First DIY tube amp project

Although I’ve built a couple tube hybrid headphone amplifiers and a nixie clock, I’ve never built a real tube amp. Something about the point to point wiring and dealing with heaters turned me off of them. Then I started getting into the guitar speaker/amp thing and you almost HAVE to get a tube amp if you’re in this space. So I started looking for kits and didn’t really find a lot that had a good kit or good documentation. However, a coworker mentioned this boutique Dumble amp as a super high end option that might be DIY-able so I started doing some research. As often happens after I do research, I spend money. In this case, I went with a kit from which is a Malaysian based company that sells clones of all sorts of classic amps. The kit I got was complete and just needed assembly. I also planned on building the cabinet myself. Documentation was semi-complete, but the proprietor of the store, Nik, was super responsive and bridged the gap where I had questions. I actually did end up finding a mistake in the layout so I feel inordinately proud of my contribution towards the documentation for folks who will build the kit after me.

In any case, here are a few pics of the amp in progress. I’ll capture further notes once it’s done. At the moment, I still have to fix something in the overdrive circuit that isn’t working quite right.

Well packaged (but not well labeled) parts:
Dumble guitar amp

Pots, switches, jacks installed:

Rear view:

PCB populated:

My poor P2P wiring skills on display:
Dumble ceriatone lunchbox

With tubes installed:

EDIT: Build tips (disclaimer: proceed at your own risk. no accuracy or warranty or other claims are made here implicitly or otherwise. blah blah blah, don’t be stupid with high voltages!)
Since the documentation on this is less than ideal for a novice as myself, I thought I could post some tips here for those who might benefit from my mistakes/lessons learned.

Process steps:
1. Arrange all the parts per the numbered baggies and check to see you have everything you need as specified in the BOM. Some of the resistors and other parts aren’t labeled, but through process of elimination you can at least figure out if you have a gross number of missing items. For example, I was short one knob (which Nik sent to me). Once you’re in front of a multi-meter, I would label all the resistors to make sure you have everything and to make subsequent installation easier. Also, in my kit, the fuses were inside the fuse holders themselves so make sure you have the right one with the right fuse.

2. Install all the pots/jacks/switches. The is pretty straight forward. Just make sure you have the right pots in the right positions since they are different. Keep in mind the bias pot goes through the top of the chassis, not the front/rear.

3. Install the tube sockets. This is pretty self explanatory. While you install them, take note of the subtle numbering to make sure you have them oriented correctly.

4. Populate the PCB. This is also fairly straight forward. One thing to note is that there are some resistor values that are the same with the exception of their wattage. Make sure to use the right ones in the right places. Process of elimination here helps. Also, study the photos on the website. That helps too for the capacitors and other wiring routing as well. Pay attention to capacitor polarity as well as the relay orientation.

5. Install all the other point to point items like the terminal strips and the items that are connected to the tube sockets.

6. Start planning on where all the transformer wiring needs to go. This part was confusing to me at first but if you study the layout diagram enough, it will become clear. Here are a few aha moments for me, personally.
– The output transformer primaries (blue and brown) go to what’s labeled OT AN on the V4/V5 tube sockets (the two larger ones on the left of the layout drawing.) Also, I ended up swapping them compared to what’s labeled in the layout. For some reason, I was getting bad hum with them the other way around so on Nik’s suggestion, I swapped them. Apparently, reversing the phase makes a difference? Also, the center tap wire of the primary (red) goes to the HT fuse holder.
– The output transformer secondary black wire goes to the ground of the output jacks. This was not labeled in the layout but I think Nik has since fixed it (see above on my inordinate pride in finding this omission.)
– The output impedance selector switch connections are not obvious. Use a multimeter to figure out what connections are made with the switch in the various positions and wire the correct colored wire to the corresponding tab. Of course, the inboard tab goes to the output jacks.
– The other transformer wiring is more obvious once you eliminate all the wires/connections that belong to the OT… you just might need to stare at the layout for a while. Keep in mind you won’t need all the primaries that go to the power switch. Here in the US, I used the 120V blue wire and just wrapped and insulated the ends of the other (red and orange) wires.
– The brown heater wires just go to F1 and F2. Doesn’t matter which brown wire you use… just keep the F1 and F2 consistent down the chain of tube sockets.

7. The footswitch layout drawing is looking from the inside of the amp. This seems obvious to me now but it wasn’t when I was first studying the layout. Note the ground connection for the middle pin. Also, be careful with how much heat you apply here when you solder. I feel like I almost melted the plastic the pins are seated in.

