Orange (Apollo) Bass/Treble Booster biasing

[gigimarga]

Hello,

I've just finished both an Orange and an Apollo Bass/Treble boosters, using these schematics:
http://www.geofex.com/FX_images/orngbst.gif and http://www.geofex.com/FX_images/apolobst.gif.

I replaced the 10K resistor from the collector with trimpot and i've adjusted the voltage on collector to -4.5V.

It's this method right or not?

I asked that because i saw a version (i forgot where...) in which the 80K resistor from base to ground was replaced with a 100K trimpot and it said there that it was "for base biasing"

For me, the biasing in a circuit like that means to set the collector voltage, not the base voltage!!!

Thx a lot!

[zachomega]

Setting the base voltage sets the collector voltage. 

Here is how.

You set the base voltage to whatever you choose.  The base voltage sets the emitter voltage by way of the base-emitter diode drop.  That is the emitter will be .6 (for silicon and .3 for germanium) volts lower that the base.  The emitter current is a combination of the base and collector current.  However, since the base current is so small, we can forget about it and say that the emitter current is just the collector current.  In that case the emitter voltage  (set by the base voltage) divided by the emitter resistor gives the emitter current which is the same as the collector current.  The collector current times the collector resistor equals the collector resistor voltage drop.  That is how you bias the collector. 

-Zach

[Gus]

The way the stock circuit is drawn is a "good" way to bias

If you want to change the bias point I would adjust the 150K before the 80K.  The 80K is the lowest value of the input bias resistor so this has more effect on the input resistance than the 150K

You could try 100K trim and a 100K fixed for a 100k-200K range

1/2 of 9VDC is not always what one might want for the collector voltage.

Drain and collector trims are not "designing"

[gigimarga]

Thx both of you for patience and kindness...i learned another interesting thing about the biasing!

In this case the next logically question is "how to bias it because i don't have the original transistors and i never heard an original?"

[gigimarga]

 

[zachomega]

What transistor are you using?  Or more importantly, is it silicon or germanium?  For a germanium transistor, bias it just as the schematics show.  I'm sure either variation will work and they will probably sound a bit different.  Either pick one and stick with it or build both and report back which one you like better.   

If you are trying to make a silicon transistor work in there, you might have to make small changes to the biasing to get it right. 

-Zach

[gigimarga]

Thx zachomega for your reply...i used a Hitachi germanium transistor, a 2SB77 as i remeber well

[JHS]

Both schems are not verified and it seems that nobody ever had analysed this booster and nobody really knows how the originals are biased.

Acc. to BSM originals have OC76 in it, so old AC122, old AC128 and old AC188 will work w/o altering any value if the hfe is between 60 - 70. Maybe, acc. to their specs., other Ge-trannies need a complete recalculation of the R-values.

I would subs the 80k with a 100k trimpot and adjust the gate-emitter voltage to 0,15 - 0,35V max.
0,15V is at least neccessary to prevent shutting off and at 0,35V the sound will be very soft and often has a mushy bass response.

There is a spot where the booster sounds fine so tuning the sound of the booster by ear is useful and if the trannie is one of the above types the collector voltage will be OK too.

JHS

[gigimarga]

Thx JHS for your answer...i like a lot this booster...one sounds very "glassy" and louder, another less louder and still enough "glassy"...i will try to replace the 80K resistor as you said and i will post the result

[soulsonic]

Both schems are not verified and it seems that nobody ever had analysed this booster and nobody really knows how the originals are biased.

I've noticed that alot of Vox circuits from this time period have Si transistors in them. I think that's something worth considering here - something like a BC109, etc...

Drain and collector trims are not "designing"

[col]

This one was definately Germanium but I can't remember which transistor. 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. I actually built a second to check it wasn't just me! The sound was right though. I had much more success with the booster that was sold through Guitar magazine, there was a schematic on here for it. That had a tone control and worked perfectly first time and lots of gain.

BTW I really like those Hitachi Ge transistors. I bought a bag full of 2SB156s, they have a gain of around 65-75 but are very quiet with no hiss and sound good whatever you put them in.

[soulsonic]

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. It's pretty much impossible to get a decent, usable sound from it with the stock configuration. When I built the Cattle Drives, I modified the circuit to be more like a Rangemaster with switchable input caps.
Drop that terrible tone control and increase the size of the input cap, and you will have a much better circuit... which is, at that point, nearly identical a Rangemaster. It's the tone control and tiny input cap that really cripple that circuit from being able to function rightly as a booster.

[gigimarga]

 

[frankclarke]

For the Apollo, R.G. has suggested that the 150k may be 750k. Worth a try, especially if the 150k doesn't work.

[Gus]

Just did some fast math on the two circuits using a Si.  The 470K should give about 5.6VDC at the collector (Funny some like RMs at 5 something volts).  The 150K did not make sense when I did some simple math

I added the input resistors together
then using 9VDC calculated the the current
took that current X 80K for the base voltage
subtact the B to E drop (I used .6) to find the emitter resistor voltage
take that and the resistor value find the current
take that current X10K for the collector voltage drop

Change the B to E drop for Ge

750K makes more sense than 150K

[soulsonic]

I almost always use 470K in my Ge boosters. The first time I tried building a Cattle Drive with the published "Apollo" values, the results were definitely not good.

Funny story about the creation of the Weber Texas Cattle Drive kit: I had built this Rangemaster clone after reading R.G.'s article about it. Anyway, I tweaked the circuit a little bit to work with an AC128 and I think I had it biased closer to -5v and it was sounding good. Anyway, this was when I worked at Weber, so I brought it into work and showed it off to the bosses and Ted thought it would be cool to do a kit of it. I gave them the schematic of my version of the pedal and datasheets for AC128 transistors so that maybe they could find an inexpensive equivalent from their Asian suppliers. A couple weeks later, a stack of boards shows up for this thing and a bunch of random transistors start showing up. I looked at the board and was confused to see that there were several additional components. Ted asked me to build one up to try out and he handed me the schematic. I was quite surprised (and a little insulted) that the schematic he handed me was a printout of J.D. Sleep's drawing of the Apollo Treble/Bass Boost circuit! And not only had he ignored my proven design, he also didn't bother to see about getting a good source for AC128-type transistors and instead just bought a bunch of random lots of untested transistors off of eBay... they weren't even all PNP, so you had to specify which kit a person was getting so he'd know which way to orient the polarized components, etc, etc, etc... And of course when I built the thing, it sounded terrible and the bias was way way off, and that's why they now include a selection of resistors to sub in there to try and get it right. Of course, good general values in the Base voltage divider and a trimmer on the Emitter would go a long way in making the biasing process much more effective, but of course that would require a new board design and Ted had already ordered a bunch of them (before even testing the circuit... ). That's around the time I realized that I didn't like working there.

[frank_p]

This is a noob question, but as I drew the Q (operating bias point) on the load line, I see that the Rangemaster is operating near the cutoff but the Apollo is operating near the saturation.  I am using the CE DC bias std equations.  Does the "upfront" "treble-bass" adjustment of the Apollo's section do play a role on the bias of the base ?  It does look like to me, but I am perhaps making a wrong evaluation.  If so (my circuit analysis class are way back in my mind), what is the benefit of being near cutoff ?  I tough that being near saturation was a better option.  Does it depends on the transistor that is used ?  The difference is "so drastic".  The base bias seems to me, so different between the two circuits.  If someone can give me a hint; I was waiting a post like this for asking that question.  Meanwhile, I was "hanging" in the lounge section making jokes, but I am a bit confused.

[frank_p]

Oh, Frank (the Other) by the way, I see you use brainmapping, have you tried to use Compendium ?

[soulsonic]

This is a noob question, but as I drew the Q (operating bias point) on the load line, I see that the Rangemaster is operating near the cutoff but the Apollo is operating near the saturation.  I am using the CE DC bias std equations.  Does the "upfront" "treble-bass" adjustment of the Apollo's section do play a role on the bias of the base ?

But you're assuming that the Apollo schematic is correct. I believe it is NOT correct, which means that any calculations based on this schematic would be erroneous compared to the reality of the actual circuit (if it even existed!).
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.

[frankclarke]

Compendium looks interesting, Open University UK.
There was a guy posting here with an Apollo guitar with a treble/bass boost and a broken fuzz, I suspect he might have the answer to the Apollo schematic.
http://www.diystompboxes.com/smfforum/index.php?topic=63422.0