Orange (Apollo) Bass/Treble Booster biasing

[soulsonic]

Someone should PM that guy and cajole him into re-opening that guitar to take a look. This is a mystery that deserves to be solved!

[gigimarga]

Very very interesting...i hope that somebody will reveal the mistery!!!

[frank_p]

That's the point I'm trying to make: I think the schematic - especially the Apollo one - is completely wrong. I believe this because of the simple fact that it barely functions (if at all) when built as illustrated.

I see what you mean Martin. That would really make sense because the Rangemaster would be biased strangely if using Si, but using leaking Ge, it would make sense.  On thee other hand, the Apollo is operating right on the saturation.  So if there is leakage, it's pushed OUTSIDE of the "3D" characteristic curve.  But even with a Si transistor, it would be on the edge of falling down in complete saturation. I guess it must fart hard.  I am just happy that I've analysed that simple thing before even thingking building it.  And you made me realise that I forgot to think about leakage, which is plain stupid on my side...

[Gus]

The 470K one looks like it should work

470K + 80K =550K 
9VDC / 550K = .016ma
.016ma X 80K = 1.28VDC

1.28VDC -.6VDC =.68VDC
.68VDC / 2K = .34ma
.34ma X 10K = 3.4VDC drop across the 10k
9-3.4= 5.6

1.28VDC - .3VDC = .96VDC
.96VDC /2k = .48ma
.48ma X 10K = 4.8VDC drop across the 10K

.6VDC and .3VDC are VBE drop

Simple calculation not taking into account leakage current

One could scale down the input resistors to make the bias more stable and predictable: however it would reduce the input resistance and load the input "tone" control and guitar

I would try a High Hfe Si like a mpsa18 as a test

[frank_p]

Thanks Gus.  I might try it now.

[gigimarga]

No good news...

[frank_p]

No good news...

What do you mean exactly ? You can breadboard it and I think you will have some results, with these values that R.G. an Gus pointed you at...

[Gus]

A mpsa18 is an NPN Si so one would need to adjust the circuit for +9VDC

[Harold]

I was very dissatisfied with the one I built as it just had unity gain and didn't improve when I tried a few different transistors.

Very true. I've built the "Apollo Treble/Bass Booster" aka "Texas Cattle Drive" several times for Weber customers and I'm convinced that either the schematics are completely wrong, or it's just a totally junk circuit.

Has anybody seen (or built) this schematic of the Apollo T/B booster?


http://www.free stompboxes.org/viewtopic.php?f=19&p=131057#p131057