Rogue RB120 head

[woodyc]

Anybody know these amps or have a schematic?

I'm looking at one with fried output transistors. The bizarre thing is that it appears to be designed for a BJT output stage, but it came with Darlingtons (TIP142/TIP147) -- which it can't produce enought volts to bias. I was thinking about replacing the fried Darlingtons with some cheap TO3P BJTs like maybe NJW0281G/0302G.

[Enzo]

Then you'll want to add drivers to make up for the lost gain the darlingtons provided.



WHat do you mean it can't bias them? I'd expect the darlingtons to have about 2v between opposing bases, but if it is lower than that, so there is some crossover distortion. They may have left that in the design. WHat sort of bias circuit is in there now?

[g1]

It worked with the darlingtons before didn't it? Wouldn't it be more likely that there is a fault in the bias circuit, rather than it not being able to work right with the stock parts?
Maybe a zener that is not providing it's correct voltage? You seem to be certain that there are no other faults aside from the outputs?

[woodyc]

It worked with the darlingtons before didn't it? Wouldn't it be more likely that there is a fault in the bias circuit, rather than it not being able to work right with the stock parts?
Maybe a zener that is not providing it's correct voltage? You seem to be certain that there are no other faults aside from the outputs?

I don't think the bias had to do with the failure. Its just curious. I made a schematic this morning...

[g1]

I don't think the bias had to do with the failure.

Maybe not, but the failure may have damaged something in the bias circuit. You only need around 2 volts between the bases of the TIP's. I don't see any reason why that circuit can't provide it, unless the output failure caused damage to some other components.
What kind of voltage are you getting across the .22R emitter resistors?

[woodyc]

Maybe not, but the failure may have damaged something in the bias circuit. You only need around 2 volts between the bases of the TIP's. I don't see any reason why that circuit can't provide it, unless the output failure caused damage to some other components.
What kind of voltage are you getting across the .22R emitter resistors?

I can't measure anything -- the power stage was fried and I haven't replaced it yet. But according to the schematic the voltage would be zero.

In particular, the bias range of the vbe multiplier is about 2.3-2.4V. Since the drivers drop half of that, whats left is insufficent for Darlingtons. So neither the drivers nor the output stage will have any bias current in this amp, and the bias trim will do nothing.

[J M Fahey]

Thanks for providing a schematic.
Funny thing is, the amp *does* have driver transistors, enough to drive regular non darlington power transistors.
The voltages you measure confirm that.
My guess is that somebody repaired it before you, and slammed unnecessary Darlingtons there.
Given the other parts used, I expected Japanese 2SA/2SC power transistors, not TIP14x.
It will work anyway, and a lot of people do not seem to notice crossover distortion that much or don´t find it so objectionable.
Those who play only full tilt besides a drummer are an example.
Bedroom rockers being the exact opposite.

[woodyc]

Thanks for providing a schematic.
Funny thing is, the amp *does* have driver transistors, enough to drive regular non darlington power transistors.
The voltages you measure confirm that.
My guess is that somebody repaired it before you, and slammed unnecessary Darlingtons there.
Given the other parts used, I expected Japanese 2SA/2SC power transistors, not TIP14x.
It will work anyway, and a lot of people do not seem to notice crossover distortion that much or don´t find it so objectionable.
Those who play only full tilt besides a drummer are an example.
Bedroom rockers being the exact opposite.

JM, you're welcome.

I believe the transistors in my schematic were original to the amp. But... this just in: there are part numbers on the board under every transistor. None of the actual parts in the amp match those numbers and all the actual parts I checked are under spec in some way. As you guessed the board calls for 2SC4468 and 2SA1695 in the power stage, which are 140V BJT, not the 100V Darlingtons they installed.

The issue may be slightly worse than just crossover. For one thing the balance mechanism relys on current in the output stage. Without it the amp could be very off-kilter every time a signal arrives. Would that be stressful or not? For another, with the extra gain of the Darlingtons (as much as a factor of 20) hf instability is a possibility.

***By the way, if anybody is interested you can always figure the bias voltage from the schematic in these kind of amps... In this case you look at Q9. Depending on the pot rotation there is 10k-11k between its base and emitter which means 54-60uA. (I'm assuming .6V for Silicon vbe.) Thru the three-resistor string that makes 1.04-1.15V, and when you add in the two diodes its 2.24-2.35V. There is an extra 40mV due to the base current in Q9, which brings the bias to 2.3-2.4V. That base current is more complicated to figure out -- you have to start up at Q4 and work your way back downstream (and also look up the current gains of Q9 and Q12, and also know the supply voltage, which is +-50.)***

[J M Fahey]

I would put the proper Power regular BJTs there , which will be both "alive" (duh !), beefier, and get properly biased.
*Maybe* you still have other issues, but I think these will solve the worst.
Good luck.

