Power Scaling the Overtone Special with London Power's SB-1 Kit

[mx]

Hello All,

Pretty juiced about a BIG mod I just finished this morning...4:30am!

I love my Overtone...but, as you all know, unless you play out, have a place to rehearse at gigging volume or you live in the woods, miles away from any neighbors, you are SOL when it comes to cranking it up.  Of course, higher MV settings is where the Overtone really sings.  So...I never really got to enjoy the amp as much as I'd wanted.  I was always looking for bedroom volume WITH all of the Overtone goodness 

OK...so I searched all over about attenuators, Iso cabs and sound reduction schemes..  IMO, either they sounded like crap or were ridiculously expensive to implement.

I stumbled on Kevin O'Connor's business, London Power, and a kit he offer for fixed bias amps like the Overtone: http://www.londonpower.com/catalog/product_info.php?cPath=7&products_id=27

I am not expert but from what I read about him and by him this guy is a freakin' amp genius.  He knows this stuff inside and out and dispels a lot of myths.  I've developed a lot of respect for Kevin.  Check out his kits and books.

So, to the project...  I tried to take some pics of this as I went along but, frankly, I was more interested in getting it done than in creating a documentary.  Also, I don't want any shit about my wiring job with that damn cloth wire  I've gotten better with it since my build of the amp but it's still messy looking.

However, I did put a lot of time into a layout specific to the Overtone that you can use.

To be continued

[mx]

1. RAW BIAS SUPPLY - First thing Kevin recommended was changing the raw bias supply from half-wave rectified to full-wave bridge.  He indicated that the kit would work better overall with this mod (smoother supply/less ripple).

You simply remove the board containing the rectifiers for Bias and Power.

Take all of the components out for the Bias supply.

You will need (4 each) diodes. I used 1N4007 because that's what I found - that's overkill as only 60-70 volts will be going through here.

What's nice is you can use the existing holes in the board for wiring up the bridge.  You might find that the Cap just barely makes the stretch between the positive and negative ends of the bridge.  Fortunately, my Cap had long enough leads...YMMV 

Be sure your diodes are heading the correct way; opposite to the diodes for the power supply.

The positive end of the bridge and the positive end of the Cap will be sent to star ground.

Take the 50v tap off the PT to feed one side of the bridge and the 0v (which is presently grounded) and put it on the other side of the bridge.

The negative end of the bridge is now your full-wave rectified raw bias supply!

[mx]

2. REMOVING STANDBY SWITCH & RELATED WIRING - In reviewing the layout Kevin indicated that the amp was wired wrong in a way that would cause a loud pop when the STANDBY was thrown to OPERATE.  Of course.... I knew that!  I didn't tell him that, he deduced it on his own.  His recommendation ?  Get rid of it and use the vacant hole for the Power Scaling pot (attenuation).

Referring to the enclosed layout:

As you know, the the rectified power supply (E-1 red) goes to the STANDBY switch from where it is then referred to as "K", in yellow, as it goes to the filter caps.  If you remove the STANDBY switch then E-1(red) simply goes directly to K at the filter caps. 

At this point let me introduce the SB-1 circuit board which is scored so that it can be broken into three pieces. This was a good call because space is at a premium and finding a single location for the entire circuit board would have been tough; especially since two of the three boards need to be mounted to the chassis so that it can act as a heat sink.
 
In the enclosed pic you will see the unbroken SB-1 circuit board with sub-circuit boards referenced as W1, W2, and W3 by white arrows.  Next to the board designation are between 3 and 5 solder terminals enclosed within a box.  Within this documentation when I refer to W1/1, for example, I am referring to the W1 board and terminal 1.  There are also solder points referred to as "Pads".  I will clearly label a Pad as, for example, W3/Pad 5.
 
I chose to mount W3 on the corner of the large board containing the filter caps (you can see it at the top right of the Power Supplies pic.

