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Fender AB763 Builderino

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jun 29, 2016.

  1. kleydejong

    kleydejong Tele-Holic

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    Context

    After completing an Epiphone Valve Junior conversion to a 5F2A Tweed Princeton circuit I officially got the itch. A true handwired AB763 has always been my holy grail and so I decided to jump head first into the deep end. I wanted to document the build and my progress as well as pose the questions which I'm sure will arise.

    Preparation

    With my first build I relied on a kit and a pretty clear set of instructions to ensure I had everything I needed at every step along the way. With this build I plan on going a little more peacemeal. Hopefully with some patience I can get some good deals and keep the costs down.

    I also spent some time doing research on the exact technical differences between all the different blackface models. I really don't have a specific AB763 amp in mind. I want something with dual 6L6's and I want reverb. Other than that I'm open to wherever the build takes me.

    In my research I found the Weber 6A40 layout to be the most clear and helpful. So I plan on using that as a guide.

    [​IMG]

    Parts

    After doing some scrounging around I located a used Classic Tone power transformer for a decent price. It is the model 40-18041. Link - http://www.classictone.net/40-18041.html. Specs - http://www.classictone.net/40-18041.pdf. My understanding is that this transformer is spec'd for the smaller dual 6L6 style amps like the Vibroverb.

    Question - Given the use of the above transformer, I can still use the above layout - correct? But it may impact my choice of rectifier tube?

    Next I located a chassis. It is spec'd for a Deluxe Reverb, so it is likely that I'll need to do a little resizing to get my transformers to fit. But the front and back plate should be good to go and it should have plenty of space.

    Now for the small parts. I decided to order through Hoffman. Their kits are basically a preset bill of materials that you can modify in the cart, as opposed to other kits that are all or nothing. To fill things out I still needed the output transformer, choke, and reverb driver. I ordered those from Hoffman as well. For the OT I got the Hoffman 40 watt 2x6L6 2-4-8 ohm output transformer. It is rated for 50 watts with a primary of 4000. I got the large fender Choke as well. Hoping these parts will all work together nicely. I've heard of folks dropping in a Bassman OT in a Bandmaster Reverb, so I'm thinking the beefy 50 watt OT will work nicely.

    More to come soon.
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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  3. kleydejong

    kleydejong Tele-Holic

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    Thanks Rob! Your website is amazing.
     
  4. kleydejong

    kleydejong Tele-Holic

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    I've been reviewing different schematics from various blackface amps. Specifically looking at the power transformer, rectifier tube, and the voltages it sends to the power section and preamp. I could use a little help here in understanding what I'm looking at.

    Let's start with my power transformer. According to the specs (http://www.classictone.net/40-18041.pdf) I see 325-0-325. Upon reviewing the schematic for the Vibrolux AB763 amp I see this (which I interpret as a match, meaning my PT is functionally similar to the on found in a Vibrolux AB763):

    [​IMG]

    Now compare this section of the schematic to other AB763 amps and there is quite a bit of variance. Deluxe Reverb is down at 305. Super Reverb is up to 360. Makes sense to me that the DR would have lower voltage because it uses 6V6's. I am quite confused as to why the dual 6L6 based amps are all over the place though.

    First Question - Is there an explanation for the range in voltages across the difference schematics?

    Possible answer - In reviewing Rob's page on 'How a DR Works' there is a section on the power supply. It states "Higher amp voltage tends to increase output power, tighten up the tone and make it "punchier." The power supply of the Deluxe Reverb is slightly under rated in current output compared to other AB763 amps such as the Twin Reverb. This under rating leads to voltage sag that adds unique touch sensitivity and playability."

    For the sake of science and documenting my process I'll leave this up as I believe I answered my own question.

    In application this means that the PT I bought is relatively under rated compared to bigger 6L6 blackface amps like the Super Reverb. This results in lower output, looser tone, less punchy. But it will add touch sensitivity and playability. Lower volume required to drive the circuit.
     
  5. kleydejong

    kleydejong Tele-Holic

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    In continuing my review of the schematics and Rob's comment above, I found another point of variation. The signal goes from the rectifier into the choke. Then I believe the filter caps and step down resistors are next.

    In the Vibrolux AB763 schematic I see two 10k resistors and 3 16 uf caps:

    [​IMG]

    The Deluxe Reverb AB763 is identical here I believe.

    But the Super Reverb is different with a 4700 ohm resistor, 1k resistor, and 3 20 uf caps:

    [​IMG]


    Again, Rob's explanation of the Deluxe Reverb is helpful: "Increasing the size of these first two filter caps is a common Deluxe Reverb modification that will tighten the amp's bottom end and reduce farting out. Replacing the first two 16uF caps with 22uF caps works well and won't strain the rectifier tube on startup." So it appears that the difference between 16 uf and 20 uf caps is just fine, and may actually be a beneficial change.

