Trace Elliot Bipolar Bear Repair

[diydidi]

Im busy repairing this amp module. Installing the recommended higher SOA transistors and ditching the TIP35/36 outputs. This is the 300W module. I noticed an interesting resistor on the bottom of the board, not shown on the original schematic. I have attached it here with the resistor added.
There are actually two resistors on the bottom of the board. The one on the right is just part of the fan circuit.
Any idea why the 4k7 on the left was added in this weird way?
i have confirmed with other stock photos on the net and they all have this resistor.

[J M Fahey]

Weird.
That is an "almost do nothing" resistor because both ends are at practically same DC voltage, but even more, both swing together so itīs not applying NFB either.

The voltage path is:
Tr11 base > Tr11 collector which is about "2 diodes" higher > Tr13 base same voltage > Tr13 emitter (you go down 1 diode) > Tr14 emitter (you lose 1 diode)

My Math says: X + 2 - 1 -1=X (original voltage)

"1 diode" means 1d ode drop, about 0.7V

If you apply same voltage at both ends of a resistor, no current flows through it, so in practice "it is not there".

That said, this applies to the ideal world.

In real world, not all "1 diodes" are exactly same, since transistors mentioned are all different, currents through them also, so maybe that 4k7 provides a *tiny* correction to Bias.
I see no other useful purpose there.

[glebert]

Well, it is some sort of feedback, and since no one ever tries to build in positive feedback in an amp it seems it has to be a little negative. Considering the reputation these modules have for self immolation I would guess this is a band-aid applied to try to prevent that.

[J M Fahey]

Band aid? Sure.

Not here but in another Bipolar Bear thread or a GK one, I mentioned I was very mislead and disappointed by TIP35/36 in the past.

They look very good on paper (consider they have been around for decades), 100Vce, 25A , 125W ... but they do have terrible SOA, at last in practice, where they blow easily on complex, real world loads, such as speakers.

Notice how allowable dissipation strongly kinks curves down, meaning it can stand less, barely above 30 something Volts, fully negating its usefulness in a +/- 40/45V rails amp.

Even worse in Musical Instrument duty, where they are guaranteed to be abused.



Given resistor goes from output to *bias transistor*base and not to amp differential input, also being direct coupled, I guess the idea is to improve biasing, maybe minimize thermal drift?

In any case, I would leave as is, doubly so if you found other amps with same patch applied.

[Luis_vxd]

I know this post is old, but it seems to me that resistor is, in fact, in parallel with R37, probably trying to increase the voltage of the fan which is quite lower than what the documents say.

[g1]

I know this post is old, but it seems to me that resistor is, in fact, in parallel with R37, probably trying to increase the voltage of the fan which is quite lower than what the documents say.

In the first post, he got the 'left' and 'right' resistors mixed up. The one on the left is for R37, like you said. The one on the right is connected as shown in the schematic picture in post #1 (drawn in red, '4K7').

[Luis_vxd]

There should have been a lot of version of that Bipolar Bear. The one I am working on is quite different.