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Vox ac10 from old radio.

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  • Vox ac10 from old radio.

    Howdy Chaps, For starters it's been a very long time since I've been near forums like this. I think it was maybe 2006, so twelve years ago, I kinda gave up and moved onto other things.
    Umm, Ampage... That was where it started for me, 2000 it would have been, a 166MHz 'puter and a dial up...

    Anyway, here I am again and I've a ac10 schematic in front of me... I like it, reasonably simple (I was looking at the ac15 with tremolo too but, my God, I'd need to take out a mortage to get all those caps at higher voltages, then (it'd started with the ac30) maybe theres something even smaller and sure enough... the ac10.

    This radio I have, though the tubes might be shot, it's basically got the power and output transformers, two 12ax7's, two el84's in push/pull, and three radio tubes so I should be covered.
    Also I bought a box of about 8 x ef86's a while back at one of my favourite second hand dealers and before I gave up had a penchant for ecl82's (6bm8) as they were quite in abundance as single ended radio output tubes.

    Okay... questions.
    1. I don't want all the inputs (4) so was thinking of paralleling up the ef86 and the triode of the 6bm8 at the inputs just through mixing 68k's to each grid from a single input. Simple enough?
    Then I'd, I suppose have the two 500k volume pots into say 100k resistors then together into the phase inverter just on the top leg. Another option possibly is each channel with 100k on the ends and across opposite ends of a single 1M pot with the wiper going to the phase inverter.
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    Also I have a few old reverb tanks from organs (I think thats where they were from) and I think I'd quite like reverb too! Referring then to this deluxe reverb schematic, I quite like the switch to bypass the tank and use it as a boost, and given one of my tanks has an 8 ohm input then is it likely, given small radio output trannies stuck on speakers usually 6", that that size of trannie would suit the paralleled up 12at7 shown on this schematic? (I know, I know, do the calcs... but I'm lazy and it's just at the beginning stages.)
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    Gosh, It's getting complicated... but given I then need another full tube to recover from the reverb I may as well use an 12ax7 whereby the second triode could be a cathode follower and throw in a set of tone controls as with all this going on I may need them!

    Typical of me getting so complicated... gosh, no wonder at least half of the amps I'd built way back then didn't work! Well, more like a third... thing was I liked the construction of them, all the metal folding a cutting of holes, the wiring and the soldering, not to mention cabinet building... it was all so much fun that if it didn't work, I'd just start another one.

    Hopefully this time around I'll be less inclined to go gung ho and spend a bit more time bread boarding stages... actual testing of circuits before committing to metal work and making special knobs on the lathe!

    Thank you for taking the time to help me see right, Sean.

  • #2
    Done a little research and the reverb driver on the deluxe reverbs is 25k/8ohms so I'd be somewhat off using an 8k/10k small output unless it had a 4 ohm secondary otherwise, and I've lots of those little radio output trannies I could take off a casing and unwind, after seeing what the impedance ratios were, what is hopefully the high current winding on the outer then calculate what might be slightly less than half the turns and rewind it to that...

    Did the calcs and if I was to use a regular power transformer. we're on 230-240VAC here, It'd be a 4VAC out... bugger, a 6V out equates to a 12K input impedance.

    But then again I'm somewhat jumping the gun as it might pay to have a look at my reverb tanks and see what gives there.

    Hammond type 4 (bit rusty and supposedly quite nice as the slight rustiness and stretched springs of old units can't be replicated by accutronics... internet folklore?)
    input 283mH 173ohms
    output 409mH 176ohms
    Italian organ pan
    input 66mH 1ohm
    output 418mH 168ohms
    Last edited by sean k; 04-02-2018, 07:39 PM.


    • #3
      Funny how we end up answering our own questions. Remembered I got a short spring reverb from accutronics ages and ages ago and built me a reverb. I'd ordered high impedance inputs on it, 1475 ohms, so I could direct drive it from the tubes. I haven't taken it apart to see what and how I did it but I'm thinking now, as I didn't like the short reverberation times, I can actually use the drivers coils on this, which are three springs, and make me up a tank with long triple springs... sorted! These valves are 6sl7's and those are pretty close to 12at7's so I think everything is fine and dandy.
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      • #4
        Since itís EL84 pp... you might want to look at an 18 watt Marshall single channel variant like a Spitfire or a Ceriatone layout before you decide. Not sure if your PT and OT are up to it, but the transformers arenít THAT expensive, especially if you find some other recyclable ones on the cheap. Also... generally a spring reverb doesnít work well being integrated into a low powered amp like this. There isnít a suitable clean breakin point. What does work (well better anyway) is a $28 Behringer reverb pedal on the input.


        • #5
          Cheers for your reply olddawg, did I mention I'm in New Zealand? I guess not. Straight off I've realised I got confused between the ecf82 and the ecl82, and that's possibly why this amp took my fancy as I'm pretty sure I did a bit of collecting of the ecl82's (6bm8) before I boxed all this stuff up and got going on other stuff. So it's 6u8a, direct equivalent, or 6gw8 (close)... which I seem to remember I might even have (6gw8).
          Anyway, the New Zealand thing means that while it'd be nice to just buy stuff, and it does happen as my Mum travels once a year to Ontario in Canada, getting various bits through US customs, they thought a hammond output transformer might have been a bomb (!), in luggage is tricky and having stuff sent here, we're so off any beaten path, is also sometimes problematic.

          I'm in the north Island and basically rely on old fellows down in the south Island, which is even further off any beaten paths, for their stashes of NOS stuff which basically means I gotta build to suit what appears as viable. That said once I get going on this stuff I might be able to generate income and theres a crew in Australia, which I incidentally bought from way back at the beginning of the century who sent me photocopied price lists back then, but now have a modern site, which weirdly doesn't seem to include New Zealand, right next door, as a destination for articles... and they now deal with a whole bunch of Hammond stuff so maybe I'll send them a note reminding them to be nice to people just over the fence.

          And the idea of playing with weird tubes and old radios actually appeals to me. My first amp I designed kinda from scratch, working out stuff on the charts, was 12at7's with p/p 6ak6's and it ended up working quite well, except it was noisey, though I'm now putting that down to the power supply caps which I used to use which were NOS and since then our suppliers of new stuff have added a whole bunch of high voltage power supply type electrolytic's...

          As for reverb? I don't understand what you mean. Surely reverb is only about enough signal getting into the tank and that signal being brought back to where it was when it was taken? That said though, yes, this ac10 does have only one stage before hitting the phase splitter so maybe instead of the long tailed pair I could do the one with just one triode, where the signal is taken off the plate and the cathode and use the other as a regular voltage amp before a split off to generate more push?