    Question - Regarding the 10k voltage dropping resistors. Is there a meaningful distinction between the 10k's used in the Deluxe Reverb compared to the 4700 ohm and 1k used in the Super Reverb? If I continue to follow the voltage after these resistors it drops to 300v on the Virbolux and Deluxe Reverb, but stays at 410 on the Super Reverb.
     
  6. kleydejong

    kleydejong Tele-Holic

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    Thinking some more about the relationship between voltage and tone / response - would it not be the same experience in a pedal compared to an amp? For example I have a few overdrive pedals that run standard at 9v but can also be run at 18v. At 18v they have increased output and clean headroom. Sounds like the same thing occurs in an amp.

    So in essence, while an amp like a Vibrolux and a Super Reverb may share similar preamps and have the same dual 6L6 power configuration - using different transformers and different voltages across the circuit changes the overall output and headroom capabilities. So Fender's overall offerings lets you find an amp with the right amount of power and headroom to fit your playing situation from bedroom to bar gig to outdoor festival to arena.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    kdj, you are on the right track. I believe the speakers and speaker arrangement in the different blackface amps makes most of the difference in their tone but their differing power supply systems do affect the tone and touch sensitivity.

    I recommend you chose a favorite blackface amp and build it to spec.
     
  8. kleydejong

    kleydejong Tele-Holic

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    [​IMG]

    Some of the parts received in the mail! Hoffman processed my order and it arrived at my door in 3 business days. Pretty lightning fast if you ask me. Stuff is looking good so far.

    The chassis I got for pretty cheap. It is sized for a Deluxe Reverb though... So I'll be wrestling with it a bit. First, I have to open the power transformer hole. I decided to keep the top left bolt hole as an anchor and then open the bottom and right side. Trying to keep it as efficient as possible. Slow and steady with the Dremel. I find I need to take breaks or it gets really hot. But it gets the job done.

    [​IMG]

    For this build I decided to swap my order of operations. Assemble the chassis first and do the component board last. Hopefully more accurate connection wire lengths and a little easier time moving around in the chassis without the board in place.

    [​IMG]

    [​IMG]


    Now some initial work on the board.

    [​IMG]

    From my last build I changed my procedure a little bit here as well. I'm not soldering anything in place until the very end. I did all the caps first. Then I went to the resistors. The Hoffman kit didn't put all the resistors in pretty little marked containers like the Mojo kit, so I used my multi-meter and simply measured each one. Then I'd check the color code chart. Then put it in its place. Then measure again.

    Strangely I believe I noticed a difference from when I first test the resistor to when it goes on the board. When I test them by hand I hold the two prongs of my multimeter in each hand and then I press each end of the resistor onto each prong with my thumbs. The resistance was kind of varied. But then when I'd check the code and put it on the board it would read much more accurately. Does the human body affect resistance?

    I also noticed what has become my second hurdle in this build. I didn't have all the values compared to the Weber layout. You can see I highlighted the ones I'm missing here:

    [​IMG]

    I do have extra resistors and caps. I'm guessing as I would find that these are relatively minor component deviations from different models. I have some 2.2k resistors on hand. One of the 820 ohm resistors is the negative feedback, and I think I'm going to install a pot on the back anyways - so that's fine.

    As I did my homework I discovered this link: http://el34world.com/Hoffman/AB763Notes.htm. Hoffman uses a totally different tremolo section in their layout from a Vibrolux 6G16 circuit. That would also explain why I have no opto-isolator. It took me a good half hour of googling to figure out what that spider looking thing is on the layout diagram.

    So I have to decide if I want to stick to the AB763 layout and order parts for the stock tremolo. Or I have to decide if I want to try and use the parts I have for the 6G16 tremolo and install them onto the eyelet board which appears to match the Weber AB763 layout.

    Another decision I have to make deals with the filter caps. I did not order a cap can. I noticed Hoffman talks about wiring the filter caps PTP inside the chassis itself, possibly using terminal strips or positioning them under the pots like this:

    [​IMG]

    Pending those decisions I'm going to keep inching forward. I'm going to start using the Vibrolux schematic to guide the power section and supplement it with the Weber layout (its just so easy to read). I'm going to keep it stock to start, but I fully anticipate I'll do some modding. NFB, 4 ohm and 8 ohm speaker outs, and a pretty heavily modified normal channel seem likely culprits at this point.
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The big downside to the Hoffman bias wiggle tremolo is it limits how you can bias your power tubes. Too hot or too cold and the tremolo will be weak. Doug said on that web page that he much prefers the sound of the bias wiggle tremolo so he changed the circuit.

    If you're going to build the Hoffman AB763 then you obviously need to use his layout diagram.
     
