GGG Brassmaster: Does anyone know the correct resistor values?

[gigimarga]

@DavidRavenMoon: I can't download your soundclip with your fixed Brassmaster clone. Can you post it on another server?

[George Giblet]

I do have some questions about this design:however they are really for the original designer(s) of the circuit.  I will post them anyway most are why some of the transistor operating points?

I suspect the Emitter resistors of the gain stages are tweaked to produce the desired gain.  The voltage drops across the emitter resistors are so low that they don't really help with any bias predictability.  [Edit:  For got to mention because they wanted higher gains in the circuit that needed small emitter resistors relative to the collectors resistors.  They probably didn't want to increase the component count by using larger bypassed emitter resistors, so they just put up with the poorish biasing - feedback biasing isn't too bad anyway.]

Regarding Q1, the low feedback resistor, while making the bias more predictable with the other 150k, is going the make the input resistance quite low and it will load the pickup - deliberately loading the pickup?   

The tap off the emitter is obviously an attempt to get a free buffered (gain of 1) signal for the straight through path, but I suspect the fact the bias point is very marginal is an oversite.

The output mixer (Q6 and Q7) appears to be a standard Maestro building block. The same circuit appears on the Maestro Octave Box, however the C-B resistor is larger (1.5M) on that one and produces a better bias point.  Presumably that was to adjust for different transistor gain but maybe not:  1.5M vs 470k is a factor of 3 and it is unlikely the gains would vary that much!

Regarding the bias shifting - that is strange.  I can only guess it's an oversite, or something disregarded.

Regarding the generally "poor" biasing - who knows, oversite vs. deliberate sound shaping?


BTW:  Using gtrgeek1's voltages I've estimated the hfe of the non-darlingtons to be about 130.

[R.G.]

I suspect the Emitter resistors of the gain stages are tweaked to produce the desired gain.  The voltage drops across the emitter resistors are so low that they don't really help with any bias predictability. ...

The tap off the emitter is obviously an attempt to get a free buffered (gain of 1) signal for the straight through path, but I suspect the fact the bias point is very marginal is an oversite.
...
Regarding the bias shifting - that is strange.  I can only guess it's an oversite, or something disregarded.

Regarding the generally "poor" biasing - who knows, oversite vs. deliberate sound shaping?

One of my favorite quotes is from Napoleon: "Never ascribe to malice, that which can be explained by incompetence."
At the time of the design, good EEs were snapped up by military contractors, and in short supply. Component sensitivity was still a research issue.

Regarding Q1, the low feedback resistor, while making the bias more predictable with the other 150k, is going the make the input resistance quite low and it will load the pickup - deliberately loading the pickup? 

Yeah, see my note from earlier in the thread. It's about 17K to 18K input impedance, and loads the daylights out of the incoming signal. Of course, as David said, it is a bass effect.
 

[DougH]

See Gus's Octave Up Sick Box for an earlier example of an op amp based phase inverter in a pedal.

[R.G.]

I have the new doco package up at GEO for checking.

For checking...

If you have a hankering to, (please, David and gtrgeek) please look at the new layout and schemo and let me know where my typical absent mind has missed something.

[Plinky]

The transistor designations follow the original factory schematic.
Q1,2,3,4,6,7 are 2N3392 factory marked 83930. All have an orange dot on the top, possibly matched or selected.
Q5 is 2N5308 factory marked 135 43C. Pinout matches 2N3392 not schematic pinout.

Uhh...according to the factory schematic, Q6 is the 2N5308. Am I missing something? I've been following this thread very closely anticipating a resolution (ie, new pcb layout). I've hacked my pcb so many times that I need to etch another one, so I want this to be correct as well if I am to start over from scratch.

[Plinky]

DOH! You beat me.

[DavidRavenMoon]

I have the new doco package up at GEO for checking.

For checking...

If you have a hankering to, (please, David and gtrgeek) please look at the new layout and schemo and let me know where my typical absent mind has missed something.

Will do!

[gigimarga]

I replaced the diodes from 1N4148 to Schottky (1N5817) as R.G. suggested and it sounds less gated obviously (all 4 diodes matched around 160mV).

But when i connected the Q1's collector from 9V to 8V it started to oscillate very strong, so i gave up and went back to 9V.

