maxkracht wrote: ↑Fri May 24, 2024 12:38 am
If you add a pair of resistors across your switch it won't pop when you change settings. Resistor value doesn't matter too much, too small and the caps will start to boost before being switched in, too large and it will take too long to charge the cap and stop the pop, I usually go somewhere around 100k.
maxkracht wrote: ↑Fri May 24, 2024 9:00 pm
Yep, it's generally a good practice to add them when you are switching caps in and out. I like the input jack drawing on webers layout much better, and the 220k bleeder resistor on the first filter cap is also a good idea.
Hmmm, well I do have some extra 100k 0.5w resistors, and an extra 220k 1w resistor lying around. It would be fairly simple to still incorporate those changes at this point.....
I'm looking at the two stroke layout, and the Weber Maggie layout, and they each have a slightly different wiring setup for the power/fuse/PT/switch. Is one design better than the other? The top one is the Maggie.
Screenshot 2024-06-03 at 2.26.06 PM.jpg
Screenshot 2024-06-03 at 2.25.46 PM.jpg
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The weber drawing is correct. You want to fuse then switch the Hot wire (usually black in the USA). Ideally, you would switch both hot and neutral if you are using a captive power cord, but I wouldn't worry too much about that.
Due to company, etc., the project has been on hold for a few days. I've got it all wired up now, and have checked and double checked all my connections.
IMG_4051.jpeg
I'm almost ready for initial power up, but I have one more question. Can I join the two grounds (see attached diagram) together and solder them to the brass plate together, or do they have to be separate?
2_stroke_layout copy.jpg
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jer_vic wrote: ↑Sun Jun 09, 2024 9:59 pm
Can I join the two grounds (see attached diagram) together and solder them to the brass plate together, or do they have to be separate?
You can do that. It's not ideal, but then Dave Hunter's ground scheme is not ideal either. Probably not gonna make an audible difference in this simple amp.
So the amp is done, now the debugging starts.... I followed Rob Robinette's startup procedures (with variac and lightbulb limiter), and everything went fine until near the end (step eight). Testing the amp with guitar and speaker (plugged into the lightbulb limiter) it emits a hum which builds to a squeal. It sounds like the same sort of issue as here: https://ampgarage.com/forum/viewtopic.php?f=6&t=37280
Looking at the layout for the Webber Maggie amp., I see they have the blue and brown output lead transformers wired opposite to what I have. Going to switch them, and see what happens.....
Last edited by jer_vic on Tue Jun 11, 2024 1:48 am, edited 1 time in total.
Ok, so switching the 2 output transformer primary leads did the trick. The amp worked after that. I was getting some fairly substantial hum, so I opened it back up and re-routed the wire from the cathode bias to pin 8 of the power tube - it was egregiously close to both the filament wires and the OT primary leads. That quieted the hum somewhat. What remains, according to the decibel app. on my phone, is a 56 dB hum at ~240HZ (measured at 1 metre away).
EDIT: A 2nd application reports 40 dB, and hum at 120 hz. I think 120 hz is correct. If I play that frequency via a tone generator app on the phone, it sounds exactly like what the amp is producing...
I'll do some research, see if I can reduce it some more.
Meanwhile, I'm happy with the amp. I feel I was pretty careful while building it, and that led to a very positive experience today when I first turned it on. No smoke or explosions.... It sounds pretty good to my ear, and if I can get rid of some more of that hum, I'll be very happy indeed. I think I'll call this a success. Thanks to all who chipped in to answer my questions, and offer some advice!!
I'll write a longer post in a few days, outlining some thoughts, problems I had, lessons learned, etc.
Edit - some voltages:
Power tube:
Pin 4: 312V
Pin 3: 387V
Pre-amp tube:
Pin 1: 146
Pin 9: 148
According to the plan/spec for this amp, those are in the ballpark, it seems.
