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Howdy!

Been a bit quiet on my side, thought I'd share my latest amp build. Pace was very slow on this one, have been working on it since July grabbing the odd hour or two whenever we got a break from the home renovations.

Anyway, My 18 watt Lite IIb has not seen much use since I finished my Trinity Plexi Brown. And there was an unused phase inverter input and space for another triode. What to do! Starting from this:



Simple design, single volume and tone control, cathode bias EL84 no negative feedback

The biggest learning curve in each of my builds has been how critical layout is. My previous Plexi 6V6 had layout issues that resulted in unsolvable noise problems. So the first month was spent with pencil, paper schematic and blank chassis trying to figure out where to put everything. This also helped me choose the design eventually.



The chassis had space for 4 input holes, five pots, an indicator and a switch. Hmm. Not quite enough for a Plexi. Decided presence control would go on the back of the amp. Plus, this helped my layout as this chassis puts the speaker jacks off to the side, which can result in parasitic oscillation with poor NFB wire placement.



At this point was thinking of doing a variation of SDM's 2203/1959 mod. But more on that next...
    A plexi with a 6v6?

    Also, how will a bad layout cause noise? Grounding issues?

    Looking forward to seeing your build! Havnt had a good build thread in a while!
      For the first time, I'm using terminal strips rather than turret board. While I saved a some cash and time doing it this way, I do prefer the look of turrets.

      Anyway, now that I knew what the physical limitations of the build were, I started on the circuit design. I have a small list of things that I've discovered work for me in an amp

      1. PI/PA VVR is a must. This means that the circuit needs some form of volume control before phase inverter coupling cap. So a plexi would need a master volume added.
      2. Bright caps on gain controls are more trouble than they're worth. If installed, must be switchable.
      3. Line out on 8 ohm tap.

      Adding VVR and a master volume would mean finding another two holes for pots. Decided to put these in place of two inputs.

      Fired up inkscape and started with control panel



      After a few iterations, took it to a trophy store and had it laser engraved. Come to about R250. Not bad that!

        Hi Psyx

        Thanks. Just trying to put controls as close to their corresponding parts on the board to keep wire runs as short as possible, especially in the early gain stages. Input on V2b after the 470k mixer resistors is very sensitive to placement and can introduce hum. If you look at the later pics you'll it's flying rather than up against the chassis. Good layout can save you from having to use shielded cable and other band aids. Negative feedback is also a pain, can introduce all sorts of mosquito whine in to your signal. Twist AC wires to cancel hum and make sure all signal and power wires cross at 90 degrees.

        T
          Having finalised faceplate, it was time to drill the chassis. Now, you know you're getting a bit amp obsessed when you ask your father in-law for a step bit for your birthday! (Mind you, he's a man who appreciates a well stocked garage, so he was happy to oblige)

          What a damn pleasure those things are to work with. Quickest, easiest and most accurate chassis work I've ever done. Drilled out the power tube sockets to take octals, added the extra control holes on the front, new mountings for the terminal strips and the choke.



          OT wired up, heaters done, PT halfway done, presence pot, Zobel filter, VVR mounted and DC elevated heater board.



          Choke, power switch, indicator, first two filter caps and VVR completed.

            Hi Thomas...hope you're well.

            WOW, looks like it's going to be a killer amp...looking forward to following the build....maybe you'll allow me to Profile it when it's done. 8)
              Power tubes and phase inverter done.



              Just a note on grounding: I try to stick to star/bus topology. Each stage is grounded directly at its relevant filter cap. Using this as a guide.



                Hey Lance!

                Anytime. I'm busy setting up the home studio/rehearsal room, so might be a great exercise. But see how you feel after the clips....

                T
                  At this point, I had two tubes to finish wiring and then the controls. Which seemed to take the longest. In between a manic time at the office, Mango Groove shows and getting a new tenant in our cottage I just couldn't find the time. So finally last week Friday I grabbed a good two hours worth of soldering and finished up. Grabbed my multimeter, tested grounds. Looking good. Time for a test with power but no tubes. Switched on to standby to test PT and

                  WHITE SMOKE!!! ARGGGGHHHH!!!!!!!! ☹ ☹ ☹ ☹ ☹

                  And only then did the fuse blow. Man, what a disappointment! Heartbroken, I started trying to find out what had happened. Here's what I knew:

                  Fault happened when on standby
                  PT was fried

                  So the problem had to be something wired to the PT. Eventually I trace it to the bias tap wire. At some point I'd reversed the orientation of my bias circuit right to left to optimise my grounding. But I'd forgot to switch the input and output wires. So was dumping 290v AC across a 65v rated cap and 200v rated trimpot and then to ground. Sheesh.

                  So, when Monday came around I dropped off the PT at P Souris for a rewind, picked up some new parts for the bias circuit and fixed the damn fault. Serious reminder of how dangerous these things actually are. And why taking your time in testing is so important. If I'd just flicked the amp on I could have taken out the output transformer and power tubes, not to mention if I'd been touching the chassis!!!

