DIY Valve Vari-Mu Adventures

I’ve recently been working on a point-to-point build of a vari-mu compressor. Unlike a lot of DIY, the circuit is the work of local professional and so not publicly available and I can’t share it. I am extremely grateful to have been given the opportunity to have a go at building it.

 

It has however been sitting on my workbench barely started for probably 3 years. I had wasted an inordinate amount of time creating a 19″ 2u case for it out of bits from other cases. Metalwork is not my strongpoint and this was an epic waste of effort. God knows how many hours I spent hacksawing, filing, drilling, threading. By the time I was done I had long lost interest in the fun part, hence it sat on the bench. Lesson – just pay the f^&kin’ money, buy a case and get building!
As is often the case, for no apparent reason I suddenly resolved to get stuck into it. The first hurdle was the external power supply – a lot of fiddling about to find a good case for the transformer, make and connect the umbilical cord, and find appropriate connectors to safely get the high voltage and heater supplies into the case (I eventually found the excellent 7 pin Bulgin connectors from RS Components).

 

Once the power supply was built and connected to the case, I still had more metalwork to do –  holes for the XLR connectors, pots and VU meter…and what to attach the valves to? They had to go horizontally to fit into the 2u height, so that was 2 sets of 90 degree brackets to be drilled for 5 valves, and safely mounted in the case. Finally with those and the input and output transformers bolted in, I was under way with the hook up.

 

I had decided largely to wing the hookup. I didn’t plan it other than I decided what valve was going where, where the I/O was, where the knobs would go, where the audio transformers were, and where the power supply caps were going. I rotated the valves so the heater supply would have the shortest, most direct path to their tags without having to go around the other pins (as per the advice in “Building Valve Amplifiers” by Morgan Jones). Once I had checked that the heaters were all receiving their 6.3v I just started at one end of the schematic and started soldering.

 

Here is what I did wrong:

  • I soldered the heater wires directly to the Bulgin connector. I should have taken them off to a tag strip and run from there, as later on there was some fiddling around with the center tap and voltage divider. It was hard to get to it to solder/unsolder, whereas a tag strip would have been easy.
  • I didn’t leave enough room around the power supply caps and dropping resistors to allow me to experiment later. I crammed it all into the corner where later on it was hard to get to to change out components.
  • I connected the circuit earth to the chassis near the PS caps. The place to make this key connection is as close to the most sensitive input stage as possible. I thought you were supposed to connect it where the heaviest currents were flowing – apparently not! (note that the actual safety earth from the mains has its own dedicated connection straight to the chassis, right by the plug).
  • I wired up the amplifier tubes incorrectly. Pretty stupid mistake – in the schematic (and any similar one) the two halves of a dual-triode are shown horizontally mirrored, ie the anode is always on the outside and the cathode on the inside. I wired the top half ok then continued as though they were not mirrored, i.e. I had the anode and cathode reversed for the second half. Luckily I figured this out before power was applied – a good demonstration of why you do your idiot check a day after wiring, not immediately after. However, because I had the tubes mounted horizontally and was doing p2p instead of using tagstrips, this was hellish to undo and fix. The result is even more of a rats nest than it needed to be, and difficult to get to to troubleshoot. Lesson – anything more complex than say an Altec 436, give yourself plenty of room or use tagstrips.
  • I should have used more tagstrips in general…bit of a theme here….sturdier, tidier, easier to follow, orders of magnitude easier to troubleshoot and change.
  • I wired a couple of pots backwards. Not sure I’ll ever get the hang of that one.
  • The case…as explained, just buy one.

Things I did well:

 

  • The heaters – I found some nice solid core mains wire at the local hardware shop (Mitre 10 for NZ locals) that was red and black and twisted nicely with the drill. I used heavy gauge for the first part of the run then split it out to lighter gauge to each valve. Tagstrips would have made that easier too but I wanted to keep it all close to the chassis. I kept the windings tight and out of the way of everything. By raising the valve mounting brackets off the case floor the only point at which the heaters are close to other other components is where they come off the case and up to the pins. Heater noise is not a problem.
  • By using p2p pretty much all circuit interconnects are very short – in particular, as I understand it, the very high impedance (and therefore susceptible to noise) connections to the grids.

Power up.
With the assistance of Clary Schollum, who to my great benefit lives nearby, it was powered up bit by bit with a variac. Nothing exploded, voltages were near enough. However it was pretty noisy and hummy, so back to the the troubleshooting bench. Well, first mistake I found was putting an ecc83 in instead of an ecc82. No comment. Changing that out got me into the “its working ok” ballpark. However I couldn’t get the second B+ into the 90-100v range, and consequently the threshold wasn’t working properly. It was doing a better job as a overdrive box than a limiter.
After exchanging a couple of emails with the circuit designer, it was off for a troubleshooting visit. I came back with a few things to do:

  • add a resistor across the powersupply to give a load to the dropping resistor, so we can get the 90-100v required
  • change the input pad – too much level going in
  • redo the voltage divider for heater elevation – it was too high at 50v, needed to get down to 40v
  • redo the grounding as discussed above
  • attach the input transformer shield and valve shields to the chassis

I am still fiddling around with the input (and output) pads, but apart from that the only problem left is motorboating on the two fastest release settings.

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