First build: Small Special

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telentubes
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Re: First build: Small Special

Post by telentubes »

I used a .1uf en route to the master Volume.
Nice schem and layout drawing by the way.
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martin manning
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Re: First build: Small Special

Post by martin manning »

IMO the only thing more wasteful than a vacuum rectifier is a vacuum rectifier that is hot, and doing absolutely nothing. You might consider something like this, which preserves your idea to use the reduced secondary voltage connections when using the vacuum rectifier.
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jam-mill
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Re: First build: Small Special

Post by jam-mill »

Switching high-voltage DC is surely going to make that switch arc.

What happens if the bias side of the high-voltage switch fails?
Will the output tubes red-plate because of no bias voltage?
John 15:12-13
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martin manning
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Re: First build: Small Special

Post by martin manning »

Standby switches are subject to that same abuse. However, I would not recommend moving this SS/vacuum rectifier switch unless the main power has been off for a minute or so to drain the reservoir. Any mechanical switch is a potential failure point, as are pots. You are right, as shown the dual voltage bias supply will fail hot, but it could be reconfigured to fail cold.
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

I've decided to abort the tube rectifier experiment. This is the new schematic (I've not had time to do an updated layout).
Power Supply v7.bmp
I'm thinking about using clamp mounted electrolytic caps, so I put them on the side of the chassis while resistors in the filtering circuit are in a terminal strip. What's your opinion. I will be using a "Two Rock style" chassis (20,5 inches wide), but I want to cut on space as much as possible, because maybe in the future I will be doing an ODS or ODRS conversion.
Also, is it a good idea to install a Dumbleator circuit or the Merlin FX Loop is better?

Thanks for your suggestions and for your help. I think I will have to shut down my computer and start building, because if not I will be changing the circuit forever :roll:
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psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Sort for my inactivity, but this week I’ve been busy ordering my all the parts, and doing this in Spain is quite and adventurous and tedious work. Anyway’s here’s a photo I took of the first shipment I received. I’ll keep you updated the following week
5E41353F-ECB7-4F83-87CC-B2919676DDDF.jpeg
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psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Hello,
After a lot of time researching all the materials in the few suppliers we have in Spain, I finally have everything I need. I have also spent some time designing the chassis, which will be machined (mainly) by my local metal shop (but some holes, because I want to fine tune the placement of some parts, will be drilled by me).
The main question I have is, with a black painted chassis, what points will I have to sand to keep them conductive? Only ground points?
Thanks.
Captura de pantalla 2018-03-12 a les 8.06.03.png
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norburybrook
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Re: First build: Small Special

Post by norburybrook »

Anywhere where you have a ground point needs to have the paint/anodising removed. things to watch out for will be the rear jacks as everything on the front connects to a bus bar which grounds, the rear jacks ground through the chassis, so be aware of that. I spent a day trouble shooting a serious hum on a build to find eventually it was the main speaker jack that wasn't grounding properly through an anodised chassis.



M
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martin manning
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Re: First build: Small Special

Post by martin manning »

But, you do want the control pot housings to be grounded to shield the elements. If you don't solder a bus to the pots (and I don't recommend that), make sure that the bushings make good contact with the chassis.
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Matt J
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Re: First build: Small Special

Post by Matt J »

psaulaferraz wrote: Mon Jan 15, 2018 3:01 pm 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.
Best of luck with your Small Special build!
I worked on one myself over a year ago. That schematic you posted was one I tried to piece together from different sources. I forgot to add the impedance selector switch on the OT secondaries, and ended up doing a few things differently as well. The one I tried to build featured a tubes-down design like most Dumble amps instead of a tubes-up head like the original pictures showed. I made several error trying to transpose the layout from one type to the other. If I did it all over again I would have kept the tubes up design since I'm still really a beginner at all this.
I never got around to completely finishing it up. I had to focus more and more time on finishing up graduate school so that became my priority. I had it up and running, but was never able to fine tune it or finish the cabinet for it. I ended up parting it out for other things. It certainly is an interesting little amp and I hope you end up with an awesome amp!
Now that grad school is finally over and I've get some extra time I can get back in to amp building. It's something I've missed dearly!
- Matt J.
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

^Thanks for your dear wishes Matt!
I'm also doing a tubes down design and I hope that, in the end, it will turn out to be a good amp. I also hope that you have good luck finishing yours!

