Bugera BVP5500 fault

[valvusmusicus]

I bet there are loads of you out there with 5500's that go into meltdown, or start spluttering and farting once they get hot!
I think this is because the quiescent (standing) current st too high for the MOSFET output devices-or even that the power rail voltage is too high for the chosen output devices
Does anyone know of a way to lower rails by about + and - 10v, without changing the secondary of the PSU transformer-which means fitting a new one
Not possble with a potential divider is it, as Amp needs to see a low impedance from the PSU??? Dunno!!
SO I guess its change the Traffo, and reset the standing current-BUT I cant see a preset to do this. it may be possible to keep the existing traffo, and just lower the standing current-this may increase xover/ other distortion a little-but this may far outwiehg a meltdown situation=or farting and spluttering.

Any thoughts??

[Steve Conner]

What kind of mosfets? What kind of power supply, switching or linear?

Regardless, there should be a pot in there to adjust the idle current, just turn it down.

But, in a well designed MI or PA amp, dissipation due to idle current should be negligible compared to the dissipation from the music signal. So, easiest cure would probably be a bigger heatsink. Bugehringer are likely to have pared the heatsink size to the bone to save money. A potential divider won't do anything useful.

If you're in the UK, we are on the high side of the "230V", and equipment can cause trouble that would work fine in the rest of Europe. I've seen mains voltages up to 250V that simply cook some marginally designed kit.

Also, maybe go up one speaker impedance. If you were using 2, try 4. That is the same as turning the HT voltage down.

[valvusmusicus]

Bugera BVP5500

Hi Steve
Sorry they are bipolar output devices
One TRANSISTOR is a 2SA1943-and I suppose its compliment-I cant see, as Ive re-assembled the heatsink Assy and did not write anything down-They are those eplastic Toshiba devices-probably cant get the heat away efficiently
Its a Linear PSU-all the caps and bridge seem high enough rated.

[Steve Conner]

OK, well the complement is the 2SC5200. They are popular high power audio BJTs. Better than MOSFETs IMO. BJT amps can be made with no bias adjust pot, they are simply biased very cold with a little crossover distortion to leave plenty of room for the tolerances that would normally be taken up by the pot. Peavey made most of theirs this way. If there is no bias pot, I can't see how the amp could be biased too hot unless there was a quality control SNAFU at the factory.

The plastic TO-3P packages can dissipate plenty of heat if clamped properly. Not quite as much as a TO3 but still pretty good. If they were poorly clamped, the amp wouldn't splutter, the transistor would just blow.

Did the heatsink look adequate to you? The amp looks like a copy of one of the Ampeg SVT-Whatever Pro series, which were known for having undersized heatsinks, maybe Bugera faithfully copied that too.

How many pairs of transistors did it have? For 550W I'd expect 4 pairs at least, maybe 6. That's what I'd use if I were building a 550W amp anyway.

I think what I'm getting at is, if used with a 2 ohm load on 240+ mains voltage, the SOA protection might well kick in, which would fit the symptoms of nasty farting distortion. I've experimented with these protection circuits on my own SS designs and they really do sound bad when they trigger. It may only happen with a reactive speaker load, the amp behaving fine when you test it on the bench with a dummy load.

[valvusmusicus]

Hi Steve
The heatsink is 170mm x 80 x 80cm-with 3cm squared fins running the length of the 180mm section-the fan blows thru the centre-seems a bit flakey. There are only 3 pairs of output devices-perhaps they are running them at their extreem spec-Ill need to measure power rails of output devices-I might try a fan if all is in spec. Do you have a schematic-I think I might need one to reset the crossover distortion/quiescent current

many thanks#

Paul

[MarkusBass]

The amp looks like a copy of Ampeg SVT3Pro with a bipolar power amp. Can you post a photo of the power amp? It's hard to believe that with such an output power there is no bias adjustment pot. Can you measure the voltages on the balast resistors (to see what is the quiescent current)? And what are the power supply rails?

Mark

[Steve Conner]

Yes, +1 on all the above, pix plz!

Sorry, don't have schematics. They are hard to get for Behringer stuff and I assume Bugera is the same.

The unit must surely have a fan already. The heatsink seems a little better than the SVT-3Pro which didn't have any fins- it was just a U-shaped channel.

In the light of there only being 3 pairs of trannies, I'm still going with my above guess: the protection is kicking in to save the undersized power stage, and the UK mains voltage is just making matters worse. The cure would be avoiding 2 ohm speakers. If it's doing it even with a 4 ohm load, then I don't know what the problem is.

Most SOA protection circuits don't know the actual temperature of the output devices, so increasing the cooling may not help.

[valvusmusicus]

OK folks-will take pictures and make all of the suggested voltage measurements and post later on this evening
Thanks all very much for continued help

Paul

Wouls putting a second fan, connected in paralell with first on the PCB fan supply connections present any problems- if they are both the same-ie just 2 pins or 3 with a control pin

[MarkusBass]

Steve,

I think that no one (except you) said that the amp is running with 2 Ohms speaker :-). The manual says that the minimum load is 4 Ohms and the output power is 550W peak (not RMS). Looking at similar amp: HD amp I'd say that the RMS power is slightly above 300W @ 4 Ohms and this is OK for three pairs of output devices. I'd like to see a photo of the power amp (and the preamp to compare it with SVT3Pro).


Mark

[Steve Conner]

On the Bugera website it says "4 or 2 ohms".
http://www.bugera-amps.com/images/ph...HART_800PX.png

550W "music power" at 4 ohms with 3 pairs of devices is OK :-) It would be interesting to measure the RMS power on the bench. If it works out much above 300W, then there isn't much SOA left to cope with reactive loads in the field. Either the SOA protection would trigger, or it would be unreliable.

If you have a variac, test it at 230, 240 and 250.

[MarkusBass]

The manual says something else. As you can see they also forgot to mention that 550W is PEAK power and not RMS.
I assume that Paul is not from UK: "valv US music US", so he has different mains power supply, right?

Mark

[Steve Conner]

Well in another thread he has germanium transistors for sale priced in UK pounds, so I kind of assumed.

It always amazes me how much amp designers get away with. I recently tried to design my first SOA protection circuit, and a strict interpretation of the SOA graphs, plus tolerances, inductive loads and so on, left me with less than 100W RMS per TO-3P pair. Either I got it wrong, or the SOA spec is conservative.

The HD Amp thing is rated at 150W into 8 ohms and 340 into 4. It's impossible for the 4 ohm rating to be more than twice the 8 ohm one. That says to me it is a typo and maybe should have been 240.

[MarkusBass]

Ampeg SVT3Pro is 400W @4 Ohms with 4 pairs, so your estimations seems to be OK. I'd say that HD Amp is 300W @4 Ohms.

[valvusmusicus]

Hi
I dont remeber saying that I ran it into 2 ohms-It was an 8 ohm 4x10 Hartke cab only. Probably my mistake!
The amp gets hot almost immediately-with no signal going thru it. And no spkrs connected. The fan gets up to max speed almost immediately-and boy, the heatsink gets hot
This is why I suspect quiescent current problem
Ill put it back together and make some measurement and post them with the picture

Thansk again

[Steve Conner]

OK. You never mentioned any of that before, and it changes things. So yes, maybe the idle current has got too high somehow, or the amp is oscillating at an ultrasonic frequency because of some fault.