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modifying ECL86 hifi tube amp for guitar use

Discussion in 'Amp Tech Center' started by telegumbo, Jul 14, 2013.

  1. telegumbo

    telegumbo TDPRI Member

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    Hello,
    i came across an old turntable with an integrated small tube amp.
    Ripped out the amp section, plugged in the guitar and played it through a 1x12 cab.
    It works and sounds nice. This will be the foundation for my no-budget-bedroom-amp project.

    But since its a hifi amp it wont break up at all. I'd like it to be a bit crunchy.
    Any suggestions where to start modifying?

    Thanks.
     

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  2. muchxs

    muchxs Doctor of Teleocity

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    There's only one gain stage in front of the power amp. It's the equivalent of one ECC83 stage in front of an EL84.

    Add an EF86 for an incredible little amp.

    An ECC83 in front of the ECL86 is one more gain stage than you need. You can make it into a trick little high gain amp.
     
  3. Harleyjon

    Harleyjon TDPRI Member

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    Or use an effects pedal and it will sound from Mesa Boogie to Marshall through Fender.
     
  4. jefrs

    jefrs Doctor of Teleocity

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    One of my amps uses a pair of ECL82. They are triode-pentode like the ECL86. These are not-quite ECC83 and not quite EL84. The ECL86 will run to 4W output and it is a hifi valve (the EL84 is not). So I suspect it is more like a 6V6 in that respect. I have a tape recorder with an ECL86 amp for when I get a tuit.

    If you want more crunch then it needs a typical pre-amp section, add an ECC83 at the vol with a vol+TMB in between. The EF86 is notably frail but works well in a low power amp (less rattle to break it up), it still needs to drive an ECC83 pre-amp though. The ELC82 amp does crunch nicely but has an MV where your vol is, and it needs the MV so you can push the gain up at the front end. It also makes a very good (as in sort-after) jazz amp.

    The front-end of your amp may be the RIAA vinyl record equalisation, you need to junk that part.
     
  5. printer2

    printer2 Poster Extraordinaire

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    No need for a EF86, standard 12AX7 will do. As jefrs said, junk the RIAA stuff. If it we me I would make a few more mods but then it would not be the same amp. Schematic I drew up has a parallel triode input, 100k plate resistor 1M grid, 1k cathode, reuse the cap from the original circuit (the arrow, and just wire directly from the pot to the grid). The 1k cathode resistor could be anywhere from 820 ohms to 3k. Might be a good idea to replace the two power supply caps, might be both in one can.

    [​IMG]
     
  6. muchxs

    muchxs Doctor of Teleocity

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    That's what Matchless said when they looked at a JMI AC15 circuit. :lol:
     
  7. BiggerJohn

    BiggerJohn Friend of Leo's

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    No need to whip out the chainsaw and blowtorch just yet.

    Yes, lose all that phono equalization stuff, go straight into the volume control.

    Add cathode bypass caps to both sections of the tube.

    see how that works.
     
  8. muchxs

    muchxs Doctor of Teleocity

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    You wouldn't say that if you heard an EF86 into an ECL86. :D

    It does all those "corny" '60s tones that were previously outta reach. Sounds like early Beatles recorded at A & M instead of Abbey Road. :D
     
  9. alnicopu

    alnicopu Friend of Leo's

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    Much is exactly right. I'm experiencing the woes of subbing in a 12ax7 where a lower gain tube was into a tiny phono chassis. They're not set up for that. Lead dress wasn't an issue back then because no one dimed a record player. If it doesn't squeal like a drunken prom date, it'll hum so bad that it'll be unplayable with both sides of a 12ax7 in there, bypassed or not. I ended up using a 12au7 and unbypassing a stage just to keep it from being an oscillator and to keep the hum at a livable level. I was really exited when I finally got it to be playable but it sounds like a guitar played through a record player. Your gonna have to make some comprises and it will never be a Matchless.

    You can't shove a Judge 400 from a GTO in a Volkswagen Beetle. I don't care how many shoe horns you use.
     
  10. printer2

    printer2 Poster Extraordinaire

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    You could always hit it with a gain pedal. Actually I was going to suggest a LND150 FET on the front end.
     
  11. BiggerJohn

    BiggerJohn Friend of Leo's

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    Man those big Ponchos don't breath all that well. If you are going to deal with the weight of a big block, may as well go with a 502 Rat motor. Or if you *really* want to go for it, the 572.
     
  12. telegumbo

    telegumbo TDPRI Member

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    I got one of those old 2xECL82 amps, too. With vol. at max = perfect distortion. I play it through two efficient 12" Electrovoice Speakers, then it's loud enough for playing with the band.
     
  13. telegumbo

    telegumbo TDPRI Member

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    Thanks for the responses, so far.

    I'll try the cathode bypass caps first. Where should I put them and what values do you suggest?


    If thats not cool enough, in a next step I will be adding a tube.
    So, you think a 12AX7 will deliver too much gain, an alternative would be an EF86.

