Added bypass cap to Harvard - WHOO!

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Oct 11, 2019 at 9:52 PM.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've thought about and talked about it, but I finally did it today. I added a 2uf bypass cap to pin 3 cathode resistor on V2. Then I changed the bias from 60%-68%, all JJ's. This thing turned into a monster. I remember a thread a while back where @keithb7 did the same or similar to his 6G2 and have been curious about it since. Well, my curiosity is satisfied!

    My pro tools kept crashing, so my computer has been defragging all afternoon, hopefully I'll get a chance to do a real mic'd recording with it. Until then, my cell on the floor is going to have to do. I changed the bias just to go ahead and see how dirty it would be. Hearing the difference the cap made, when I do a demo, I'm also going to cool it back again and see if I can get some more definition back without it being to clean.

    This is what the original purpose of this build was for. I had a 6.6k Deluxe Reverb OT on hand and used it, and it ended up being the cleanest amp I've ever built. It honestly needs a switch now because the cleans were so great it would be a shame to lose that. But, damn this thing can growl now! I was wearing earplugs. Still no rug in my office yet and hickory floors in a closed up room -- it was kind of nauseating with the volume.

     
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  2. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Sounds great. I can tell you are having a load of fun with it.
     
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  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Thanks, I am! Only trouble is that it will peel paint in my office/music room. I took it into the guest bedroom earlier - carpet, bed, furniture, etc. - and it wasn't nearly as loud as it is in here. I've got to get a rug and maybe some sound treatment for the walls in a few spots. It is just insanely loud in this room. But, a 1258 doesn't pull any punches.

    Open ended question for anyone who listens to the track: What does it sound like if you had to guess without knowing? Just for curiosity on my end. This is a 115mA PT vs I "think" 70mA Champ PT for an original. Deluxe Reverb OT. B+ is upper 350v range (instead of 305v), but I think the bias is too hot and mushy as is -- I'd like a little harder break in the notes. NFB is 68k I think vs 56k. I added a grid stopper as suggested on Rob's site, and I can say I did notice the difference in smoothing things out. I spent a long time playing with voltages (dual secondary PT) and trying lots of stuff because of the original brightness. The higher voltage is what cured it. It's obviously a LONG way from a stock Harvard. So, even though this is a cell phone and I still have to do the real recorded demo, I'm interested in anyone who says, "That sounds a lot like a ......"
     
  4. Nickfl

    Nickfl Tele-Afflicted

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    If you put a bypass cap on v2, pin 3 you don't have any NFB, that cap is going to send the feedback signal straight to ground. Thats a big part of why it is adding so much volume and drive.

    Edit: I realized you said you used a 2uf bypass cap, so you probably aren't removing all the global NFB, just most of it. A cathode bypass cap functions by reducing or removing inherent local NFB, but it will have the same effect on global NFB if you apply it to the cathode where the global NFB happens to land. So, in this case you are getting double the bypass effect since you are bypassing both the tubes internal feedback and the global negative feedback.
     
    Last edited: Oct 12, 2019 at 2:14 AM
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  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I knew that...but not today! I completely forgot about the NFB relationship with that bypass cap. So, that definitely explains some of the huge boost I got today.
     
  6. danlad

    danlad Tele-Meister

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    That might be worth looking into as an alternative feedback switch, good idea
     
  7. rze99

    rze99 Poster Extraordinaire

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    Sounds massive
     
  8. rjtwangs

    rjtwangs Tele-Afflicted Silver Supporter

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    My Harvard, they are in my opinion, the most overlooked monster of Fender tweeds....yours sounds wonderful!!! Hope you have a lot of fun, and it certainly sounds like you are!! Congratulations on your new toy!!!!


    RJ
     
  9. Wally

    Wally Telefied Ad Free Member

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    Imho, it would be better to place a 25k-50k pot in series with the 22k NFB loops resistor for a more versatile approach. Ommv.
     
  10. EsquireBoy

    EsquireBoy Tele-Meister

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    Sounds glorious indeed, have fun and take care of your ears!
     
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  11. keithb7

    keithb7 Friend of Leo's

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    It does sound awesome. Congrats.

    I did experiment some with different cap values. It was interesting to customize the amp specifically to a guitar. I found that with higher gain pups it was easy to go too far and end up with mush. A 3 position micro switch might be cool. Stock position, a position for cap option one another for option two. Might be good for different guitar pup outputs.
     
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  12. reckless toboggan

    reckless toboggan TDPRI Member

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    Sounds amazing. Huge. Leave it as is. That sounds like an absolute tone MONSTER.

    If you are going to change anything, put in a switch or pot, so that you can have this sound anytime you want (and keep the hotter bias for sure), but can switch or blend to another cap/signal path to get closer to the original sound too.
     
  13. LetItGrowTone

    LetItGrowTone TDPRI Member

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    "What does it sound like if you had to guess without knowing?"
    I see we've been shy about answering specifically. It sounds like a lot of great music from that era and beyond. Congratulations!

    Edit: The highest notes (for example 5:06-07) and the slide tones (after 5:18) are especially great. Some tube hobbyists/guitarists would be tempted to try to tighten the bass, but NO! just put a bass guitar under it!
     
    Last edited: Oct 12, 2019 at 11:23 AM
  14. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Without thinking about it usefully or helpfully or anything, the NFB note from @Nickfl reminded me of why Rob put a bypass cut when he added NFB to his 5e3. Would some inverted-logic version of that work here?
     
  15. jsnwhite619

    jsnwhite619 Friend of Leo's

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    If I were to do it again - and probably will at some point - the Princeton Reverb type with a 47R tail would solve the no NFB issue. That's how I did my 5e3 NFB and it worked great - extra gain and tighter sound. Kinda crowded in there right now, so I don't know if I'll try it. I think that I could disregard a switch if I could rig it to get the NFB and the bypass like my 5e3.

    Sent from my moto g(6) using Tapatalk
     
  16. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I had a thought, but it's too late to try it tonight even if I do it. I played it more today, and yeah, it's just too bright with no NFB. I think I'm going to remove the bright cap and see what that gets me.

    Any suggestions for taming the brightness with no NFB and a single tone control are welcomed.

    Sent from my moto g(6) using Tapatalk
     
  17. RodeoTex

    RodeoTex Poster Extraordinaire

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    Cool sound.
    I've always wondered why the Harvard gets no discussion here.
     
  18. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Well, test it in a room with carpet and drapes. ;-). Your current room might make any amp, turned up, a bit too much.

    Seriously, I like your idea to retain some NFB. Maybe R/C frequency calculations too?
     
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  19. keithb7

    keithb7 Friend of Leo's

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    Do you have any other speaker options to try? Perhaps a smooth cone speaker like a G12C, or something similar. The smooth, non-ribber cone is designed to roll off some top end.
     
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  20. Nickfl

    Nickfl Tele-Afflicted

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    Removing the bright cap might help, but remember that it isn't doing much at higher volume settings anyway, so it may not do enough.

    The above suggestions to change the room and speaker make sense. Another option would be adding a snubber across one of the plate resistors. Somewhere in the range of 500-2000pf should roll off the icepick without deadening things too much. Try clipping some in at different values and see how it sounds.
     
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