                    Fast forward five days later, I get the call that my PT is done!



                    Once again, testing step by step I fire up the amp. Looking good! Bias was a bit high, so had to replace the 10k resistor with something a bit bigger to bring it down.



                    Am thinking of doing Marshall Silver Jubilee type styling, so going to order some silver Marshall style knobs. For now using the silver Boss style ones, but not mad about them.

                      Final amp is as follows:

                      Plexi 6V6 with the following mods:

                      PI/PA VVR
                      Master Volume (Used in tandem with VVR so as not to overload PI)
                      Negative Feedback variable from JTM45 values to '71 Aluminium Panel spec (22k to 120k)
                      Dark channel cathode dropping from 220uf to 22uf. Plenty meat, but no flabby ultra lows.
                      Stage 1 cathode switchable from '66 plexi style shared cathode to '68 style split cathode.(First half of DPDT)
                      Bright channel coupling cap switchable from 22nf ('66) to 2.2nf ('68) (Second half of DPDT)
                      Input jacks wired like an 18 watt. Bright channel input is internally jumpered. If plugged in to Dark channel only, bright channel is muted, which solves the low pass issue with the 470k/470pf mixer resister bleeding to ground.
                      Phat/Brown switch puts 680nf bypass cap on V2a, and drops V1a cathode to 820r if in '68 mode. This gives me four settings:

                      1. JTM45ish - shared cathode, big coupling caps, super tight NFB
                      2. '66 Plexi - shared cathode, big coupling caps, V2a mid boost, 47k NFB
                      3. '68 Plexi - split cathode, bright coupling caps, 47-100k NFB
                      4. Brown Plexi - High gain, split cathode, bright coupling caps, V2a mid boost, loose and gainy NFB.

                        *Edit* Can't quite seem to get sound cloud to embed. Links will have to do.

                        Of course, this thread is worthless without clips! Please forgive noodling. Clips recorded DI'ed in to focusrite interface, Redwirez IR for speaker modelling, light verb and delay in Logic.

                        '66 Plexi mode



                        '68 Plexi mode



                        Haven't quite found the right setting in Brown mode yet. But my Trinity Plexi Brown nails that. Can't wait to try them in stereo!
                          Magnefique!

                          Pack it in your hand-luggage when you come to CT ?
                            Thanks Alan

                            Hehe, Ewald suggested the same. He's been telling me about how wicked your Tokai is too! I think an afternoon of mighty tone is in order. Plus beers.

                            I'm pretty happy with the sound, the fixed bias VVR works MUCH better with PI/PA. And very little noise, even when everything is dimed. And great power tube breakup at TV volumes. I will say that the build would have been neater with a turret board, and that the shaft of push/pull pot for the bright input/66/68 switch is a bit small for the input jack hole where it's mounted. But aside from those niggles it works.

                            Honestly, it's not that different from the 18. Fixed bias, NFB and 6V6 and SS rectification each add their own flavour, but it's a similar pre amp and tone stack. At the end of the day, I'm using the same guitar with the same cab and the same set of hands.

                            Go figure.

                            T
                              Yeeeaaaahhhh, that's what an amp should sound like.

                              just one question:

                              WHAT FOOTPEDLAZ DID YUO uz FR DISTORTION!?!?
                                Ooo-La-La thats very tasty Thomas well done man sounds great

                                BTW: I hope youre going to do some more Shut Up 'n Play Yer Guitar interviews
                                  @Ewald - Thanks man! Pedals? Those are things in my car. Guitar, cable, amp. DAW. I can haz teh Br00talz.

                                  @Attila - Good question. We just need to find another venue since Tanz closed. We're keen to do something with Mauritz, Andy Innes, Nic Rush, Werner Rokstarr, Johan Rautenbach etc. We really have an abundance of world class guitarists here in SA. And I'm not even scratching the surface.

                                    Thomas, you never told anyone about this gem,




                                    Its Thomas playing his dimed 18 watt (like mine and Singemonkey's) showing all the sounds one can get from a dimed amp without the floorpedals ?
                                      Oh dear.

                                      I made that one when I was considering selling the amp. I'm not happy with it, gain control should have been one notch lower throughout. "Just got paid" sounds more late ZZ than early ZZ. But yeah, everything happening from the guitar in that clip.

                                      Actually, it's not dimed. Volume and Tone on 7 if remember right. The bridge PU in my PRS is about 14k, so it does all the heavy lifting as far as gain is concerned. Neck is closer to 9k. But the hot bridge is why the split coil tones work.

                                      *edit for settings*
                                        Thomas Selmer-Olsen wrote: Oh dear.

                                        I made that one when I was considering selling the amp. I'm not happy with it, gain control should have been one notch lower throughout. "Just got paid" sounds more late ZZ than early ZZ. But yeah, everything happening from the guitar in that clip.

                                        We're our own worst critics. The video is great.

                                        PS. Very few people know what the early ZZ Top sounded like on vinyl, before they re-amped everything...