Guitar amplifier wierdness

[Frazzled]

I purchased a guitar amp yesterday and opened up the chassis today to clean out the dust and crud and I found two resistors that have had one leg clipped to remove them from the circuit.

If you look at the schematic HERE look at R102 and R105 on the base of Q1 and Q2 respectively.

Any ideas why one side of the resistors would be lifted from the circuit?

Thanks,
Don

 

[anatech]

Hi Don,
Believe it or not, the over current protection circuit was removed! Check the emitter resistors for the correct value and reinstall those resistors.

-Chris

 

[audiofan]

This is a protection circuit modification to get more power before protection get in and limit current around 150% of initial rating

 

[anatech]

Hi audiofan,

This is a protection circuit modification to get more power before protection get in and limit current around 150% of initial rating

I figured as much. It's still a stupid thing to do. The original designer had the limit designed in for a reason. I could see the protection being backed off a little, possibly a time limit out in to delay action. Disabling the circuit entirely is the wrong way to go about it.

-Chris

 

[Frazzled]

GEEZE!

I wonder if this was done to hide a problem with the amplifier or strictly for more power...

Well as you can see they're 3.3k resistors... I'll be re-installing them shortly, I'll let you know what happens.

Thanks,
Don

 

[djk]

"I could see the protection being backed off a little, possibly a time limit out in to delay action. "

The 100µF caps should delay the VI limiter long enough so that it doesn't activate on any musical signal, but still protects in the event of a short.

At least Fender used three sets of outputs, not like the newbies here that think they can run one set and have the amp hold together.

Grounded output stage, makes for a simple drive circuit.

Some current feedback, the later Carver M1.0T used a similar scheme.

 

[anatech]

Hi djk,
Yes, but Carver found the current feedback affected the gain greatly with even 1% components in the feedback loops. They therefore used a pot which caused them more grief further down the line. Just did an M 4.0t for that. 😉

I still think disabling a protection circuit completely is not intelligent.

-Chris

 

[Frazzled]

OK but looking at the circuit it is not completely disabled only partially right?

 

[anatech]

Hi Frazzled,
It is for the one pair. The proper solution would be to either change all those resistors to another value, or fix the specific problem that caused the amp to go into protect (assuming the tech couldn't find the problem due to high or open emitter resistors). There is no guarantee that those transistors will current share given how that stuff usually gets fixed.

djk,

The 100µF caps should delay the VI limiter long enough so that it doesn't activate on any musical signal, but still protects in the event of a short.

Maybe not. These amps do not reproduce music. Often they are run into sustained clipping. Therefore what we instinctively have a feel for does not apply to guitar amps sometimes. I had to think about this a little.

-Chris

 

[djk]

The delay looks like 500mS or so, and the current about 10A peak. Looks like it could pass a 400W squarewave at 4 ohms for an indefinite time, but go into limiting on lower impedances.

 

[anatech]

Hi djk,
That may be, but as we all know the average guitar player may just add stacks until it blows.

I still say that exposing the last pair of transistors without protectin is foolish. How many guitar amps do you see repaired properly from other shops and roadies?

Maybe the supply collapses before the long term limits are reached (thermal for one).

-Chris