TubeNexus

At the end of this step we should, in theory, have a working amplifier. The step mainly involves straight one-to-one wiring of the main-board leads to the pots and tube sockets, as well as wiring the power supply leads to the appropriate main-board turrets.

This photo shows the final wiring, after I upgraded the yellow Mallory 150s supplied in the kit to Orange-Drop 600V polyester capacitors. I found the 150s bright/brittle sounding and noticed in my tweaking that they are quite microphonic as well. The orange drops should be the 6PS type from Vishay/Sprague or SBE or the PVC type from Mallory. You don’t want the 715 series polyproplyene capacitors.

You might also notice a grid resistor on the 2nd triode of V1. In my debugging I had played with a grid resistor here to help stabilize the amplifier a bit (keep it from easily breaking into oscillation). A grid resistor is a common tweak seen in real Trainwrecks as well, a common result of the tweaking process. I don’t think I need this resistor anymore, but felt it did no harm to leave it in place.

Another, earlier, overview shot of the main-board wired in place. Definitely refer back to the Francesca wiring pictures for this phase of the project (see Part 1)! You want to make sure that you get the lead dress (wire routing and shaping) correct here!

After completing all the amp wiring, check and double-check the finished product against your layout diagrams and wiring photos. It may help to take a break from the project and come back to do your final checks with fresh eyes. Resist rushing to plug it in and ’smoke test’ the device… you might get smoke! Next time we’ll talk about power up and debugging.

Some of the most critical wiring in a high-gain amp like the Trainwreck is around the 1st tube stage… including the Input Jack, the Volume Control, and the route to the grid of the 2nd stage triode. In my project I found the placement of the bright caps and damping microphonics associated with them to be a challenge. The lead dress in this area, including the ground wire and the blue wire leading from the volume control is very critical and I tacked the wires and caps in place with hot glue at several points on the chassis.

It is very convenient to pre-wire all the pots and get the ground-bus established before fitting in the main board. One of the hardest things was to get a good reliable connection from the back of each pot to the brass rod provided as a ground bus. The brass rod takes solder well, but the pot cases are another story. I filed the pot cases to roughen the surface and take off any lacquer, but I think you also need a really powerful heavy-tipped soldering iron to get enough heat to take solder well. If you have pots with stainless steel cases (like mil-spec clarostat or PEC) you wouldn’t be able to solder at all and you’d need to find special lug rings that go over each pot shaft to make the chassis ground connection. Separate wires go from the pot connections to the ground bus as required. The yellow wire is the negative feed-back to the presence pot circuit from the 8-ohm speaker lug on the impedance selector switch (see other posting).

The coax lead from the input Jack to the grid of the 1st amplifier stage has its shield grounded right at the jack and left open at the grid to avoid a ground loop and potential noise from that.

In the overview picture we see the chassis now complete except for the main board.

Wiring up the Power Supply

The trickiest part of this project is probably the installation of the power supply board and the filter capacitors. Careful consideration must be given to terminating the various grounds and some care in routing the DC voltages as well. The published layouts provide a good guide and hopefully the high resolution versions of these pictures (click on the picture) will help some more.

I used a hot-glue gun to bond the capacitors together and attach them to the chassis. Others swear that you have to use silicone and worry that the chassis interior may get hot enough to melt or at least soften the hot-glue.

One of the changes I made was to short the bias wiper to the unused leg of the potentiometer in order to ensure that an intermittent wiper wouldn’t open circuit the bias pot completely.

Mounting Components and Wiring the Power Tube Sockets

You were half finished when you hooked up the output transformer and heater wires. The remainder of the power tube wiring is straight forward.

A spare tube pin is used to mount the screen resistor and the screen supply, direct from the power board, is daisy-chained to that pin on both power tube sockets.

I found a couple of precision 1-ohm resistors in my kit so I wired up the cathode and suppressor to ground on each tube through a 1-ohm resistor that is useful for checking and setting the bias.

The little stack of diodes is used to protect against fly-back voltages and supposedly keep a shorted tube from taking out the output transformer… a safety modification Ken used to do in his repair business.

The grid resistors are mounted ‘flying’ off of the grid pin on the tube socket… keep grid resistors close to the pin. The purple wires shown above are the two phased output from the PI tube. Keep the two PI wires symmetrical at the tube so that you can easily swap them if you get positive feedback (squealing) when you first power up. There is a 50% chance of feedback because apparently the transformers aren’t wired with the phase consistent.

If you can read this, the DNS changes have propagated and you are reading the blog from the new web host. Continue reading »

Ken Fisher gave us the Trainwreck(TM) amplifier legacy and his Express amp is what the “Expression” Kit is inspired by.  But what does a Trainwreck Express sound like?

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The Strat-O-Blogster Guitar Blog bills itself as: Continue reading »

Another great development on the AmpGarage Forum… Ron Worley has compiled a lot of the key reference materials for a Trainwreck Express build into one large document and written a very comprehensive “Builders Guide” around the materials… and very well done, too. You can download the result from this thread: Express Build Guide. You need to register with AmpGarage.com to see and download attachments. Continue reading »

A lot has happened on the AmpGarage Trainwreck Discussion in the 1+ years since I originally built my Ceriatone. DrHulsey had a thread in which he shared his cleaned up schematic (last update: March, 2008)… finally a schematic that completely aligns with Francesca… here is the DrHulsey Schematic Thread. In this thread is also a layout that DrHulsey derived from Richie’s corrections to the original Ceriatone layout. Continue reading »

Peavey recently announced a new product offering… tube-amp modeling from a company that makes tube amps… Continue reading »

This series of postings will detail the construction and debug of a Ceriatone kit.  Along the way I hope to capture some of the key considerations & stumbling blocks and give you a good feel for the process. Continue reading »