First build: Small Special

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psaulaferraz
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First build: Small Special

Post by psaulaferraz »

Hello,
I've been a long time reader and after working with pedals I've decided I want to make an amp. I've collected lots of information about the Dumble Small Special and so far I've been analyzing this two schematics I found here:
Small Special (Draft) (Ampgarage.com).jpg
Posted on this thread: http://ampgarage.com/forum/viewtopic.php?f=4&t=29979
Small Special Layout (Draft) (Ampgrage.com).jpg
I can't remember where it was posted.

Overall, the main differences are the tonestack, the 68k resistor into the grid of V1, the cap going out of the plate of V2 (in the layout is 0,01 uF), the resistor out of the master volume (in the layout is 1k2), the feedback resistor (in the layout is 100R) and the resistors going into the grids of the power tubes (1k5 instead of 2k2). Also, the power supply is completely different.

I understand how tube circuits work and I'm going to use the schematic to build my amp, but I have a few questions:
  • Do I have to add a 68k resistor to the grid of v1 (as most fenders do)?
  • Would it be better to use a 100R feedback resistor?
Apart from this I'm going to make some modifications, such as adding and Fx Loop (I was thinking passive, but would it be ok for a dumble?) and changing the tone stack for an Skyliner with mids. Would it loose a lot of gain?

Thanks for your help. As soon as I resolve all this doubts I will start the building. Probably I'm going to record the construction and I promise I will post lots of pictures and media.
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xtian
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Re: First build: Small Special

Post by xtian »

Welcome!

The input grid blocker’s purpose is to eliminate radio reception and noise. You could leave it out as per schematic, and add it later if noise is an issue. 10K to 68K are common values—I typically use 22K there.

This schematic has a 3K3 feedback resistor. 100R would provide much less neg feedback, making the amp livelier, angrier, and noisier. Worth a try! You can use a SPDT switch to provide a couple of options. Good mod.
I build and repair tube amps. http://amps.monkeymatic.com
telentubes
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Re: First build: Small Special

Post by telentubes »

Here is another schematic to look at. Should there be a coupling cap going into the Master Volume?
Screen Shot 2018-01-15 at 8.46.54 AM.png
I used a 40-18005 Transformer. Not the 40-18004
I used the 68K input grid blocker.
820R for NFB
Lowered values of treble cap to taste as well. The first two of the three I built had spiky treble.
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didit
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Re: First build: Small Special

Post by didit »

Schematic has secondaries on the output transformer connected -- a serious error, though not repeated in the layout.

Recommend values between 10K-33K for grid input resistor on a single jack design. Direct jack-grid works, but prone to noise issues that I find unworthy of the hassles.

Best .. Ian
telentubes
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Re: First build: Small Special

Post by telentubes »

Also used 10K and 10K dropping resistors to lower B+ and plates on V1 and V2
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Thank you all for your helpful responses.
I've done a quick photoshop with how the schematic will look (later on this week I will draw it with some kind of CAD software. BTW, what do you recommend me?):
Small Special (Draft) (Ampgarage.com) v2.png
I took that tonestack from the internet. Maybe I'll change it or modify it.
The final doubts I have are:
  • Is it correct to use blackface bassman transformers? That's what seems easier to get for me (I want to build a 50W SS)
  • Where do I place the FX loop jacks? After the 10k resistor that comes out of the master or before? Will I need any kind of buffer?
  • What's better, an accent control or a presence control?
Thanks another time. I hope that next week I will start designing and building the chassis.
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psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Quick update!
I finally decided how the schematic will be. I decided to put a John Mayer Signature Tone Stack in there, as I want a really clean and kind of mid scoopy amp.
Small Special (Draft) (Ampgarage.com) v4.png
Do you see anything wrong? I will start to work on the layout tomorrow so this weekend I can start the chassis!
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didit
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Re: First build: Small Special

Post by didit »

See one error. NFB connection should be on 8 ohm secondary (off that tag on the switch would work) versus jack where the drive will vary dependent on the selection, which is wrong and unlikely make you happy.

Consider a built-in active loop approach following the well-worked Small Special with Merlin effects loop.
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

^So it would be better to just put a jack for every tap on the OT (4 Ohm, 8 Ohm and 16 Ohm) and just connect the NF to the 8 Ohm's tap?
Thanks for the suggestion on the buffered loop. Following this schematic:
s FX Loop -- Input-Output circuit.JPG
  • Where would I have to place it? After the master or after the 10K resistor that goes into the P.I.
  • Should I power it from B+4 (246V)?
  • What's the purpose of having two inputs?
  • What's the purpose of the knob? Is it a parallel/blend circuit?
Thanks for your help
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didit
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Re: First build: Small Special

Post by didit »

psaulaferraz wrote: Tue Jan 16, 2018 9:37 pm So it would be better to just put a jack for every tap on the OT (4 Ohm, 8 Ohm and 16 Ohm) and just connect the NF to the 8 Ohm's tap?
Prefer the switch personally. Simple wiring onto the same tab on it that you'll solder down the 8 ohm secondary tap.
Thanks for the suggestion on the buffered loop. Following this schematic:
s FX Loop -- Input-Output circuit.JPG
  • Where would I have to place it? After the master or after the 10K resistor that goes into the P.I.
  • Should I power it from B+4 (246V)?
  • What's the purpose of having two inputs?
  • What's the purpose of the knob? Is it a parallel/blend circuit?
Thanks for your help
Where to put the active buffer along with how specifically would be something to defer to others. Hopefully telentube can give more detailed advice on it since that's his build. Will offer this limited view. I'd put it right before the 'level' control, plus build a separate B+ 'drop' off the choke-fed B+2 using a suitably sized dropping resistor into another 20uF cap to get into the range of 250-300V. Bit of space plus ~$20 is small cost to have clean isolation - at least in my experience. There are several other threads on that loop here on TAG too. I'd recommend Merlin's book "preamp" book as well worth the ~$40. His loop is covered in it.

Best .. Ian
telentubes
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Re: First build: Small Special

Post by telentubes »

I will come up with something presentable. Unfortunately, all my schematics and layouts are on napkins and the backs of envelopes and the like so wouldn't be much help. The loop is inserted before the master volume. B+ supply is the same as V1, if I recall correctly.
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Thank you for your suggestions. I will definetely add it and I will probably buy Merlin's book.
BTW, I remember seeing in your thread that someone suggested changing the value of C3 and value of the cap of the second cathode. Did you ended up doing it?
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Ok, after some redesigning I've finally done the schematics and the layout. In the end I've ended up doing 2 mods:
  • Using a Super Reverb PT so I can add a rectifier tube and select between tube and solid state rectifier.
  • In regards to the FX Loop, I've used a layout I found somewhere here instead of the schematic that telentubes used.
This are the schematics and the layout. I also attach the Expres SCH file for your enjoyement:
Layout v1 Crop.png
The layout
Circuit.bmp
The main circuit
Power Supply.bmp
The power supply schematic

You can download the schematic here: https://drive.google.com/file/d/1A_Qh9H ... sp=sharing

This week I will start with the chassis. I will keep you updated!
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telentubes
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Re: First build: Small Special

Post by telentubes »

So sorry. You beat me to it. I was just going to post this drawing I did and discovered you were way ahead of me. Here it is anyhow. Yes, with updated values from Martin.
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psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

^That's exactly what I used. The original drawing I followed is this:
Screen Shot 2014-07-04 at 3.04.50 PM.png
The only thing that's important to take into account is that, after the OUT pin in the layout, there must be a coupling cap (I used 0.02 uF)
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