My SLO Clone 50 watt build

dinkyguitar

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Hi All,

I've been working on my clone for a few weeks now.

I'm getting ready to install the power board so I can start taking some voltages aided by a variac.

Here's some progress...





This is what I used for bending lead....adjustable compass.



This is a little tool I make to hold the components while I soldered them. I used a plastic pen, cut it to size, put the spring in inside and used a dowel as the plunger.



Preamp board.













Spray painted the PT & OT bells due to scratches and such.



Pots are cut to length.





I'm doing elevated heaters...I still have to install the resistor that goes from the cap/resitor over to pin 6...I'm mounting it between the terminal strips.



I also took the wires apart because I didn't like the arrangment, plus a added a different color wire (red) going from the IEC hot side to fuse 1.

My next item will be buttoning all the wires, leaving a service loop in case the powerboard needs work in the future, hooking up all the powerboard wires, them plugging it in and taking some voltage reading.

Hopefully, everything will go well.

Let me know if you guys see anything unusual.

dinky
 

dinkyguitar

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Problem I have now is that the ac bias in voltage is too high.

It's supposed to be around 60vac, but I'm measuring 100vac.

There's 2 datasheets on the O'Netics power transformer..

The one I got with the PT says the red/yellow wire (bias in) is 60v.

I searched through my doc's and found a datasheet listing all the O'Netcs transformers, and on there that wire is marked 100vac.

Here's the one that came with the PT


And here's the one I dug up:


My voltages align more so with this...for example, both my AC in voltages are 346vac each, which is closer to this doc.

Not sure how to fix it....

I email the site I got the kit from...perhaps I was shipped the wrong PT.

dinky
 

coolidge

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You need to adjust R67 and or R53 to give you an adjustment range of roughly 20-50ma. It's not uncommon to have to adjust resistor values I just did this bias dance on a couple of amps.

220k is typical for R67 IF you are tapping one leg of your 346-0-346 HT for your bias. Many PT's don't have a dedicated bias tap and that's how it's done. Since you have a dedicated 100vac bias tap you will need a lower value in R67. Try a 56k for R67. 27k is typical for a Marshall with a bias tap. So somewhere in that lower range is likely.

Once you have the bias range roughly close its common to fine tune with R53. Typical values there are 47k and 56k with 47k being more common. I swear whichever I guess 47k or 56k its always wrong and I have to swap that resistor. lol

Some good news is the Slo specifies a 50k bias adjustment pot vs 25k for Marshall so you have twice the adjustable range for biasing.

If your bias range is off one way or the other you will either not be able to bias the tube hot enough, or cold enough, that 20-50ma range should suffice for tube variances. I was recently rolling a bunch of EL34's through an amp, the tubes themselves varied by +- 20ma from tube to tube at idle. So depending on the particular set of tubes you are biasing even this may force you to nudge your bias resistors. Fortunately if you have 20-50ma of adjustment you are very likely to be good regardless of the variances in tubes.

slobias.jpg
 

dinkyguitar

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Still reading you post.....

The powerboard I'm using is based on that schematic but doesn't have those those 2 resistors.


On the rights side you'll see R52 nect to D5. (R52 is 10k)

The bias in wires goes to the turret which goes to D5.

I've emailed C3amps for thier advise...

The datasheet I got with the PT has the bias wire at 60vac.
 

coolidge

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I looked at your board just now, confirmed both those resistors are omitted, weird. Your bias supply comes straight into the diode, through the lone 10k resistor across the two caps to the bias pot. Not sure why they did that other than to save the cost of 2 resistors but likely requires that lower 60vac bias supply vs the 100vac.

Now that said you could simply solder a resistor onto the end of your bias supply wire and attach the other end of the resistor to the Bias IN solder pad that will give you R67 for adjustment.
 

dinkyguitar

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Right....

I think the whole issue with this is I got the wrong PT.

The datasheet I got with the PT says red/yellow bias is 60vac.

And mine is 100vac, which aligns with a datasheet I have from o'netics....

If I had the the right PT, this would not be an issue.....
 

dinkyguitar

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I'm sorting out some PT voltage differences, but I've made some progress.

Bus bar is in and soldered to the pots along with the 2 blue jumpers.

You can't really see them, but I also soldered in the 2 coax brackets on the left 2 pots.



Slave in hooked up, speakers ohm switch in, and speaker are jacks are wired.



And a shot of the elevated heater mod.



I rearranged lots of wires on this build.

On some I didn't think the wire color was right, or I didn't like how it was routed etc.

I'm hoping to sort out those voltage issues with my PT soon so i can finish this thing.

dinky
 

coolidge

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Those look like PEC pots? If so grab your multi meter, set it to continuity test and confirm you have continuity from the pot rear cover to ground on all of those pots and that none are loose.

Details: See those orange colored conductive wafers on the pots, PEC pots have a real bad habit of not fulling crimping the pot covers so that they reach over those and are in electrical contact with the front of the pot. Instead they hang on that non-conductive wafer and are hence not grounded and not providing pot shielding.
 
