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Jolly's Tube Amp in a Tele Build


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I split the body back into three pieces to do some general deburring and cleanup using a razor blade and sandpaper.  After that I put the top on and finished the neck pocket taper using chisels and sandpaper. I had to put the back on to screw in the neck or else the screws would have been too long but luckily, similar to the mill set up, the top is still removable with the neck in place. With the top off I screwed on the speaker, bride, and control plate. The screws that came with the bridge were too long for the plywood and I wasn't confident they'd hold so I added a piece of scrap pine as a "bridge plate" for the time being. The speaker screws also poked through a bit so I covered them with cardboard, again temporarily. Finally I installed the pickup and wired it along with the speaker to jacks in the control plate. 

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Alright, here she is! The first video is just a quick overview of the setup. I have the pickup going straight into the amp and then the amp output going straight back to the guitar speaker. I have a 1

With it this close to playable I couldn't help but install the tuners and throw some strings on it. I have a lot of setup issues to start chasing down so it currently plays like garbage but... I obvio

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With it this close to playable I couldn't help but install the tuners and throw some strings on it. I have a lot of setup issues to start chasing down so it currently plays like garbage but... I obviously had to plug it in to get a feel for how it sounds! My prototype amp was being finicky so I started by plugin into my 5W Kustom Defender tube head. It confirmed my wiring was good but started uncontrollably squealing with any more than 1 - 1.5 on the volume knob. I moved back to my 12V amp and did a little bit of debugging to get it functioning again. The results.... Surprisingly good! The wattage on this amp is low enough that it didn't squeal at all, its very "touch sensitive" and has all the sweet tube compression you could ask for. Like I said my setup is awful right now and I have the saddles really really high, I have the pickup maxed out in height as well but I think it has more to give once I get it closer to the strings. It had a little bit of break up but I know from testing the amp with other guitars that there's more distortion to be had. Its a little shrill as well, something that I think will clear up when I wire in the tone pot. I'm going to do a little setup work first but I'll try to post a sound clip in the next few days. :hyper

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The tubes are currently on the bench but the idea is to get them on board after a little more test and tune. The battery will take up the bottom part of the slot where you can see the "bridge plate" now. When its finished it should be an all in one package; pick it up, turn up the volume (its a switched volume pot) and get to rockin' :rock

At this point I'd say its far from impossible! I've certainly got a few big hurdles left but I've proved out enough of the individual elements that I'm confident it will come together. I also weighed the guitar as it sits with a box of remaining loose components. Came in at almost exactly 9 lbs. I wouldn't be surprised if the tweed/finish/whatever else I forgot adds a little more weight but I'd be amazed if it ended up over 10 lbs. Maybe a little heavy for a Tele but far from a total back breaker.

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Alright, here she is! The first video is just a quick overview of the setup. I have the pickup going straight into the amp and then the amp output going straight back to the guitar speaker. I have a 12V supply running the tube heaters and the 18V battery running the rest of the amp. It has almost no headroom right now so the demo's are both fully cranked. This is just my cellphone camera and mic to give everyone a rough idea of the volume level. Let me know what you think!

 

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As a quick update on progress I pulled the back off and made a 1/4" plywood spacer so I could reattach the neck. With the back now open I mounted the transformer. It was my last heavy component with location options so I mounted it low and as close to the back as practical as the guitar is a little neck heavy.

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This schematic was mostly to figure out signal flow but it should give a little insight into the amp workings. The values below are what I used in for the demo; minus the tone pot, "effect loop", and selector switch. I also had no negative feedback. I used a combination of borrowing from other schematics and what I had laying around so they probably aren't final values. Figured it was worth chasing out packaging and signal flow issues before I got too hung up on designing for tone.

C1: .022 µF (Tone)
C2: .1 µF
C3: .022 µF
C4: .015 µF
C5: 2.2 µF (electrolytic)
C6: .1 µF

R1: 1M
R2: 47k
R3: 27k
R4: 220k
R5: 220k
R6: x
R7: x
R8: 3.9k
R9: 1M
R10: 22
R11: x
R12: x
R13: x
R14: x

Tone Pot: 250k
Volume Pot: 1M
V1: 12U7 (Pre Amp)
v2: 12k5 (Power Amp)

I basically cut all of the parts off my first amp mock up and soldered them back on to a new board that fit in the depth of the body. Its a huge mess still but it worked, the amp is all on board now. I'm using two batteries because I still don't have a way to step down 18V to 12V for the heaters.

