My New 5X3

Discussion in 'Shock Brother's DIY Amps' started by Keala, Aug 21, 2014.

  1. Keala

    Keala Tele-Meister

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    The amp sounds amazing with the 6V6's, you're exactly right. I actually like the sound of the 6V6's and WZ34 together too.

    The 6L6's are JJ 6L6GC. And yes, there isn't much grid drive. And they sound a bit shrill.

    Just for reference, I'm using the classic tone 40-18002 20W output tranny. And I just realized the primaries are 6.6k ohm vs. the normal 8k ohm primaries on most 5E3's transformers.
     
  2. tubeswell

    tubeswell Friend of Leo's

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    There is a limited amount you can do with a 5E3 circuit in terms of adapting it for 6L6s, because there are only two inverting stages of pre-amp gain in the circuit (the cathodyne has unity gain), and hence limited overall gain. So you will never get the grid clipping that you get with 6V6s unless you use a gain boost device (like a pedal) in front of the amp. Otherwise 6L6s in this circuit will at best sound 'jazzy' as opposed to 'rocky'

    Even so there's a bit of room for improvement in terms of the overall tone. In this regard the 6L6s really need to run hotter, like about 2 x as hot, which means paralleling another 250R with the cathode resistor. This will drag the plate voltage down, but will also decrease the bias voltage, and give the pre-amp more of a show at driving the 6L6s.

    The optimal load resistance for centre-biased Class A for 6L6GCs at 320VAC is about 3.4k, so you could halve the speaker load on your 6k6 OT by swapping the existing speaker to the next secondary tap down (if the OT has multiple secondary taps), or paralleling another speaker of the same impedance as the existing speaker, with the existing speaker (like if you have an extension cab of the same impedance).
     
  3. FenderLover

    FenderLover Friend of Leo's

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    While there is limited drive with two gain stages and 6L6's, 'jazzy' was what I was after at the time and it is anything but shrill, so that part is a little curious.

    Adding input boost can provide more drive, but tends to overload the preamp as well, giving you what many amps already do (amplify a distorted preamp/phase inverter signal).

    My current project using the 5E3 chassis (5C3, actually) is to use diode rectifiers and replace the ill-gotten tube slot with a post-inverter driver stage. That allows all of the above, good preamp headroom without crashing the phase inverter, ample output drive, and another gain stage to get 'mad' when conditions are favorable. Review the Dumble Odyssey power amp circuit and you'll get an idea where I'm going with it. It was also used by McIntosh, Genelex, and Weber in his JP25 amp.
     
  4. Keala

    Keala Tele-Meister

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    Do you have a diagram or schematic of this? I'd definitely like to try that out.
     
  5. tubeswell

    tubeswell Friend of Leo's

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    Local manufacturer, Beverley Bruce and Goldie, did this with their 'Jansen' brand 'copies' of Fender amps in the 1960s. In the heyday of good old trade protectionism, when you couldn't buy Fender tube amps in NZ, local people started making their own versions.

    'Blackface' (hehe)
    [​IMG]

    'Silverface' (hehe)
    [​IMG]

    Note that some of the stuff he says about Fender's amps typology in that blog I linked to above, is just plain wrong, but that main message is that Jansen 'Bassman' amps used a 12AU7 post PI gain stage on each side of the inverter (which often was also a 12AU7) to increase the drive to the output tubes.

    [​IMG]

    They also used a separate PT winding for the screen supply (as he says in that blog article)

    [​IMG]
     
  6. FenderLover

    FenderLover Friend of Leo's

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    Thanks for the post, Tubeswell. I think it's a very cool circuit and I can always use more evidence that I never invented anything.
    My 5E3/5C3 conversion makes more deliberate use of all the holes and silkscreening, so there is a 6SC7 handling all the inputs, two per triode, then the standard Vol-Vol-tone, on to 6SN7 LTPI and 6SN7 drivers. I'm considering fixed bias at this time for the 6L6's.
    Keala, yes, I do have a schematic and 5E3 chassis/turret board layout that is nearly done. It all fits, topology looks functional, and I'm just fleshing out the last component values. I wouldn't consider posting it until I hear something out the other end first though.
    I am also just finishing a 4-gain stage version more similar to Tubeswell's last post for a paying customer that is first in line. It also uses the LTPI, the EQ was lifted from Dumble, and there are added Hi Cut and Master Volume controls. That one is built on a repurposed Champ chassis.
     
