Output Transformer Primaries - doing the math(s)

The only information I had about the output transformer was the following:

OT:

8K Ohm primary impedance
4 Ohm secondary (yellow)
8 Ohm secondary (green)
16 Ohm secondary (orange)

Which is helpful enough for the secondary wiring going to the speaker jack, but wasn't much help for the primary wiring.  On the other side I had three wires: red, brown and blue.

Knowing a little bit about push-pull amps, I know that one of these wires is a centre tap that's going to go to the can capacitor and then back to pin 3 of the rectifier valve (via the standby switch).  On most of the layouts I've seen, that's the red wire.  The other two primaries need to go to pin 7 of V5 and V4.

Knowing that the primary impedance is 8k ohm, I broke out the trusty multimeter and measured the impedance between all three pairs of wires.  (I knew it was never going to be bang on 8k for various reasons, but the connections to the centre tap would be half the impedance of the other pair.)

If you can read my notes, the results were:

Brown - Blue  0.588
Brown - Red  0.283
Blue - Red  0.303

So red is definitely the CT.  Good to know.  I believe there's a chance I'll get an out-of-phase squeal if I get the other primaries on the wrong power tube, but it's a simple enough job to swap them round.  I've got a 50% chance (and Vin, the kit designer, has pointed me in the right direction - brown V5, blue V4).

How to twist wire properly

Greenback (part two)

Simple, but needed doing.  The speakers were in the wrong orientation for the most effective wiring so I removed the back panels from the cab and rotated the speakers by 90 degrees to line up the contacts.  I meant to put one of them on the scales while I had them out - they certainly live up to their "H" designation.  Interesting to note that they're made in Thames Ditton (about ten miles down the road from me) but have been all the way to the Netherlands and back (and who knows where else on the way...)

The speakers are 15 ohms each, so by my calculations, if I wired in parallel should be good for the 8 ohm output on the OT.  I think.  I got some square white speaker wire from Barry at AmpMaker (one wire in the pair is identifiable by the raised braiding down the side which I used for positive).  As Barry points out on his site, if you always use square wire for speakers and round wire for instruments, then you'll never get the two mixed up.

While I was there I put the chassis with valves in place to see how it all lines up.

And with the back panels in place:

The valves look a little exposed, but the preamp ones will have the covers on when it's done.  Looks almost finished!

Greenback (part one)

Rectification

Like me, when you're contemplating rectification, you probably only think of solid state or valve, right?  Something like this:

Or maybe this:

But there is another type.

Mercury arc rectification is by far the most impressive:

The two glowing orbs on the right are the mercury arc rectifiers at Kempton Steam Museum and convert 3 phase 415VAC into 200VDC @ 100A.  Impressive, but possibly a bit big for my amp.

Completed turret board wiring

A flurry of activity on Sunday afternoon started with the completion of the turret board wiring:

 

And the view of the bottom:

I've allowed plenty of wire for connecting to valves, pots, etc.  Probably too much as when I was estimating the lengths I assumed I'd be running to edges and going at right angles to the relevant terminal - I think that the wires may take a more direct route.

Tinning

Several times during the build I've been tempted to cut corners by not bothering to tin the stripped ends of hookup wire.  The results have generally been crap and have often resulted in me having to redo the whole join because it looked untidy or wasn't a good bond.

The easiest way to tin wire is to hold down the soldering iron using something heavy (like the stand, or a trusty pair of pliers):

Then simply touch the wire and the solder to the iron simultaneously and draw the wire across the iron towards the cut end.  Remember to leave that thermal gap (of approx 2mm) between the tinning and the insulation, or 80s "sodderman" will come and find you.  (Not so important with PFTE wire as it's pretty much impossible to melt.)

And here is an example of a nicely tinned wire (the OT centre tap).  Note the insulation gap at the bottom.

 

 And safely soldered in place: