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valvusmusicus 27th March 2011 05:40 PM

Bugera BVP5500 amp problem
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 is too high-or even that the power rail voltage is too high for the chosen output devices
I dobt this-as Ive not heard anyone else complain about rail voltages.
This is a BJT amplifier with 2sa1943/2sc5200 output trannies 3 of each) preceeded by a tube pre. Tubepre is fine.

There are 3 presets 2 at 1K and the ither unmarked-the 2 1K's could be a standing current adjustment-its diffiacult to trace the bd as its double sided.A
ny thoughts??
Its a Linear PSU-all the caps and bridge seem high enough rated
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

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

Tested with a scope at the input of the power amp section and the speaker output-No DC ofset or oscillations-must be the idling current
Will keep all posted-hopefully this will help some other poor sod who bought one of these
They seem to have at least a 1 in 10 failure rate out of the box as Ive read in other posts and forums!!
Lets put it back together and see if anything goes bang!!

Ive put it back together now. There is one high amp fuse after the transformer AC feed to the output amp bd (no dc fuse on each rail after rectification). So I stuck an amp meter in place of the fuse-starts off at about 0.9 amp from cold-then as the heatsink heats up, it goes down toa stedy 0.37amps when hot. This with the fan connected
Is there a similar topology of power amp with these devices - I shall have a look around



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