  7. FenderLover

    FenderLover Friend of Leo's

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    To be fair to your observations, I'd have to admit that my 5X3 project was actually closer to a 5C4 Super because I used a 5D3 turret board which has the paraphase inverter. Warm and woody tones. Wide panel tweeds aren't as popular as the narrow panel designs, but I think the paraphase inverter is the essence of the wide panel sound. Though I own an original '55 5E3, I prefer the 5D3 hands down.
     
  8. tubeswell

    tubeswell Friend of Leo's

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    What Fender Lover said. I too dig the wide panel tweed amp sound with the paraphrase inverter.
     
  9. Keala

    Keala Tele-Meister

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    Can't go wrong with some lacquered tweed from John Mergili. Here she is all buttoned up, ready to rock-n-roll!
     

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  10. fretman_2

    fretman_2 Friend of Leo's

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    Very nice!!

     
  11. Amazed

    Amazed Tele-Meister

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    Looks great.

    Did you ever buy a black chassis from Mable by the way?
     
  12. tubeswell

    tubeswell Friend of Leo's

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    Sweet lookin' little amp!
     
  13. Keala

    Keala Tele-Meister

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    Thanks guys!

    Hey I have question as I'm still working the kinks out on this guy. But you'll notice in this video a slight cruch to fizz at the end of the overdrive trail.

    What could be the cause of that? I've read different articles regarding bad solder joints and such, but I can assure you, my joints are pristine. Any ideas guys?

     
  14. Keala

    Keala Tele-Meister

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    So with the 6L6's and GZ34 installed. I removed the 300R cathode resistor and installed a 125R. I kinda like the sound, but here are some interesting voltages that have me pretty confused :(

    My plate voltage is at 311v now on both 6L6's , and my voltage drop across the 125R is 11v.

    Does that sound correct? When you decrease the "Cathode Resistor", it lowers your plate voltages? It would be nice to get this amp running between 25-30 watts...if that's currently possible. :) Or my other thought is trying to find some NOS 6L6's that like to run at 310-350v plate dissipation. Like some Sylvania 6L6WGB's.

    Tubeswell also mentioned trying two parallel 250R's so that the preamp can hopefully drive the 6L6's a bit more. Is that the same as running a 500R resistor?
     
  15. screamin eagle

    screamin eagle Poster Extraordinaire

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    Don't forget that you can halve 1 or both of the dropping resistors to get more ma to the 6l6. This can limit your ability to use 6v6s but if it's on a switch them no big deal. I halved both on mine. I think I'm around 58ma on the tubes, 400ish b+ and about 380v on the plates. Obviously 400v and 58 ma with kill just about all 6v6s, but it sure makes a 5e3 take advantage of the 6l6s. However I'm also running 25w xformers instead on the usual 15.

    It clock's out at about 22 watts. And doesn't sound cold.
     
    Last edited: Oct 3, 2014
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Tying two 250Rs together in parallel will give you what you are running now, 125R.

    With 311v of B+ and 11v of voltage drop across your 125R cathode resistor shared between 2 6L6GC's you are only running them at 43% when your target for cathode bias should be around 97% (calculated using this web page bias calculator)

    Your 30w plate dissipation 6L6GC's are running very cold but they were even colder with the 300R cathode resistor. You might be better off running normal 6L6's with 19w of max plate dissipation.
     
  17. Keala

    Keala Tele-Meister

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    Yup, that's exactly what I was thinking. I figured the amp would sound a lot better (tonal wise) with standard 6L6's or 6L6wgb's.
     
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