AC30 - Please help me get to my power amp tubes.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hamfist, Jan 16, 2008.

  1. hamfist

    hamfist Member

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    I opened up my new Heritage AC30HH head this morning, to have a look inside, and try a few new tubes.
    Taking the back off was easy, exposing the pre-amp tubes.
    There were also 2 bolts on each side which I removed. expecting to be able to slide the chassis out. But there seemed to be no budging it.
    I didn't want to use much force in case I damaged anything.
    Here's a picture of the rear of the amp without the back panel.
    [​IMG]

    To access the power tubes, I need to get the chassis out, as they are on the "other side" of it, so can anyone help me get to them ?
    The chassis seems to be sitting on a black painted piece of plywood. Is the chassis attached to that, and will it all slide out together ?
    I've only been inside Marshall-style heads before, and this is very different.

    If anybody is interested I did a few gutshots through the rear of the amp, to be found at :-
    http://www.box.net/shared/static/87n4uebk00.jpg
    http://www.box.net/shared/static/0havpglc0k.jpg
    http://www.box.net/shared/static/roqvvw5oo8.jpg
    http://www.box.net/shared/static/yo9xkiw00o.jpg
     
  2. ECase

    ECase Member

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    There are two screws UNDERNEATH the chassis (going UP through the plywood that it sits on). Unscrew those, and you should be in business.
     
  3. hamfist

    hamfist Member

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    there are no screw or bolt heads on the bottom of the head shell to undo. How would I access these screws ?
     
  4. Mastervolume

    Mastervolume Member

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    Nice to see that they are using the wrong (for vintage anyway) values for the power supply filtering and using the same board as the ac15.

    removing the 4 screws on the sides of the cabinet should allow you to pull out the tray.

    you might have the give the cab a little bump to break any glue that may have still been drying when the head was assembled.
     
  5. Backstage Kent

    Backstage Kent Member

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    Feel around on the bottom side outer edge of the plywood shelf the chassis is sitting on--on the combo models there are 2 bolts per side running through the shelf into the chassis. Haven't seen the head version yet, so I don't know for a fact that they're there. If they are, I'd say it would take a right angle ratcheting screwdriver to get them out.(I've got one that's got a hex bit holder that I picked up cheap somewhere--also very handy for removing and reinstalling reverb tank securing screws.) Just a continuation of the proud Vox tradition of cabinet assembly using extremely-difficult-to-access screws.
     
  6. hamfist

    hamfist Member

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    thanks guys. i'll have another look tommorow - when I'm sober again !!
     
  7. John Phillips

    John Phillips Member

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    If so this is completely ridiculous. Power tubes are a routine service item - you shouldn't have to strip down the entire amp using specialist tools to get at them.

    And I thought this model was supposed to be hand wired?

    Obviously the individual resistors and caps are soldered into turrets (presumably by hand) but it looks remarkably like a PCB that the turrets are mounted on :). It even says so at the left-hand end... PCB_VX15HW, which is an interesting way of mixing abbreviations. If one of the arguments against PCBs is the large plane conductors leading to additional capacitance, that board is a perfect example.

    I don't have anything at all against PCB BTW - in fact I think the whole argument is misguided, and both my amps have them - but this amp is certainly not built the same way as it's marketed. I also have no idea how it sounds - it might be great - but I'm really not impressed with the design and build at this point.
     
  8. hamfist

    hamfist Member

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    I thought Vox were quite well known for making tube and speaker access quite awkward in some of their amps.
    As for the hand-wired/PCB construction issue, there was a huge discussion over at the Plexi-Palace about it - http://www.vintageamps.com/plexiboard/viewtopic.php?t=64810&highlight=ac30+heritage .
    Apparently current Matamps are constructed using exactly the same technique but (and it's a very big BUT), they use brown PCB board, not green. We all identify PCB's as being green, so Matamp made the smart move. I've not heard a single word criticising Matamp for including PCB's on their "hand-wired" amps.

    BTW, this Heritage AC30 is the best amp I've ever played, period !
     
  9. John Phillips

    John Phillips Member

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    Yes, recently - old ones are much easier, you just undo the four bolts at the ends of the back panel and the whole thing slides out.

    It doesn't excuse it though - in fact, they should have learned from the criticism of the earlier reissue amps and the CCs and done something about it.

    I never go there, so that's probably why I missed it ;).

