Hi Folks! I am very excited to show off my restoration of the amplifier. In part I, I went through all of the considerations and most of the math necessary to find a transformer for this bad boy, and I mentioned in passing a few things I’d be doing with it when the parts came in. The parts are here! So it’s time for us to install an isolation toroid from Triad Magnetics which cost about 30$ including shipping.
In order to make the space we need to install the toroidal transformer, we need to do some house cleaning inside the chassis. First thing is first: remove what was added by other technicians that doesn’t belong. What, specifically in this case am I removing? Some misguided fool of a technician put a 330uF/250v capacitor on the PSU circuit. Why is this worthy of removal? Isn’t more capacitance always good for power supplies? The answer is mostly extra decoupling capacitance is always good for PSUs, except for when the PSU uses a tube, like the 35W4 used in this circuit as a rectifier (diode.) The tube’s job is to convert AC to DC by only conducting during the appropriate part of the AC cycle.
The tube needs to be “loaded” by another component in order to rectify AC to DC correctly. In HiFi design, there is really only one option (for those of us with the $$$ and know-how) for a tube power supply to be loaded: with a big, expensive, heavy, high voltage low frequency inductor capable of passing all of the current the PSU produces while powered. This is a non-starter for guitar amps in most cases, which are made cheaply to keep them afforable to the customer. There are a few like Fender, Vox and Mesa, where they’ve done designs that use chokes for “clean-tone” amps, but they’re usually not the kind a rookie can purchase to practice with.
While omitting a choke from the PSU doesn’t actually make the quality worse, per se, it does mean that you can’t pass as much current through the tube as you could with an inductive load. Without a choke, most tube rectifiers are loaded with a capacitor instead. This part is mathy: how much current will you draw at idle, and then at peak? Once you know that, you can look at the datasheet for the tube and it will tell the max values for capacitive loading the tube under the conditions where you are drawing X amount of current. You have to choose a capacitor that will discharge somewhat between each AC cycle, thus allowing it to conduct current. So small capacitive loading values are common with tubes (some as low as 4uF!) Too large a capacitance value can actually hamstring a PSU with a tube in it, and prevent it from conducting current properly. Too big a capacitor can also kill your tube.
In this case, 330uF of capacitance is more in that one cap than the whole stock circuit had when it was built! That means someone wanted to make it have a “stiffer” PSU, less likely to sag under load (like a big chord,) and probably thought, “well, it works in solid state amps!” And went ahead and put the cap in. Big mistake- the tube rectifier can’t work properly. The guy working on it was likely trying to compensate for the leaky caps left and right, which adding capacitance to the PSU will not effect. The reason the last owner experienced a “loss of power” playing this lil amp is simply because the old paper-in-oil caps were trashed and leaking. He also told me that the headroom and breakup for the distortion were all F’d up too, which of course, your coupling caps are totally critical to. Let’s get to cuttin’ shall we?
Above you can see the space left by the old multicore capacitor, which I unceremoniously cut out with diagonal cutters. The old capacitor was totally trashed, and one of the three chambers hadn’t even been used by the manufacturer. Pretty funny, because they were paying for the whole thing, they might as well have added it to the PSU. Oh well. The “firecracker” caps, the kind of multicore caps with colored wires instead of solder terminals, are all long ago garbage at this point in the year 2020. Most were rated for “one year in circuit” which I find amusing. So the old one got cut out, and I cut out it’s constituent wires too. I kept the cheapo-ass looking pop rivet in the chassis, as it is better than a hole, but I HATE pop rivets. They look so fucking cheap and crappy. I digress. This space we’ve just made will accomodate our new transformer quite nicely, I believe. The new toroid has rubber pads to prevent it from rubbing against the rivet while the amp is dragged around, after I bolt it in.
Above you can see the Standard brand capacitors which are leaking PCB oil (God I hope it’s mineral oil, but probably not.) I have heard people call that substance “wax.” Wax is a misnomer, though, as it implies that the material seen dripping from these caps is meant to seal the electrolyte inside. In my experience, that “wax” is usually the electrolyte itself, cooled to a cool enough temperature to begin to solidify. Most old capacitors used nasty ass shit for electrolyte, things like PCB oils that cause cancer. In the photo above, the only cap saved from the restoration will be the blue one stretched between the sockets; all the others in this circuit need replacement. So bing, bang, boom. We cut em out, and solder some new ones in.