I didn't include cathode bypass caps when I put it back together and lost a ton of volume. I haven't checked everything yet to see if I missed something else or have a bad connection but it also cleaned up a lot. I have one lug of the selector switch in mind for switching V1B's cathode resistor/capacitor pair and with this new data point think that will control how much distortion it has pretty well. The other leg I was thinking about using to vary negative feedback but it could also switch another cathode if need be.

Anybody see any glaring issues with my schematic? Any suggestions? At the bottom left of the schematic I have a possible heater set up, can I just use resistors to drop my voltage as shown?
 

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4 hours ago, Jolly said:

Any suggestions?

Maybe consider rearranging the cathode resistor switch on the bottom of the 12U7 such that one resistor is always present to ground, and the switch adds other resistors/capacitors in parallel with it as it is operated. If the switch you're using is a break-before-make type, as you change from one position to the next you might get a bit of a thump between positions.

 

5 hours ago, Jolly said:

I have a possible heater set up, can I just use resistors to drop my voltage as shown?

You can, but you're also wasting heat and battery run time by warming up a couple of resistors. Any scope for reducing the battery voltage down to 12V instead of 18V?

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19 hours ago, curtisa said:

Maybe consider rearranging the cathode resistor switch on the bottom of the 12U7 such that one resistor is always present to ground

Solid idea, I'm not sure exactly how the switch operates but it still couldn't hurt to set it up this way. 

 

19 hours ago, curtisa said:

Any scope for reducing the battery voltage down to 12V

https://frank.pocnet.net/sheets/127/1/12U7.pdf

http://www.r-type.org/pdfs/12k5.pdf

The tube data sheets list the heater voltage range from 10 - 15.9V with a drop in life expectancy operating on the high side.

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What I'm getting at is that currently you're running the whole thing at 18V including the heaters. To prevent over-stressing the heaters you're dropping the excess through a resistor for each tube from 18V to 12V. That's about 4 watts of power being wasted just to keep the heaters alive. However if you can run the whole thing on a 12V battery pack you can do away with the dropping resistors altogether and squeeze a bit more life out of a battery of equivelent physical size to an 18V one.

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3 hours ago, curtisa said:

However if you can run the whole thing on a 12V battery pack you can do away with the dropping resistors altogether

That would be ideal, I'm only searching for other options because of sunk cost in the 18V battery I have. When I originally sourced it I was thinking more about packaging than electronics, all of the 12V drill batteries I found had stop tabs that increased the thickness past what I was looking for. In hindsight that wouldn't have been a big deal and would have simplified everything else but... here we are. If there's ever a next one it will be 12V for sure!

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A benefit of working with low voltage tubes is obviously safety. With no possibility of deadly high voltage floating around it made it a lot easier to tweak the circuit on the fly. I drilled the hole for the output jack and wired it up as shown in the schematic. I don't have a stereo cable so I haven't tested the return feature yet but its seems to function as intended; the onboard amp runs with no cable and I can still get a passive signal out when plugged in. Switching back and forth between those options I started swapping around components and signal paths to try and dial in my 3 "channels". I honestly lost track of the actual wiring and will have to reverse engineer it to get a schematic but for now I'll try and talk everyone through the controls.

The tone pot pretty much just bleeds signal to ground, I have a few different caps hooked to it through the selector switch.

I removed the volume pot. In previous tests I got a more useful control with the guitar volume, I didn't have the right value pot laying around to replace it so for now its omitted.

The pickup selector switch sets the "channel". I'm shooting for a clean, rhythm, and lead channel. With some minor differences this should work for both the internal and any external amp. I've got a few videos to post, I'll add more explanation there.

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The first demo is though my Line 6 Flextone III, no crazy settings or effects. It's hard to see the controls in the video so here is what I did. I start on the clean Channel, tone 10. Leaving the tone at 10 switch the the rhythm then finally lead channel. I roll the tone back to O and then work up through the rhythm and clean channels. With the tone still at O I head back through the channels with a little bit of soloing. All of the passive components are switched by leg one of the selector switch

Clean: .02uf  Tone Cap

Rhythm .047uf  Tone Cap

Lead: Cocked Wah Cap/Resistor Pair (did a lot of guess and check here, will have to get component values/wiring later)

Its not perfect but I added a little orange tape to the controls to make them easier to see. This video is the onboard amp and starts on the clean channel with the tone at 10. It's kind of just playing around with the different settings but should give you an idea of what the controls do. The passive components are obviously still switched the same but the second leg of the switch is as follows:

Clean: Nothing!

