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My first custom tube amp, out of a fender Vaporizer

Discussion in 'Shock Brother's DIY Amps' started by tb37, Nov 9, 2020.

  1. tb37

    tb37 TDPRI Member

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    Let's have a bit of context: I began playing guitar 2 years ago, and I'm very into gear and tweaking things, more than improving my playing .

    While I had just an harley benton guitar and a random 90s transistor amp, I bumped into a second hand Fender Vaporizer for sell, pretty cheap, but not in great conditions.
    [​IMG]

    I bought it because the "tube amp" thing was very appealing to me back then, though the amp sounded bad to my ears, compared to other transistor amp I had. (too mid focused, lack of top and low ends)

    I started modifying things in it according to a post in another discussion page, but the changes didn't make the difference I was expecting.

    That's why I decided to desgin a whole new amp, using the vaporizer cab and chassis, plus some of its parts (speaker, transformer, etc.)

    Cab mods :

    That's the first thing i've done, though it's not the most relevant. I removed the original tolex and grillcloth, that I honestly found pretty ugly (and they were in bad condition anyway).

    I replaced it with some sort of couch cloth (I'm not sure), on which I applied blue paint and a bit of lacker. it's tough and rough, but not that bad.

    I also replaced the grillcloth and painted it with gold spaypaint. It gives it some sort of gold foil vibe.

    IMG_20201014_093633.jpg

    (that matches my MIJ 70s teleish guitar)


    Circuit desgin :

    To desgin the circuit, Robrob website has been really helpful!

    I wanted to have a good clean tone, somewhere between blackface and bassman amp, which could also be pushed to give some glorious distortion.

    I stick with the EL84, for multiple reason (not much room, cheap, stock power transformer could not stand bigger bottles, etc)

    I wanted the amp to feature :
    -Tube driven reverb
    -Bias wiggle trem
    -Late tone stack in the preamp circuit, driven by CF

    So I came up with that circuit (which actually contains the later mods I made) :
    IMG_20201109_114700__01.jpg

    To explain some details :
    -All preamp tubes are 12AX7, except LTP and reverb driver being 12AT7
    -First thing on the preamp is the volume control + bright switch
    -Then we have a fender ab763ish reverb circuit
    -A dc coupled cathode follower drives the tone stack
    -Finally, the power amp is driven by a LTP, which has nothing special except a 100k resistor across the outputs of the LTP, which helps preventing blocking distortion.
    -I decided te elevate the heater voltage, because it's supposed to be better for the cathode follower, and also help reduce hum.
    -I added a 5e3 tone stack to the reverb, to make it a bit more versatile
    -I used the original solid state power supply to power the bias circuit
    -I added a 220k resistor between the trem oscillator output and the bias power supply, in order to add a dc component to the induced wiggle. This gives a colder average bias when the trem intensity is increase.

    Here is the assembled circuit board (every thing fits in!):
    IMG_20201013_122051__02.jpg

    Compared to the original design, I added 3 preamp tubes, but that didn't seem to affect the power supply.

    I swapped the tiny original output transformer for a Hammond 1750PA.

    I added a MASSIVE choke found on Ebay (as big as the output transformer). It does a fine job though.

    Caps are random chinese high voltage film caps. Resistor are metal film, except some carbon load resistor in the preamp. Nothing fancy here.

    IMG_20201107_173922.jpg

    Wiring was a bit boring but turned out well. I past a few days having toubles with what seemed to be blocking distortion, until I realized I forgot to solder one side of a jumper cable on the cathode follower tube socket.

    IMG_20201107_173958.jpg

    Tests:

    Right from the first startups, the first thing I noticed was that the amp is dead quiet, even witch volume and reverb fully cranked. All the hum and hiss comes from guitar and pedals.

    I checked how behave the amp with a sine at the input, to see how it clips and at what stage. The first clipping comes from the power amp, and the preamp start clipping just a few dBs later.

    While clipping, I checked the screen grid current, and it's within specs. The absolut max ouput I got was 16W RMS, mesured across a 8ohm resistor replacing the speaker.

    I tweaked the tone stack with a variable tone slope resistor. This hepled balance the tone, given the fact that the vaporizer speaker are pretty bassy and scooped in the high mids. with the right circuit voicing,
    those speakers sound very pleasant to me.