There we go. The .01uF 400v caps were replaced with .022uF/630v metal film capacitors usually used in powered pickup amplifier circuits for electric guitars. The old Good-All cap was replaced with a new one, also metal film. The electrolytic caps are a burly 400v rating although this circuit likely never sees over 200v. It should make the PSU happier, too, now that it only has 96uF of capacitance in the whole power circuit. The old firecracker style capacitor had a green wire, which was a 30uF cap at 25v. I replaced it with a 47uF/35v cap. The last things to do, after these steps, will be to add the transformer, and then to wire up a new 3-pronged plug that will have the chassis grounded to Protective Earth, and wire the power cable to the transformer so that the on-board on off switch still works properly.
To add the toroid, I simply drilled a new hole in the chassis, making sure not damage the nice blue paint. I measured it’s distance from the edge of the chassis to make sure it fits, and it does. Yay! Toroids are totally fucking awesome compared to EI core transformers: only one hole to mount them, less stray flux bands in the electrical field, smaller, quieter, and far more resilient to over-voltage and over-current circumstances than their older E+I core counterparts. They’re usually expensive, but Triad makes good quality and cheap priced ones for isolation. I chose the model that can supply 440ma.
This amp design is weird: since it was designed to run off of mains with no isolation, the designers had to run the AC through a bunch of the circuit that normally doesn’t see AC in an audio circuit- at least no hifi circuit I’ve ever seen looks like this. One phase of the AC is run through the filaments, cathodes, the screen of V1, and the suppressors of the tubes. Holy Cow. Personally, this amp’s schematic looks like a dog’s breakfast of a mess to me. You can see it below. The isolation transformer I installed was the same as in the drawing. I even forgot to add a fuse, just like the drawing. The 120v mains fuse belongs on one of the two lines going to the transformer from the wall.
Why would the designers take the AC mains neutral, and introduce it to the cathodes and suppressors of the circuits? It acts as a “ground,” although it oscillates with the mains. Ok. That’s pretty straight forward. We only have a single rectified DC phase bouncing up and down to run the DC supply on to begin with, so having this AC neutral ground should be sound OK. They “ground” all of the components in the output stage to neutral. This is ok for the output stage, as it is more immune from transients than the input stage. The input stage “ground” is again an AC secondary, this time from the filament transformer secondary. The two separated ground rails are divided by an RC network comprised of a .047µF/600v cap and a 130k-170kΩ resistor. I assume it’s to attempt to isolate the ground for the input stage, which will be more susceptible to noise from the ground rail.
V1 and V2 also have their screens connected through a 2.2MΩ. The Anode of V1 also feeds the screen of V2 through a resistor, in addition to feeding signal to the grid of V2. Like I said before, it looks like a dog’s breakfast. Lots of noise is possible in this circuit. But hey, I don’t do guitar amps. This may be a perfectly effective solution for signal noise in such a simple circuit. I don’t know because I’ve never had any experience with such a cheap design. I can’t wait to see how it performs.
One end of the isolation transformer’s parallel cores is the red and yellow wire, shown above going to the on off switch on the front panel.
Above you can see the brown wire, which connects to the other end of the isolation transformer’s parallel cores. This way, no AC ever touches the circuit without going through the transformer first.
Above is a photo of the strain relief for the power cable, which is a plastic cable tie and a zip tie working together to prevent catastrophe if someone kicks the power cable out of the wall, as happens at gigs and in studios from time to time.
You can see I opted for the “fool’s strain relief” inside the chassis, by tying a knot in the power cable. The secondary plastic cable tie is a must in this case, to prevent a real problem in case the knot isn’t good enough. Anyhow, all is well with this little 3W amp. We’re ready to put it all back together and see how the owner likes her sound.
