Dumble Mid-Boost question

[Colossal]

Hi All,

This is my first post in Dumbleland although I follow the threads here with great interest.

My question is regarding the mid-boost (Deep) switch as shown on the #124 schematic. Steve of S2 Amps built a killer custom 100w amp for Joe Bonamassa based on a '68 Plexi but incorporated a Dumble Mid-Boost as Joe apparently likes a bit more mid presence to bring the tone up front. I am trying to sort out what is happening in the 124 schematic and how that may be applied to the Marshall tone stack. From what I can make out from Steve's photos on his website of the gutshots from JB-100, there appears to be an additional silver mica cap linked to the tone stack. I'm guessing that this may be a 300pf cap as shown on #124, but can't be sure of the path or the value. It looks like the switch adds/removes this cap from the path, but there may be more to it; I just can't tell.

Can anyone lend a hand?

Thanks, in advance,
Dave

[glasman]

Look at the schematics in the files section. There are a LOT of examples

Normally the mid boost is acccomplished by putting an .002uf in series with a 390pf(or sme other value) for the treble cap. The Mid switch shorts out the 390pf switch which drops the Treble break point (a lot).

Gary

[Bob-I]

It simply adds another cap in series with the treble cap. Most cases the standard treble cap is a 390pF, the mid boost adds a .001uF in series. The 4.7M resistor between them is to prevent pop with switching.

[Colossal]

Look at the schematics in the files section. There are a LOT of examples

Normally the mid boost is acccomplished by putting an .002uf in series with a 390pf(or sme other value) for the treble cap. The Mid switch shorts out the 390pf switch which drops the Treble break point (a lot).

Gary

Thanks for the explanation Gary. Yeah, I have all of the schematics, just wondering if it's as simple as applying the same methodology to the typical Marshall tonestack. Seems pretty straight forward.

It simply adds another cap in series with the treble cap. Most cases the standard treble cap is a 390pF, the mid boost adds a .001uF in series. The 4.7M resistor between them is to prevent pop with switching.

Hi Bob, thanks for the help. I'll consult the #124 schematic again and make a drawing.

I appreciate the fast response guys, thanks a lot.

Regards,
Dave
_________________

[Tonegeek]

Look at the schematics in the files section. There are a LOT of examples

Normally the mid boost is acccomplished by putting an .002uf in series with a 390pf(or sme other value) for the treble cap. The Mid switch shorts out the 390pf switch which drops the Treble break point (a lot).

Gary

Thanks for the explanation Gary. Yeah, I have all of the schematics, just wondering if it's as simple as applying the same methodology to the typical Marshall tonestack. Seems pretty straight forward.

It is easy enough to try. Just clip a cap across the Marshall's treble cap and you will get an idea of what is going on with the Dumble. Try a 500pf cap for starters and work your way up to where the total (original cap + the one you are clipping in) is .002 which is a typical Dumble value with mid boost engaged.

[Normster]

It simply adds another cap in series with the treble cap. Most cases the standard treble cap is a 390pF, the mid boost adds a .001uF in series. The 4.7M resistor between them is to prevent pop with switching.

I know that you know how this works, but the way you explained it isn't quite correct.

Normal mode uses .0022uF plus 390pF in series resulting in a treble cap value of 331pF. Mid boost simply bypasses the 390pF resulting in a treble cap value of .0022uF.

[Colossal]

It simply adds another cap in series with the treble cap. Most cases the standard treble cap is a 390pF, the mid boost adds a .001uF in series. The 4.7M resistor between them is to prevent pop with switching.

I know that you know how this works, but the way you explained it isn't quite correct.

Normal mode uses .0022uF plus 390pF in series resulting in a treble cap value of 331pF. Mid boost simply bypasses the 390pF resulting in a treble cap value of .0022uF.

