Finally, here are some pics of the finished amp. Building the enclosure was a lot of work, and equally a lot of fun. It was built in the Stadswerkplaats Utrecht together with David.
The wood is 15 mm birch multiplex, could have been better quality but it's good enough. Finger jointed using a home built jig! It's very strongand I'm quite pleased with the end result. The grill cloth was purchased at Banzai.
After almost two years, I'm finally building a cabinet for my Champ!
I'm going for finger-jointed, 15 mm birch multiplex. The look I'm going for is the un-tweeded narrow panel tweed Deluxe. The Deluxe has a 12" speaker where mine is only 10", so my cabinet will be 4/5th the size.
I've built a 1:3 scale model to see if everything would fit. I'm pleased with how it came out but the front angle could be a little less steep so the speaker will point up some more.
Material thickness is 15 mm, outside measurements are 230 x 470 mm for the top, 250 x 470 mm for the bottom, and the sides are 380 mm high. I'm thinking about making the top 215 x 470, but I'm not sure yet. The baffle will be 10 mm thick and the speaker off-center, just like the Deluxe has. I really like that look.
The white box inside the model is the actual amp enclosure. The black box on top of the is the power transformer. Don't say I didn't try hard.
Click the pic for a larger view.
Do tubes, transformers, capacitors, speakers need "burn-in"? I've been reading up on it and it seems that in the digital age this is no longer needed. Some people still do, either as part of the mojo or to test for component failure.
Speakers do need a break-in period, sometimes also called burn-in, it is generally agreed that speakers will sound better after a while, about 10-20 hours.
Tubes have a settling period. I've read on the 18 watt forum that the bias can creep up after the first week or so of use. This may explain my amp getting more "settled down", or something.
"Official" builders stress test their amps for as much as 2 days straight, volume halfway up, into a dummy load (ears say "Thank you!"). I will not be doing this.
Capacitors: The disappearing hum you experienced was likely from the filter caps; until they "formed up" they were not working correctly and hence not "filtering" out the 60 hz hum.
I poked around the amp a little bit, and the hum practically went away. So this is why they sell chopsticks as "Amp Tech Tools"!
Perhaps it's also the tubes getting comfortable or the transformers settling in. Is that what burn-in does? It's still not a dead silent amp, but for a semi-stargrounded class A vintage replica I think it's OK. When you turn it up really high there is some audible hum, then again, the amp is really loud at that point. I'm wondering how well it would hold up against a drummer.
Next up: the case! I've decided to go with a Tweed Deluxe model, because I really like the rectangle shape of it. I think it's a lot better looking than the square-ish champ. The Deluxe model has a 12" speaker, this amp a 10", but I'll have the baffle replaceable so I can change to a 12" speaker in the future. My brother who is learning to be a furniture maker/woodworker will create the box. He has access to all kinds of interesting woodworking tools. It will have swallow tail joints!
I haven't decided on a wood sort yet, it should look nice because I don't plan on actually tweeding the amp. I really like the look of this 18 Watt in S2's amp gallery.
I was taking special care not to touch the B+ line, draining it each time I turned the amp on, treating it as live and charged each time I was away for a while, discharging and measuring those volts. And forgetting the wall plug was still plugged in when I was re-arranging some leads to get rid of hum... Ouch! 220 Not good!
In other news, Noortje's picture of my cap discharging tool made it on the Makezine blog!
It works! Fired up the first try! Voltages are a little high and the power tube is running really hot. The power transformer is hot as well, which is worrying. I'm going to lower the power tube's bias voltage a bit, hopefully that will cool it down some.
The transformer is more of a problem though. It's not red hot but it's also not comfortable grabbing around it, which makes me wonder if there is some stray current running somewhere. At least it sounds pretty good already, without breaking in the speaker.
After a long Sunday of soldering, I've managed to get pretty far! The board fits in the chassis and all the components are attached. Here's a picture of the speaker with lead attached:
As you can see, everything fits and it looks pretty neat, too. The push-back wire is pretty easy to work with, since instead of having to strip bits of insulation off, you can push the insulation back to uncover the wire. Also, it doesn't melt like plastic cover, so when it gets hot the insulation doesn't fall off.
