Monday, 17 March 2014

Smoke, mirrors and a very powerful wizard

The "wizard" in the title is the phenomenal Justin Johnson, who's currently on his Smoke and Mirrors World Tour. And, as luck would have it, he's been staying with Mr Peter Blind Kiwi Murphy. So it was only a matter of time before Justin got his hands on the CBA and gave it a seeing to. ;-)

Justin and Peter very kindly sent me this movie file and granted me permission to post it.




Saturday, 19 October 2013

More CBA mods

Found the treeware diagrams

The T'Watt VIP has the following tone stack, but bear in mind it has two extra stages of gain in the pre-amp to make up for the loss in the stack.


The 860pF cap shown at the input to the circuit can be left out. It was a throwback from an earlier tone circuit that didn't decouple the DC between the pre and power stages.. With this tone stack the decoupling between pre and power stages happens with the three caps in the circuit. This drawing was done from the actual amp layout a while ago. I've since physically removed that cap.

This tone stack is the one I've been happy with for over a year now in the current T'Watt. It does everything I want it to.

The original T'Watt had no tone stack at all, and didn't suffer because of it. If I wanted less trebles, I simply wound down the tone on the guitar. ;-)

However, I started experimenting.

With a cathode follower in the circuit to drive the tone stack...


...I found that this bass and treble circuit gave me the best results.


And in that circuit, the 1uF cap is there to decouple the DC between pre and power stages. You can fiddle with that value, just don't leave it out. I tried a version with a middle control, but it was too fussy. The controls were just too interactive. This one gave good results for the lowest component count.

I would strongly recommend downloading and playing with Duncan's TSC (tone stack calculator) to get some ideas and to "see" what the response curves look like when you tweak the values.

So, CBA mods...

Apologies. It's been a busy week in the office, and this project had to go on the back burner. The other reason is that the physical build of this one-off for Peter is now complete. I'm just waiting for him to call in (all the way from Australia) to collect it next Sunday (27 October 2013) so, in my head, the pressure is off.

However, my other awesome friend, Bob 1B King, has started down his own path to valve amp addiction. Bob's amp is already going to be a little different.

One of the mods Bob's looking at is purely cosmetic (but fun) and he's adding an EM80 "magic eye" power indicator. I have these on my WTF Evo X and the T'Watt VIP, so here's the circuit.


The EM80 is fed from one of the later B+ nodes to get the voltage down below 250 vdc. On Evo and T'Watt, I added an extra node just to isolate the EM80 from the pre-amp circuit a little more. Not needed, but I had the components and the physical space. The supply is fed direct to pin 9 and through a 470K resistor to pin 7.

Pin 2 goes to signal ground.

Pin 1 is the signal feed. The signal is "read" from one of the anodes of the output valve (either pin 1 or pin 6 of the 6N1P). The 0.1uF 400 volt cap blocks the DC from the output valve's anode. The 390K to 470K resistor and the 220K preset are used to adjust the deflection of the magic eye beam. I found that a 470K fixed resistor was just fine in my own amps, but you may want to tweak the values if you're getting too much or too little deflection for the preset to cope with. The pair of diodes just rectify the signal to get a DC level for the EM80 to work with. The 0.1uF cap marked "Hold" in the diagram determines the rise and hold time of the display. A bigger value gives a slower reaction speed. Feel free to tweak.

BE CAREFUL! There is a temptation to tweak the deflection preset with the amp powered up and under load. Remember the warnings about LETHAL voltages and currents. DON'T do this unless you are ABSOLUTELY sure about what you are doing! It's far better to be patient and take your time to power off the amp, tweak, power on and test than to electrocute yourself. I cannot be held responsible for you poking your finger/screwdriver/butter knife into a live DC circuit!

It's fun and gives a wonderful moving light display, but I wouldn't say you could calibrate it to any degree of accuracy for recording and what not.