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coolidge

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I see you have some blue painters tape. Tip those make great solder and flux spatter picker uppers tear off a piece and go around your chassis sucking up all those spatters.
 

coolidge

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Other dirt cheap wonder tools -

1. 6 inch wood shaft cotton swabs. Great with a bottom of isopropyl alcohol for cleaning flux off. They also make great wedges during soldering. I frequently cut one in half and use them to wedge under wires, components to position the leads in the center of tube sockets in conjunction with blue painters tape. 100 for $6 https://www.digikey.com/product-detail/en/chemtronics/CT200/CT200-ND/240941

2. These Mueller flap tipped smooth alligator clips for heat sinks. 10 for $4 I use these to keep heat away from the component body during soldering. The are quite small so good in tight spaces and the flat tips will not damage component leads. https://www.digikey.com/product-detail/en/mueller-electric-co/BU-34/314-1017-ND/304581
 

dinkyguitar

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Thanks coolidge!

Yep those are PEC...not cheap either!

1. But yes, I checked for continuity so they are good..I'm going to start a list of things to do...this being 1 of them.

2. Yes again...blue painters tape...use it quite often on this build...There's a whole lot of dust, solder, flux spatter...all that will have to come out. I was going to use compressed air this past weekend, but never got around to it.

3. I'm currently using q tips with alcohol, but I'm finding the cotton gets caught on every little thing and I wind up pulling it off everything....I remember having something similar to a q tip but having a sponge at the end but I don't remember where I got it from.

4. Those flat alligator clips seem good to have. I'm using regular ones to hold components while soldering...but sometimes the jaws move the part around...still good to have though.

Thanks for all the suggestions....

Have you build a slo clone?
 

dinkyguitar

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coolidge, that's awesome!

That slo clone...I know how much all that stuff costs, and just sitting...maybe you should do little at a time???? Or re-purpose for another amp build????

I was going to build a mojotone JCM800, but then I'm not sure what happened.

I guess I didn't want to drive he front of the amp with an OD pedal....I don't know.

Where did you get the boards from? They look really nice...

So..........some more forward movement today.



The color of those bus wires are actually blue...nt sure why it shows up as teal.

Probably becuase of the cheap camera..

Well, I think I got 1 of problems with the PT solved...

To recap, I have a O'Netics PT with 100vac Bias circuit instead of 60vac & 15.3vac switching voltage (blue wires) instead of 12.6vac.

So, I'm now going to use the orange wires, which are 5vac, and putting a jumper in place of R61 15R...as per notes in sloclone schematics.

Which stinks because I have to reorder & re bundle all the PT wires.

The other 100cav Bias issue, I'm still working on.
 

coolidge

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I repurposed a lot of the smaller parts on other amps. Still have those 100 watt 6L6 transformers sitting on a shelf. I took several years off amps and when I came back to it I decided I didn't want to build anything larger than 50 watts. I may build a Dumble style amp next. Kind of have the Marshall spectrum covered now.

I designed the boards with DesignSpark PCB (a free download) and had PCBPrime work with their Asian PCB manufacturers to manufacture a small run of the boards for me.

Randall Aiken (Aiken Amps) planted that thought into my mind. A hybrid turret/pcb board spec'd to the hilt. This is .125 inch thick FR4 board. 2 oz thick copper vs the typical 1/2 oz. Plated through holes (look inside the holes) that connect the top and bottom pads together. Black solder mask with white silk screen labeling.

I stole this idea from my Marshall SC20H. These input jacks still bolt to the chassis but have a massively thick PCB anchoring the back side together. Plus solder pads for the tipical 1M, 470k/470pf, and inter connect wires.

cb03.jpg


Here's the main board. Turrets mechanically staked into the plated through holes and soldered to the board on the bottom. 100 years from now when someone pulls the chassis on this amp they will go, whoa! lol I also added solder pads for many of the inter connect wires vs wrapping them around the component turrets.

cb04.jpg
 

dinkyguitar

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Got 2 coax cables done...kind of a pain.

The wires are so small, I had to redo 1 cable since I cut 1 or 2 wires from the core.

It was not my favorite...I have a few of these to go.



 

dinkyguitar

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Not sure if I like that :dunno:

Lack of feel and control of the wire....

I did by extra wire for everything so if I mess up I have more...

@coolidge
You were right about those PEC pots not being grounded...I thought I checked them like I said, but I went over them again when you suggested it, and sure enough 2 pots, the "mid" & slave back covers were not grounded. The slave was also a little flakey...one minute it's grounded, then it's not. I'm not going to use the slave anyway, but I used a pair of needle nose plies to further crimp over the tangs and tightened it more.

Also, speaking of not using the slave out...would I be able to use that for a volume boost?

I saw a Brit-tone slo clone using the slave out jack as a volume boost along with the pot to control the output.

Not sure how it's done, but I'm looking for schematics or instructions.
 
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