"Arfa Daily"
"Phil Allison"
** 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.
Interesting. He starts off talking about tone, tone, tone, and that is
true. But the 'wanted' tone to one guitarist can be a mile different from
that wanted by another, and that is often achieved by altering the way
that the output stage is biased. Most of the guitar players that I know
don't give a **** about valve life, as long as they are getting the sound
they want. There are plenty of respected repairers and amp gurus giving
on-line tutorials about biasing for sound. So what is this 'ideal' bias
point that he is talking about ?
** 34mA with a 500V B+ supply equates to 17 watts dissipation per tube.
That is a safe maximum and very typical of most guitar amps that operate
EL34s or 6L6s in class AB.
The next version of the Infinium might allow for some user adjustment -
ie cool, warm and hot settings.
.... Phil