Hi Norm,

Thanks for clarifying which mode is which! I've been studying Steve's (S2) photos of both Joe Bonamassa custom Super Lead-style amps that he built which incorporate the Dumble mid-boost. The placement of the sliver mica treble cap is laid out slightly differently in each amp but I suspect the functionality remains the same. It looks like he has a 3 way switch for the boost. I'm working on a drawing which I will post shortly to incorporate the mid-boost into a Marshall tone stack (hopefully I'll get it right). This exercise is largely academic but I am thinking of adding it to my 6V6 Plexi for kicks. I find I like the mids cranked, it really projects nicely so am wondering how the Dumble boost might affect that. Bonamassa states how he likes to blend amps with differing degrees of mids. I've been on a blues binge lately and really dig his tone.

Thanks again for the help.
Dave

[mat]

It simply adds another cap in series with the treble cap. Most cases the standard treble cap is a 390pF, the mid boost adds a .001uF in series. The 4.7M resistor between them is to prevent pop with switching.

I know that you know how this works, but the way you explained it isn't quite correct.

Normal mode uses .0022uF plus 390pF in series resulting in a treble cap value of 331pF. Mid boost simply bypasses the 390pF resulting in a treble cap value of .0022uF.

Hi Norm,

Thanks for clarifying which mode is which! I've been studying Steve's (S2) photos of both Joe Bonamassa custom Super Lead-style amps that he built which incorporate the Dumble mid-boost. The placement of the sliver mica treble cap is laid out slightly differently in each amp but I suspect the functionality remains the same. It looks like he has a 3 way switch for the boost. I'm working on a drawing which I will post shortly to incorporate the mid-boost into a Marshall tone stack (hopefully I'll get it right). This exercise is largely academic but I am thinking of adding it to my 6V6 Plexi for kicks. I find I like the mids cranked, it really projects nicely so am wondering how the Dumble boost might affect that. Bonamassa states how he likes to blend amps with differing degrees of mids. I've been on a blues binge lately and really dig his tone.

Thanks again for the help.
Dave

Very interesting !

I'm building a marshall silver jubilee 2555 and would also like to include the proper (JB one) midboost in it. Looking forward for Your drawing.

Thanks,

[Colossal]

Very interesting !

I'm building a marshall silver jubilee 2555 and would also like to include the proper (JB one) midboost in it. Looking forward for Your drawing.

Thanks

Sliver Jubilee eh? That's awesome! I'd like to see how your amp comes out. I've been really studying Joe's rig and reading as many articles as I can where he discusses his approach to tone. He says that he uses the 2555 as the "always on" amp and it has a scooped mid EQ. He adds to that the Category 5 Super Lead clone with the Dumble Mid-Boost for one of three possible tones (the Van Weelden Twinkleland/Carol-Ann, or a Two-Rock are the other two as you probably know). The Dumble clones have a much more pronounced mid presence.

As for the mid-boost on the JB-100 Super Lead Clone, I don't know what value was chosen as the stock treble cap, but I have a layout from Metroamp for the '68 SuperLeads which shows a 560pF. On Steve's/S2 website, he has gut shot photos of both Joe's American and European Category 5 Super Leads (with the boost). There is a slight difference in the layout of the mid-boost between the amps, at least from one I can discern from the photos. The American amp has an extra silver mica cap in series with the standard silver mica cap on the tone stack. I'm guessing that the mid boost switch just bypasses the first cap as Normster, Gary, and Bob-I have pointed out. On the European JB-100 Super Lead, there is a sliver mica cap in line with the two mixing resistors. I've enclosed a picture contrasting the two as well as a very quick attempt at trying what Normster mentioned (I may have gotten it wrong).

Dave

[mat]

Hi Dave ! Thanks for the picture. I'm also big fan of Joes sound and playing. I've also followed his setup quite closely. I know the mid boost is not on the jubilee but I would like to try it in there because I like those mid heavy sounds too
I'm building the 2555 first as 2550 (50w) because I have the irons from TW-build I gutted. When I have enough to spend I'll buy Marshall irons.

The chassis is quite small that I'm using. I'll have to put the can caps inside the chassis to have room for the power tubes + 2 trannies and the choke on the top of the box.