You will notice that the negative Raw Bias Supply (blue) goes to W3/5.

Since the Raw Bias Supply is going to W3 on the filter cap board and the Power Supply is also headed in the same direction I twisted them together (blue/red in the pictures too though layout colors and actual wire in pics DO NOT always match).

[hywelg]

I have heard mixed reports from amp techs about how they 'expect' powerscaling to work with the OTS. One says it will work well and allow the power section to get 'cooking' and another says that this type of amp, essentially a clean power section, won't respond well to PS.

I' have PS in my Dc-30 and love it and would like to fit it to my OTS so I am very keen to hear how you get on with it.

[mx]

Kevin provides some excellent Q&A on Power Scaling here:
http://www.londonpower.com/pscaling.htm

I am not an EE and I'm new to amp building so please excuse me if I use any wrong terms or if my explanation isn't completely accurate about what is happening electrically.....  I can assure you that the layout I created is correct, however - Kevin checked it and my amp is up and running beautifully.


3. WIRING UP CIRCUIT BOARD W1:

W1 manages the Va power source (junction of 100uF Cap/200K 3W resistor) before passing it off to W2 and will physically reside between the Filter Cap and the Pre-amp boards.  Conveniently, there is a hole in the chassis right there.

If you are running a 100W version with four 6L6's this might require some tight work but it is an ideal location due to the proximity of the sources and short wire run lengths.

On the stock Filter Cap board (large board), the Output Transfer Center Tap (red), labeled OT CT, is soldered to the board together with one side of the Choke. I've highlighted it in green in the enclosed pic.

a. Remove the OT CT from the Filter Cap board. It's going to W1/terminal 3.  You may need to splice in a length of wire...

b. In the stock setup, the OT CT is directly connected to Va .

c. As noted above,  we've disconnected the OT CT from this Va junction.  To supply power we will run a line from this junction to W1/terminal 1.

d. Twist the red OT CT wire (let's call it W1/3 for its destination) and the Va source (W1/1)from C above (in the pic, my Va source W1/1 is blue). 

e. W1/2 (yellow) goes to the wiper of the Power Scale pot (1M).

f. The wiper on the Power Scale pot also has a 22n/630V cap

g. A jumper is run from Pad 1 of W1 to Pad 3 of W2.  This is the green wire of W1

Next...wiring up W2

[JD0x0]

IMHO it looks like too much trouble for me to do. but also this isnt a power tube distortion amp so there is not really a need for power scaling although i do think the amp sounds much better louder but i think its more because im moving air and pushing the speakers rather than working the power section harder.

Needless to say if my OTS came with power scaling i would NOT be upset about it
...unless it degrades the natural tone of the amp from the extra circuitry

[mx]

4. WIRING UP THE POWER SCALING (PS) POT:

Rather than jumping directly to the wiring of W2, I am interjecting this step because the PS pot and W2 are related in managing Vs and wiring the PS pot first makes more sense from a wiring standpoint.

a. I chose to mount my Power Scaling pot in the hole vacated by the STANDBY switch

b. Solder W1/2 (yellow in both layout and pic) to the center/wiper terminal of the PS pot with the 22n/630v cap

c. Disconnect Vs (PS-1 on layout in orange) from pin 6 (I also disconnected PS-1 from the board to add a longer length of wire to run to the PS pot.

d. Leave the W-11 connection(s) between the #6 pins

e. Run a length of wire from the negative end/ground of the screen caps (PS-5) - green in the pic (before you start twisting note how PS-5 ground needs to be a few inches longer than PS-1/Vs.

f. Twist PS-5 (green in pic/layout), above, with PS-1 (blue in pic/orange in layout..sorry)

g. Add the 34k8 resistor in series with the PS-5 ground wire and shrink wrap

h. Solder the PS-5 ground/resistor and the other lead of the 22n/630v cap onto the PS pot terminal as shown

i. Don't solder the PS-1/Vs to the other outside terminal of the pot just yet....