    Last edited: Jul 1, 2016
  10. 5F6Animal

    5F6Animal Tele-Meister

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    I messed around with power supply dropping resistors and negative feedback resistors on a 67 bandmaster I owned. If you use the 10k's it'll break up a little sooner.

    I used 10k's and lifted the nfb, great tone!
     
  11. kleydejong

    kleydejong Tele-Holic

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    Listened to some demos and spent some time comparing schematics and layouts over the holiday weekend. I preferred the sound of the bias modulating trem in the 6g16. So I decided to try and go for it in this amp. I have the parts, but my eyelet board matches the AB763 bug design. The Hoffman information that I've found is all based on a turret board which is layed out very differently, so I'm going to have to build the bias mod circuit using the eyelets available.

    Weber has some nice layouts for the 6A40 (Super Reverb AB763) and the 6G16. I believe I've focused on the components for the tremolo here:

    [​IMG]

    There are also some differences on the intensity control and the interaction with the bias circuit:

    [​IMG]
     
  12. kleydejong

    kleydejong Tele-Holic

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    Here goes nothing. I pulled out the components related to the tremolo and put the 6G16 components in their place. The layout doesn't match, but the connections should be correct.

    [​IMG]

    Question - On the vibrato channel the footswitch capability of the trem and reverb will not be useful for me. Can I wire it up so both are simply always on? I'd like to have access to the trem without the footswitch connected.

    I also starting work on the normal channel. Went to the hardware store to get some non-conductive washers for the 4 input jacks. I plan on running a bare ground wire across the preamp controls. It will be stabilized primarily by the input jacks with connections running from the pots and the eyelet board. On the far right I have a black wire that will get its own bolt on the chassis. Shielded wire running out of the volume controls to V1 and V2.

    [​IMG]

    In posting this I noticed the bass control is wired wrong. Will review.
     
    Last edited: Jul 6, 2016
  13. kleydejong

    kleydejong Tele-Holic

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    The wife and 2 year old daughter were gone tonight and so I ended up spending the night hunkered down in the basement with the sweet smell of solder in the air. For some reason this build feels quite a bit more tight compared to my previous 5F2A project. The preamp tubes in particular are feeling very close quarters and I'm feeling the need to be more surgical. I am trying to be more cognizant of my lead dress.

    The progress tonight:

    [​IMG]

    With my previous build I did things in a very segmented approach. First I wired the board with general leads. Then I assembled the chassis. Then I dropped in the board and made the connections.

    With this build I prepped the chassis as much as possible. Then I placed the components on the board. Now I'm going through the entire circuit from preamp to power amp (right to left) and am soldering the connections.

    I like how this approach gives me a better understanding of where the connections are made. It is easier to visualize how the schematic relates to the layout. But progress feels quite a bit slower and it feels a bit more complex and I have to double and triple check often to make sure I am not missing anything.

    [​IMG]

    Made it up to the customized 6G16 tremolo section. Phase inverter next.

    [​IMG]

    Normal channel controls. Some of these things I feel I'll need to go over with a fine tooth comb to make sure the ground wire or the floating resistors on the input jacks can't accidentally make contact with anything they shouldn't be touching.

    [​IMG]

    Back side connections. I currently have these controls wired per the diagram, but I would be interested in possibly wiring them in such a way as to not require the footswitch having both reverb and tremolo always on the vibrato channel.

    [​IMG]

    Speaker output jacks. My OT has 2, 4, and 8 ohm secondary outputs. Am I allowed to wire these speaker output jacks like this? The green wire is 4 ohm and the green / yellow is 8 ohm. So my understanding is that this wiring means I have one dedicated speaker out at 4 ohm and the other at 8 ohm. Or should I use a SPDT switch and a single speaker out?

    [​IMG]

    OT wiring specs:

    [​IMG]
     
    Last edited: Jul 7, 2016
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The advantage of using a single switched jack is you can use a shunt jack to protect the output transformer.
     
  15. kleydejong

    kleydejong Tele-Holic

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    Ah I see. So if I use multiple jacks I cannot wire them with a shunt as all but the one plugged will short. However having the shorting jacks is beneficial because if a speaker is ever not connected them it will pop the fuse instead of the OT. An open circuit vs a closed circuit.
     
    Last edited: Jul 7, 2016
  16. kleydejong

    kleydejong Tele-Holic

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    To leave the tremolo always on can I just ground the wire that goes to the vibrato pedal connection on the back?
     
  17. Thin69

    Thin69 Friend of Leo's

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    Yes, grounding it turns it on.
     
  18. kleydejong

    kleydejong Tele-Holic

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    Another pretty lengthy evening of soldering last night. Going out of town for the weekend so I'm posting lots of pics for review. No way I didn't make at least a couple of mistakes. I'll be going over these with a fine tooth comb and hopefully I can come back next week with a list of things to correct, give it another once over with fresh eyes, and begin some startup procedures!

