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Project Tweed-O-Verb - squeezing a Vibroverb into a Tweed Deluxe...

Discussion in 'Shock Brother's DIY Amps' started by buddy_coles, Apr 12, 2021.

  1. buddy_coles

    buddy_coles TDPRI Member

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    Hi Guys, hope you're all keeping safe and well.

    I thought I'd start a thread to track my latest project in the "not really necessary but quite interesting anyway" category.

    I bought a 'mystery amp' off ebay a while back with 6L6GC (actually 7027, as it turns out) push-pull power stage and transformers big enough to anchor the Ever Given, with the intention of re-mounting the circuit inside a Deluxe Reverb chassis and cab. Turns out it wasn't quite what I was after (620V of B+ was too rich for my blood!) so I got hold of a nice vintage power TX that gives 350 - 0 - 350 before rectification, which should end up near enough 465VDC using a GZ34 - a perfect match for the AB763 Vibroverb circuit.

    One of the main drivers for this project was to repurpose as many parts as possible, so although it will look a bit rough and ready, this is deliberate! Assuming it all works OK with sufficiently low hum I'll get a local fabricator to make me a nice new shiny chassis, but for now a chrome front panel will do the job I think.

    So, part 1 was to assess the 'mystery amp' which involved several evenings mapping out the circuit to create an 'as-built' schematic (attached). It looks like someone took a bit of care with the design and build, though I have reason to think the power transformer was better suited to something a but more monstrous than my project :D

    Also, I wonder if it was meant for European 220V mains, rather than UK 240V - all my measured heater voltages were in the 7.0 to 7.4VAC range, which would suggest either there should be a lot more valves, or that the input voltage is about 10% too high...

    TBC...
     

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  2. buddy_coles

    buddy_coles TDPRI Member

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    OK, straight in with part 2 as I am still on my lunch break! Here are a few photos of the original amp in all its glory. Thanks to some forum members I managed to confirm the output valves are 7027s, i.e. a beefier version of 6L6GCs.

    s-l1600 (3).jpg s-l1600 (6).jpg s-l1600 (7).jpg s-l1600 (8).jpg

    I ended up stripping out all of the components and setting the chassis to one side. Then putting in a big order from Mouser for some parts...
     

    Attached Files:

  3. buddy_coles

    buddy_coles TDPRI Member

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    And here's the lucky intended recipient of my labours - a nice tweed 1x12 cab that currently houses a 5W single ended amp built from a kit from ampmaker.com (I have to say the quality of the kit and instructions was exceptional, and Barry who runs the website is a great guy to deal with).

    The cab was built by Chris Uff (cabsbychris) and again is of very high quality. Next time I'll probably do one myself, following a good first build for a Blues Jr extension cab, but for now this is where the amp will be going. The 5W amp sounds amazing and is almost silent when I'm not playing, but it just can't cut it next to a drum kit.


    20210324_202628.jpg 20210324_202636.jpg 20210324_202642.jpg
     
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  4. buddy_coles

    buddy_coles TDPRI Member

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    And so the search for scrap materials began - I found a nice sheet of aluminium in the shed, which I dutifully marked out and cut with tin snips as that's all I had! It's not my best job by any stretch but given the available tools and the time constraints it is adequate for a prototype :D I made a makeshift metal folding press on my workbench using a couple of bits of angle iron and some old door hinges - didn't really perform as well as I'd hoped but I ended up with a chassis that will at least be useable.
    As an aside, I wondered (whilst making some toast) whether a suitable place to find a nice chrome front panel might be an old electrical appliance at the tip - our toaster appears to be made from chrome plated stainless steel at about the right thickness (22 gauge ish) which would probably do the job if I printed the control markings on to a clear vinyl sheet and stuck it on...


    20210410_102657.jpg 20210410_102702.jpg 20210410_105147.jpg 20210411_141016.jpg 20210411_141030.jpg 20210411_141047.jpg
     
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  5. buddy_coles

    buddy_coles TDPRI Member

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    And with the chassis beaten into shape (it's far from beautiful but good enough for now), it's time for some marking out. I want to squeeze in a rectifier, 2x6L6GCs, and 6 dual triodes into the back panel, plus a couple of speaker jacks, reverb send and return, and a jack for a tremolo footswitch. I never use a reverb footswitch... I also wondered if it would be worth getting a pot with a push/pull switch to activate the trem if a footswitch was not required.
    There's not a lot of room for all of this, especially given the transformer sizes! The power TX, choke and reverb TX are reclaimed from old valve gear courtesy of a good friend of mine who has an Aladdin's cave full of this sort of thing!
    Next task is to start drilling holes for the valve sockets. I plan to get the transformers and valve sockets into place, then think about how to squeeze the necessary circuitry inside. I have a load of turret board stuff on its way so will try to plan a layout in the next week or so. I may consider putting the reverb send/return RCA jacks on the top of the chassis (same side as TX mountings) to save a bit of space on the back panel where the valves go), as these won't need to be accessible during normal use...

