micro amp power tubes - ef80 vs 12au7 - compare?

Discussion in 'Shock Brother's DIY Amps' started by jtcnj, Aug 3, 2021.

  1. jtcnj

    jtcnj Tele-Holic

    Age:
    57
    Posts:
    621
    Joined:
    Feb 2, 2015
    Location:
    N.J. USA
    THoughts on how a pair of ef80 sound vs a 12au7 in low watt push pull amps, such as Robrob's micro series?
    Youtube videos really dont translate well, in my experience.

    I built the RR JCM800 Mirco 1w 12au7 version.
    It sounds pretty good; I'm not disappointed nor WOW'd.
    I omitted the Lo input; should have included it I think.

    I play mainly 70's classic rock into early 80's metal.
    Think Bon Scott era through Back in Black AC/DC to early Iron Maiden/ through Powerslave.

    Just wondering.

    20210718_172322-2.jpg
     
    eslover likes this.
  2. NSB_Chris

    NSB_Chris Tele-Holic

    Age:
    56
    Posts:
    565
    Joined:
    Sep 2, 2017
    Location:
    New Smyrna Beach Florida
    What speaker are you playing it through?

    I agree! I would build the ef80 version also and compare.
     
  3. eslover

    eslover Tele-Meister

    Posts:
    207
    Joined:
    Nov 17, 2009
    Location:
    Ellicott City, MD
    I hope to post a clip of my jcm 800 micro EF80 soon. It’s a great sounding amp.

    hopefully someone can compare the sounds of my amp to the 12au7 version.
     
    jtcnj likes this.
  4. jmp81sc

    jmp81sc Tele-Meister

    Posts:
    449
    Joined:
    Feb 6, 2015
    Location:
    Orange County
    Bump as I am also interested in the difference in a Micro Bassman design.
     
  5. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,534
    Joined:
    May 24, 2010
    Location:
    Canada
    What voltage are you running, what is the OT impedance?
     
  6. eslover

    eslover Tele-Meister

    Posts:
    207
    Joined:
    Nov 17, 2009
    Location:
    Ellicott City, MD
    The line voltage in my home fluctuates pretty wildly, but lately has been sitting at 123v. The plate voltage is basically 2.3 x line voltage.
    So, about 283vdc in the plates.

    im using the fancier 5watt OT which has 22500 ohm primary impedance.
     
  7. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,534
    Joined:
    May 24, 2010
    Location:
    Canada
    I had good luck with 6AK6 tubes for the output on a low power 5E3 amp. I used 230-240V as a supply but with a lower impedance, possibly 10-12k. It is a real pentode output tube and in SE makes 1W, so P-P 2-3W. I had a file of a buddy playing it online but it seems it is not available from Microsoft anymore, was up half a year ago. I took a quick peek and this calculator seems to think with a 22k OT at 250V, 260V screen and a bias of 9 mA it will make 4.5W. I think realistically a watt shaved off that is more doable after the transformer losses. Might be more power than you want though.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
  8. jtcnj

    jtcnj Tele-Holic

    Age:
    57
    Posts:
    621
    Joined:
    Feb 2, 2015
    Location:
    N.J. USA
    Playing the amp as is, it really muddies up when pushed hard, preamp past noon and cranking the master. dialing back the pre-PI master doesnt help all that much.

    The treble is there - it can be overly bright and i usually have the treble pot at around 10:00 or less and the guitar tone pot down around 2-3 with a Super Distortion bridge pickup.

    The 12au7 triodes were not red plating but running at about 3.25w.
    All my voltages were about 10-20v or so higher than robs schematic, at wall voltage of 120.1- 121.4. I played around with dropping that with the variac.

    I finally decided to increase the shared power tube cathode resistor, which I had changed from 470Ω to 1kΩ earlier, by adding 250Ω in series. This brings the power tube dissipation to around 2.5w per triode.

    It sounds much, much better.

    Still considering the ef80 swap, but dont feel I need to do something.
    I may try some of the mods rob has on taming the treble.

    Earlier, the best tones I was getting was more low gain crunch with the master cranked and the pre-amp down at 9:00 or below, sometimes with a clean boost or OD used as boost.
     
    Last edited: Aug 26, 2021
  9. dougsta

    dougsta Tele-Meister

    Posts:
    393
    Joined:
    May 25, 2015
    Location:
    Edinburgh, Scotland
    I built my push pull EF80 amp back in Dec 2020 with the goal of getting rich pentode output valve distortion for use miked-up in a home studio/bedroom without the ear splitting volume.

    I kept it very simple, I wanted to hear the output section so I just used 2 triode stages for the pre amp, very little distortion from the pre amp stage, no tone stack. A single triode stage for a zero gain cathodyne phase inverter to drive the push pull pair. The triode stages are also EF80 valves but triode strapped mode, they are great for clean preamp stages, very linear too.