    [​IMG]

    Circuit board. Running all the connections on top of the board maybe was a bad choice? Little messy.

    [​IMG]

    [​IMG]

    [​IMG]

    Now the tubes. Lead dress could use some fine tuning. For whatever reason I battled with these preamp tube connections. Having the heaters installed presented me with some troublesome angles.

    [​IMG]

    [​IMG]

    Power tubes. Didn't realize until it was kinda too late that the keys are oriented opposite of one another. Sorry if you love symmetry. Also I noticed that I believe the 470 ohm resistor on the left is actually a 4.7k. Will fix.

    [​IMG]

    Rectifier:

    [​IMG]

    Rear stuff. Went to a single output jack. Didn't have the SPDT switch I thought I had, so I'm just running the 4 ohm tap to match my favorite 2x12 that will house this bad boy. The pot to the right of the on/off switches is a 250k pot in series with the 820 ohm nfb resistor. Perhaps 250k is too big?

    [​IMG]

    Front controls:

    [​IMG]

    Vibrato channel:

    [​IMG]

    Reverb and Tremolo. I think the speed control is missing a jumper from the left to middle. Will check:

    [​IMG]

    Bias circuit got a little tight so I used some terminal strips. Used a couple washers to make sure the bias pot has clearance.

    [​IMG]


    I need to wire up the filter caps yet. Will be wiring them like this. I didn't order an eyelet board for the filter section, so I'm using the stiff cardboard back that came with my 5F2A eyelet board. I'll drill the holes and wire it PTP.

    [​IMG]
     
  19. kleydejong

    kleydejong Tele-Holic

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    Wrapped up a few things last night and today. Double and triple checked the wiring. Things looked good to my eyes.

    Went through some initial fireup testing procedures. With no tubes or anything the pilot light comes in nice and strong. Tried a GZ34 recto tube and a speaker cab next. Nothing burned or sparked or did anything funny. Tried a full set of assorted 12AX7's in the 6 preamp slots. Everything okay still. Then I tried an old set of 6L6's from the drawer and after about 15 seconds the light blinked and went out. Fuse blown. Pulled the 6L6's. New fuse. Fired up just fine and held. Took these voltages:

    GZ34

    1 -
    2 - 440
    3 -
    4 - 330
    5 -
    6 - 330
    7 - 440
    8 -


    6l6 Outer

    1 -
    2 - 3.3
    3 - 454
    4 - 453
    5 - .57
    6 - 432
    7 - 3.2
    8 - 0


    6l6 Inner

    1 -
    2 - 3.2
    3 - 454
    4 - 454
    5 - .8
    6 - 443
    7 - 3.2
    8 - 0

    v1

    1 - 260
    2 -
    3 - 1.9
    4 - 3.2
    5 - 3.2
    6 - 263
    7 -
    8 - 2.1
    9 - 3.2

    v2

    1 - 262
    2 -
    3 - 2
    4 - 3.2
    5 - 3.2
    6 - 260
    7 -
    8 - 2.1
    9 - 3.2

    v3

    1 - 256
    2 -
    3 - 2.08
    4 - 3.2
    5 - 3.2
    6 - 258
    7 -
    8 - 2.05
    9 - 3.2

    v4

    1 - 161
    2 -
    3 - 1.2
    4 - 3.2
    5 - 3.2
    6 - 256
    7 -
    8 - 2.08
    9 - 3.2

    v5

    1 - 161
    2 -
    3 - 1.2
    4 - 3.2
    5 - 3.2
    6 - 442
    7 - 164
    8 - 170
    9 - 3.2

    v6

    1 - 287
    2 - 42
    3 - 74
    4 - 3.2
    5 - 3.2
    6 - 282
    7 - 46
    8 - 74.8
    9 - 3.2

    The phase inverter does not seem to light up compared to the rest of the preamp tubes.

    I tried a second set of JJ 6L6GC's. A pair that I know is good and has been running well in another amp. They did not blow the fuse, however upon installing them and letting the amp warm up for a bit the GZ34 started to pop flashes of bright light every 5 to 10 seconds. After about two or three I pulled the plug. Did not get any voltages. The fuse did not blow however.

    Any thoughts?
     
  20. Pup Tentacle

    Pup Tentacle Tele-Holic

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    Your bias circuit looks incorrect. The positive end of your bias cap needs to go to ground unlike all your other electrolytic caps. It also appears to be under rated. What is the voltage rating on it. I can't tell but it looks like it's only a 25 or 5o volt rating. Even though the Weber layout shows a 50uf/50v cap there, that circuit can typically exceed 50v. I would recommend a 100uf/100v cap there.
     
    Last edited: Jul 12, 2016
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