    Oh, and the staggered 12A*7 valves are to give as much breathing space as possible given the constraints of the chassis! I will probably do the same with control knobs on the front panel too, as I want to have both Vibroverb channels available if I can.

    Thanks for reading thus far - more in a few days!

    20210411_210751.jpg 20210411_215936.jpg
     
  6. Chicago Matt

    Chicago Matt Friend of Leo's

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    Sounds like a worthy project, Sir!
    Would love to hear it when you're done.
     
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  7. dougsta

    dougsta Tele-Meister

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    I'll be making the same DIY brake to make bends (a 90˚ one and a 30˚) the chassis from a Frontman 212R, I had to cut it into 2 parts, at least mine is aluminium. Main issue is the chassis is 620mm long (Twin Reverb size)

    IMG_8391.jpg
     
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  8. buddy_coles

    buddy_coles TDPRI Member

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    Hi Dougsta, I did have a quick read of your Frontman thread last night, and it looks like a perfect cabinet for a Twin Reverb. 620mm is a long length to bend, but probably still achievable. I think if I had had some heavier duty hinges my folder would have performed a lot better. Have a look on 'instructables.com' for some good ideas for metal folding. I drilled a 3mm hole at the corner points of the bends to give a weak spot so they at least started and ended in the right place. All those years studying Engineering were not wasted!! Good luck with the Twin build - the best sound I ever got from my Tele was through a Twin Reverb that was provided with the backline at a festival :) I just can't carry one up the stairs any more, hence 1x12 is about my limit!
     
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  9. dougsta

    dougsta Tele-Meister

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    Cheers Buddy, I've just been calculating (with high school maths) the angle I need on the angle I eyeballed as 30˚, it's actually 25˚.
    Should result in a 70mm (2 3/4") height chassis with a flat part of 150mm (6") deep, 620mm (24 1/2") long. Lots of scope to fit a decent amount of electronics, not decided what amp yet but a twin reverb like clean channel and a separate foot switched crunch channel, both into a valve driven spring reverb tank and effects loop for 'surf's up'

    I'll get a strong coffee and work out the best way to get a 25˚ bend, probably lots of use of a rubber mallet over a wooden former cut with my mitre saw...

    Doug.

    IMG_8392.jpg
     
  10. buddy_coles

    buddy_coles TDPRI Member

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    Looking good - I've posted a comment on the thread for your build to keep stuff in the right place (hopefully!). Eyeballing 30 deg and measuring 25 deg is a pretty accurate estimate in my opinion!
     
  11. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Nice work! Can’t wait to see the end result.
     
  12. dougsta

    dougsta Tele-Meister

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    What's the weight of that monster transformer?
     
  13. buddy_coles

    buddy_coles TDPRI Member

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    10 1/4 lbs, according to the data sheet!! My bathroom scales reckon 4.8kg as a second opinion, so about right... :) My original plan was to make a separate chassis to mount the transformers and power section at the base of the cab, and then have the preamp and reverb driver in a chassis at the top, but I figured I could squeeze it all in the top and then put the reverb tank in the base.

    The mains transformer has 4 mounting bolts at each end, and it will end up about 1/4" away from the speaker baffle (and be quite close to the side of the cabinet) so I plan to add a decent support bracket to take its weight directly into the cab. I am hoping that way the chassis won't get overly stressed!

    I think I said before, part of the aim of this was to avoid buying new parts if I could avoid it, and the spec for this transformer is just about spot on: 350-0-350 at 250mA (and I bet that is extremely conservative!), three 6.3V windings at 2A, 3A and 4A, and 5V at 3A. I plan to take the bias off the HT with a couple of silicon diodes and some resistors, as per Merlin Blencowe's website examples.

    I just weighed the mains transformer that came with my 'mystery amp' out of curiosity, and it is even heavier - 5.4kg!!! I would have bet against the replacement one saving weight, but there you go!
     
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  14. dougsta

    dougsta Tele-Meister

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    That is a biggie.

    I'm going to rewind the Frontman 212R power transformer for use in a valve amp, should be good for a 60 watt build. Then build the OT depending on what amp I build.
     
