GGG Brassmaster: Does anyone know the correct resistor values?

[MikeH]

Ok, I've done a search, but information on this issue isn't exactly definitive.  Perhaps an exact solution is yet to be found?  On the GEO Brassmaster, RG's layout, from GGG, the Brass selector switch produces no audible difference.  There are a lot of threads that discuss changing the resistor values in the twin-T filter.  It seems that one thread might have suggested that the labeling of the resistors in that area might be to blame.  And that maybe the layout has some in the wrong position?  I'm not sure.  Does anyone have the answer to this problem?  I built this for a friend, and the poor guy has been waiting for it forever.  I just want to get it working right so I can give it to him.  Can anyone help?

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=144&Itemid=171

[mountainking]

Its funny you should ask this question. I've spent the last week sick in bed with nothing to do but watch tv and look at schematics. Well I was looking at the brassmaster schematic(R.G. Keen's version) and decided to try and figure out why everyone seems to have problems with the filter/brass switch. It took me about two minutes to find the problem. The problem is not the resistor or cap values, its connection errors. Use Keen's schematic as reference so me explaination makes sense. First take the output of the filter drive and connect it the junction of C10 and R25. Next connect R22 and the .01 cap to ground. The filter switch should work great with these changes. The original schematic is wrong as is R.G. Keen's revision. I'm really suprised that no one has made these corrections already. Now of course any PCB's out there will need to be modified to accommodate these changes.

[R.G.]

The original schematic is wrong as is R.G. Keen's revision. I'm really suprised that no one has made these corrections already.

Hmmm... the original schematic is wrong? That would account for a lot of problems if I accurately reproduced an incorrect maker's schemo.

I always did think that setup looked screwy, but that's what the schemo said.

Do you have an actual device to compare it to?

[MikeH]

First take the output of the filter drive and connect it the junction of C10 and R25. Next connect R22 and the .01 cap to ground. The filter switch should work great with these changes.

Thanks for the reply!  But just to clarify.  The "output of the filter drive", would that be the collector of the "FILTER AMP" on the schematic?  So, at C12?

[moosapotamus]

 I would think that Q7 is the filter drive. So, like this... maybe??



~ Charlie

[mountainking]

First take the output of the filter drive and connect it the junction of C10 and R25. Next connect R22 and the .01 cap to ground. The filter switch should work great with these changes.

Thanks for the reply!  But just to clarify.  The "output of the filter drive", would that be the collector of the "FILTER AMP" on the schematic?  So, at C12?

No the "filter drive" is before the filter and the "filter amp" is after. Output of the filter drive comes from its emmiter/R18.

The original schematic is wrong as is R.G. Keen's revision. I'm really suprised that no one has made these corrections already.

Hmmm... the original schematic is wrong? That would account for a lot of problems if I accurately reproduced an incorrect maker's schemo.

I always did think that setup looked screwy, but that's what the schemo said.

Do you have an actual device to compare it to?

No, I don't have an actual device to compare it to, but really all you need to know is that its a twin t filter and without the changes I mentioned it isn't really a twin t filter since it won't really do much filtering and most of the components a basically bypassed. R.G. your updated schematic did correct another thing that I think was a mistake that was also in the original schematic and you certainly made the filter circuit much clearer. I've seen other Maestro schems that had obvious mistakes on them so I guess who ever was in charge of drawing them wasn't paying very close attention to what he was doing.

[mountainking]

I would think that Q7 is the filter drive. So, like this... maybe??



~ Charlie

You need to move the ground to the junction of R22 and the .01 cap.

[mountainking]

and don't forget to reconnect R22 back to the two .005 caps.

[puretube]

never...

[MikeH]

No the "filter drive" is before the filter and the "filter amp" is after. Output of the filter drive comes from its emmiter/R18.

Ahhh.  I see.  That's reading a schematic on a whole new level.  As in, reading the actual words on a schematic  .

Like this?

[mountainking]

Yes, correct. Sorry for my lack of visuals, but I'm at work right now.

