S.E.X. or Crack2A - which is better for learning and experimenting?

[Doc B.]

Second choice. Star ground it.

[Deke609]

Many thanks Doc.

[2wo]

I actually built one of these back when. I'll dig it out and post a pic...John

[Deke609]

John - that would be cool. Even cooler would be if you could help me understand the circuit. That way I'd not always be pestering PB - I prefer to spread my pestering around 

From the so-called "conventional" view of positive electricity flow, I think I Have a really basic understanding of the circuits.  But when I analyze it in terms of negative flow, the circuit seems more complex - but at the same times makes more sense this way. 

[2wo]

Here you go, the case is a Stainless Steel tissue box IKEA use to sell. The power supply is in a separate except for the big oil cap you can see which is the last cap of the supply.

To better understand the circuit, redraw it a little. Remove the transformer and add 2 resistors, one end to each plate, the other to B+. Now you have a conventional differential amp stage, good circuit descriptions can be found in most any tube book...John
     

[Deke609]

Thanks John. That tissue box makes for a pretty good looking amp! Do you use it?

[Deke609]

@PB - How do I wire the pot? I am using a dual deck A100K that I took out of the S2 when I upgraded to the MourQuiet.

Please see attached diagram with pot (bottom view) and pot wiring schematic.  I have labeled the 3 lugs 1, 2, 3, (left to right, bottom view) and the pot connections on the schematic as A, B, C.

I am confused by the schematic b/c I would have thought that the signal input to a pot is wired to the wiper and would be represented by the wire with the arrow pointing to the middle of the resistor.  But that doesn't seem to be the case here.

Can you tell me which solder tabs goes with which schematic wire?

Many thanks,

Derek

[Paul Birkeland]

If you wire the wiper to the incoming signal, then the pot will present a variable input impedance to your sources and a variable output impedance to the grid of the 6J6. 

I would suggest reading up on resistive voltage dividers, as a potentiometer is just a variable voltage divider.   

[Deke609]

Thanks PB. I'll do that.

More practical question: Is the center solder lug on the pot connected to the wiper?  I can't find a datasheet that specifies which lug connects to what inside the pot.

Many thanks,

Derek

[Paul Birkeland]

In order to figure out which terminal is the wiper, set the potentiometer to the center.  Now measure the DC resistance between various lugs.

The DC resistance between two of the lugs will be the value of the pot (100K), then you'll see a different lug that measures two lower resistances to that other pair of lugs, and this will be your wiper. 

[Deke609]

Thnaks PB. I should have thought of that.

[Deke609]

The "Nickel Wonder" is built and it works!

Thanks PB for suggesting it.  I've already learned a lot from the experience ... but about 95% of that was moving things from the "unknown unknowns" column to the "known unknowns" column.  The actual differential circuit still confounds me, but that's ok, at least now I know what I find confusing and it gives me something definite to figure out.

In more practical terms, I learned that layout is H-A-R-D. I failed to take into account all the ground wiring that is involved in "star grounding" when planning the layout; and trying to find a good path for the signal wire away from the transformers is all but impossible!  But I had a lot of fun.  And now I have something to experiment with.  The first thing I want to look at is DC rectification and ripple smoothing -- in no small part b/c it's the only important part of the circuit that I sorta understand.  The Nickel wonder uses 2 diodes to strip out -ve current, but I've seen that many amps use 4 in what I think is called a "bridge" structure.  I might play around with that and additional/different ways of smoothing the DC.  This will give me a reason to learn to use my o-scope. It should be pretty simple to run a sine wave tone through the preamp and actually see the rectification and smoothing -- which will be cool.

Listening wise: I was expecting the Nickel Wonder to sound pretty poor, but it is actually quite listenable when put in front of the S2.  I particularly like what it does to treble -- it seems fuller-bodied and livelier -- but also a lot less detailed. The two observations -- fuller/livelier and less-detailed -- may be related.  I've previously noticed that upgrades/mods that give greater detail often make the sound seem smaller and thinner. The mids of the Nickel Wonder are fine.  But the lower bass is absent.  I have high hopes for the BeePre in front of the S2. 

[Paul Birkeland]

A bridge rectifier will change the voltage output of the power supply.  Once you blow up an electrolytic capacitor and have to live with that stench, you'll tend to tread fairly carefully on stuff like that in the future! 

One of the features of the differential circuit is that it's less picky about power supply noise. 

This is a powerful tool that you can use to simulate power supplies:
http://www.duncanamps.com/psud2/
One hint that is helpful is that when you use a full wave rectifier in PSUD, the transformer voltage is half the total winding.  In the instance of the Nickel Wonder, you would use a full wave solid state rectifier and the transformer voltage would be 125V. 

[Deke609]

Once you blow up an electrolytic capacitor and have to live with that stench, you'll tend to tread fairly carefully on stuff like that in the future! 

Very good to know! I will start treading carefully now.

And many thanks for the link to PSUD2 and tips.

cheers,

Derek

[Doc B.]

Pick up an old Radio Amateur's Handbook for an approachable discussion of power supply design.