The Futurama FU-3 (part 3)

For more aluminium welding practice I decided to weld up the knob holes in the original Corvette front panel. The front looks OK but you don’t want to see the other side. I then made a new layout using MAD- permanent Marker Aided Design.

Next step was to finish wiring up the Marshall 2204 preamp.

See earlier posts on the Ninja Corvette Hybrid if you’re puzzled by all the extra transistor stuff.

Yeah it makes the master volume Marshall racket 😀 This was recorded at the 1W power output setting.

Got some matching knobs from Thonk. I will get round to making a completely new panel one day using the MAD one as a template.

1950s Belling electric fire restoration

This winter I discovered that blocking up the fireplace makes the living room much warmer and cuts down the gas bill substantially. All good for the Conner Labs carbon footprint. 🙂 However I guess an open fire has some sort of primeval appeal. Before I knew it I had bought this “vintage retro” piece of junk on Ebay.

I immediately regretted it in case it turned out to be full of asbestos.

It was :/

As it was also caked full of dust, and to be honest smelt a bit suspect, I decided to take it outdoors and wash it down with soap and water. Everything including the wiring, to get the asbestos wet for safe removal.

It was hardly the most complicated assembly so I stripped it down to the last nut and bolt and cleaned everything. The reflector was polished using T-Cut.

Rewiring with bare copper wire in modern high temperature fibreglass sleeving. The switch marked X had somewhat melted contacts. I couldn’t find a replacement so I retensioned them as best I could and used that switch on the lowest powered heating element.

Modern toggle switch doesn’t have 1/10 of the vintage mojo. Doesn’t fit the panel hole anyway.

Red fireglow lamp is arguably the most important part 🙂 25W filament ones are still available.

About now I realised the error of using WD40 to free off the nuts holding the heating elements in place, as great clouds of WD40 flavoured smoke belched forth. 🙂

Once the fumes had dissipated it turned out to heat the living room better than the old gas fire and cost less to run.

The Futurama FU-3 (part 2)

With the chassis suitably butchered it was time to start on the electronics. I used Mark Huss’s schematic as a guide.

Soon the phase inverter and power amp were finished and working.

One big difference is that the 2204 uses negative feedback around the output stage while the original Corvette didn’t. So I decided to go with the NFB, and include the transistor output stage in the feedback loop too for an extra challenge.

It was perfectly stable first time! LOL just kidding… It suffered from high frequency parasitics-

And these comically chaotic LF oscillations could be provoked by overdriving it at low frequencies.

After much trial and error I ended up with something like this. The 0.68uF/5 ohm RC snubber killed the HF oscillations, and removing C17 and C18 (this schematic) stopped the motorboating. With these values it was just barely stable with the load disconnected and a 220k NFB resistor (vs 100k in the original 2203 circuit)

Note that when the transistor output stage is in play, the OPT secondary becomes bootstrapped and flies around with the speaker output, so the NFB takeoff point I used sees the output voltage of the transistor stage plus the output voltage of the valve OPT.

Removing C17 and C18 demanded quite a lot of extra current from the bias generator, but it seemed to deliver it no problem, so no changes were required there.

Resistor values were also changed to reduce the current gain of the transistor output stage, due to the increased output of the valve part of the circuit.

The Futurama FU-3 (part 1)

I got bored of the Ninja Corvette Hybrid and decided to transform it into something with a little more “FU”.

The plan I came up with was to strip out the valve part of the circuit and replace it with a clone of a Marshall 2204. This is a classic rock amp that I hadn’t had much experience with.

I decided to use 6AQ5 power tubes running off 250V, for a modest apartment-friendly power output. The 3 position power switching would be retained, giving power levels of 1W, 10W and 40W.

The Marshall 2204 circuit has 5 valves, but there are only 2 holes in the chassis…

A new output transformer was also required, as the original one was single-ended. I used the cheapest PP one I could find at TAD. I also TIG welded a bracket for it, as I’ve been watching way too much Project Binky.

TIG welded speaker stands

I was looking for an aluminium welding project that wouldn’t kill anyone if it failed. And also some speaker stands that wouldn’t take up any desk space.