I'll let You know how it turns out. This one I'm building slowly because the rush at work..

btw. I don't know where You live but are You planning to go to Joes RAH concert ?

[JimiB]

That is not the traditional d style mid boost in those amps.
Looks like the switch is activating the .68 bypass cap on V2A and is also passing the signal from the mixer resistors thru that silver mica cap to V2A which would roll alot of low end off.

[Tonegeek]

Very interesting !

I'm building a marshall silver jubilee 2555 and would also like to include the proper (JB one) midboost in it. Looking forward for Your drawing.

Thanks

Sliver Jubilee eh? That's awesome! I'd like to see how your amp comes out. I've been really studying Joe's rig and reading as many articles as I can where he discusses his approach to tone. He says that he uses the 2555 as the "always on" amp and it has a scooped mid EQ. He adds to that the Category 5 Super Lead clone with the Dumble Mid-Boost for one of three possible tones (the Van Weelden Twinkleland/Carol-Ann, or a Two-Rock are the other two as you probably know). The Dumble clones have a much more pronounced mid presence.

As for the mid-boost on the JB-100 Super Lead Clone, I don't know what value was chosen as the stock treble cap, but I have a layout from Metroamp for the '68 SuperLeads which shows a 560pF. On Steve's/S2 website, he has gut shot photos of both Joe's American and European Category 5 Super Leads (with the boost). There is a slight difference in the layout of the mid-boost between the amps, at least from one I can discern from the photos. The American amp has an extra silver mica cap in series with the standard silver mica cap on the tone stack. I'm guessing that the mid boost switch just bypasses the first cap as Normster, Gary, and Bob-I have pointed out. On the European JB-100 Super Lead, there is a sliver mica cap in line with the two mixing resistors. I've enclosed a picture contrasting the two as well as a very quick attempt at trying what Normster mentioned (I may have gotten it wrong).

Dave

On your schematic you would want your switch to bypass the 390p cap, not the .0022 cap.

[Colossal]

Hi Dave ! Thanks for the picture. I'm also big fan of Joe's sound and playing. I've also followed his setup quite closely. I know the mid boost is not on the jubilee but I would like to try it in there because I like those mid heavy sounds too

Sounds cool Mat. Yeah, I'm just trying to figure out what's going on in Joe's Super Lead clone with the mid-boost. Super Leads have kind of a flat EQ relative to the Jubilee or the Carol-Ann or Van Weeldens. For what it's worth, I added a 50K pot in series with the 33K mid slope resistor on my Plexi and you change the character of the mids with it. It's pretty cool although I have been just been leaving it around 33k with the mid pot turned all the way up.

btw. I don't know where You live but are You planning to go to Joes RAH concert ?

I'm just south of Houston, TX. Unfortunately, Joe isn't playing anywhere around here on this leg of his tour Guess I'll just have to watch the videos on his website for my concert fix...

Regards,
Dave

[Colossal]

That is not the traditional d style mid boost in those amps.
Looks like the switch is activating the .68 bypass cap on V2A and is also passing the signal from the mixer resistors thru that silver mica cap to V2A which would roll alot of low end off.

Jimi,

Your explanation is interesting and I think you are onto something. In looking at the photos of the mid-boost switch on Joe's amp, there is more at work than just switching a mica cap in and out of series with an 0.0022uF cap as shown on the #124 Dumble schematic. I don't even see an additional 0.0022uF cap in the tone stack on Joe's amp either. I'm confused though why the layout is slightly different in his American-based amp versus the European-based amp (unless it is achieving the same result with just a slightly different layout and I am making more of it than there is).

Thanks for your input!

Dave

[Colossal]

On your schematic you would want your switch to bypass the 390p cap, not the .0022 cap.

TG, yep, you are absolutely right. I realized it as soon as I posted it and unfortunately I was traveling the last couple of days so couldn't make the change. I made the update to the original post.

Thanks for pointing it out.

Dave