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1970s-1980s Vox AC30 problems

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  • #16
    Originally posted by voxrules! View Post

    P.S. Hi HTH! I think your Vox collection is FANTASTIC! I already knew you owned a "copper panel", now I learned you also own a "gray panel" - WOW! Congratulations!
    Bob, my Vox collection used to be MUCH better...

    * red-panel JMI ('63/'64) non top boost head (normal version), matching 2x12 cab loaded with original alnico blues. This still had all the original tubes too (Brimar/Mullard) and had never been touched inside. It's been sold for a good few years now though. That amp could nail the Whats The Frequency Kenneth tone and I think it was the closed back cab that did it, gave a tighter, more aggressive tone.

    * grey-panel JMI ('64+), non top boost combo (treble version). this was a fixer-upper. had the wrong transformers installed (Drake ones that actually were perfect for a 50w Marshall), no choke and the speakers were total rot too - oh, and it had been recovered, badly. When I started looking inside, it struck me that it was a minor miracle that the amp worked at all.

    The actual PTP board was ok, just dirty - I got to use pretty much 90% of the original components on the board. Since it was a total refurb, I decided to add an EF86 channel which absolutely screamed - I can see why those early AC30s are so sought after.

    The PT was a repro from Drake - got it direct from them, they said it was the last one they had as they were no longer making PTs for the AC30.

    The OT and Choke were OEI from Speedracer (anyone remember Joe ???) and were fantastic units.

    The cab was recovered by Matamp in the correct basketweave cloth and I got some diamond fretcloth from Vox (£80.00 for a piece of fretcloth was/is daylight robbery though).

    I found a pair of early 60s alnico blues in a second-hand bass guitar shop (a total dive actually), paid £90.00 (bargain and a half).

    With the whole thing assembled, recapped and fully checked over it was an absolutely cracking amp - beutifull tone that went from chimey cleans to really agro overdrive.

    Again, this one is long gone - was skint and it paid the rent

    * last one I still have is the copper panel, non top boost, transformers leaking wax and most likely in need of replacement. Got the original alnico blues, but they need reconed. cab needs recovered, but I've got some NOS fawn to use up on it - will be a smart-looking amp once its done.

    This will be a keeper since I'm pairing my collection down. It will be my main amp along with the JTM45/100 Metroamp kit I've still got to build up, my homebrew AC15 head (top boost preamp with many tweaks/mods running into a pair of cathode biased 6V6s, very sweet amp using a custom Shinrock OT). Oh, and I've got the rack gear too which is perfect for home use - the Cream Machine and the Marshall 9001 preamp.

    Sorry for the threah hijack
    HTH - Heavier Than Hell

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    • #17
      Re the measurements for the missing back panel, see
      http://www.geofex.com/Article_Folders/AC30/ac30dims.pdf
      I would measure mine but forgot tape measure when going to rehearsal rooms, then noticed RGKeen had already published a full set of dimensions - Peter.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #18
        well, i haven't read this article for a while. the email notification doesn't work. i did the mods voxrules! suggested, only to find them wrong now...

        the amp works better, definitely, even with those. the 51k cathode resistance probably works. the tubes are much cooler, they glow much less, and tubes in pairs glow evenly. there's no crossover distortion. i think ill try using 270 Ohms on each el84 cathode, as pdf64 suggested. what do you guys think about that?

        i also happen to have some money, so i'm thinking of completely redesigning the pcb for output. the chassis also has two extra spaces for tube sockets, so what do you think about making a pcb for a tube rectifier version? i guess i can always go back to solid state, by using an ss rectifier in a tube socket package (like weber copper cap WZ34). i also plan on changing all the electrolytic caps, and getting some nice new AlNiCo speakers, probably Celestion.

        the tremolo channel is still broken, it's even worse now. It has almost no gain, and there is absolutely no effect. what could be the problem? broken cap? and, yes, i know that the vib/trem switch is not original, it was replaced by the previous owner, since the old one broke. and the speed control isn't a pot, as suggested in the schematic i gave you, but a sp4t switch with resistors. anyone know what to do?