Wiring up W2 up next.....

[mx]

5. WIRING UP W2:

This is fairly easy...

a. Twist up a length of two wires sufficient to run from the PS pot to the placement of W2

b. In my case, I placed W2 between the PT and bias point and ran the wiring above Ground, Fuse and Mains

c. Vs for W2/1 (blue in pic/orange in layout) comes from the PS pot and through the jumper

d. W2/3 is wired to Pin #6

e. Two jumpers are run; W2/Pad3 to W1/Pad 1 and W2/Pad 4 to W3/Pad 5

[mx]

6. WIRING UP W3:

a. W3 doesn't need a heat sink so I put it on the corner of the filter cap board

b. Run a wire from W3/2 to the negative end of the screen filter caps at PS-5 (green on layout)

c. W3/5 comes from the negative Raw Bias Supply covered earlier (blue on layout and pic)

d. As I noted before, if you plan on removing the STANDBY switch as suggested then twist up the Raw Bias (blue in pic and layout) and Va power (red in layout and pic) and run them up near the PT.

e. W3/Pad 5 is wired to W2/Pad 4 as covered in the previous step

f. W3/3 and W3/4 come from the new Bias pot which I'll cover next

Bias pot..final step

[mx]

7. WIRING UP THE BIAS POT

The final step involves replacing the stock 10K pot with a 25K high grade pot

a. Obviously, de-solder the old pot and remove

b. I repositioned mine to the small lower hole on the back of the chassis in the Full/Half power switch hole

c. The only thing the new Bias Pot has in common with the old one is that the center/wiper terminal is still wired to the red wire going to the preamp (labeled E in layout

d. Looking at the back of the pot with terminals on the bottom:

e. W3/3 (blue in layout/yellow in pic) is wired to the left terminal

f.  W3/4 (pink in layout/green in pic) is wired to the right terminal

g. These two outermost wires W3/3 and W3/4 should be twisted together as shown in the pic

[mx]

THE RESULTS!

As noted...this was a fair amount of work.  You'll be removing your Rectifier and Filter Cap boards and enough wires to make you think you are going backwards for awhile    But with my having already done the layout, planned the wiring and step-by-step plans here you should be able to knock it out in a few hours.

I have gone to the trouble of writing this all up for TWO reasons: 1)  my amp NOW sounds incredibly better at very low volumes and 2) I want to make this mod as easy and as accessible as possible for others who think it seems too difficult (it isn't).  This mod renders the long-awaited 5 watter SE Overtone unnecessary.

I realize that in citing my OPINIONS about something as subjective as "tone", I might be treading on sacred ground. I have no intention of starting a holy Dumble tone war.  If I could actually play well I'd send clips but I refuse to be in the company of some of the people on this list.  If you ever visit the Boston area let me know, you can have a test drive (bring an axe, I'm lefty).

Already, there are questions about whether the legendary Dumble circuit can be improved upon, that most of the tone is preamp generated and it doesn't need its power section driven hard etc...

Frankly, I don't have much love for pre-amp distortion. That's why I own THIS amp.  To me, and I have the ears of a mortal only, pre-amp distortion sounds gritty and doesn't get me the smooth, bluesy/jazzy sustain and tone I want when cranked up... It's the Robben Ford, Larry Carlton etc fans who love this amp's tones and singing overdrive and sustain on the verge of feedback.

Stock 12AX7's sounded too gritty to my ears.  And even with 12AX7's in V1 and V2, I needed to crank the amp to get the sustain and feedback I wanted. So I tried every combination of higher and lower gain tubes in the V1 and V2 positions.  Ultimately I found that NOS 5751's in V1 and V2 took the edge off and created a mellower tone but I still couldn't overdrive the output section without ear-piercing volumes; same deal as with 12AX7's.

THE CONUNDRUM: Volume = tone.

