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All pentode EL34 SE build.

Discussion in 'Shock Brother's DIY Amps' started by Bendyha, Apr 19, 2019.

  1. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Here is a first draft of the intended schematic, let me know if you have any ideas, criticisms, suggestions or tips on modifications before I start working on a layout.

    upload_2019-4-19_19-57-17.png

    Some things, like the actual EL34 cathode resistor value, will have to be determined when the thing is up and running, and measurements can be made.
     
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  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What do you use to draw those schematics?
     
  3. MadJack

    MadJack Friend of Leo's Silver Supporter

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    I don't know enough about circuit design, but it looks interesting.

    I'm watching this one...
     
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  4. FenderLover

    FenderLover Poster Extraordinaire

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    With an EL34, I'd want to keep the voltage up and forego the tube rectifier if you're using 300-0-300 PT. My first approximation would be to use a solid state rectifier, 5K OT and 250 ohm cathode resistor. The PT should put out about 150mA. Looking forward to it!
     
  5. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Yep... my first draft for the amp was using SS rectifiers, but then I thought of using the EZ12...because it was there in the spender amp that is suppling the transformers for the build.
    For the few watts of extra gain, there is not so much in it. As to the difference in sound between the two in a SE amp, maybe also all not that much of a difference...sort of just a mojo thing I guess. So I will hold both options open for now...or what do you think the pros and cons might be? Can't say I've done an AB comparison on such an amp...you maybe?
     
  6. FenderLover

    FenderLover Poster Extraordinaire

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    The difference in output power is nothing that will be too notable. Getting the most power out of a big bottle in class A is fun. I'm working on an SE using a KT88 and have the current to bring it up to to full dissipation. And that's the balancing act - giving up power (or taking advantage of all you have) depends on the iron you use. In the end, the best plan is try and use what you have because they are the expensive bits. A few watts either way won't be noticed at all.
     
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  7. Nickfl

    Nickfl Friend of Leo's

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    If you have room and are willing to add another control, you might consider the valve wizard pentode/triode morph control on that second EF804 stage. I used it in a recent build with a 6SJ7 pentode in the driver position in front of a single ended 6v6 power section and it provided a nice way to adjust the dynamics of the amp and especially to adjust how hard the power tube was being driven.
     
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  8. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Gimp 2.
    I tried a few dedicated schematic drawing programs, but only feel comfortable drawing my own diagramatics. Copying and addapting from all over the place, I now have a smallish self-made library of components that I can move around to find the balance I like. It's maybe not the quickest way, but it gives me time to ponder.
     
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  9. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    It is rated for 200mA, so no problems there.

    I'd be interested to know what your thoughts are conscerning NFB in a guitar SE amp?
    And which NFB method to use is a factor that intrigues me. The options seem to be either taking the signal off the OT secondary, and feeding it either more globally - to the cathode of the tube preceeding the output tube (this would in my case place the treble bleed in the loop).
    Then there is taking the signal off the OT secondary, and feeding it back more locally, to the the top of a tail resistor on the cathode of the output tube.
    Then there is the very local version of feeding back a portion of the AC from the output tubes plate, back to its input grid, leaving the OT and speaker to do what they want in adding to the sound, purely cancelling portions of the output tubes induced distortion....
    I guess each has its own pros and cons.
    As I intend to use this amp mostly as a slide guitar, swampy thang, I thought I'd first try no NFB.


    There is plenty of room in the chassis for adding it later perhaps, and I'm sure it could be usefull....but first off, I want to try the simple three knob option.
     
  10. fastedtex

    fastedtex TDPRI Member

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    Are you using Adobe Illustrator or something else?
    I have re-draw some of the Fender BF schematics in AI for my own use and was interested in sharing libraries.
     
  11. Zipslack

    Zipslack Tele-Meister

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    The Gimp is a raster image editor that is an open-source alternative to Photoshop. I would think it would be easier to do vector graphics using AI or the open-source Inkscape alternative.

    His libraries/brushes probably would not be the best option to import into AI.
     
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  12. fastedtex

    fastedtex TDPRI Member

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    I was on @robrob site and he was printing PDFs of his DIYLC files for framing and display. Cool idea!:cool:
    However he does not redo the schematics and the Fender schematics are kind of crappy for framing.
    Seem to be easier to put original on a layer and trace it in AI in place of DIYLC.
    I have re-drawn a few in AI so they can be scaled and printed any size.
     

    Attached Files:

  13. FenderLover

    FenderLover Poster Extraordinaire

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    I always start without NFB and often leave it that way. It would probably suit slide guitar perfectly. I'm a fan of Matchless, Dr Z, Vox, the 5E3/5E4/5E5, all good examples with no NFB. Works for me, personal preference.
     
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  14. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Easter gave me a bit of free time to get on with this build, so here's a bit of an update.