Update. So my hum is due to a shit soldering job I did on the 2 grounding wires I mentioned a few posts up. I suspected as much, so that was the 3rd thing I did, after checking if having no input cable made any difference (it did not) or if removing the pre-amp tube helped (hum stopped). Poking the aforementioned bad ground has temporarily fixed the issue - the hum is at a MUCH more acceptable level now. I'll have to go in and fix it soon, though.
Some pics:
IMG_4056.jpeg
IMG_4057.jpeg
IMG_4055.jpeg
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Coming along! One thing to keep an eye on (I can't tell from the picture): You'll want to make sure the pilot light connections don't short to one another. Probably just an optical illusion, but the picture makes them look awfully close.
johnnyreece wrote: ↑Tue Jun 11, 2024 6:48 pm
Coming along! One thing to keep an eye on (I can't tell from the picture): You'll want to make sure the pilot light connections don't short to one another. Probably just an optical illusion, but the picture makes them look awfully close.
Yeah, optical illusion. I twisted the rings for the 2 tabs so they weren't vertically aligned, so they're a safe distance apart. Appreciate that you pointed it out, though.
I've fixed the crappy solder work on my problematic ground connection, and the hum has substantially reduced. With my crappy phone app., it measures 35dB at idle from 1 metre away. Background noise in the room (ie. with the amp off, and me standing still/holding my breath) is 30 dB.
I'm calling it done. So far, happy with how it sounds. I'm going to spend a few days with it, twiddle the knobs and voice/boost switch, swap in different pre-amp and power tubes, try some different guitars, etc. I'll report back my impressions and such in a wrap up post.
Eye candy:
IMG_4064.jpeg
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Recap:
I built the 2nd edition of Dave Hunters Two-Stroke amp, basically a modified Princeton (5F2A) design. I started with
a Weber 5F2A kit with no speaker (I had a Celestion Greenback) and sourced my own resistors and jacks, and changed
a few other parts as well. Suggestions from this forum led to a couple of other small mods.
All things Weber:
I was very happy with all aspects of my interaction with Weber. The kit was decent and definitely worth what they
charge for it. I'll leave arguments as to the quality of the components to others. Webers kits are very flexible, in
that they offer lots of options for cabinet covering, handles, head vs cab vs straight chassis, etc. There is also
the ability to remove various components (ie. PT, OT, resistors, tubes, capacitors, jacks, switches, etc.) if you want
to source your own parts. I source my own resistors and jacks and swapped out the standard 5F2A OT they include for
a larger one that better matched the Hunter spec. I also added a different (Fender style) pilot light and some
carbon comp resistors to my Weber order. The kit arrived in about 4 weeks, well within the advertised 4 to 5 weeks.
It was well packed, and everything was there. Funnily enough, shipping to the west coast of Canada was cheaper than
shipping to the US (ie. my brother in Seattle). Weber customer service was absolutely top notch. Fast, friendly, helpful,
I really couldn't have asked for more. They provided good advice on upgrading the OT, and charged a very reasonable
fee for swapping the OT and the add-on parts I ordered. The cabinet in particular seemed very well built. In short, I
would not hesitate to order from them again.
The amp and the Dave Hunter book:
I found the book a great introduction to the whole process. There was some interesting info about how amps worked, etc.
and a fairly well documented amp project. It is definitely an introductory text. People with experience or some basic
knowledge will probably find they already know much of what's here. I decided to base the build on the Weber 5F2A. I
swapped out the resistors, as the 2-stroke had some different values, a few of the capacitors, and the jacks. Also the OT.
I went with a much beefier one than what comes with the 5F2A - the Weber WSE15. This matched the 2-stroke spec pretty
much exactly. Interestingly, their Maggie kit is supplied with the big brother of "my" OT - the WSE25. That OT would have
been physically too large to fit in my cabinet - a 5E3 with cutout for the 5F2A chassis. In hindsight, I maybe should
have started with the Weber Maggie kit, and made my modifications to that. I was unaware when I ordered that the Maggie
was their version of the Two-Stroke. It seems to be a hybrid of the 1st and 2nd Two-Stroke designs, with a few mods thrown
in to boot. I printed out both the 2-stroke and the Maggie layouts to work from.
