Roland JC 200 & Bugera G5 Infinium

[oc disorder]

Thank you very much to sugeda and J. P. Bass for the "schemes".
Teemuk yes they are and yes they are drivers - I assumed incorrectly they wouldn't be claiming 5 watts from a 12BH7.
I will post again when I have answers.
Oh for the simple life when a one tube (valve in Aussie land) amp was just that !


None of this Infinite Blackhole complication ! ! !

[Jazz P Bass]

I really would like to see the 'redesign' of the Blackstar circuit.
Sigh.

[nickb]

<rant>
The amount of the circuit that has been duplicated by Behringer here is astonishing, just as they have done to others in the past. They clearly have no shame and somehow still get away with it.

And the torrent of marketing drivel is equally astonishing.
</rant>

[teemuk]

Oh no, here we go again...
Personally I don't see how what they (Bugera) do is any different from what other manufacturers do when dublicating the age old Marshall, Fender, Vox, Soldano, Dumble etc. amps.

Here Bugera took a popular HT-5 amp design (which is pretty close to Valvestate Marshall amps BTW), added their own tone controls (in order to not infringe a patent, nothing else in the circuit has a valid patent BTW). Then they added their own channel switching scheme, and their own version of automatic biasing with microcontroller (which is similar to Fender amps, or Marshall amps, or H&K amps, or DV Mark amps, or KBO Dynamics TubeSync module, or... so on..) - and a feature not found from original HT-5, BTW. Finally they devised their very own circuit boards, chassis and assembly lines for the unit, which is also moderately inexpensive. Yes, it's a clone but huge percent of amp products are clones of popular designs so why care?

[gaztech]

Hi guys,

I have a faulty G5 at the moment. Came to me with the symptom - no output.

I found quite a lot wrong with it. Front end was not processing any signal at all due to a blown TL074 chip. SMD so a bit of a pain to put a replacement in there. I now have signal all the way to the output stage. Standby unmutes the signal to the power amp section.

However, I found a bad output driver FET (leaky) and a blown 22K (3W?) resistor. To be sure, I replaced both FETs and both 22k's. Everything else seems ok. Still no sound coming out of it at all. Tried a new output tube too as I had one to hand.

But... measuring the primaries on the ouptut transformer between the centre tap results in readings of around 450 ohms (DC resistance). Seems too high I think. Does anyone know what the actual measurement should be across these windings?

Secondary output winding measures 1.0 Ohms. Could be correct maybe...

And a question... Does anyone know if it's possible to get hold of a new transformer.?

Behringer (Music Group) are as useful as a chocolate fireguard when it comes to getting either parts or tech info out of them. Luckily, I do have the schematic... I'm wondering if I could get one from Blackstar. Yeah, a bit cheeky but at least the schematic is very similar

Anyone have any comments about this?

[Helmholtz]

But... measuring the primaries on the ouptut transformer between the centre tap results in readings of around 450 ohms (DC resistance). Seems too high I think. Does anyone know what the actual measurement should be across these windings?

Secondary output winding measures 1.0 Ohms. Could be correct maybe...

DCR values look ok for a tiny OT.

[gaztech]

Sorted... It wasn't the transformer at all... Board was slightly conductive and was tracking high voltage between the FET driver pins! I found this by checking the drive before the tube grids. One part of the board was worse than the other. Odd fault.

The only way I could make it work was to drill some of the board so that there wasn't any board between the pins. Now works fine.

This might help others with similar symptoms.

[camjenkins]

That seems to be an pretty common failure mode for amps that use the Blackstar mosfet phase inverter/driver. It's surprising that they don't seem to have done anything to address the problem.

[vintagekiki]

sugeda