    I made some sound recordings, guitar straight into amp/12" speaker miked-up to a Focusrite Saffire firewire audio interface, here's some sound clips:

    edge of breakup


    medium breakup


    bit heavier


    back to the '50s


    Add a short demo idea with drums/bass in a mix



    Doug.
     
    Last edited: Aug 26, 2021
  10. jpsthecelt

    jpsthecelt NEW MEMBER!

    Age:
    68
    Posts:
    2
    Joined:
    Aug 27, 2021
    Location:
    albuquerque, NM
    Doug: Nice sounds - Can you post some voltages?

    Reason being, that I built RobRob's EF80 JCM800 (with tone stack, preamp1 & 3, Cold-clipper, and Cathode follower) and it sounds Nothing like this.

    So, of course, I’m using different triodes (12ax7s), but I have WAY-WAY less volume than I’d expect. (It's way less than my Vox S/S DA-5 on 1.5W setting, e.g.). Checked all voltages, connections, etc. (e.g. checked PI ‘tail'resistor @10k, and 470ohm to make sure no error there. I’m using a Hammond 270CAX with dropping-resistor, bringing B+ to 300v.) I’m also using a 125C OT @22.5k/8 vs. a B.
    I’ve got 6.7v on filaments (raised to 50v for noise, as per RobRob schem.)

    I tried my Strat-fluence, stock LP, & stock Carvin DC500 - all anemic, at best.

    So, I have dunno why the low volume. [once I get home, I guess I'll put an O-scope on it and also post some voltages]. Anyone have thoughts on what else to check?
     
    Last edited: Aug 27, 2021
  11. jpsthecelt

    jpsthecelt NEW MEMBER!

    Age:
    68
    Posts:
    2
    Joined:
    Aug 27, 2021
    Location:
    albuquerque, NM
    (Built the cab, put in an old Fender 8-ohm; repurposed my Angela SuperSE chassis, etc.) Amp generally has a nice crunchy tone with both MV & preamp gain ‘dimed’, but b/c of low-volume don’t get the subtlety of tone that I hear in your recordings.

    Oh, and besides the chassis being ‘rough’ from being repurposed, this ‘gut-shot’ is not the latest (has a wire or two ‘hanging’, that I’ve already soldered). Thanks for any thoughts on what to check - jps.
     

    Attached Files:

  12. eslover

    eslover Tele-Meister

    Posts:
    207
    Joined:
    Nov 17, 2009
    Location:
    Ellicott City, MD
    When I dime the preamp and master volumes it is just way too loud, but sounds great attenuated. Strangely it doesn’t sound remotely like Doug’s recordings…might have the same dynamics though. my hearing sucks these days.
     
  13. dougsta

    dougsta Tele-Meister

    Posts:
    393
    Joined:
    May 25, 2015
    Location:
    Edinburgh, Scotland
    Sure, will need to dig it out of the pile
     
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,898
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Doug, very nicely played, recorded, and *designed*. That is the sound I think a lot of folks would like in a micro amp, and gives real credence to the output EF80s. Seems like you got the recipe right, while some other PP EF80 implementations aren't quite as nice? I tried to hunt up your documentation of this build, but you've contributed so much to so many of these threads, I couldn't link back to any layouts, schematics, or pics of your actual amp. Can you give us a steer? Or re-post a few?
     
  15. dougsta

    dougsta Tele-Meister

    Posts:
    393
    Joined:
    May 25, 2015
    Location:
    Edinburgh, Scotland
    Sure thing, here's the thread I made back in Dec2020.

    https://www.tdpri.com/threads/3-4-watt-small-signal-pentode-push-pull-amp.1056788/

    I wouldn't bother with the early sound samples, I didn't get the recording workflow right.

    The current schematic is on the last post by me. It includes a final tweak to get the extra gain n goodness from one of the EF80 valves switched between triode/pentode mode. I also added a bright switch as a test tho not sure if it's in the schematic, just a standard cap after the 1st stage to brighten up humbuckers.

    I think the cathodyne PI helps get the best out of it

    Doug
     
    King Fan likes this.
  16. dougsta

    dougsta Tele-Meister

    Posts:
    393
    Joined:
    May 25, 2015
    Location:
    Edinburgh, Scotland
    Here are the readings:

    I wound the PT with an extra tap on the HT so I could add a switch for a cut/boost, roughly 10%

    I also added a triode/pentode mode switch on V2

    Low HT 223v
    OT Anode 1 221.9v Anode 2: 222.1v

    V1 anode 43.4v
    V2 anode triode mode 41.4v and pentode mode 123.8v
    V3 anode 175v

    High HT 246v
    OT Anode 1 244.3v Anode 2: 244.4v

    V1 anode 47.8v
    V2 anode triode mode 43.1v and pentode mode 136.8v
    V3 anode 193v

    OT DCR
    Anode 1 to CT 708Ω
    Anode 2 to CT 478Ω
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.