I’d like to take this chance to point out something: There are only 7-pin tubes, or septal based tubes, in this amplifier. That is pretty cool. Why? Because I always wanted to know what an amp made with only septal based tubes was capable of, and now I know, at least approximately: 3W for a guitar. If I made a hifi based septal system, I could use solid state rectifiers and get some more efficient power delivery out of the amp, but less wattage due to how Class A amps tend to be much lower power than their guitar amp counterparts. It cuts down on distortion. I could probably do a 1 watt per channel stereo push pull hifi amp with septal tubes and it and still have it rock pretty good. I should design that…
I got distracted and forgot to add the fuse to the unit. Oops. After drilling another hole and securing the fuse holder with a 4-40 screw and nut, we should be good to go. You can see above it’s lookin good.
OK. Now we have the approximate circuit from the schematics above, except we are using a 10Ω resistor where the “7.5Ω fuse resistor” should be, and a real fuse inline with the mains input to the isolation toroid. I’m not sure what the F*&% people were thinking, making a device that uses no isolation and a resistor as a fuse. A resistor is NOT an adequate fuse by any means. A real mains fuse of 125v at 0.5A should do, and although slow-blow fuse would be fine (tube amps are pretty robust to a short circuit condition) I’ll start with 0.5A fast-blow fuse to make sure the amp can handle it before I button it all up again. A 1A fast blow fuse might be necessary to accommodate any current inrush that might occur after the on switch is thrown.
Why might there now be an inrush of current in the circuit power supply if we have removed the large sized capacitor that was swamping the tube rectified PSU down? Shouldn’t less capacitance result in less inrush current? Yes is the answer. But we’re forgetting to account for a critical design aspect of a toroid power supply: the initial magnetizing current needed to magnetize the core. With a toroid, there is more magnetizing current needed to magnetize the core than would be required to magnetize an E+I core transformer. This can cause a sensitive fuse to blow prematurely from the current needed to magnetize the transformer initially. Another alternative is a slow blow .5A, which should be able to withstand the momentary transient current spike of the core magnetizing.
The last thing I would want done to this amp, if it were mine, would be to add an indicator light to show the unit is powered on. I like those. Indicator lights make me warm and fuzzy inside because I know something is turned on and working. An amp without an indicator, too, is likely to end up being left on and forgotten about in the mad studio rush of recording and multitracking. Fortunately, though, tubes like not to be switched on and off, so leaving tube amps on over night (or occasionally for days or weeks,) usually doesn’t “harm” the circuitry at all. It just puts hours on your tube life clock, which is of course somewhat undesirable. You want your tubes to last. But leaving this lil guy on for weeks, even, I can’t see anything inside the chassis failing- the tubes are of questionable lifespan, but they’re dirt ass cheap to replace. I’ve made it all pretty robust, so I’m not worried. Just more things to consider.
Speaking of considerations, I missed the rear wood piece that seals the little cabinet together when I placed my toroid. Fuck. So after I realized the chassis was not going to fit back in the cabinet, I considered cutting a little wood out to fit it. Then I came to my senses, and realized I could move the toroid around inside a little bit, cut out the old ceramic input capacitor and replace it with a polypropylene one soldered in a slightly different place, and toque the transformer down in it’s new location making the cabinet and chassis fit again… Just barely. Phew! So relieved. Now even the input cap is a nice boutique amp cap, so I guess that’s cool, though probably not necessary.
I saw these amps going for about 200$ or so online in a few places. An amp like this, with the “widowmaker” architecture taken out and replaced with a properly isolated power supply, should actually be worth the pain and expense. If we take into account the parts used, it costs roughly 2 hours or so of wiring, stripping, drilling, and assembly in addition to the approximately 50$ for the components used. So if I were to find this amp used for 50-150$, the whole job would add up to about 200$ plus a couple hours. Super cheap for an authentic point-to-point wired amplifier full of glass tubes with a Jensen custom speaker. My conclusion: well worth it. Triad makes other isolation transformers, too, so one has some flexibility with power transformer choice when doing similar “widowmaker” amps. Hope this post was interesting. I really hope the owner likes the new amp, because electrically at least, it is far better designed now than it was when I got it. Cheers. Go play some music, even if you suck at it.