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Bugera Infinium amps ??

** 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:

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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

Reply to
Phil Allison
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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.

Reply to
William Sommerwerck

"William Sommerwerck"

** 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.

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

Reply to
Phil Allison

"Arfa Daily"

** 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

Reply to
Phil Allison

"Phil Allison"

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

Couple of bucks each at Digikey.

... Phil

Reply to
Phil Allison

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