I've installed a small mod, the negative feedback is switchable. The sound is supposed to be less distorted with negative feedback, so this should be a nice add-on to the design. The switch may be in an awkward position though, it's located between the pre- and power amp tubes, right next to the speaker out. Oh well.
Next up: installing the final two parts - the two 100 Ohm resistors on the tube heater lines. After that, it's testing all the connections, powering up with the current limiter attached, measuring the voltages, installing the tubes, and keeping my fingers crossed...
OK, next step is done. Here I've measured all the wires that run from the board to the various off-board components. The wires are soldered to the underside of the eyelet board, which makes is harder to debug (can't see where those leads are leading) but it looks so much neater! The space in the chassis is so tight that it would need major soldering and bending and shifting to get the board out. Not very moddable...
Pay attention! In any tube amp there are large capacitors that smoothen out the ripple on the DC that comes out of the recitifier. These capacitors hold a possibly lethal charge of voltage, even after the plug has been pulled from the socket. To safely work on an amp, these caps need to be discharged.
To this end I have constructed a low tech but very effective tool. I got some nice wire in pretty yellow and a 5 Watt 15K Ohm resistor. The wattage could be less, I guess, but it's better to be safe than sorry and this is all that Conrad had. In combination with a pair of alligator clips and some shrink tube it looks like this:
Started wiring up the chassis. It's just like with pedals, there's never enough room. The way the wires run is more important here than in effects because of the much higher currents.
Here you can see the front panel with some wires hanging out. Note how shiny and chrome it is! You can see the reflection of Noortje's camera.
The rectifier tube is wired up. The yellow wires are the high voltage ones, the reds are for the heaters. I've measured a bunch of other wires as well and have connected them to the eyelet board. I hope it doesn't get too messy.
Anyway, it's coming along nicely! I need to go and get the current limiting light socket and a high wattage resistor to decharge the power supply caps on Saturday.
This seems important. To reduce hum, the circuit needs to be star grounded instead of grounded to wherever is convenient. I'm not sure how much I can use this advice, or need to use it, but I'm keeping the option open.
Further hum reducing will come from connecting the two filament resistors to the cathode of the power tube instead of to ground. Choice quotes: The idea behind elevating the voltage of the heater CT is to get it further from the cathode voltage of the preamp tubes
and PAUL RUBY SAYS: 'A better solution than grounding the CT is to connect it to some clean, positive DC voltage source. If the power stage in your amp is cathode biased with a nice big bypass cap on the cathode resistor, you're set! Just tie the CT to your power tube's cathode. There's somewhere between 9VDC and 35VDC at this node. It's just as free and easy as ground, so use it instead of ground if you have it.'
Those eyelets seem large at first, but stuffing 4 or 5 wires through them makes it pretty hard. Well, the top side and two of the bottom connections are done. It's not so easy to cut the cloth covered wire, you need a really sharp tool for that. I combined my pliers and scissors and that worked OK as well. I could have removed the cloth entirely but decided to leave it on for now.
From left to right: the big black parts are the electrolytic capacitors that level out the DC in the power supply. Then in green and black cathode bias resistor and filter cap for the power tube, the orange parts are coupling caps between the two pre-amp stages and between the pre- and power amp.
You can't really tell from the pictures how large the board is, it's 15 cm wide and 7,5 cm high. That means that the parts I'm working with are a lot larger than the ones I'm used to for guitar effects; but then these need to be able to cope with far larger voltages.
Well, here we go. The first resistors are soldered to the eyelet board. It looks neat and very much handwired and point to point and all that, I feel good about it.
The plan said to drill a hole in the chassis, which is quite heavy steel. Since the only drill we had was a dull HSS, it took about 30 minutes to go through. Yes, I should have bought a new one the next day, but I was just too eager to get started. The hole is a little bit ragged due to the heat but I've smoothed the sides and it's pretty much OK now. You won't see it anyway.
I've received my kit! Bought it from Tube Amp Doctor, who are a tad (haha) expensive, but the service and the product are both excellent.
There were two missing resistors, but after a quick email they were sent over within a day or two.
The kit is a Tweed Champ 5F1, no cabinet, with a Jensen P10R speaker. The speaker is getting some mixed reviews so I'm going to see how it sounds. I can always sell this one and get a Weber or an Eminence, instead.
A bunch of links to some of the sites that I find helpful.