I started with a 1m length of 3″ x 3″ x 1/8″ box section.

After a whole afternoon of hacksawing, jigsawing and filing it was reduced to 2 columns and some wedge shaped pieces.

The pieces were then welded together into a giant C clamp shape and a 3mm plate was welded to the top to support the speaker. I decided to only tack weld the plate because I was worried the heat of a full weld would warp it.

I find the fillet weld the most difficult. This doesn’t exactly look great but it’s my best yet.

The stand base screws to the underside of the desk with some hefty wood screws.

The desk is completely clear and there is plenty of room underneath for oscilloscopes, soldering irons and so on.

Tiggy Weldrix Wah Pedal

I got fed up with the thumb controls on the torch that came with my welder and decided to try a foot pedal. They are extremely expensive to buy so I decided to make one out of an unwanted wah pedal.

First, a bracket 🙂

The main difference between a wah pedal and a TIG foot pedal is that the switch engages as you first apply pressure to the pedal, telling the welder to open the gas valve and go through its preflow and ignition cycle. Depressing the pedal further then controls the welding current.

To allow this operating mode I threw away the existing stomp switch and replaced it with a microswitch operated by a cam.

The other difference is that it needs a rather strong return spring to avoid igniting the welder by accident. I found this out by trial and error. D: Keeping with the musical instrument theme, I used some Strat tremolo springs from eBay.

I connected it to the socket for the hand controller as a spare plug for this was supplied with the welder. It thinks it is a torch with 1 button and thumbwheel. This seems to work fine, but I will try the foot pedal socket if I ever get round to figuring out the mating plug for it.

And the bead goes on…

PFC Part 10: Integration test

It was time to put it all together! (This actually happened in March- these are post hoc posts 🙂 )

Tesla coil driver converted for 24V DC control power.

First Odin’s control electronics had to be converted to run off 24V DC instead of the original 240V AC. (And mounted in a Eurorack while I was at it…) This wasn’t too difficult as they already used 20-something volts DC internally, derived from the mains with a traditional iron cored transformer and rectifier, and regulated to 15V.

24V DC input module. It is essential to test this on a furry rug.

I added DC input sockets to the driver and gate drive amplifier modules, and changed the fan for a 24V one too. The original 240V AC inputs are retained in case the PFC breaks down and I need to change back to the old power supply.

The PFC will be situated at the operator’s position with long cables for 750V and 24V running to the coil. This made everything simpler, as there was no need for remote control and the circuit breaker on the PFC could be the emergency shutoff for the whole system. But it did leave the 24V cable vulnerable to strikes and general pickup of the extreme levels of EMI around a Tesla coil. My solution was to make a DC input module using a surplus Traco 40W DC-DC converter to give galvanic isolation, and lots of EMI filtering on both input and output.

The red module is the receiver for my Teslink system that sends multiplexed control signals over a Toslink optical fibre. I finally got round to completing it (and making a Eurorack mounting transmitter too)

PFC provides 750V DC output (on left) and 24V DC (on right)

The idea is that the PFC accepts single or 3 phase power at anywhere between 208 and 415V, and supplies 24V DC to the Tesla coil electronics from its own control power supply. I didn’t want the hassle of having to change taps on control power transformers, or rather the carnage of connecting it to 415V with the taps set to 240. (I have done this before- it was messy)

Setup for integration test

The Tesla coil primary was set up using a water-filled steel pan as a dummy load.

It didn’t explode! 😀

The next step would have been to take the PFC and immersion heater bucket to a lab with 3 phase 415V power. Unfortunately this was made impossible by the COVID-19 lockdown. The debut was to have been the Nottingham Gaussfest, but this was also cancelled. Insert corona joke here 🙁

PFC Part 9: This case I’m working on

Penn Elcom rack enclosure
Trial fitment of components
Is this what they mean by a multi-level converter?
Trial fitment 2
Bleed and ballast resistors attached to heatsink
Trial fitment 3 with EMI filters
Rear panel
Metalwork nearly finished
Testing the precharge circuit
And it’s done