Heaters, power and wiring the basics up and the magic of a step drill

Power transformer through holes with  grommets

So, last post I ended with a tinge of frustration seeing as i couldn't finish the last of the drilling holes. The holes for transformer wires i couldn't get big enough, and didn't have the right sized grommet.

Power transformer feedthrough

Frustrating indeed. SO on advice I ordered a step drill bit, a drill bit with many levels used to drill nice round holes. And boy do I wish I'd got this earlier. It would have made everything so much easier. For the m3/4/5 screw holes I'd still use a drill bit though, but anything larger and this is the answer. Perfect nice round holes with minimum effort. I wanted to find more stuff to drill through, but figured I shouldn't get carried away lest everything I own ended up looking like cartoon swiss cheese...

Anyway, I drilled two nice 14mm holes, and added freshly delivered grommets to them with minimum effort, result looks cool and will stop the wires getting chafed and thus electrifying the chassis, which is not something that is in the best interests of my health and longevity to happen...

Checking everything fits through the holes

Anyway, a quick check to see if everything fits nicely and after confirming that, a quick bit of cosmetics adding grommets to the unused holes in the bell ends either side of the transformers. This has no bearing on the sound (no, the tone won't leak out before you crack that joke) but it's nice, and will satisfy a sense of inner pride.

sans grommet

with grommets

That done, time to do heater wiring, I'll mount the transformers later, but for now I want the chassis to still be at a level where it is easier to work on, ie, desk level.

Right then, heater wiring. For this on both sets of tubes on this amp the heaters are wired in parallel. A lot of the other wiring I'd looked at used twisted heater wires (to cancel out noise in each other and to help the amp remain a bit more noise free). But on the slo they are parallel which should achiever much the same effect. Plus it looks cool. Anyway, to achieve this I used bus wire, feeding it through the pin, then adding some clear heat shrink cut to the length of the distance to the next pin (suitably rated for high temperature, you do not want this stuff to melt). For the power valves this involves twisting pins 2 and 7 so there's a clear line across all the valves, and feeding one wire through all of pins 2, and through all of pin 7. For the preamp valves, same deal, except through pin 9 and twist 4+5 so they allow a clear run through them (as pins 4 and 5 are attached to the same heater run).

Next, hooking the two pairs up. For this, I did need twisted wire, and found the best way to achieve it was to put two wires into a drill instead of the drill bit, then hold the other end with a pair of pliers and give it a quick (very quick) spin. Took a few attempts of not being quick enough and it slipping from the pliers forming effectively a strimmer beating the crap out of me like a madman with a whip. You learn pretty quick...

So armed with twisted wire, i hooked up the two heaters and also the bus wire to the tag strip. First bit done. Next, connect the individual pairs of tubes up, and wire the bias points. Pretty simple and not too bad. Then add a ground bus wire, and hook that up to the speaker jack sleeves, I'll be adding the "0" tap from the Output transformer to this as well later. Also, same tactic as before, bend the socket pins so they form a nice straight line out of the way of the other nice straight lines.


Next up, and the elevated heater mod, which is a cap+resistor in parallel to ground which is then attached to the center tap of the heater supply, or to the faux center tap if you don't get one from the transformer. This is not essential, it's a nice to have, but is another way of minimizing noise from the heaters leaking to the cathode (seeing as it's 50hz/60Hz hum you'll be getting if things are bad. This is also the reason for twisting your heater wires).

That done, time to mount the trasformers, no getting away from it anymore, the rest of the amp will be built with them on. A lot of "difficult" time later (they take a while to mount) and with wires fed through, I can now solder the 4,8 and 16 ohm taps to the selector switch, all good. At this point I muust mention that the negative feedback (which goes to the preamp/deptyh mod if you have one) should be soldered to the 4ohm tap, and the slaver output is soldered to the 8ohm tap. Forget these and all that nice neat heatshrink will be for nothing (like ibn my case) so it's wise to solder some nice long bits of wire to them at the same time. A

Anyway, once the impedance selector is hooked up, the output soldered to the speaker jacks (starting to feel more real now) it's time to hook up the anode wires.

Now, hopefully I've hooked them up the right way round, one wire goes to the left set, the other to the right set of power valves, get them the wrong way round and you allegedly get massive squealing. It'll also mean messing up the nice wiring... anyway, same deal, run into the correct pin, then make it run parallell.

At this stage the B+ wire (high voltage supply) can now also be run across them and with that all the parallel lines reach saturation point. Looks nice though. And is satisfying too.

Add the brick sized resistors between the high tension supply and the anodes, and then solder it all up (the B+ wire).

Following that, it's time to connect up the heaters to the power transformer heater ouput, and finally the pilot light...

Them wire in the on/off and standby switches (as much as can be done without the power board) to complete this rather long section...

 

So now the transformers are all mounted up, next step will be to wire up the power board, followed by the preamp board, but that's for another post.