Rhythm: Adds Clipping diodes after V1b

Lead: Adds A bypass cap to V1b

I'll probably mess around a little more before finalizing it but I'm pretty happy with the tonal options this set up gives. When I tear it apart to clean up my wiring I'll get a schematic put together as I'm sure my explanation was hard to follow.

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  • 4 weeks later...

I left it hacked together for way too long because of how fun it was to play but I finally got the nerve to tear it apart and get back to work. I pulled the electronics out and took detailed notes on the wiring, still haven't turned it into a schematic but I'll get around to it sooner or later.

I also mixed up some shellac to test finishing options. This is my first time working with tweed, and shellac for that matter, so I wanted to try a few configurations to see what looked best. Board 1 has straight shellac, board 2 has a little amber dye mixed in. On both boards the left tweed sample was coating side up and the right was fabric side up. The fabric side up samples really soaked up the shellac and still have a little more of a fabric feel than I'd like. I think the final product is going to be coating side out with a touch more amber dye than the test sample.

I've got a trip planned this weekend to finish up a few woodworking loose ends and then I'm going to start assembling it for real!

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  • 4 weeks later...

After some slight delays I've finally made a bit of progress! I used a hole saw to begin turning the pickguard into a "grillguard". I used the cutout piece to test a few different processes and settled on this method. First I scuffed the entire surface, when I left it shinny in the test it was easy to see unevenness in the glue. I cut an oversized piece of grill cloth and stretched it a little bit by taping it to a flat surface. I then spread a thin coat of superglue onto the scuffed pickguard and placed it face down on the stretched cloth. After the glue dried I went around the perimeter of the guard and made cuts at approximately .125" to .375" intervals. I wrapped these strips around the back, pulled them tight and secured them with additional super glue. There are a few edge spots that are a little rough but overall I'm pretty excited with how well it turned out!

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I finally have enough stuff figured out to start moving towards permanent assembly. I super glued dowel pins into the top and back plates for locating. I then used wood filler putty to fill all the in process holes along with a few other minor defects. With everything filled and in place I gave it a final sand with 180 grit. I tend to like the look of blocky Teles vs deep round overs so I just broke the edges with sandpaper. The tweed cloth makes a bit of a radius when bent over a sharp corner so between that and the slight edge break it should end up about where I want it.

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The last loose end as far as "woodwork" was something to hold the battery in place. I modeled a receptacle in fusion 360 and 3D printed it. This one is just a test, I have a little bit of wood filament left from my 3D printed guitar that I'll use for the real deal. It includes a wiring channel so I can hide some of the wiring that needs to run between the top and bottom sections. The placement of the battery is such that you can see the built in charge indicator.

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I've started covering it in Tweed! I used hot hide glue for this so there are some gaps in process pics but I'm going to try and give a pretty detailed explanation of how I did it. Hide glue is pretty forgiving and allowed me to soften spots with a hairdryer and fix minor mistakes along the way.

I started by drawing a centerline on the back side of the tweed along with locating marks so I could quickly flip the back and position it so the fabric pattern was where I wanted it. I covered the back with glue, flipped it into place, and kept pressure on it for several minutes. I repeated the same process for the top.

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Once the glue had dried I moved to cutting out the internal features. I made cuts into the fabric perpendicular to the wood. After brushing in some glue on the edge of the plywood I pulled the excess fabric tight and taped it in place.

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I repeated this on the speaker hole and control cavity. The pickup hole was a bit tight for clearance so I just cut that off without wrapping it around to the back.

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With all of the internal features done I couldn't help but see how my overall hardware scheme/look was coming along.

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For the outside profile I basically repeated the same steps, however I traced the outline ~1/4" out from the plywood and started my perpendicular cuts from there. With the outside edges being visible on the final product I wanted to try and maintain a consistent tweed pattern. I had to make a few of the perpendicular cuts all the way to the wood at some of the particularly tight radii but these few exceptions should be easy to hide.

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Some mad science going on here, really like it, but the tweed kicks it up a notch further... should have "bookmatched" it to make it in true guitar building fashion. :D

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