Watts Happening Now
June 30th, 2011Amp Virtually Finished
This is too cool not to blog
Last night I got my Fender Bassman clone running! Here’s a photo. It’s gorgeous. I call it “Thick Al.” I asked for advice on a forum, and I referred to it as a “thick Al chassis,” referring to the aluminum, and someone thought I was giving the amp a nickname. Sounds good to me.
This is a 40-watt amp powered by 6L6 Sovtek Svetlana tubes. It has a 12ay7 preamp tube (V1), and then the signal goes through two 12ax7s (second stage and phase inverter). The rectifier tube is a Chinese GZ34, and it’s backed up by 4 4n1007 diodes.
It’s the exact circuit Leo Fender used, except that I didn’t follow his practice of always using the cheapest components, and I omitted the ground switch, added variable outputs, and used a 125V pilot light. If I had it to do over again, I would omit the standby switch, as a noted authority has convinced me that they are worthless.
The chassis is a 20″ length of 6″ Aluminum Association channel. I machined hollows in the back to make it easier to mount things. I made the circuit boards using a table saw and drill press. I polished the chassis with an orbital sander and 220-grit sandpaper.
It still needs something to cover the bottom.
A day or two, I did the “smoke test,” firing it up to see if it worked. I kept blowing fuses, and that’s when I learned that the cases of electrolytic capacitors are electrically connected to the innards. I had the filter caps mounted so the leads touched the cases, and that caused shorting. Hey, I don’t make capacitors. How would I know?
I got that fixed, and I built a current limiter to conserve my remaining fuses, and I found that the amp was very quiet and distorted. It took two days to find out that I had put 470K resistors where I needed 470 ohms. Two were on the screen grids, and one was on the phase inverter. I also had a 250pF capacitor where I should have had a 250uF, and I had the volume pots wired so they turned backwards.
Last night I got it running, but it was too late to try it out. Today I turned it on, and I had lots of noise.
I learned a lot about grounds. I had to move a whole bunch of them. In the process, I rewired the volume pots and also the presence and EQ pots (which, I now know, were wired correctly at the time). I replaced various bits of cheap stuff with good stuff, I put a tube shield on V1, and I turned the amp on again.
Still noise.
Then I turned on my Fender Blues Jr. Guess what? It was considerably noisier. That told me I had made it. I got some great suggestions for reducing the noise further, but it’s great right now.
I played the amp using my Epiphone Riviera P93 with Lollar pickups, using a Holy Grail reverb and the following distortion pedals: Plimsoul, Blues Driver, and Fat Sandwich.
The amp is amazing. The sound is very sweet, hot, and detailed. It has lots of punch, so you get a big reward for picking louder or softer. It’s less muddy than the Blues Jrs. Apart from the residual noise, it’s the perfect amp. And the volume actually works! A lot of amps go from 0 to 10 in ten degrees of rotation. On this amp, 3 means 3! It sounds good at practice levels! I don’t need my Firefly now!
I play it through a homemade cabinet with a 12″ Weber Signature speaker (super cheap). I can’t turn it up because the speaker is rated at 25 watts, but so far, it sounds beautiful. I just received a 10″ Eminence Ragin’ Cajun, and I plan to make a cabinet for it tomorrow. That will give me 75-watt capability, so I’ll be able to fire the amp up for real (probably with ear plugs).
I never expected to be able to use this amp. I built it in order to learn. But it’s extremely useful. A keeper. The ideal blues amp.
I just wish the selector switch on the Epiphone wasn’t going out. I have to get that fixed.
I love the way the amp looks. I was going to put some kind of roll cage on it, but I don’t really need one at home, unless I plan to get drunk and sit on the amp.
Now I need a new project. The guys at my church complain that their Vox AC30 amps are too loud; they can’t crank them and get good tube sound without blowing up the church. Maybe I should make an 18-watt EL844 based amp and see if they like it.
It’s amazing how God is taking the old threads I dropped and weaving them into something meaningful. This is a blast.
July 1st, 2011 at 2:31 PM
Dang, that’s a pretty amp! There’s something magical about tubes. They’re almost mechanical in the way that it’s possible to envision the electrons bubbling off the filiment, rushing toward the anode, only to be slowed down — sometimes more, sometimes less — by the bias on the grid as they make their journey.
One question though: do you know why there are four diodes, presumably serving as a bridge rectifier, in addition to a rectifier tube?
July 1st, 2011 at 3:18 PM
Thanks for complimenting the appearance. I was somewhat startled to see how nice it looked.
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I put two diodes on the rectifier because these tubes tend to die, and when they do, the amp quits, and it’s my understanding that you can get wild AC current running into the amp and messing up other components. The diodes do the actual rectification, and they continue working even if the tube croaks. They will see to it that the amp continues to receive DC. The tube is still there because it may contribute sag, adding to the amp’s compression.
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I now have 4 diodes in there now because these are not the ideal size, and I thought it was best to avoid taxing them. There are two diodes in parallel on each hot wire from the transformer.