    Here is a quick demo (as I said, my playing is pretty bad, sorry for that)


    And there it is ! I'm very pleased by that build ! Just got to improve my playing skills now
    IMG_20201108_162249.jpg
     

    Attached Files:

    Last edited: Nov 9, 2020
    jjlemon, Piotr, AAT65 and 21 others like this.
  2. goonie

    goonie Friend of Leo's

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    Wow, great job. My first build is going to be a 5f2-style SE amp in a Pro Jr cab. About 1/10th the complexity of yours!
     
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  3. James Knox

    James Knox Tele-Holic Silver Supporter

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    Welcome to the Shock Brothers Community!

    Great job on a very ambitious First (re)Build. Lots of creativity.

    Thank you for sharing your build with us.

    peace,
    James
     
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  4. ukepicker

    ukepicker Tele-Afflicted

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    That looks and sounds fantastic!!
    Well done!
     
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  5. zook

    zook Friend of Leo's

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    Very nice job! I like the finish too.
     
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  6. Coloradotwanger

    Coloradotwanger Tele-Meister Silver Supporter

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    Very nice build!
     
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  7. That Cal Webway

    That Cal Webway Friend of Leo's Silver Supporter

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    Wow!!!

    Nize
     
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  8. ronzhd

    ronzhd Tele-Holic Ad Free Member

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    Sweet mod bro!
     
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  9. Mongo Park

    Mongo Park Tele-Afflicted

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    I’m sorry to say you might have trouble improving on your next build. Amazing first project
     
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  10. glenlivet

    glenlivet Tele-Afflicted

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    Wow....well done...


    unnamed.gif
     
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  11. archiemax

    archiemax Tele-Meister

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    Cool!
     
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  12. tubelectron

    tubelectron Tele-Afflicted

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    Excellent ! :)

    If it is your 1rst attempt to work on a tube amp, I wrote it : that's excellent ! Congratulations... :cool::cool::cool:

    -tbln.
     
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  13. El Marin

    El Marin Friend of Leo's

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    Like it
     
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  14. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    A first build?! Very impressive. Cab, schematic, soldering, even your English. A Frenchman that writes English this well deserves a compliment: chapeau! :)
     
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  15. Theorage

    Theorage Tele-Meister

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    Excellent build -- looks great!

    Perhaps some of your noise reduction may be ascribed to your negative feedback loop. An 820ohm resistor seems very low off an 8ohm Xfmr. If you were to increase the NFB resistance, you may still retain low noise but gain some volume and add another flavor of overdrive. It would be very easy to switch out resistor values as RobRob describes:

    http://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch

    Again -- nice sounding, attractive amp!
     
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  16. Pick_n_Strum

    Pick_n_Strum Tele-Meister

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    That's a very impressive build. I'm curious, do you have a background in electronics? If not, what resources did you use (other than Rob Robinette of course)?
     
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  17. AAT65

    AAT65 Poster Extraordinaire

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    Nice work!
     
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  18. tb37

    tb37 TDPRI Member

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    Thank you all!

    Is it that low? ab763 amps seem to have very clos NFB resistor values. Oh, and I didn't correct my schem about the output transformer, but the NFB comes for the 4 ohm tap. During the first tests, I didn't wired the NFB, still it was very quiet.
    I tried different NFB resistor values, and the values I choose give a nice clean headroom, and a sweet breakup to my ears.

    I think the noise reduction is mostly due to other factors : grounding, heavy PS filtering (big chocke, 22uf electrolytic + 1uf film filtering caps for the preamp), tight twisted wires (heaters, reverb output), elevated heaters, transformers position, etc.

    The only electronic background I had was wiring lowpass filters on breadboard at school. Back, I had just little understanding of what I was doing. I just learned one thing that I use now all the time while designing a circuit : a capacitor is "equivalent" to a resistor of 1/(2*pi*capacitance*frequency) ohm . I know this is not how it really works but this formula is really handy when it comes to calculate the right capacitance for the frequency response I want.

    About 2 years ago, when I wanted to understand how my amplifiers work, I assembled my first circuit following that scheme :
    [​IMG]
    Then I learned how BJT and FET transistor work, to make my own preamp circuits.

    And thanks to Robrob website, the valvewizard, TDPRI and other discussion page, I learned to design and troubleshoot tube amps.
    For the tone stack, I used the TSC tool.
     
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  19. Piotr

    Piotr Tele-Afflicted Silver Supporter

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    That's a good sounding amp! And no electronics background? Impressive.
     
  20. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Have you checked your power transformer can handle the additional (heater) current of the added tubes?
     
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