    Like I said I have nothing whatever against PCB - at all, in fact in many ways I prefer it - but I do have something against misleading marketing, and this is a good example. If people don't realise that Matamps are built the same way because the boards are brown, that perfectly illustrates the stupidity of the anti-PCB snobbery though! :)

    I believe you! I haven't played one yet. It will probably disappoint me because I love the original AC30/6TB with the Vib/Trem channel and I don't think any of the variations that stray far from there have 'the tone' (to me), but I'll reserve judgement.

    FWIW, if it does require you to undo screws in the underside of the shelf, I would consider carefully drilling holes in the bottom of the head box directly in line with them, so you can at least get a standard screwdriver in quickly.

    I still wouldn't own one and rely on gigging with it though...
     
  10. hamfist

    hamfist Member

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    OK, I've been a complete doofus (again !!).
    There are NO other screws or bolts coming up from underneath holding the chassis in place.
    What I had not been doing was actually unscrewing the 2 bolts on each side enough. Yesterday, I didn't actually remove them. I just undid them enough that I assumed they were not holding the chassis in place any more - WRONG !!
    Today, after removing the rear panel, I took all four side bolts completely out and the chassis slid out nice and easy. Phew !!
    What I discovered was some pretty sizeable transformers (for a 30W amp), a Sovtek rectifier and Electroharmonix EL84EH's.
    I think I 'll stick with the EL84EH's as I like the tone, and they are reasonably priced. I''l just have to see how long they last before I decide fully though.
     
  11. John Phillips

    John Phillips Member

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    Nice to find I was wrong :).


    I still really can't believe that some people think that the definition of whether a board is a PCB is if it's green, BTW ;).
     
  12. hamfist

    hamfist Member

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    Yup, it's the old "hear with your eyes " thing again ! :(
     
  13. Bonenut

    Bonenut Member

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    That board looks like turrets on this side, and traces on the far side (ie: PCB). I guess the handwired part is the pots and switches...:BEER
     
  14. hamfist

    hamfist Member

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    The turret board is basically industrial grade PCB. Instead of the turrets being connected with wire underneath the board for circuit connections, as in a standard turretboard, the turrets are now connected by wide PCB traces. All components are still handwired onto the turrets. And obviously all the pots, switchs and tube bases etc are also handwired into the circuit as well.
    For component replacement and troubleshooting, it would seem to be a pretty good design. Where it will become less handy is if people want to start tweaking the circuit, which would be easier with a fully handwired turret board.
    As to whether the PCB turret board will be more or less reliable than a traditional board, will be down to how the PCB has been constructed and implemented within the amp. If there are stresses and strains on the board, my understanding is that unreliability will result. But if the PCB board has been implemented well, and the amp designed to reduce any stress on the board, then my understanding is that it should be extremely reliable.
    I've never built or even tweaked an amp in my life, but I'm pretty sure that's a fair description of the construction method. Someone please correct me if I'm wrong.
     
  15. John Phillips

    John Phillips Member

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    That's correct, although it doesn't look any more 'industrial grade' than any other cheap single-sided PCB.

    The only real problem I can see for maintenance is that if you aren't careful with the amount of heat you put into the turrets when soldering, you could damage the joint on the back side of the board, which will then need fully removing to get at and fix. This is no worse than a conventional single-sided board - where this can happen if people snip the old component leads above the board and solder the new one to them, which is why it's usually bad practice - but definitely worse than one with through-plated holes... the type used by Mesa, Soldano, Rivera, THD etc. where it's usually possible to get at the joint directly and avoid the need to work on the back of the board at all.

    That's the basic truth about any PCB design, which is why the argument about "PTP" vs PCB is ill-informed. A well-designed amp using a properly-mounted and stress-isolated high quality PCB is at least as reliable as a "PTP" (any form of non-printed board) one - in fact ultra-reliability-critical, cost-no-object systems (eg avionics and medical) are built that way because it is the best method.

    I would think the turret-PCB board will be slightly less so because there is more mass to vibrate the board around, and the component leads are slightly longer without the component being pulled down against the board, which could also cause vibration fatigue.

    I strongly suspect this method has been used for marketing purposes rather than any real technical advantage. That doesn't make the amp bad - or sound bad - but I don't think you can regard it as a significant factor in the quality of either.
     
  16. R.G.

    R.G. Member

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