Boost switch explained

[Tdale]

I'm having trouble figuring out the boost switch. It's supposed to bypass the EQ, after what I understand..?

I look at the Hybrid-A schematic, but I just can't figure out what the boost switch does... or how it works at all.

It probably has to do with that I don't get the tone stack completely... I understand partly how the tonestack work, but not completely.

I have also seen several different ways of wiring the boost switch (I think)

Could anyone explain what the boost switch does to the signal flow in the Hybrid-A schematic, and how it does it?

Tommy

[jaysg]

You have to step back to the Blackface amp tonestack lift. Put a switch at the bottom of the 6K8 or Mid pot. The tonestack knocks the level significantly and that's why the next stage is called the recovery stage. Open the circuit and you get a huge boost, however, the knobs don't do much. More subltely, you can put a 500K or 1M pot there and dial in the amount of lift, then the pots have diminished effect, depending on the lift.

Dumble, always one to tune things, doesn't completely disconnect everything. I think the treble pot still works...can't recall since I just woke up.

If you don't have Duncan Munro's tonestack program, you want it. The power supply app is fun too. http://www.duncanamps.com/

[Bob-I]

It probably has to do with that I don't get the tone stack completely... I understand partly how the tonestack work, but not completely.

I'm with you on that one.

I have also seen several different ways of wiring the boost switch (I think)

Could anyone explain what the boost switch does to the signal flow in the Hybrid-A schematic, and how it does it?

Tommy

The one that I use is the one on the Hybrid-A. By disconnecting the treble from the bass the signal now passes through the treble cap. I also don't understand why the signal doesn't go to ground through the mid cap.

Confused too.

[Luthierwnc]

The boost disconnects the tone stack between the treble and bass pots. Attached is how the schematic would look if there were no mid-boost, Jazz/Rock or Deep switches. The bass and mid caps still reference ground but the signal path of least resistance is across the treble pot straight to the volume pot.

Take a red pencil and trace the Hybrid schematic across the hodgepodge of switches. The connections shown are the boost in the off position and the rock side. When the boost is off, the #1 lug (read right to left if you are looking at the back of the pot) of the treble pot joins the bass cap on lugs 2 and 3 of the bass pot.

I hope this helps, Skip

[Tdale]

OK.. Let me see..

If the switch is closed, the signal have two paths to the volume pot:

1 - Through the treble cap, and through the treble pot.

2 - Through the slope resistor, through the mid cap, through the treble pot

I still don't really get why this would bypass the tone stack. If the mid pot is in the end position, won't the signal go through the 10K resistor, to ground in stead of through the treble pot? Won't it all depend on what position the tone pots are in, when the boost switch is activated?

Also, I might be wrong, but it looks like the layout is different from the schematic... should the bass and middle pots change names in the layout....?

For instance, in the layout, the mid pot is grounded at the left lug, while in the schematic, the same pot is grounded through the 10K resistor.

Or is it simply the two caps (0.1 and 0.01) that have changed places in the layout?

[Tdale]

You have to step back to the Blackface amp tonestack lift. Put a switch at the bottom of the 6K8 or Mid pot. The tonestack knocks the level significantly and that's why the next stage is called the recovery stage. Open the circuit and you get a huge boost, however, the knobs don't do much. More subltely, you can put a 500K or 1M pot there and dial in the amount of lift, then the pots have diminished effect, depending on the lift.

Dumble, always one to tune things, doesn't completely disconnect everything. I think the treble pot still works...can't recall since I just woke up.

If you don't have Duncan Munro's tonestack program, you want it. The power supply app is fun too. http://www.duncanamps.com/

I was kind of wondering _why_ it gives you a boost...what actully happenes in the circuit...where does the signal flow when the boost is activated, compared to when it's not.....

Tommy

[Tdale]

Also.. I looked at the hybrid schem again.. It seems that the position of the rock/jazz switch affects the signal flow through the boost switch... Or am I reading wrong?

Tommy

[Pete]

I was kind of wondering _why_ it gives you a boost...what actully happenes in the circuit...where does the signal flow when the boost is activated, compared to when it's not.....

As I understand it, in boost mode there is less signal going to ground than in normal mode when much is being lost (to ground) through all the caps, resistors & pots (i.e. tone stack stuff).
That is the simple answer I guess but the point is (in boost mode) you aren't getting gain/boost from some place, you are eliminating much of the (normal) loss.
Kinda reminds me of making it more "Fender Tweed amp" like. An example of an amp that I think has less tone stack loss (easy breakup of the front end). not sure of my clarity but hope this helps some.
Pete

[dogears]

You must also break the wiper of the bass pot. Otherwise, you have alot more signal going to ground and a much less full PAB tone depending on bass pot setting.

[Tdale]

back to basics..

Is the boost switch active when the connection to the other side of the treble is broken or when it's connected?

I'm looking at the hybrid-a schem, and the rock/jazz and boost switch arrangement confuses me more and more.

The way it's drawn in that schem, the bass wiper goes through the boost switch, then through the rock/jazz switch, then on to the right side of the treble switch.

If the boost switch is switched, both the bass wiper and the right side of the treble pot are isolated.....

I'm having trouble sleeping now... I can't figure this out!


Tommy

[Luthierwnc]

S#*t! I put up the wrong one! Sorry. Try this:

[Luthierwnc]

And here's the layout. I haven't use a Jazz/Rock switch so this is the one that went in my two amps. It is pretty effective. FWIW, I broke the two hot legs off the relay so I didn't have to install extra turrets. sh

[HiGain]

The switch breaks the cold lug (1) of the treble pot and the wiper of the bass pot. The signal then travels mainly through the treble cap and lugs 3 and 2 of the treble pot. It also leaks a little through the slope resistor and bass pot (the maximum value of the pot).

Please point out errors in the drawing.

-Jake

[tictac]

Try this: Note the difference, better tone with this...

[tictac]

whoops not wired right, try this: The .1 should be part of the circuit in boost mode.