July 3rd, 2011 at 9:36 AM
Dude, that’s amazing, well done. I’m looking forward to hearing your take on playing it through a 10″ speaker.
I picked up some of the 10″ speakers Fender uses (I think they’re made by Eminence, and they’re 30 watters w/ Alnico magnets) for around $75 each a while back to replace the JBL’s in a vintage Concert amp I have, and they’re really good. If you can’t get that 75 watter to break up right at lower volumes it might be worth playing around with one of those, too.
July 3rd, 2011 at 10:46 PM
Get a Brother label printer, and make some nice looking printouts for your chassis. Or, take it up a step, and either go with acid etching, which you can then fill with beautiful paints of your choosing, or go to the ‘net and find a good, used engraving setup, like the gift and trophy shops use.
As good looking as that chassis and rig is, it deserves that last little touch of class.
Congrats on the awesome build!
Jim
Sunk New Dawn
Galveston, TX
July 3rd, 2011 at 10:51 PM
Thanks, guys. I took it to church, and it was used for 3 services. The guitarists raved about it, and I have to tell you, they were right. The sound is warm, detailed, dynamic…you name it; this amp has it. They said it was better than the church’s $1000 Vox AC30s.
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The 12″ cheapo speaker is nice for home practice. It’s somewhat warmer and less sensitive. But the 10″ speaker is better, all-around. The bass is much better. It’s more detailed and somewhat harsher, but in anything but a small room, the harshness disappears.
July 4th, 2011 at 12:44 AM
That’s a beautiful piece of work. I said it before and I meant it: building amplifiers is a noble endeavor and should be pursued with gusto. You’ve made the world a better place.
All the best,
Lee
July 4th, 2011 at 8:01 PM
Is Flip Wilson your pastor/ The Church of Watts Happening Now?
It looks sweet.
I like what Jim said about etching. That would be the frosting.
July 4th, 2011 at 8:23 PM
I didn’t think anyone would get the Flip Wilson reference.
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I would love to etch or engrave the amp, but I don’t know how to make the etching look professional. I don’t want it to look like some idiot grabbed a nail and tried to scratch letters into it.
July 5th, 2011 at 6:19 PM
What he describes reminds me of photoetching a circuit board.
What if you took a similar strip of aluminum or brass and took it to a trophy shop to have that engraved and glue it under your pots?
July 5th, 2011 at 7:09 PM
I found an engraving machine for $99, but it weighs a hundred pounds and would take up room. I have two other options, though. One is to use letter punches and pound the letters into the aluminum one at a time. That’s very cheap. Another is to look into silkscreening. I think the punches would work well, and I could fill the letters in with paint.
July 6th, 2011 at 3:54 PM
Maybe try etched (or laser-cut) nameplates from here:
http://www.emachineshop.com/
July 7th, 2011 at 12:03 AM
Steve, take a look at how the car dealerships (actually, the roving vendors who do the work on subcontract) do the VIN etching on automotive glass.
It’s a computer driven font transfered to an acid film, and it works. I think if you do some research, and contact the companys that make the stuff, you might find that some of them offer it up in nicer fonts than the “dot matrix” used in the VIN etches.
As for the engraving machine, that’s what has my vote. Find the room! Not only will it serve you for many amplifiers yet to come, but think of all the other things you can do with it, such as engraving proper brass estuchians for parts on your machine tools, reloading equipment, bits on your HD and Guzzi, and the list goes on.
The punches? NO way! You’ll not get a nice, neat row of letters. It’ll look like monkies on meth withdrawls got into your tool room and ruined your amps. Punches tend not to go to the same depth, letter to letter, and they seldom go in flat and perpendicular, either.
Find a way to mount the engraving machine on a swing down, counterbalanced jig which will disappear under your workbench when you’re not using it.
It’s worth doing right. You’re on to something with those amps. Don’t cheeseball it.
Respectfully,
Jim
Sunk New Dawn
Galveston, TX
July 7th, 2011 at 10:59 AM
I had the same concerns about the punches. I have considered punching the legends into small metal plates and discarding them if they don’t look right.
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Right now, you couldn’t swing an ant into the space under my workbench. Stuff has to go somewhere, so that’s where a lot of my stuff went.
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The emachines idea will work, but if I were going to go the expensive route, I’d just load the amp into the truck and drive it to a professional. I can get nice legend plates if I don’t mind adding a hundred bucks to the cost of each amp.
July 7th, 2011 at 1:35 PM
OK then. Do what I did in my 1 car garage.
Suspend the engraving machine from pulleys mounted to rafters above the ceiling sheetrock.
Winch it down when needed.
I have my B&D miter saw stand hung from the ceiling that way, as well as an 8′ fiberglass ladder.
You can thank me later 🙂
Jim
Sunk New Dawn
Galveston, TX
July 7th, 2011 at 1:43 PM
Well, I already have 3 storage platforms up there, plus two bikes, and I’m planning to hang a 180-pound rotary table.