The whole circuit can be built directly on to the EM80 valve base. It's fiddly, but it can be done. Just keep an eye on the orientation of the two 1N4148 diodes (that's where I usually mess it up first time round).




To come - the dreaded tone stack - and the need to add another valve!

Saturday, 12 October 2013

Where to get parts


Please bear in mind that this is solely down to me hunting around on various forums. I don't endorse (with one exception), nor do I have any connection with, any of these suppliers, I'm just posting some links. And I'll try to indicate the relevant country.

And this is NOT an exhaustive or definitive list by any stretch of the imagination. If you have a favourite supplier for valve amp parts, use the comment box to let me know and I'll add the link to this blog. :-)

Hoffman Amplifiers - US

Apex JR - US and Canada

Digikey - US (but has global delivery option)

Mouser - Global

Ampmaker - UK (but will ship globally)
Barry is the one exception to my "don't endorse" statement. I buy 99% of my parts from him!

That'll have to do for the moment. I've a living room carpet that isn't going to vacuum itself.

Thursday, 10 October 2013

Okay, last lap for the actual build. But there's more to come in this blog. Over the next few installments I want to take a brief look at some of theory behind what makes the CBA tick.

But for now, I'm going to post tonight's little diversion.

I want to give Peter something that is pretty much self-contained so it was always my plan to add a speaker cabinet to the package. And this would also be built into a cigar box.



So, we have a speaker from a car stereo system, simply because it's rated at 30 watts RMS and is small enough to fit in the box I've got to hand. And you can see the box and socket in the pic too.


Not only do I prefer my amps to be old school and use retro technology, I'm the same about my wristwatches. And this spring-powered beast didn't deserve to be magnetised while working on the speaker so it had to come off.

So we find a centre point by drawing across the diagonals.



Then measure the mounting diameter of the speaker. There is a cutting template in the box, but I always like to check anyway.


Out with the trusty old pair of compasses.



And then I hit it with a large hole saw. No pics of that sadly.

Speaker mounted and socket soldered into place.



Time for testing (with the lights off so you can see the glow).


And with the lights on so you can get a clearer view.



It sounds better than I expected, but it's more an exercise in completeness. The amp sounds stunning through a big cab, so I'm guessing it won't be too long after Peter gets back to Australia before he sources something more appropriate. Oh, and tonight's "ah bollocks" moment... I've fitted the speaker grille upside down. There's lettering that reads "Mutant X" on one of those curved sweeps and that kind of gives it away. And it was a sod of a job getting the nuts on the bolts too! Still, I've got until 27 October to correct that little error though.

Tuesday, 8 October 2013

And Bob has come through with the schematics! He's even included a parts list. All that's missing from the parts list are the transformers, the power sockets, valve sockets and valves. They're all things that are pretty freeform and depend on what you can source locally.

For builders in the UK, the transformers and valve bases can be bought from Ampmaker. I'll do a bit of research for other countries and post up some suggested sources here.

But before we go any further, here's the disclaimer!

Building and modifying amplifiers is DANGEROUS (like most fun things). Valve amplifiers invariably contain both very high voltages and high currents, capable of killing you. Never work on a live amplifier!

This blog is here to share information about a particular amplifier project. It is up to you to exercise caution and common sense at all times to avoid electrocuting yourself, and make your amplifiers safe to use. I am not responsible for your negligence. Building valve amplifiers is also more addictive than nicotine; you have been warned.

Now, here's a link to a PDF that has the schematics and suggested parts list.

And the pics below are what you get in the PDF. If you'd rather not download the PDF, clicking on any of the pics will open them full size.




Would you like to see my etchings?

I thought I ought to make a special, one-off nameplate for Peter's CBA. So the copper sulphate and power supply came out this evening.

It's difficult to photograph, but I'm really pleased with the way this has turned out!




I've glued it in place for the moment, but tomorrow I'll be hunting through my watchmaking bits to see whether I've got four screws small enough to just add the finishing touches.