    But first a bit of back-ground story.

    upload_2019-4-25_20-36-21.jpeg
    I picked-up this P.A. Amp recently, somewhat speculatively, not knowing what it was, or even if it was in fact an audio amplifier, but was pleasantly surprised, not only by the price of $23 ( – including the postage ! ), but also by what it turned out to be; a single ended EL34 amplifier with two small signal pentodes as the preamp. Probably made in the early 1960's. It's certainly a home-made amp, but whoever made it seems to have known their stuff.

    Firstly, I drew up a schematic of it's current lay-out, just to workout what is going on. Although I certainly need to alter several things to make it into a guitar amp, I decided to base the new schematic along similar lines to the original.

    The original amp had an output-level meter, being driven from the OT secondary, via a small tube rectifier. I was tempted to retain it, but for the sake of simplicity, I have chosen to drop it.

    The two pre-amp tubes are EF804's, essentially the same as EF86's, but with a somewhat different pin-out. I haven't tested them yet, but as they are Telefunken, they should be good for a couple of thousand hours of use. If they do prove to be duds, I have a few spares, and also some Telefunken EF86's that I can easily change by swapping over a few socket wires.

    upload_2019-4-25_20-37-43.jpeg

    The chassis is a very stable steel construction, that has the added advantage of being able to be turned and laid on any of the six faces ...very handy whilst building or repairing …..but it's very heavy, and has no carrying handle. Okay if you don't intend to move it around, but a drag as a portable guitar amp.

    Then I thought of the cab from my Super Champ that is lying around unused, and the idea came that it would be ideal to use as the basis for this new amp.

    The only real hassle is the open-build form of the transformers with its exposed high voltage terminals. Forget an open backed cab, I'll have to make some form of protective cage around them, or maybe just screen off the whole back.

    upload_2019-4-25_20-38-58.jpeg
     
  15. Wally

    Wally Telefied Ad Free Member

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    Subscribed. Bendyha, I have two amps right now that impress not only me but everyone who hears them. Both of them use the 7199, which has a pentode and a triode in the bottle. Although they use them in different manners, both amps have a liveliness that is desirable to my ears.
    One of them is a little California P.A./record player. I removed the r3cod player and applied the one control to the mic input. It runs a 12AX7 for the input stage with the triode of the 7199 being the stage that feeds the pentode that drives the single EL-84. It is a magnificent little amp....it makes a Champ wish it could be half of what it is.
    The other is an Alamo Fury from 1966. In that circuit, the pentode of the 7199 drives the triode, which is used for a cathodyne PI to drive 2x7868 power tubes. Again......a very impressive amp that does th8ngs differently from any other amp I have heard. There is no known schematic for this amp. The bass tone control was so interesting that I had to draw the circuit how just to see it....very different tone control.
    At an rate, I am liking some pentode action in the preamps.
    And....I am looking forward to watching your project here.
     
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  16. tubeswell

    tubeswell Friend of Leo's

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    Wow - cool old amp! Those pots!
     
  17. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Yep, certainly reacts differently from the standard triode set-up we all know so well. The last all pentode amp I built is an amazing match for my slide guitar, and a pleasure to play, it is just way to big with it's matching Theil EVM15 cab. Thats why I am building this one, as a small combo....but more to that later.
     
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  18. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    After stripping down the old amp, and preparing a new chassis for the Super Champ cab,

    upload_2019-4-30_21-0-20.png

    I put a EVI 10” speaker in the cab and then started to see where the transformers would be best positioned.

    upload_2019-4-30_21-2-58.png

    As you can see.........................It ain't gonna work!..........and there is bugger-all room for the rectifier tube and the big EL34.............unless I shift the mains transformer down to the bottom of the cab..........so I have to make a secondary chassis, and in that case I will skip the rectifier tube as well, and go for SS rectification like FenderLover suggested.

    upload_2019-4-30_21-4-13.png

    So as not to have to build a cage around the PT, I have chosen to lay it down and have all the connections on the underside. Rather than running the mains switch up to the top chassis for the power switch, and then down again to the PT before running it back up again, I will keep the switch, fuse, humdinger-pot, rectifiers, and first filter-cap in the lower chassis. The pilot lamp will go up top, and I will put that on the filament supply.

    upload_2019-4-30_21-5-49.png upload_2019-4-30_21-7-35.png


    The silicon umbilical is topped off with an octal plug, and contains a twisted-pair of 6.3V, a partially smoothed high-tension supply, and a 0V rail/grounding wire. Now I have plenty of space in the top chassis to achieve a relaxed and open layout.

    upload_2019-4-30_21-8-38.png


    upload_2019-4-30_21-9-31.png

    A bell cover for the PT would have been nice, but I didn't have one to hand.....no matter, the cabs lower back strip should offer enough protection.
     
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  19. tubeswell

    tubeswell Friend of Leo's

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    Awesome sheetmetal skills there Bendyha
     
  20. Nickfl

    Nickfl Friend of Leo's

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    Very interesting, you wouldn't happen to have a schematic for that first one would you?
     
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