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 Post subject: Hot Rodding the sIII
PostPosted: Mon Jul 03, 2006 10:15 am 
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Holy Ghost
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Here's some simple tweaks to 'hot rod' an sIII. If you find the TMB channel too 'clean' and want to increase the gain, it is easily tweaked with a change in V2 cathode resistor/bypass capacitor addition. Here is some information you might find useful.

Go here for the sIII schematic & layout: viewtopic.php?t=297

The schematic shows the changes in Red from a stock TMB.

To increase gain on an sIII:
From a stock sIII, you can start by adding another 2K7 in parallel with the V2 cathode resistor; go to 820 if you want even more gain (Richie TMB). Then if you want more crunch, add a 1uf (starting point) bypass cap across it. You can add a switch (call it a boost if you like) to connect them into the circuit.

JA used 3K3 on V2 with .68 bypass cap on the 'boost' switch.

Most people I heard from, found the stock sIII values were just about right for their tastes but these tweaks are dead simple to do if you want to experiment.

The sIII can get a clean to grind on the TMB channel using the Vol & MV controls. If you jumper in the Normal channel, it's a great tone.

Enjoy! 8)

Anybody have some other suggestions or comments?

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 Post subject:
PostPosted: Mon Jul 10, 2006 1:29 pm 
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The boost mod is highly recommended.


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PostPosted: Thu Jul 13, 2006 8:47 pm 
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So, if I were an idiot and wanted to try this boost, that would be an 820 - 2.7K resistor and a 1uf cap in parallel with R9? From V3,pin3 to ground?

Thanks,
Andy

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 Post subject:
PostPosted: Thu Jul 13, 2006 9:58 pm 
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Start with one 2K7 in parallel with the V2 cathode resistor and a 1 uf as well both in parallel with R9 on the turret board. This is from V2 , pin3 to ground but just solder it across the turret board. there is an extra turret there to use if you want.

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 Post subject:
PostPosted: Fri Jul 14, 2006 2:52 am 
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V2 pin3, too many 3's, my bad. I am an idiot.

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 Post subject:
PostPosted: Sun Sep 10, 2006 3:10 pm 
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I put a boost switch in and I added 2K7 in parallel with V2 cathode plus a 22 uF cap across the extra resistor.

A 1 uf works fine, but accentuates the high end. 47 is also fine but allows a lot more signal through - good and dirty. 22 uf seems to be just right for lbet & my tastes. I'll see what others think in the coming week.

It's a switch tho, not a foot switch. That would be an enhancement I think, for some players anyway.

:?: I'd like to hear about people using foot switches. Did they use shielded cable? What kind? Did it add hum? Did they build the pedal themselves and how???


Pic showing sIII Boost Mod. Same value bypass cap as a Trainwreck!

Image

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 Post subject:
PostPosted: Sun Sep 10, 2006 4:03 pm 
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I have added the 47uf cap with a footswitch type switch mounted in the chassis between V2 & V3. Its a combo, so I can reach around the back and depress the switch to change sounds. I have a footswitch attached but, that's for switching between 6V6 & EL84's. I sent you a photo Stephen. I'll have to try differant values of cap and resistors but, so far I like the 47uf.


Edit by coco. Picture added.

Image


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 Post subject: MO gain
PostPosted: Tue Sep 19, 2006 10:42 am 
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I am very new to this, so excuse any stupidity. Other than these 'boost' mods, is it possible to really add some substantial gain, possibly another pre amp tube?

I don't own a Trinity(yet) and the clips I have heard sound great, but I'm one of those guys who occasionally likes a little gain on my gain and I'd like to do it without a pedal.


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 Post subject:
PostPosted: Tue Sep 19, 2006 11:03 am 
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Yes !! You should check out the thread called sIII One. It's a High Gain sIII amp just rolling out.


viewtopic.php?t=701

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 Post subject:
PostPosted: Wed Oct 18, 2006 11:19 am 
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coco wrote:
I put a boost switch in and I added 2K7 in parallel with V2 cathode plus a 22 uF cap across the extra resistor.

A 1 uf works fine, but accentuates the high end. 47 is also fine but allows a lot more signal through - good and dirty. 22 uf seems to be just right for lbet & my tastes. I'll see what others think in the coming week.