The forum here (thanks, @maxkracht) suggested a couple mods that coincided with the Maggie build (some resistors across
the voice/boost switch, and another across the first filter cap), which I incorporated. I also wired the mains power and
fuse/switch as defined in the Maggie layout. The build went well, and I ALMOST feel like I didn't need to buy the variac
and build the lightbulb limiter. However, they're most valuable when things go wrong, which, fortunately for me, they
didn't. I made a few mistakes, almost all of which I caught before flipping the switch for the first time. I felt this
particular project was a good level of challenge for me and where I was at with my skill set, knowledge and confidence.
I've been playing the amp for about a week now, rolling some different power and pre-amp tubes through it, and different
guitars, etc. I quite like how it sounds. I can't say how it compares to an actual Tweed Princeton, or even another
2-stroke, given the mods I made and my choice of OT. It seems very, immediate? A bit raw, but not TOO much so. I'm not
sure the voice/boost switch is a necessary feature - I think I need to play with it some more. As someone somewhere noted,
this amp is fairly bright to begin with, and the 2 boost settings seem to mostly make it more so. I can say that I have a
Bad Cat Cub Player Series, a Mad Professor Old School 21RT Head (into a home built cabinet with a Celestion Neo Creamback)
and a Mesa Boogie Lonestar Special, and I feel my 2 stroke comes off well in comparison. Different sounding than any of
them, and fun to play.
Lessons learned:
1) I had some issues with my cheap soldering station. Cranked up to 8 (ie. maximum) it was barely getting hot. I finally
realized that the 2nd tier of plastic threaded coupling (closer to the body of the pencil) was loose, and it wasn't getting
a good connection to the heating element. Turns out the threads there are almost stripped, so it's easy to over tighten
them and have them pop loose. Once I got it fixed, I had it too hot for a bit. Luckily, I didn't seem to damage any
components. Check your tools.
2) Because I bought a kit and modified it, I missed a few small components that I needed. ie. some ring terminals, some
insulated wire, the correct switch for the voice/boost switch.... Check and re-check your bill(s) of materials...
3) My shellac was not as dark as I maybe would have liked. I was going for an aged tweed sort of look, and didn't really
achieve that. It looks like a slightly richer, shiner version of the original colour. I'm not unhappy, just, you know....
The shellac itself was easy enough. I used 99% rubbing alcohol as the medium, which worked fine. Seemed to take a little
long to dissolve, but no big deal. I put 8 light coats on.
4) I ordered most of my other components from Digikey, and somewhere along the way I didn't notice it got switched from the
Canadian site to the US one (I had my shopping cart open for around 4 weeks), and ended up paying a bunch of duty/taxes that
I probably didn't need to. I doubt this will be an issue for most people, but if I can save someone else $40.....
5) Check, check, re-check and check your work again. Test fit, cold fit, measure twice cut once.... I think this saved me a
bunch of headache and heartache.
6) If you found this post, on this site, then it's a good chance you've already found Rob Robinette's site as well. His amp startup
procedure was invaluable for walking me through the scary first time switching on. Lots more on his site that I haven't got to
yet.
7) Be careful, take your time, don't get cocky. I can see that last one being an issue - getting complacent about the amount
of juice running through these things. Are amp builders like pilots? There are the ones who have gotten shocked, and the ones
that haven't gotten shocked YET?
Next steps:
This was a lot of fun, and I'm super happy with both the process and the outcome. My friend has a Jensen speaker which he said
he'd loan me. The Celestion is a great speaker, but I'm curious to see how a "more appropriate" one will sound. I'm curious to learn
more of the tech/science behind the amps. It's one thing to know that you connect this wire from point A to B, it's another to understand
WHY this resistor or that capacitor is a better choice for a particular application.... In other words, I want to understand a schema, not
just a layout....
I can see that amp building is addictive - I'm already thinking about the next one!! (Maybe an 18w Marshall. And then a Dumble clone
and then a.....)
