either your two output tubes are the input tubes. Yes, the input tubes can effect the
bias reading on the meter as well since they are coupled to the output tubes.
O W N E R S M A N U A L
Decware High Fidelity Engineering Co., East Peoria IL, USA www.decware.com