I tested it on a Squier guitar, not on a bass guitar (i liked the clips with B:assmaster used on guitar...)

@DavidRavenMoon: After a lon battle I downloaded the clip...

Thx a lot all!

[DavidRavenMoon]

@DavidRavenMoon: After a lon battle I downloaded the clip...

Are you on a dial up connection, or was there a problem downloading the file?

[George Giblet]

> Component sensitivity was still a research issue.

Actually that's a good point.  What we see as obvious today probably wasn't in those days (hence all the dodgy early designs).  Transistors were new strange beasts at the time.

From our era it's like switch mode power supplies. Once mystical beasts studied by researchers, now throw-away items that come with your mobile-phone.

[George Giblet]

> Uhh...according to the factory schematic, Q6 is the 2N5308. Am I missing something? I

You have to watchout what the Q designators are in this thread.  Some posts are using RG's mark-up because it matches the layout most people have built,  others use the original schematic (which is that maestro bb-01 link that appears a few times in the thread).

[R.G.]

The original schematic had no part numbers for the resistors and capacitors, only transistors and some of the controls. The version I did that's being sold by GGG has numbered parts; I back-annotated the factory schematic with part numbers matching my PCB layout - getting one pair of them reversed in the process.

I have redone the layout, and the schematic. This has yet a third, different set of part numbers.

Part numbers have no meaning outside the context of the schematic and/or PCB to which they refer or link together.

So yes, you must be scrupulously careful about which schematic and/or PCB or both you refer to when referencing part numbers.

[gigimarga]

@DavidRavenMoon: After a lon battle I downloaded the clip...

Are you on a dial up connection, or was there a problem downloading the file?

No, i have a good cable connection...but IE cann't open the file...only with Firefox using a proxy

[DavidRavenMoon]

@DavidRavenMoon: After a lon battle I downloaded the clip...

Are you on a dial up connection, or was there a problem downloading the file?

No, i have a good cable connection...but IE cann't open the file...only with Firefox using a proxy

That's odd... it's just an MP3 file.  I'm on a Mac and I can play it from the browser on both Firefox and Safari.

MS sure likes to complicate things.   

[R.G.]

Few people appreciate the extents to which microsoft went to protect digital media rights in Vista. Many are not apparent in any way except "gee, that sure is a funny way for that to (not)work."

If Vista is involved, it may be working as designed.

[gigimarga]

I use Windows XP, so no Vista here .

I think i have some problems with the Internet provider (some security settings) because i can open http://topopiccione.atspace.com only using an proxy server (and the same thing for analogguru's site and another DIY site).

Thx both of you!

[Gus]

Little off topic but take this
http://www.diystompboxes.com/pedals/ousb.gif
    10uf and to to the right remove increase the 270 ohm resistor value and add
the 2.2ks,  diodes and  transformer  from the Bass Ackwards and a volume control at the output of the transformer

Bass Ackwards looks like it might be a fun build.

[majormono]

O.k., since it's oh-so-bitter-cold outside and I had to wait half an hour I could not resist to play around with the Brassmaster circuit... just more or less random mods with possible errors in thinking and drawing thrown in for free. Anyway I'd call it food for thought and occupational therapy  . Well, no - I seem to have nothing better to do, but thanks for asking... be gentle with a fool in the cold.

[DavidRavenMoon]

O.k., since it's oh-so-bitter-cold outside and I had to wait half an hour I could not resist to play around with the Brassmaster circuit... just more or less random mods with possible errors in thinking and drawing thrown in for free. Anyway I'd call it food for thought and occupational therapy  . Well, no - I seem to have nothing better to do, but thanks for asking... be gentle with a fool in the cold.

Interesting... I like the improved input impedance.  I was thinking of doing that to mine, though I'm probably going to build a second unit and mod that. Just so I have a unit fairly stock.

Two thoughts though, the Sensitivity control shouldn't be a trim.  They shouldn't have called it Sensitivity either, it should have been labeled "Drive".  It's key to some of the tones you can get from this pedal.  I often have it almost off, and then sometimes all the way up, or half way.

I'm not sure a blend control would work well either, since some of the fuzz settings are much quieter than others. So sometimes you have to really crank the fuzz while keeping the dry signal about the same.  A blend would probably need a master volume control.