        Thanks! Andrej

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        • #19
          Andrej, what is your schematic? Have you measured your B+? If you have a sag resistor there's no point in a copper cap, just try different resistor values to get the result you want.
          Have you got a scope, assume you have as you think there's no crossover distortion (btw 51k means 51000, assume you meant 51R cathode resistor). If so, is the vibrato oscillator working? Peter.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #20
            Originally posted by andrej View Post
            well, i haven't read this article for a while. the email notification doesn't work. i did the mods voxrules! suggested, only to find them wrong now...

            the amp works better, definitely, even with those. the 51k cathode resistance probably works.
            Thanks! Andrej
            Andrej, I edited my "wrong post" almost immediately after reading Peter' s post, OTH my suggestions about the 51 Ohm cathode resistor and the formulas were correct and seem to have borne fruit; I have already apologized for the wrong voltage drop I stated, I'll try not to go wrong next time; It happened only because in that moment I was doing more than one thing at the same time. I'm sorry you didn't read the corrections in time and wasted some efforts and time. I just wanted to help and answered in a hurry. My mistake.
            Hoc unum scio: me nihil scire.

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            • #21
              Revisiting the project, since the amp blew a tube a month ago. It destroyed the (same) socket, above the pin connected to the output transformer. The amp makes a loud crackling noise when run with the other pair of tubes only, which wasn't the case earlier. I decided to do a quality job fixing it. I'm going to replace the whole PCB which has power section resistors, diode rectifier and stuff like that with a quality, ptp made board, because the copper on the old pcb started separating from the board. Sticking with the diode rectifier.

              Pictures and schematics:
              1) http://img230.imageshack.us/my.php?i...calo024jv6.jpg
              The power section PCB is the one below. That's the one I plan on replacing with a ptp board.
              2) http://img207.imageshack.us/my.php?i...calo022po4.jpg
              The power section PCB again
              3) http://img207.imageshack.us/my.php?i...calo007kp5.jpg
              Top panel
              4) http://www.freeinfosociety.com/elect...oxac301986.pdf
              Schematic to the amp.

              Here are the fixes I'm planing to do:

              1)Redo the PCB in ptp technique.
              I'm going to replace all of the old components on it. I know I can rely on shipping only when ordering from Banzai Musical Products (a great company from Germany, they are well supplied), and I don't know which turrets for the ptp board to order. Here is their turret list page http://www.banzaieffects.com/Terminals-c-670.html. Could you tell me which to order? I also don't know how to insert them into a board. How? What should the board be made of? What thickness? I'm getting new wire too. What gauge, material, should it be solid core, what kind of isolation? The wire is used connecting the power supply to the pcb and the tube sockets to the pcb. What wire should I use for interconnecting the turrets on the board? Should it be bare? Gauge?
              So, what are the final values to the resistors on this board? What wattage and composition?

              2) Replace the power tube sockets.

              3) Get some handles

              4) Make the back pannel. What is on the back panel? Power coord connector, footswitch jack? Could somebody take a picture of the back panel from the outside and the inside.

              5) Revise the speaker wiring. My speakers are just soldered to the wires. How should it be done? Any jacks? Anyone have a photo?

              6) Fix the tremolo/vibrato. Gonna get the original rotary switch. But, first of all, I need to understand how it works. Could anyone explain? How does the oscillator work and how does it control the effect? The channel is really low-gain too. Almost inaudiable even when set to the highest level, so I bet it's broken. Anyone know the solution?

              7) My amp doesn't have a standby switch. Which 3-pos switch do I need? How to wire it? I want to have an OFF-STDBY-ON switch, like on an Orange amp, because i don't have enough room for another switch (see the picture). Is this a big complication?

              8) Maybe replace some caps. I wonder if I should replace the sp4t switch (with four resistors) of the vib/trem speed with a pot, as suggested in the schematic.

              9) Replace the tubes. Which ones to get? I plan to use this amp for practices and gigs, no studio for now, and it's used a lot.

              10) Replace the big dual 32nF/32nF can caps.

              Anyone has any other suggestions?

              EDIT: added the pics again
              Last edited by andrej; 01-23-2009, 10:04 PM.

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              • #22
                theres a PTP AC30 layout on Hoffman.com - it's not vintage correct, but it will work. do a google search, I'm sure there will be some project pages where people have made PTP AC30s with vintage-correct layouts.

                I dare say that Voxrules will drop in on this thread - he's your go-to guy for Vox's.
                HTH - Heavier Than Hell

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