8. Solder in all the pieces that go on the volume pot and rock/jazz and mid boost switches. Be careful here with leads and make sure you don’t have any shorts. It’s quite cramped in there but I was able to do all the soldering with those items installed in place.

9. For me, I soldered all the wires starting from the pots/switches/etc and made them long enough to get to the board plus some in order to have slack since I wanted the wires to connect from the bottom. In retrospect, I would have started the wires from the board and then installed the board before cutting the wires to length and soldering to the pots/switches/etc. I think this would have made for shorter wire lengths. TIP: I didn’t have any problems with my solder connections, but Nik does recommend that you wrap the leads/wires around what you’re soldering them too in order to establish a mechanical connection prior to soldering. I would likely follow that reco if I were to do this again. TIP: Nik includes shielded wire which is indicated in the layout by a circle around a wire intersection. What that means is that you’re supposed to strip the wire first to the shielding layer and make a connection for that to go to ground. Then you strip the remaining signal wire insulation and connect that to where the wire goes. I was very confused by this at first and couldn’t understand why you would want all those wires connected to ground. The photos on the site show the ends shrink wrapped so you can’t see how it’s supposed to be. But if you use some wire cutters and just strip away the first layer to get to the shielding, you’ll see the signal wire insulation directly under. I just have never seen such thin shielded wire before so I think that’s what was throwing me off.

10. Some of the PCB standoffs and I think one terminal strip screw are trapped under the transformers. Just loosen the transformers and get the screws in the holes before retightening the transformer down. It won’t sit quite flush, but it’s fine.

11. There is one PCB pad that is not used. On the layout, it’s the fourth from the bottom right corner.

12. I tried to generally match the wiring colors that are in the layout just to help keep track of what goes where. Not necessary per se, but at least try not to use ALL the same color all the time.

13. Once everything is soldered together and you’ve double checked the layout to make sure nothing is missing and everything is where it’s supposed to be, go ahead and install the tubes. They fit quite tightly and I was a little afraid of how much force I needed to use to get them to seat, but I didn’t break any. Just be sure to use a paper towel when handling so you don’t transfer oils to the glass. I believe that makes the tubes last longer as the oil tends to create temperature gradients that could weaken the glass over time.

14. Now you’re done and just need to set bias! You measure this at the 1R resistor that goes to ground from the V4 or V5 tube sockets. Remember, there’s a lot of voltage there so be careful what you touch! It didn’t drift at all for me and it biased up right away.

Trouble shooting:
1. I had a problem with the overdrive channel (red light footswitch button) where it would mute once pushed. I swapped the relays to see if that was the problem. No change. Then, on Nik’s suggestion, I checked for one of the shielded wires in the OD circuit potentially being shorted to ground. Turned out one of the ones going to the OD trim pot was indeed shorted to ground. It looked fine, but my guess is that I may have cut too deep when stripping the insulation away from the shielding. In any case, a quick fix. If you’re checking continuity on the pots from the lugs to ground, just make sure that the pots are close to the middle position.

2. As mentioned above, I had a hum problem that was fixed by swapping the primaries on the OT.

Operational tips:
1. I’ve read that you’re not supposed to power up without a load attached. Not knowing this at first, I did my bias setting and some trouble shooting without a load. Didn’t seem to have any issues, but better safe than sorry.

2. You’re supposed to power on the main switch for at least a minute or so before flipping the standby switch. Apparently, heating up the tubes for a little while first helps them last longer.

I think that covers it for the moment. If anyone has additional tips they’d like to share, let me know in the comments.

EDIT: cabinet work
I took a good part of the labor day weekend to build an amp and matching speaker cabinet.

First stop: Home depot for some African Mahogany and some maple for a bit of a mini stack.

About 12 feet lengths of each:

Biscuited together:

Still getting better at dovetail joints. These are slightly proud of the surface.

But coming together with the kids’ help cleaning up sawdust:

Parts installed and one coat of hand rubbed poly:

In the room next to the 2×12 cabinet on end:
photo 2.JPG

This entry was posted in projects. Bookmark the permalink.

2 Responses to First DIY tube amp project

  1. Guilherme Marques says:

    Good morning , I am looking for the board pcb board layout of this amp ( ) Its me through photos of the bottom plate or the ready scheme?

  2. jason says:

    Sorry, don’t have it. The board was purchased, not DIY.

Leave a Reply

Your email address will not be published. Required fields are marked *