[DavidRavenMoon]

The original schematic is wrong as is R.G. Keen's revision. I'm really suprised that no one has made these corrections already.

Hmmm... the original schematic is wrong? That would account for a lot of problems if I accurately reproduced an incorrect maker's schemo.

I always did think that setup looked screwy, but that's what the schemo said.

Do you have an actual device to compare it to?

I just built one of these today using your circuit board.  I had the real thing back in the 70's.  This is real close, but as you have noticed, the Brass 1 & 2 switch hardly does anything, and the Harmonic switch doesn't work like the original.

So I started looking though the two schematics to see if there was an error when the twin-T filter was rearranged.  After drawing it out a dozen different ways, I couldn't find any difference, except that you are hooking up one resistor parallel to the other instead of switching them, and there is a missing connection on the schematic from the left side of C10 to the junction of R28 and C12, but it is on the circuit board.

So I was puzzled since your board is exactly what was in the schematic.   Then I saw a post at TalkBass where the guy who makes the Barker Assmaster clone said

i've built one, and i've compared it to a real brassmaster.

I will say this. The brassblaster, is based off of the Maestro Brassmaster schematic.
And the Maestro schematic is not a correct schematic for the Brassmaster.

So it makes sense now. 

So now I'm going to try this mod and see if it's the right tone. 

But I love my Brassblaster... I've been longing for my old Brassmaster and they go for a tone of money these days!  Unfortunately I had the original circuit board until recently when a box of parts went missing when we moved.    (it's a long story, but my Brassmaster stopped working one day, and I was never able to get it repaired)

[R.G.]

So now I'm going to try this mod and see if it's the right tone. 

Do that. I'll update the schematic and layout if it's the right way.

[R.G.]

... preparatory to which I dug out my originals and started looking at the schemo.

Which brought back my original concerns. I thought "Twin T" when I saw that too. I was in the process of digging out the ground point when I realized that the original circuit as drawn drives the "ground point" with the output of an emitter follower and uses the T as a feedback across a voltage gain transistor - just like a proper bootstrapped twin T.

The obvious thing to do was to plop it in a circuit sim and try it out, but as I remember (that being 11 years ago now) I didn't have a handy dandy simulator to plop it into.

That being said, it's probably drawn incorrectly and needs to be regrounded for a non-bootstrapped Twin T config. But I'd sure like to hear from someone trying it both ways.

[DavidRavenMoon]

No the "filter drive" is before the filter and the "filter amp" is after. Output of the filter drive comes from its emmiter/R18.

Ahhh.  I see.  That's reading a schematic on a whole new level.  As in, reading the actual words on a schematic  .

Like this?

I tried this mod, and although it does make the switch work better, and more like the real thing, I get oscillation when the sensitivity control is more than a third up and the brass switch is in the open position (I think... it's one of the positions).

Also it should be noted that the above schematic is wrong, as there is a connection between the junction of R28 and C12 up to the junction of C10/R25.

Has anyone does this mod and not gotten oscillations?

[R.G.]

I think I just found the biggest problem with the Brassmaster schematic, and it's not necessarily the Twin T setup.

Take a look at the base of Q7. I believe that R29 should come off the T1 side of C13, not the Q7 side. It fouls the biasing of Q7 up tremendously to leave it as the schemo suggests.

I was plopping part in the simulator and when I got to there, the first question in my mind was "... and how does Q7 ever bias up?" It can't. With R29 as shown, very little signal gets through. Very different if you put it on the other side of C13.

It's an easy cut and patch on the PCB; alternatively, you can simply leave off R29 in the position shown and take a new "R29" off from the transformer or C13.

Of course, there can be other bugs in the Twin T setup, but clearing this one should make those easier to expose.

[DavidRavenMoon]

I think I just found the biggest problem with the Brassmaster schematic, and it's not necessarily the Twin T setup.

Take a look at the base of Q7. I believe that R29 should come off the T1 side of C13, not the Q7 side. It fouls the biasing of Q7 up tremendously to leave it as the schemo suggests.