I BELIEVE that the sweeter tones, sustain and namesake harmonic "overtones" of this amp come from an overdriven output section and not exclusively or even largely from the pre-amp section.  If the pre-amp contained ALL of the magic, then I could simply turn down the MV and pretty much get the tone I wanted sans the compression, sustain and feedback but I could not.  This is what led to the search for Iso cabs and attenuators.

SO WHAT DID I GET FOR MY EFFORTS?

Kevin O'Connor's SB-1 Power Scaling Kit definitely lives up to the promise.  And for $80 USD 

Check this out...with the Power Scale at 0% (max attenuation/minimum voltage & volume) I get bedroom level volumes (defined as being able to hear your hands and string/fretboard noises over the amp)

I can have my Preamp Volume at 1-2!, EQ at noon or PAB'd and the Power Amp section cranked up with MV at 6-10 (the incremental change in volume beyond 6 on the MV is not linear and increases only modestly).  I get a beautiful overdrive I could NEVER get before at lower volumes. Finally, I have the tones everyone has been raving about with this amp. Because there is barely any noticeable "grit" and "distortion", the "overdrive" MUST be created by the output section with the Preamp volume at only 1-2.

Here is where is gets interesting:

Once you start to turn up just the pre-amp volume the distinguishable "grittier" distortion comes on and there is plenty on tap even with 5751's.  Logically, I can only conclude that the output section DOES HAVE significant impact on the tone of this amp and unless you are cranking it up LOUD or using Power Scaling you aren't getting your money's worth.

I fell in love with this amp hearing all the great Ceriatone players.  I didn't appreciate that an adequate environment to play it loud enough was necessary to really hear the tonal capabilities that sold me on it.  Because it had MV I thought I could simply crank the MV way down and get most of the tone I wanted  (I know you don't get the interaction with the speaker at VERY low volumes as you must push some air).  Once it was built my ONLY disappointment was that it did sound beautiful when played loudly but I rarely have a chance to let a 50 watter run...my mistake but what's the alternative?  A 20 watt Brown Note isn't much quieter.

I am strongly encouraging you guys to check out Kevin's Power Scaling.  Even at gigging volumes it would have application when you are in smaller venues but still want the overdriven tone.  For the more advanced tech's, this guy is a wealth of information and knowledge and has many more sophisticated mods that would afford even more control and possibilities.

Best of luck with your modding,

Matt

[mcinku]

Nice thread... 

...to be honest here, I never had an urge to do something like that. To me MV on these amps works better than most of the MV amps out there... but like you said, it gets even better when you crank that MV pot.
For now I'm staying with my Champ... but I'm glad this worked out for you... nice mod.
 

[bluesfendermanblues]

Nice thread... 

...to be honest here, I never had an urge to do something like that. To me MV on these amps works better than most of the MV amps out there... but like you said, it gets even better when you crank that MV pot.
For now I'm staying with my Champ... but I'm glad this worked out for you... nice mod.
 

+1 - Don't really see the point in adding power scaling to a Dumble, which has a great low gain overdrive tone for a master volume amp. However, on a Marshall or Fender amp, I would find it usefull.
As always beauty is in the eye of the beholder - and if you're happy with your amps tone then thats great.

[hywelg]

Having noted the diference in tone between running clean with Volume 2 and Master 8 and Volume 6-8 and Master 2 I think PS has something to offer.

I do know that poweramp distortion isn't part of what the OTS does, but, what does occur is the addition of harmonics when driving the amp harder and its this that I like. I'm still not sure about retro fitting PS to my 50watter, however when I get around to doing a 100watt HRM it will have PS fitted (in addition to 5k OT tap to properly run 6V6's)

[brickcues]

Ordered an SB-1 today for my Ceriatone Overtone S&M 100 W, after looking at your post.  Has been awhile since I soldered anything,  I can probably manage, like riding a bicycle I hope.  Used to do board level repair on cash registers.

Thanks for the info!!!