  15. buddy_coles

    buddy_coles TDPRI Member

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    OK, another small instalment - I've been trying to work out how to squeeze everything in to the available space, and I think I have a solution :confused:

    For clarity, what I call the front panel is the one with the controls, and what I call the rear panel is the one with the tubes - as if it were mounted horizontally (BF/SF style) rather than vertically.

    Two rows of knobs on the front panel should hopefully not look too bad, if I use black and ivory colours, and the Davies 1510 series look pretty nice at 3/4" diameter, which should still leave room for some lettering so I know what they do! I reckon using different colours for the vibrato and normal controls will help to separate them aesthetically. The master volume is the Lar-Mar Type-2 as per Rob Robinette's website. I think I can fit all the necessary valves and connectors on the rear panel, which will be a bit cosy but there should still be enough room for air to circulate to prevent anything cooking.

    I can get all the transformers on the top of the chassis (power tx, choke, o/p tx, and reverb t/x) so there should be plenty of room inside for the circuitry. Next task is to drill all the holes in the chassis, and then start thinking about the front panel detail (lettering, etc.), and circuit layout - I don't think I'll follow the Fender board layout rigidly, as I want to use multiple star grounding and some of my components are of slightly different sizes from the classis Fender ones. Some head scratching to be done there I think, but it's a quiet job (unlike drilling the holes) so can be done later in the evenings!

    Front_and_rear_plate_layout.jpg
    I did consider removing the Normal channel, but for the sake of one extra valve I think it would be worth keeping - plus I plan on using this for pedal steel as well as Telecaster so two channels may turn out to be a good thing when gigging.

    More soon...
     
  16. buddy_coles

    buddy_coles TDPRI Member

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    A bit more progress over the weekend - Swiss cheese chassis (well, the top and rear panels at least) and a quick once-over with the polishing mop and some Autosol. This was only supposed to be a prototype and now I'm getting all bothered by the vice marks...!
    The front panel actually came up to an acceptable finish, which was good news. Now I need to decide on the front panel layout and drill some holes, but I should probably get going on the circuit board first, to make sure it'll all fit! 20210419_132140.jpg 20210419_131719.jpg
     
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  17. buddy_coles

    buddy_coles TDPRI Member

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    Updated schematic attached - deviations from Vibroverb AB763 circuit are in red for clarity. I'd greatly value any feedback (especially suggestions for improvements) if anyone has time to have a look. Note that control panel space is tight, so I don't think I'll be able to squeeze any more knobs or switches in there. Looking at push/pull switched pots on the treble controls in place of the bright switches, and possibly another push/pull pot to turn on the tremolo if I don't use a footswitch... I considered stacked concentric pots for treble/bass but I'm concerned they would protrude vertically above the top of the cab which would make them potential targets for being squashed!

    One circuit-based thought, would anyone be able to suggest the 'best' place to put a fuse on the HT please? I don't think it would make a lot of difference to the overall operation, but would the best location be between the transformer and rectifier, rather than after the rectifier?

    Thanks as always for taking the time to read these posts - I'm very much enjoying this project and I am extremely grateful to the forum members for their collective wisdom.
     

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  18. 5F6Animal

    5F6Animal Tele-Meister

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    I’ve only installed a HT fuse between the rectifier and the first B+ node. It’s a fairly low(under 500ma)current rating and protects both the rectifier tube and transformer windings. Adding a couple of helper/protection 1N4007 diodes to the rectifier socket is a good idea too

    I love what you’re doing with this project. I just finished up a “high end” AB763 Deluxe Reverb build and that isn’t the easiest circuit to put together.
    I would definitely have a mid control and a NFB switch(off/on or different resistors).

    Good luck stuffing all of that stuff in a Deluxe cabinet!
     
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  19. Ten Over

    Ten Over Tele-Holic

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    You need to eliminate the 47K voltage dropping resistor in the bias supply. There is already a 100K dropping resistor on the other side of the diode.

    It would probably be better to connect the tremolo jump start to the filter capacitor that is closest to the diode. This way any noise created by the tremolo switch will be filtered out better at the tube bias.
     
  20. buddy_coles

    buddy_coles TDPRI Member

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    Well spotted, Ten_Over! The 47k resistor was a remnant of my earlier bias setup and somehow managed to hide!

    Regarding the 100k voltage dropping resistor, is there a reason some designers put it either before or after the diode? The only reason I could think of was that it would permit a lower spec diode as the AC voltage would be lower at the point of rectification but I wondered if there was a more technical reason...

    Also thanks for the point re. trem bias supply - I was thinking the wrong way around, i.e. that connecting to the second cap would provide a smoother DC supply to the trem. I didn't consider the effect the trem might have on the power valve bias!

    Thanks again for the feedback, really appreciate it
     
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