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modifying ECL86 hifi tube amp for guitar use

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

  1. printer2

    printer2 Poster Extraordinaire

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    Yes you are right on the typo, meant 2500, Too lazy to put on my glasses when I checked for mistakes. The computer did not highlight it so I skipped over it. The parallel triodes, I did not bother to do the math, just went with roughly what I recall seeing when I tried it. Did not try to optimize anything, I really did not care if I was of on the gain, more to let the OP know that the gain of the two in parallel would not be in the range of the two triodes in series.
     
  2. printer2

    printer2 Poster Extraordinaire

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    And why not run them in parallel? It is a simple amp, he is not going to put a TMB tone stack in.

    [​IMG]
     
  3. Mat

    Mat Tele-Meister

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    The ECL86 (6GW8) was a popular valve in Australia in the 60's.

    Goldentone made a series of small combos utilising one 6GW8. These are sweet sounding, not particularly high gain but a good round tone.

    Maybe try one of these for a different flavour from the 12AX7 - 6V6/EL84 combos.
     

    Attached Files:

  4. BiggerJohn

    BiggerJohn Friend of Leo's

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    I wonder if anyone has tried something like this but with 7591 outputs? Should be a little more powerful than with EL84s, assuming the OT and voltages were optimized.
     
  5. jefrs

    jefrs Doctor of Teleocity

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    Have you run 12AX7 in parallel? - you do get more oomph but the sound is more mushy, not as crisp. The 18W Lite runs an input into each half of the bottle, not parallel, but common cathode can do this mushiness too. The ECC83 has plenty of gain to do the job in one half.

    I believe Telegumbo is planning a tone control -
    The ECL86 is a very clean valve, it will really need an MV to get crunch from the pre- and an MV allows you to have clean or crunch on demand by adjusting vol and MV. To get crunch out of my 2xECL82 with the MV wide open, it has to be cranked up very loud indeed. The mid control on the Westminster-type circuit I posted can be replaced with a fixed resistor, the even earlier Westminsters only had a single tone control. Without the MV you will have a pretty good jazz amp.

    Did you spot the snubbing capacitor and resistor on the output of the ECL amp? The EL84 amps don't have that but the ECL seems to fare better with it.
    Ime the ECL may red plate quite easily and of course an SE amp has them permanently on, there's a lot going on inside what is a rather small bottle: they do not like being overloaded, made to clip.
     
  6. BiggerJohn

    BiggerJohn Friend of Leo's

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    Snubbing cap and resistor ? You mean the conjugate filter across the OT primary?
     
  7. jefrs

    jefrs Doctor of Teleocity

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    Never heard it called that.

    You can conjugate a verb? - I snub, you snub, he snubs ...

    Only seen it on a 2xECL82 between the anodes (which is the OT primary), not EL84 pair on similar amp. We call that a snubbing capacitor, presumably the resistor is there to limit current, "plate snubbing capacitor", common on old wireless radio receivers.
    http://www.radiomuseum.org/forum/returning_the_plate_snubbing_capacitor_to_the_cathode.html
    "The purpose of this capacitor is to absorb the energy that is stored in the core of the output transformer, in the event that the plate current driving this transformer is suddenly cut-off. The purpose of the capacitor is to "snub" excessive voltages."
    Basically stops RF splitting out and perhaps stops fireworks inside the ECL, there's a lot of stuff going on inside those little bottles.

    When the capacitor on mine blew through it toasted the resistor and red-plated a nearly new set of Mullard ECL82.
     
  8. alnicopu

    alnicopu Friend of Leo's

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    I wonder if he meant grid stopper.
     
  9. printer2

    printer2 Poster Extraordinaire

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    No, conjugate filter, also known as a Zobel filter.

    Speakers have a rising impedance, an 8 ohm speaker is only around 8 ohms in the midrange and the impedance rises in the treble and 40 ohms would not be unusual at 5kHz. This gives less loading to the amp at high frequencies and with the nature of a tube amp the voltage out to the speaker rises also. This has the effect of delivering more power to the speaker at high frequencies as compared to amps that have a low output impedance.

    In some cases the rise sounds OK but then there are others where the amp sounds 'fizzy' with the speaker. Depends on the frequency response of the speaker, the impedance rise of the speaker (some do more than others) the amount the amp is distorted (bias shift in the output stage causing the amp to have crossover distortion), the amount of negative feedback (NFB), probably forgot a few, just off the top of my head.

    So what does the conjugate filter do? It is a variable load, basically a shorting filter. You select the components so that they start loading down the tubes around the same frequency that the speaker starts to unload the tubes. So as the speaker impedance goes up, the filter's impedance goes down.

    Mainly have seen them on EL84 amps, especially ones without NFB. The Pro Jr. has one, Dr. Z amps use them. RCA tube manual calls it a corrective filter.


    See 'Experiments with Conjunctive Filtering' about one third down the page, some good sound files showing the effect.

    http://www.cs.cmu.edu/~ehn/ax84/
     
  10. 6942

    6942 Poster Extraordinaire

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    An ECL82 tube is generally much cheaper, than an ECL86 tube.
    Cost-wise, it would be cheaper in the long run, to mod the amp to accept them.

    Steve
     
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