EDIT: I *VERY MUCH* like the latest trend of the OP's providing schematics and precise voltage readings.
Way to go

[g1]

What a bizarre situation (regarding all different transistors). As I understand this brand is only available through one retailer and is very cheap.
If you could do some A/B samples and it makes a big improvement to the sound, maybe you could market it as a "mod" .

[R.G.]

I don't think the bias had to do with the failure. Its just curious. I made a schematic this morning...

Interesting schematic.

The bias generator is problematic. It makes no provision for what happens if the wiper goes open. This particular setup OPENS the bias voltage if the wiper goes open, and immediately fries both sets of outputs unless the current limit circuits save them. Sometimes that even works.

It's far better to put the pot and any limiting resistors in the base-emitter side of the bias transistor. If the pot wiper opens, the bias transistor is forced full-on by the collector bias and the bias voltage goes down. You get crossover distortion but not fried/exploded outputs.

The temp switch in the bias generator is interesting. Presumably that's to let the outputs cool down by lowering the bias. OK idea, I guess, but I don't know how you'd prove that would save things if the amp was overheating from heavy use or a heat sink problem.

I'm guessing Q9 is on the heat sink for thermal tracking. Is that right? They didn't put the two diodes on the heat sink did they? In a cascaded-Darlington output stage (this refers to the whole emitter-follower cascade, not just the output devices) thermal tracking needs to be to the final output devices.

It would work better IMHO if they'd put a current mirror on the collectors of the input diffamp. It would take only one more transistor, and would be a lot more stable and predictable, as that lets you put degeneration resistors in the emitters of the input transistors to make it more device independent.

And the TIP devices are inadequate in terms of safe operating area for this design/power supply. I agree with the concept of putting in beefier BJTs; in fact, if you put in two pairs in parallel, the thing begins to be downright sturdy. The second pair splits the output current roughly in half, so the power devices have about half the power dissipated internally, and that results in much less internal temperature rise, which keeps the outputs happier.

I have a Rogue 100W combo that I bought for $40 (!) from Craigslist. As you note, the insides are cheaply made. However, the mechanics are OK and roomy. So it's acting as a 'victim' for a soul transplant. I'm putting the front end of a UK Vox Supreme in it and reworking the reliability items. The chassis, power transformer, and heat sink are by far the hardest things to do on an amp, and these are OK. The electronics inside the Rogue is junky, but easy to replace.

[R.G.]

Something like this:

[woodyc]

...However, the mechanics are OK and roomy...

I agree, the chassis is an excellent mod platform. There is even room for a tube preamp.

The mods you suggest are good ideas, but the only really practical one is the extra two optput transistors. The board already has solder tabs for them. There must be a higher power Rogue and they used the same board for both?

Only the output transistors, drivers, and thermal switch are on the heatsink. The bias current is clearly going to rise as the amp heats up -- welcome to the world of Rogue.

[R.G.]

I would be tempted to strip the bias generator parts off the PCB and insert some kind of adjustable and safer circuit by attaching to whatever holes were there that could be used and gluing the other stuff on top of the PCB. Then run the temperature-sense transistor out to the output heat sink on a three-wire cable. Maybe put a BFC across the whole bias generator back on the PCB, and perhaps a string of limiting diodes across the whole bias generator to catch it from exploding if some genius cuts the bias generator cable.

I'd rewire the thermal switch to short the input or cut it by about 20-30db maybe on overtemp instead of dinking with the bias with it. Or hook it to the gate trigger circuit of a Triac in the main AC power so overtemp shuts down the AC power feed. I have some temp switches I bought surplus to do just that with on some retrofits.

You're right about the other mods - as long as you stick with that PCB...

[R.G.]

Just after I sent that last, I reflected that I've become very casual about PCBs over the years.

It hit me that the entire front end of that amp, right up to the driver transistor stage, could be hand-soldered onto a small chunk of perf board and simply replace what's there. It's some small signal and TO-126 or TO220 transistors at no great power, and the existing circuit could even be left there, just pulling out the connecting components to 'neuter' it. All the changes would stay on the nominal PCB, just attached to a hand-wired perfboard flying above it on standoffs.