I tried the boost with a paralleled 2.42K resistor (I had some 1% precision resistors on my workbench), which I had also used at that point in the layout. That along with a 47uF cap, gave it a definite boost. But, when I turned the Volume up past 8/9, I got a warbly, hissy sound and really awful distortion (the tone went away). I pulled the extra 2.42K and the 47uF is still giving me that sound above 9. So, I'll probably try 820 to 1.2K resistor and a 1 - 22uF cap. Sound right?

Anyway, I do thank Stephen for providing some fantastic transformers (The RS clone from Heyboer) and a great schematic and layout. My base was a Wxxxx-brand TMB, that I rebuilt to the sIII with a JJ can cap and tube recto. Stock, there was such a difference! Much more sustain and grind! I hope to get the hot rodding for more grind and gain, but as I said, stock it sounds great!

BTW: I had originally placed a 180R on the EL84s cathode (that' what I had when I started the build!) but just replaced that with a 120R. What change "should" that have made? Should that have lowered my voltages?

Thanks,
dgately


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 Post subject:
PostPosted: Wed Oct 18, 2006 1:22 pm 
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I'd start with 22 uf & work your way down to 1 uf or whatever you like. Then add the 2K7 in parallel if you want more gain.
I would want the final cathode resistance to be no less than about 1.2K in the above scenario.

On the Power Tubes Cathode Resistor

A 120 ohm cathode resistor when compared to the 180 resistor will increase the current flowing through the power tubes so that could lower the voltage.

The cathode resistor along with the Plate resistor control the bias and gain of the tube stage. The cathode resistor controls headroom and linearity, or distortion level, of the stage. As the bias point is shifted (i.e. resistor value changed), the amplifier will clip more on the top or bottom portion of the waveform.

For the 18 watt amp at 350 V B+ typical values are anywhere from 100 ohms to 150 ohms. Smaller values = more gain.

We recommend 120 ohms for the B+ & plate reisitor chosen.

The cathode bypass cap connected in parallel with the cathode resistor rolls off various degrees of bass in this stage (generally higher = more bass). The cathode bypass cap also gives this stage more gain.

We have chosen a low roll-off frequency because our stock RS Clone OT can handle the bass energy and not get flabby. Others using different transformers may want to reduce the value by a factor of 10 or 20.

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 Post subject:
PostPosted: Tue Jan 23, 2007 2:55 pm 
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but what voltage is the 22uf?


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 Post subject:
PostPosted: Tue Jan 23, 2007 4:09 pm 
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For the cathode bypass cap you can go with 25 volts I think.


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 Post subject:
PostPosted: Tue Jan 23, 2007 5:05 pm 
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On the above labelled picture, it says 35V but I'm sure 25 would work.

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 Post subject:
PostPosted: Wed Jan 24, 2007 2:44 pm 
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oops, saw that after I wrote in the pic. :roll:


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 Post subject:
PostPosted: Wed Jan 24, 2007 3:03 pm 
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The layout shows a jumper and one wire going to the switch and the picture shows no jumper and two wires to the switch. :shock:

Which is it or which is better :?:
Thanks


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 Post subject:
PostPosted: Wed Jan 24, 2007 6:49 pm 
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umm... need this answer to finish the board. I went romright to left and then left to right. This is what's left.
Thanks


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 Post subject:
PostPosted: Wed Jan 24, 2007 9:01 pm 
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jac,
Either way is right. I used the layout version. The wire from V2 pin 3 to R9 can be tied to on either end by the switch. What the switch does is place an extra 2k7 resistor and a 22uF cap in parallel with R9. In effect giving a 1K35 resistor and a bypass cap. With the switch off the cap and one resistor are removed and you are back to 2K7.


LeeMo


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 Post subject:
PostPosted: Wed Jan 24, 2007 9:31 pm 
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Thanks LeeMo,
Using the layout version, it does not show which turret will attach to what on the switch. Which is hot and which one is ground.
Thanks again
Jac


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 Post subject:
PostPosted: Wed Jan 24, 2007 9:56 pm 
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Jac,

You're correct. The sIII layout doesnt show the connections. Refer to the v6 layout. It shows the connections.

viewtopic.php?t=763

Or just follow the picture above ^

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