Thanks to all the other help regarding the chassis. Tomorrow I will order it. When I have it I will post some photos, as the trannies are now waiting at home!
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Hello everyone!
After working all this past week, I have finally finished. I'm sorry for not posting more information but later I will upload lots of pictures.

I have some questions regarding the startup procedure:
  • In my multimeter, do the filaments and the high tension wires have to show continuity with ground? I assume it's because of the grounded center tap in both secundaries.
  • First I used a light bulb style current limiter, but with a bulb of 100W only (which didn't even let the pilot light to power). That lead me to really weird measurements, so I just stopped using it (except for when I turned it on for the first time for the caps to slowly build). Is it safe to go straight?
  • Also, the transformer is making a steady 100 Hz noise (harmonic of 50 Hz, the frequency of wall AC here in Europe). Is it normal?
  • Also, the transformer, sometimes, after being turned on after a minute, starts making a soft crackling noise. It doesn't seem like burning, and it's working fine, but is this normal the first time?
Thanks for your help. As soon as I can I will upload some demos.
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pompeiisneaks
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Re: First build: Small Special

Post by pompeiisneaks »

psaulaferraz wrote: Tue Apr 03, 2018 7:00 pm Hello everyone!
After working all this past week, I have finally finished. I'm sorry for not posting more information but later I will upload lots of pictures.

I have some questions regarding the startup procedure:
  • In my multimeter, do the filaments and the high tension wires have to show continuity with ground? I assume it's because of the grounded center tap in both secundaries.
  • First I used a light bulb style current limiter, but with a bulb of 100W only (which didn't even let the pilot light to power). That lead me to really weird measurements, so I just stopped using it (except for when I turned it on for the first time for the caps to slowly build). Is it safe to go straight?
  • Also, the transformer is making a steady 100 Hz noise (harmonic of 50 Hz, the frequency of wall AC here in Europe). Is it normal?
  • Also, the transformer, sometimes, after being turned on after a minute, starts making a soft crackling noise. It doesn't seem like burning, and it's working fine, but is this normal the first time?
Thanks for your help. As soon as I can I will upload some demos.
Ground - Yes they are center tapped, you'll get continuity.
Light Limiter - Don't measure based on it, just use it to make sure nothing is shorting to ground or drawing excess current. Once that test is done, remove it and go straight into the wall
Noise - that depends on how loud. If you're not inputting any signal, it could be a bad sign, but also you'd get a bright light on the limiter
I don't think crackling in the transformer is a good thing either, but again, the limiter should light up if you're drawing too much current. If it's just as shorted winding, you'll end up with much different voltages than expected, so testing voltages is a good step.

I'm a bit wary that a new build's got noise coming out of the transformer... be careful, re-test on the Limiter. If it's too high wattage above expected of the amp, it may not light up too much and can be a poor indicator of anything BUT severe failure.

~Phil
tUber Nerd!
psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Hi!
I finally found out that the problem was a faulty transformer. It shows continuity everywhere it should. I spoke to the manufactuer and they said that they will change it. Now let's wait only three more weeks :roll: . To keep you entertained, here there are some pictures of the chassis:
IMG_7174.jpg
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psaulaferraz
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Re: First build: Small Special

Post by psaulaferraz »

Hi!
After receiving a new transformer I finally could test the amp! Everything seemed to work fine, but before putting any tubes, I tested the B+ Rails. I discovered that there was almost no difference in voltage between different B+ Rails. Is this normal? I think that it could be because there are no tubes in the circuit and therefore there is no current draw, which following Ohm's Law, will lead me to no voltage drop.
Am I right?
Thanks!
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