    Since I've got some cool old ECL86 laying around, I could also use one of those.
    Can I use only the triode section and leave the pentode dead?
    Where do you think would lead this sound-wise - compared to 12AX7 or EF86?


    p.s.
    I'm not a pedal guy (i have 0 pedals) and this amp shall be plug+play for home use (vol+tone, that's it).
     
  14. Teletubbi

    Teletubbi Tele-Holic

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    Your analogy is apt and vivid but in actual reality quite untrue.
    --------------------------------------
    We have the tools, we have the budget and we have the will.
    We are car crafters and we can do this.
    The same might be said for some deranged tubehead.
     
  15. Teletubbi

    Teletubbi Tele-Holic

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    ------------------------------------
    1. breath well enough, plenty of 400+ cfm heads available.
    2. there is no such thing as a Pontiac big block.
    3. 572 BBC? decent but you better find a bigger bullet.
     
  16. printer2

    printer2 Poster Extraordinaire

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    The triode and the pentode both have resistors on the cathodes. You want to put the cap across both resistors. You want to use 12-22 uF capacitors, 25V or up.

    Now if you do not find it enough gain you can use a EF86 or a 12AX7. The 12AX7 run in parallel like I showed will have less gain than the EF86 (using the normal guitar values). It should have enough to get some hair on the signal, when people say it has more gain than the EF86 they are talking about running one triode into the other.

    Say a EF86 has a gain of 100 in a circuit one triode of the 12AX7 may have a gain of 50. But if you run the two triodes in series you have a gain of 50 x 50 = 250. Running the two 12AX7 triodes in parallel you might get a gain of 70. And with EF86's being more expensive and sometime microphonic, I suggested the 12AX7.

    I would not use the ECL86 as it needs 0.7A for the heater while the 12AX7 only needs 0.3A. The power transformer may not notice the addition of the 12AX7 but it probably will notice the ECL86 and your heater supply may be dragged down.
     
  17. telegumbo

    telegumbo TDPRI Member

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    Thanks.

    And thanks again for your schematic above.
     
  18. BiggerJohn

    BiggerJohn Friend of Leo's

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    I question your math.

    1. You say a 12AX7 gain stage may have a voltage gain of 50. Fine, close enough. That is about 34 dB. Now if you put two of those stages in direct cascade with no loss in between, the gain would be the sum of the individual stages, or 68 dB. 68 dB is numerically 2512. Not 250. I think you dropped a decimal somewhere.

    2. You say two 12AX7 sections hard wired in parallel might have a gain of 70. Please go through the math and show how you arrived at that number. Hard wiring 2 12AX7 sections in parallel would double the transconductance. However you would have to run different value plate and cathode resistors relative to single section operation since currents would double.
     
  19. jefrs

    jefrs Doctor of Teleocity

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    No, but you can slip a flat-6 Porsche in there. Oh yes, the old Spyder lump slipped into granny's Beetle, we done that. Same with a record player amp, they can make great little unusual guitar amps.

    If the output of the ECC83/12AX7 is too great (no, don't run them in parallel) then you put an attenuator between the stages.
    The first stage V1b is for input sensitivity, which is not quite the same as gain, into the TMB and vol. The second stage V1a feeds the MV and the driver triode with the signal voltage it wants.
    You have to bias the triodes appropriately and drop the output signal voltage back down to a sensible level each time. You even can set, or control one or more triodes to soft-clip, one hot one cold maybe.

    The EF86 is very sensitive and was used for vinyl record pickups, but it is easily damaged by heavy vibrations as found in a loud guitar amp and failed there with monotonous regularity. But a record player ECL amp is not loud enough to shake it to bits. Why be normal?

    But I would get an ECC83 front-end working first. Do check out a circuit diagram for the 2xECL82 "Watkins Westminster", these are/were great little 'club' amps, a scaled down version of the 2xEL84 17W WEM Dominator (18W Marshall). The later ones were single-channel with vol, TMB and MV. And all the component values are right for feeding an ECL8x bottle.
    The diagram of the "WEM Dominator 25 Mk.II Bass" (which is a Westminster in reality but is perhaps unique in doing everything we want here up to the MV) is very hard to find. I previous posted up a PDF of mine (slightly modified for guitar) here -
    http://www.tdpri.com/forum/shock-brothers-diy-amps/283190-beginner-marshall-18w-2.html#post3464601
    This one of mine, with corect lead dress etc is almost as hum-free and quiet as a modern hifi amp. WEM built some good little amps. The 17W Dominator itself is what Marshall copied for the 18W, but WEM actually did it better in terms of build, you can still find them in good working order.
     
  20. alnicopu

    alnicopu Friend of Leo's

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    Mines in a 4x8 chassis. Both trannies, all 3 tubes, the input and ext. speaker out are within an inch of one another. If He had room for a TMB and MV it wouldn't be from an old record player. I had 3 lying around and this one was the best candidate. Like muchxs suggested to me. Best to gut it and put it in a 5f1 chassis to eliminate the very issues I mentioned.
     
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