A Fender 5B6 Tweed Bassman Build

Discussion in 'Shock Brother's DIY Amps' started by Dennis Perusse, Nov 11, 2014.

  1. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Hello,


    As promised I said that I'd do up a build thread on my attempts of doing an early octal tweed Bassman for my friend Gary who wants to learn the bass. He had been saying it for at least a year and on his birthday that he wanted to learn so his family and I got him an Aria Bass guitar and I tasked myself with building him a 30 watt bass amp to practice with at home. His in law was going to be teaching him the instrument so the amp wasn't crucial at the moment but I knew I needed to get the project underway.

    Now this is not my first rodeo in the foray of amp building but to be honest I have never built a bass amp before and have also never done a push pull configuration either. But, hey, you have to start and succeed or fail somewhere so this seemed like a perfect time to do so. After asking this list for suggestions since I was going to modify a PA-26 amp as a Bass amp the list said don't do that and use a conventional circuit for the build. I decided then to do the 5B6 circuit as I had many of the parts on hand and wanted to reuse my empty Marshall chassis and Head cabinet which was pristine and untouched. Most of the tubes I had on hand with the exception of the power tubes at the time and most of the other parts that I needed left would be all new.

    Now I was going to show all of the mistakes that I have made along the way in terms of wiring and such but it would be much better in my opinion just to give a truncated look at what has gone on so far. As this is now out of the way I present what schematic that I had in front of me as well as layout. I do have some questions for people as this build goes onward but for now let's begin at the beginning. :)
     

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  2. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Okay, as you can see the design itself dates somewhere in the 50's as you can see octal tubes being used. Also note that the amp was using a split chassis as well just by the octal socket that would connect to the separate chassis up at the top of the amp. I realized that if I was going to use the Marshall chassis and Head Cabinet then I'm going to have to translate that circuit into something that I can use that will fit inside the chassis. So with pencil and graph paper in hand I set about to get from point A to B from the original depictions of the circuit.
     

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  3. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Back to the build.

    I felt quite sure what I was doing, almost too sure as I was doing it. My past experiences with single ended amps as well as translating the Kalamazoo model 2 circuit on another build thread gave me the impression that it shouldn't be that bad. In a sort of Top Gear "How hard could it be?" kind of moment. Let me just say that that is not quite the mentality that I have when doing these projects but that is how you learn sometimes so off I went further down the rabbit hole. :)

    As you can see from these pics that I then just went and populated the board with the parts that I had purchased at an electronic surplus store in NH. With trusty soldering iron in hand I went about putting the parts in as I felt confidence in my handiwork. Take a look at these pics for reference. I also put the board onto the chassis so I could do reference points for where the tube sockets would have to go. :D
     

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  4. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Once that got done the next stage was the Chassis and all of the planning needed to get that right. It took a while and I'm not sure if I was too conservative on my measurements and placement but I went through it anyway as I then ordered my OT and PT for the project. Plus I also took the time to use the loudest color for the chassis as most of my other projects have just been the original metal color that they were made with. I rather like it myself and many of my friends who have seen some of these pics like it too.
     

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  5. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Back onto the project and soldiering on.


    Once that got done it came to the task of putting the OT and getting the PT holes prepped so that I could start to put the guts of the amp together. You can see from the pictures that I started to get everything coming along nicely but something was nagging me. I kept checking my layout and something didn't seem right upon initial comparison. Putting the thought out of my mind once I had done the holes for the PT and OT and started mocking the chassis up did I go to the head cab and prep that up. That at least gave me a break from things for a little bit.
     

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  6. Telenut62

    Telenut62 Poster Extraordinaire

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    Facepalm coming up.....
     
  7. Ricky D.

    Ricky D. Doctor of Teleocity

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    Great build thread! Thanks for sharing.
     
  8. Dennis Perusse

    Dennis Perusse Tele-Meister

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    And onto the head cab. :)

    For a change of pace I then decided to prep the Head Cab. Not much was needed here and the pictures show this. The only thing officially left that I need to do is to drill the holes in the bottom to mount the chassis so I can also put the food pads on the bottom. In fact I'll have them in the same holes as the chassis is so that only four holes will need to be used to hold both items together as one unit. A little bit of Danish Oil that I had around the garage made it look real nice and the handle feels really solid too. Good to last my friend a long time and has t nuts under it so that the handle should never come off of the cab either due to time.
     

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  9. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Back to the chassis portion of the program. :)

    As I compose this when I was putting the board as well as other stuff I kept having a nagging feeling about what I had put together. So I decided to re-examine my work and I discovered that I had made quite a few mistakes, circuit wise. I then took the time and went by the schematic and just took my time with this and came up with new versions of my layout.

    I also dug out my Tino Zottola book, which was his first one which has this amp in it albeit in a Point to Point layout. I then took a look at his schematic and the back to the original one that I had and had then drew up my own version of the schematic. It looks almost identical to Tino's in many respects so I won't put his version up next to mine. Plus I also am putting up my newest revision of the layout. If you compare the original Fender one with my original version and then look at the one that I am posting here you can see a few changes involved. My condolences about the converted pdf if it is too light but I was scanning this today and didn't darken up my lines. This is version .5 of my layout and I hope that I have this one right as I'll further discuss this build thread. But, hey, take a look at it and tell me what you think about it. Good or Bad, let me know as I can only improve with other people looking at it.
     

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  10. Dennis Perusse

    Dennis Perusse Tele-Meister

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    The long road to wiring Hell, or Dante's Inferno as Club Med! ;-)

    A little bit silly to be honest with you but anyone who has seen my wiring knows that Fender Silverface amps start to look really clean and good after a while. I was determined to do it really well but them having to rewire this a few times after catching many mistakes just means that I had to do my best and make sure I get it right before I get it pretty. :)

    As you can see from the pics posted my wiring got done albeit from a few problems like bad soldering tip and constant re doing stuff that I thought was good. As that was going on my order for my power tubes came in. I purchased them from Tubedepot as my local vender of tubes had just ran out when I had arrived in there establishment. Just my luck but that's okay to the Net I go. :)

    I had ordered a matched set of Sino 6l6 WGS tubes and although I hadn't really heard much about them I figured that I'd role the dice and try anyway as my metal can RCA 6l6's I did not want to use for the project. Besides, the 5U4GB rectifier tube, the 6sc7 pre-amp tube are Nos as my original 6sl7 tube I wanted to keep so a new one was also ordered just in case. Where I have heard that the 6sl7 tubes can be microphonic so I ordered a new one as a spare.

    Things seem to be going on quite well, messy but well. I did change out my shielded cable back to wire as I realized that that shielded cable was for pickups not for amps. Next time I get a moment I'll order some proper RG-174 shielded cable, probably from Hoffman Amps but for now I'll be okay apart from hum should everything work okay. Take a look at the pics as it goes together.
     

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  11. Dennis Perusse

    Dennis Perusse Tele-Meister

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    As one can see it is a rats nest only a mother rat could love. ;-) You'll also notice that the OT has multiple taps to it. I had asked the list about that in a previous question so I ended up putting the negative feedback onto the four ohm tap as that is what the original OT was set up.

    The OT that I have in this, for those who wish to know is a Classictone 40-18008 which is a 50/60 watt OT with a 4 ohm tap and an 8 ohm tap. I decided to use this since it gave my friend Gary a wider option of bass cabs to use. Even though the amp itself is a 30 watt amp according to Tino Zottola's book I figured that having almost double the watts in terms of OT iron shouldn't be a problem where it is going to be used what it was originally designed to be which is a bass amp. ;-)

    The PT is a Hammond 274 AX Power Transformer which I was also informed on this list as being what I need for this project. One thing to note is the fact that I am using a 330 Ohm 10w Resistor in the cathode biasing portion of the biasing section. Since this amp was designed for the bias to be self adjusting and I had heard that between 250 ohm through 330 ohms could be used on some sites I went ahead and used the 330 ohm one. Since I had it already and did not have a 250 one it saved me one trip to the surplus store. :) That is it for right now and I will continue on in the next few days. Please let me know what you think so far on this build and what mistakes you can see from my schematic/layout if any. TTFN
     

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  12. FenderLover

    FenderLover Poster Extraordinaire

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    Don't take this critically, but just for ideas 'next time around'.

    I do my own layouts too, and lately I've been paying more attention to the location of circuits, both horizontally along the chassis and also from front to back. My effort is to keep all leads as short as possible by an improved layout, despite vintage drawings. Not all ideas apply to all circuits, but here goes:

    move filter caps used for each tube to each tube stage

    pay attention to the tube pinout, and mirror that on the layout, preferably without needlessly crossing wires

    maybe move the EQ components to where the EQ is on the front panel

    jumper under the board if needed to 'turn components around' and wire the front of the board to the front panel and the back of the board to the back panel/tubes

    do not dismiss point-to-point wiring. If there is a short point-to-point route to take from tube to board, or pot to board, it may be cleaner than wiring the part on the board with more flying leads to their destinations

    My latest build in progress is a P-P 6L6 with 3 preamp tubes on a Champ chassis. It took some figurin' to keep it neat. I'll post it here eventually.
     
  13. andyfromdenver

    andyfromdenver Friend of Leo's

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    Way to go Dennis!
    I like the orange, makes me think Broncos :)
     
  14. Telenut62

    Telenut62 Poster Extraordinaire

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    That I gotta see ;)
     
  15. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Hello all,


    Thanks for all of the comments, I really appreciate it. I should give some clarity though, but let me elaborate a little for everyone here. I'm not so much dismissing PTP wiring. I'm more making fun of my own wiring skills right now. Anyone who has seen my previous builds will know how my wiring skills are slowly improving. :) It's just that when I wired this the first time most of the leads were really good and clean with very little crossing over of leads and most of them weren't flying. When I had to rewire it due to my mistakes then the Silverface look went into vogue for me just so that I can get it to operate. Like I've mentioned in the first post I'm keeping the pics to where it currently is and letting people know the mistakes that I made along the way.

    My hope is that no one has to go through the nightmare trying to make sense of what Fender was doing with the original design. The split chassis, with how that board is set up, gave me the shivers when I first set out to transcribe it to something manageable for me going only to one chassis. I almost seriously entertained the idea of using Tino Zottola's PTP wiring for it but was very uncomfortable with the idea since the last time I did a PTP wiring job I failed miserably and I knew that I didn't want to use a cap can after reading Merlin Blencowe's book. I almost thought about doing a hybrid idea. Having the power section on a tag board but the rest using PTP but then I thought that I was better off sticking to one way or the other. Where this is my last amp build for a short while (have waaaaaaaay too many projects to finish up at home at this point in time) I want to make sure that this one goes off correct electrically then any small issues I will deal with then. ;-)

    I would have done another update as I have it all together but I need to build me a current limiter then test it out. If anyone sees anything amiss in my layout give me a shout out as I think it looks okay. I am open to correction so if one exists then let me know. Now I haven't put in any grid stoppers in yet as the schematic doesn't call for one but I'll probably get around to doing it once I know it works. Same goes for the wiring cleanup too. Stay tuned... :)


    Dennis

    Fenderlover: I can't wait to see your project when that comes out. Sounds like it's gonna be awesome and looking forward to the build thread.
     
  16. Dennis Perusse

    Dennis Perusse Tele-Meister

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    Quick update for everyone.


    Hello all, we've gotta heartbeat after all. :) When last we left our amp in distress I was having problems with a few things, one being me needing to rebuild my current limiter. I found out that I had built it wrong and had to rewire it to work right. It took me a while, not that that project is hard, as I just didn't have the time to go over to Home Depot to the the proper stuff to do the job last week.

    Well, I have it working and the current limiter works fine. I then decided to see if I had any shorts in the wiring as I felt confident in it so I set it up for a mild test. I have one of my cabinets that I used that contains a celestion g12 vintage 30 speaker at 8 ohms so I figured that I was on stable ground so I used that.

    I then set up everything, connecting the current limiter to the amp and proceeded to power it up. Well, when I did this I noticed that their didn't seem to be any shorts in the unit as it powered up, no sacred blue smoke nor any of the usual trappings that most of us suffer for at initial power up. After waiting a minute or do as the bulb dimmed I then shut it off and took it off of the current limiter.