I was plopping part in the simulator and when I got to there, the first question in my mind was "... and how does Q7 ever bias up?" It can't. With R29 as shown, very little signal gets through. Very different if you put it on the other side of C13.

It's an easy cut and patch on the PCB; alternatively, you can simply leave off R29 in the position shown and take a new "R29" off from the transformer or C13.

Of course, there can be other bugs in the Twin T setup, but clearing this one should make those easier to expose.

OK that sounds like you are onto something.  I thought it odd that R29 is on that side of C13.  Last night before I went to bed I switched everything back to the "stock" schematic version.  The only thing I left was swapping R22 and R27, because they are indeed mislabeled on the component layout.  They are correct on the schematic, but since I followed the layout when I populated the board, they were not installed in the right place.  With those parts where they belong the Brass switch does something at least.  It's very subtle, the original did more.

So this morning I plugged it in and played with it a bit.  The biggest thing that stands out is the huge volume boost when I switch the harmonic switch.  Yesterday I thought the lower volume tone was the correct one, but now I'm convinced it's more like the louder tone, since that has more octave harmonics, so saying that very little signal gets through the filter makes sense.  I also notice that the harmonics come out better if I turn the volume on my bass down a bit (regardless of the Sensitivity control setting).

The mod that added the ground between R22 and C11 and also moved the connection of Q7's emitter isn't right either. While this does give more change when you switch the switches, it's way too buzzy and seems to have increased the gain through the fuzz.  As I mentioned on one of the Brass settings I would have uncontrollable oscillations as soon as I connected the power, even with no bass plugged in.  Turning down the Sensitivity would stop it, but that's not how the original is. 

Speaking of the Sensitivity control I don't remember it turning all the way off.  It's admittedly a long time since I played a real Brassmaster, but I seem to remember you could use the sensitivity control all the way down, and still have some fuzz signal.  I might be wrong though, and it works fine this way too, as very low sensitivity settings often get cool tones.

I'm going to try moving R29 now. If that brings the filter signal level up to the lo harmonic setting, that might be the fix. (fingers crossed) 

[DavidRavenMoon]

OK, I did the R29 mod.  I lifted the one end out of its hole and ran a jumper from the T1 side of C13.

I think it made an improvement to the low Harmonic setting, i.e., with the switch closed.  It's still louder than the high setting, but it has more of the envelope tone the Brassmaster does.  I like it.  The Brass switch still does pretty much nothing, slightly brighter in one setting.

I'm quite happy with the tone now, and if we never figure out the T filter issue, could live with this tone, as the Sensitivity control really gets a lot of variation.

Now I'd like to make one with tone controls for the clean and fuzz cannel... maybe a variable low pass filter.  In the mean time I might change the Brass switch to work like a tone control to roll some highs off.

[R.G.]

Ah, OK. It pretty much had to be that way. I can hypothesize a couple of different connections for the Twin T setup.

One is with the grounding rearranged and the Twin T filtering what goes into the amplifier. I'll go see about drawing that up.

[R.G.]

Interesting stuff from simulation.

The filter transistor has its bias changed all over the map when the filter hi-lo switch is flipped. Q7's collector is isolated from the DC conditions on the base of Q6 by the T-capacitors, but the base of Q6 is connected to DC ground through the T, the 150K, and the 10K volume pot. This is somewhat ugly, putting DC across the pot as it does. The change from 47K+47K to 6.8K+6.8K when flipping the filter switch changes the transistor's biasing significantly as well. Moving the "forgotten connection" of the junction of R25 and C10 from the 150K side of C12 to the collector side of C12 helps, and in one of the filter switch positions produces a nice resonant peak in the response. What should happen, I think, is that you should get two resonant peaks, one lower and one higher in frequency. The biasing problems are keeping this from happening.

I have no problem making the circuit do that on my own, but not ever having seen a real one limits my ability to make it "authentic."

I may rework this into an opamp version. That would be far more predictable.