** 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

"Phil Allison" wrote in message ...

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

Based on what Phil says... The name suggests what the amp will do to its
owner.

Even without his analysis, viewing the description of how the Infinium system
works would have made my hair stand on end.

Tubes deteriorate slowly, so there's no point in continuously checking the
tube bias. It should be sufficient when the amplifier is turned on, or the
user initiates a rebias cycle. And, as Phil points out, how do you keep the
biasing system from being "confused" by the audio signal?

I'm not an expert on tube output-stage design. But it should be obvious that
the correct idle current for one tube type is not necessarily optimum for
another. The promo indirectly acknowledges this by pointing out the mixed
tubes will produce "interesting" harmonic and IM distortion. And if the tubes'
characteristics are sufficiently different, you might even wind up with a
"healthy" (not!) level of DC through the output transformer's primary.

The guy can't even pronounce "potentiometer".

I wonder what David Hafler (who came up with a simple and cheap way of
biasing Dyna amps) would have to say about this...

I posted the following, but it hasn't been approved yet. (I expect it to be
censored.)


Bugera? The name "says it".

I'm a degreed EE and hold an Amateur Extra license (KA3QXL). Most of this is
utter baloney. Tubes deteriorate slowly, so there's no need to reset the bias
more often than the start of a session. And you don't want to be doing it
while amplifying a signal!

As for mixing tube types... Yes, you'll get "interesting" distortion -- along
with the potential (joke intended) of amplifier damage due to mismatched tube
characteristics.

This is a bad, bad, bad idea.

"William Sommerwerck"

Tubes deteriorate slowly, so there's no point in continuously checking the
tube bias. It should be sufficient when the amplifier is turned on, or the
user initiates a rebias cycle.

** Oh, but tubes often DO draw more current as they get hotter and this is a
sample to sample thing too.

PLUS the AC supply voltage has a big effect as well.

Imagine a band using a basic generator or on board a party boat - the
Bugera servo could be lethal.


And, as Phil points out, how do you keep the biasing system from being
"confused" by the audio signal?

** Well, that one is just quietly swept under the carpet .............


I'm not an expert on tube output-stage design. But it should be obvious
that the correct idle current for one tube type is not necessarily optimum
for another. The promo indirectly acknowledges this by pointing out the
mixed tubes will produce "interesting" harmonic and IM distortion. And if
the tubes' characteristics are sufficiently different, you might even wind
up with a "healthy" (not!) level of DC through the output transformer's
primary.

** Yep, poor symmetry in a push-pull stage does just that.

The Bugera servo only sets the DC balance condition at idle, which is gonna
be OK at higher current levels ONLY if the tubes are all closely similar.

Same type number, maker and age etc.

I tried one EL34 in place of a 5881 and the DSP servo bias worked as
intended - but the output sine wave became was asymmetrical at high
levels.

But a pair of EL34s, one on each side of the push pull tranny, would have
been pretty much OK.



.... Phil

"Phil Allison" wrote in message
...

** 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



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 ? Presumably a 'technical' ideal, because for guitar
players - particularly muso ones - the 'ideal' point is the one that makes
the output stage sound like they want it to sound. I know one muso that has
his amps' bias set very 'soft' to get the fat blues-y sound that is right
for what he plays. I know another that always has his set very 'hard' to get
a harsh screaming tone for the metal rock that he plays.

As to subbing the valves with anything that comes to hand, I can't help but
think that is a bad idea. If I had one valve out of four fail just before a
gig, I think I'd rather see the amp with that valve's opposite number
removed as well, and the volume turned up a bit - if that was even required.

Arfa

"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

"Phil Allison"


The reality is very, very different.

The model referred above has a DSP based bias servo system ...

** The Atmel chip used (ATmega 8A) is in fact a "micro-controller".

Couple of bucks each at Digikey.




.... Phil