** Hi to all the amp techs.

Bugera is an alias for Behringer - used for marketing their valve amps.

A model " 1990 Infinium " is on my bench now - first time I've seen one.

You need to see this Vid to get the background:

http://www.youtube.com/watch?v=P8GxgtLSadw

That slow talking dude makes some pretty wild claims - like 20 times normal
valve life, successfully mixing up different output types and care free user
maintenance. All are purest marketing fantasy.

The reality is very, very different.

The model referred above has a DSP based bias servo system that attempts to
set the idle current in each output tube at 34mA. Each cathode circuit has
a 4.7ohm resistor to ground to aid this process - shunted by a 3A diode to
protect it.

The DSP trims the four grid voltages to get 155mV DC across each resistor -
but only when there is no signal. The grid voltages become locked at the
previous idle values when even the smallest AC voltage component appears at
the cathodes.

There is enough bias voltage range available to cope with any EL34, 6L6 or
5881 ever made, from -65V to about -25V.

But this is complete madness, simply matching the idle bias current does not
make a class AB output stage work perfectly - not when the tube's
characteristics are quite different.

But there is worse.

The DSP bias servo adjusts for variations in AC supply voltage too. If the
AC supply voltage drops low when the amp is silent and then rises to normal
during playing, all the output tubes becomes seriously over biased. Using a
Variac, it is easy to create a situation were the amp self destructs in
seconds.

But there is worse.

The negative 75V DC bias supply is generated by a voltage tripler working
off the same transformer winding as the pre-amp tube heaters. All six
heaters are wired in series across a +/- 18V DC supply fed from a 20-0-20 AC
winding on the mains tranny.

A pair of T1A fuses protect the above winding - but are fatally undersized,
fail easily at switch on and disable the bias supply completely.

That is how I received the near new amp I have now - the AC fuse kept
blowing since the four Chinese made 5881s ( as originally fitted ) drew max
possible current as soon as they warmed up.

BTW 1:

There is a F6.3A fuse in series with the 6.3V AC heater supply for the
four output tubes - which is simply nuts. Even the T1.6A AC supply fuse
is under sized, since the amp draws 2.1 amps RMS at full drive.

BTW 2:

There is a plethora of utter nonsense posted about these models on net
forums, alleging bad tubes were fitted and / or fuses were defective.

The folk who know the truth are mostly contracted to Behringer / Bugera and
cannot say a thing.





..... Phil