    Once that happened and then I then powered it up again and then one problem emerged. One of my Sino 6l6 tubes had this weird almost short circuit like sound to it happening from one of the tubes. I then switched it off and then let the unit cool down. I know that my wiring scheme, albeit messy, works and is correct at least on paper so I pondered for a few minutes and then went to my tube stash for other 6l6 tubes to try out and see if I could replicate the effect.

    Now before I did that, once the amp was powered down and tubes were cold, I decided to swap the tubes over and see if this was just in the tube or just in the circuit. I figured that if it did it using a different tube then it was the circuit, but if it happened in the same tube but in a different socket then it was the tube. I say this since the allure of coffee whilst waiting for the tubes to cool down provide a moment to reflect on this type of wisdom/epiphany. ;-)

    Ok, once I did this sure enough the same tube but in the other socket did the same thing so it was the tube. Now I have a friend of mine who fixes this stuff as a side business so when I get a moment I'll use his tube tester and see if he can detect shorts in this tube. I didn't find any on my tester but mine is an emissions tester whilst his is a transconductance tester. I still plan on putting in the grid and screen resistors but where this circuit wasn't designed with them I felt that it was a good test of the original circuit in my opinion.

    Setting this up again, except this time using a pair of 7027a that came out of an Ampeg V4B I tried it again. We have a heart beat, and it sounds awesome, albeit for using a guitar speaker instead of a Bass speaker. No shorted tube sounds, but a bit of hum to which I'll get some RG 174 shielded cable to counteract that from Hoffman Amps. Overall it works and I couldn't be happier. I also tried it out using a pair of sovtek 5881 tubes in it and again no problems at all. Like I mentioned before I still wish to put in the screen/grid resistors but now that I know that it works that task will be easier knowing that I can eliminate them as a variable should something go wrong. I can't thank everyone enough who has helped me with this project as well as the other separate threads on certain issues that I has asked questions about. I will do one final update once I get the other stuff completed on it (wire cleanup, resistors put in, cabinet finished, etc) but over all this thread is mostly done and I hope that everyone who wishes to construct such an amp has an easier time about it. :)

    Dennis

    ps here is the picture of it on the current limiter at the beginning.
     

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  17. R. Stratenstein

    R. Stratenstein Doctor of Teleocity Ad Free Member

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    Congrats! Cool build.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Before you install that RG 174 shielded cable push your preamp tube wires around with a wooden chopstick. Interaction between plate and grid wires is pretty common unless you are careful with lead dress. A small change in wire placement can suddenly make the amp nice and quiet.
     
  19. '64DlxRvb

    '64DlxRvb TDPRI Member

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    I can sure go +1 on that! :D
     
  20. Dennis Perusse

    Dennis Perusse Tele-Meister

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    ****semi-final update*****

    Okay all when last we left our heroes I was in the process of getting a few things done. I had gotten my shielded cable and then proceeded to put the shielded cable onto the areas that I felt needed to be shielded. Now did it reduce some hum, well, yeah it did but it still has some hum to it. It isn't completely quiet but I didn't entirely expect it to.

    As tube amps go this design was a pain to deal with and a pain to get done but overall I'm personally happy with the results. I just have to give my friend Doug a call and have him goof with it and see what he thinks. Where he plays bass he can give me the yea or nay on what he thinks my friend Gary would find okay with.

    I also had acquired a 4 ohm 175 watt speaker so I'm comfortable in saying no worry about issues with speaker distortion. Considering that this is a 26-30 watt amp with a 50-60 watt OT I feel comfortable that this should be a good fit for my friend's first bass amp. :)

    If I can get or find a way of getting a recording of it whilst being played with a bass I'll try to have one done but I don't have any way of getting that done at this point in time. Considering that I haven't found yet a recording of it done on a bass amp is rather surprising to me but no worries.

    Looking back on this year's project I probably should have done this in true PTP style using Tino Zottola's book as the amp would have been easier to do but at least I know that if anyone wants to build one of these amps and have it put to a single chassis they know that they can. :)

    Anyway special shout out to all who helped me along the way on this forum. I appreciate all of you for both the comments and critiques as this rolled right along as this is my last project for a little while. Too much stuff to dig myself out of with my brothers PA to deal with plus a tube pedal that I want to try out and make. I'll see ya when I see ya on the next build thread. :)

    Enjoy the pics presented.


    Dennis
     

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