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Yet another 5E3 thread - with questions

Discussion in 'Amp Tech Center' started by JSMac, Jan 9, 2021.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I bet that OT would be perfect for a 5E7. The three speakers would bring in the primary impedance right near spec. for the 6L6.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Try winding your unused chanel volume up.
     
    Last edited: Jan 9, 2021
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Most of the 5E3 will break up near 6 on a 12 scale. The channels and tone are interactive as Dacious pointed out. The low B+ may be giving yours a little less headroom. The guitar signal strength will also push it into distortion so the guitar volume knob is used quite a bit.
     
    Last edited: Jan 10, 2021
  4. JSMac

    JSMac Tele-Holic

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    Interesting. Like I said I don't have any experience with a 5E3.

    I tried what Dacious suggested. There is a sweet spot on volume control of the unused channel that really helps balance out the low end. That's just past half way. Anything on either side of that and there is too much low end. This is especially so when plugged into the normal channel but it has the same effect on the bright channel.

    BTW, I only installed one input for each channel if that somehow makes a difference.
     
    Last edited: Jan 10, 2021
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You may want to experiment with jumpering the channels so maybe you could research one more input ack or a jumper switch.

    The 1M and 68k input resistors, sometimes parallel to 34k, have an effect with the EQ and signal strength. What did you end up putting in?
     
    Last edited: Jan 10, 2021
  6. JSMac

    JSMac Tele-Holic

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    I will probably install a switch to tie the channels together rather than add another input jack.

    i've tried most of the suggestions made and each one seems to have made some difference so i don't think I'll change them back.

    I lowered the coupling cap values to .022 with a .0033 in front of the PI on V2. Replaced the 25uf V1 bypass cap with a 1uf. Added a 470k grid stopper to v2.

    i've ordered some 1 meg pots.

    I took it to a guy who has built a lot of 5e3s. He even offers classes to build them from kits. When he played mine it sounded great and didn't seem to have as much of the low frequency problem. We compared it to some of his builds and mine sounded pretty close.

    I thought maybe the beefy OPT may have been an issue so I ordered a proper 5e3 tranny from carl's Custom Amps. I didn't hear any difference so I left the original in.

    I think a lot of what I experienced with the bass problem had to do with the environment that I was Testing it in. I was working on it in the small furnace room. The speaker sat on a concrete floor, lots of exposed ductwork and plenty of thing on shelves to rattle. And turned up too loud.

    I ordered a frequently recommended Weber 12a125a speaker.

    It's not pretty but it works. If I were to do it again I would probably built it on a turret board but it was fun doing it the way I did, using mostly parts that I had on hand and learning a lot along the way.

    Thanks for all the help!

    004.JPG 005.JPG 006.JPG 007.JPG
     
  7. timfred

    timfred Tele-Meister

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    Are *all* the pots 500K? The tone control depends on the values of these pots. If you are reluctant to replace them with 1M you could try halving the values of the 0.0005u and 0.005u capacitors (esp the 0.005u, which is bleeding a lot of treble to ground).

    With an oversized OT you might want to try reducing the size of the coupling caps right before the power tubes down to 0.01uF.

    If overdriving too early, try temporarily removing the 25u bypass cap from the first 12AX7 triode.
     
  8. JSMac

    JSMac Tele-Holic

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    All three pots a currently 500k. I've ordered some 1 meg to replace them.

    I did reduce the bypass cap on V1, which is a 12ay7. Is that the one you mean or the V2 12ax7 as well?
     
    Last edited: Jan 18, 2021
  9. bblumentritt

    bblumentritt Tele-Afflicted

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    A 5E3 amplifies the bass frequencies about 2x of the treble frequencies (in reality, it's the highs that are attenuated 2x). As LowerLeftCoast has stated, the bass dominates, and one of the fixes is the cathodyne grid stopper.

    As you have already found out, there is little headroom. That's also a characteristic of the 5E3.

    Here are the "fixes:
    1. The lack of headroom is cause by early cutoff at the second preamp stage - too much overall amplification in the first two preamp stages. Removing the cathode bypass capacitor allows the voltage at the cathode of the 2nd preamp gain stage to swing in
      conjunction with the voltage at its grid. This is a form of local negative feedback and thus it reduces the gain and allows more variation in the volume control before cutoff distortion occurs.
    2. Changing the first stage coupling caps from .1µF to a lower value is a huge help, and you've already done that. One of the things I do it to introduce a variable, switchable negative feedback loop, using a 22k NFB resitsor and ad 250k presence pot with a rotary on/off switch, that ties into the cathode of the 2nd stage preamp (with the cathode bypass cap removed). This allows you to dial in more headroom/cleaner sound, at the cost of less gain and volume.
     
  10. JSMac

    JSMac Tele-Holic

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    i did install a grid stopper.

    I'm confused about which bypass cap to try removing. V1 or V2?

    I've reduced the one on V1 to 1uf.

    Edit. I see that you said V2.
     
  11. JSMac

    JSMac Tele-Holic

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    I lifted the bypass cap on V2 and that made a big difference in the gain and thus in headroom. Then I clipped in various different NFB resistors and didn't notice any difference. Should I?

    Edit: Putting a 1uf bypass cap in seems to get me closer to where I'd like to be than no cap. With a 25uf cap it started to break up around three and was unusable much above 1/2 volume. With no cap I can crank it all the way and wish for a little more volume. The 1uf seems to put it somewhere in between. Nice break up beginning at about 1/2 volume.

    Still not sure about the NFB.
     
    Last edited: Jan 20, 2021
  12. JSMac

    JSMac Tele-Holic

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    it seems that I should hear a difference when switching some NFB in and out but I don't. What should I expect from that?
     
  13. JSMac

    JSMac Tele-Holic

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    After considerable parts swapping I think I've got it where I like it.

    I ended up with reduced values on all of the coupling caps to .022 with a .0047 at V2

    I reduced the value of the cathode bypass cap on V1 from 25uf to 1uf. I installed a switch to select between different values of bypass caps on V2. 25uf, 1uf and no cap. This change seems to have made the most difference in taming the gain and thus the bass. I messed around with some neg feedback but couldn't hear any difference.

    i replaced the 500k pots with 1 meg. I felt that made it too bright so I went back to a 500k tone pot.

    Installed a 470k gird stopper.

    A 5751 in V2 is nice too.

    Before these changes it would begin to break up and get loud with the volume at about 3. Anything much above 1/2 volume was unuseable to me. Now I have plenty of clean headroom and with it switched to no bypass cap on V1 it is useable with the volume dimed. And now it's more humbucker friendly too.

    I'm still not sure what the root problem was. I still suspect it may be that big output transformer but when I put in a proper one for a 5E3 I didn't think that I heard much difference. I do recall that when it was and audio amp it really had a great bass response.

    Not sure if I ended up with a true 5E3 but it's much more versatile now and I'm pretty happy with it.

    Thanks to all for the help!

    003.JPG
     
    Last edited: Feb 2, 2021
  14. JSMac

    JSMac Tele-Holic

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    I did one more mod that made it much more versatile.

    I swapped the 1uf cathode bypass cap that I put on V2 back to the stock 25uf. i installed an on/off/on switch to take that cap completely out of the circuit and the third switch position adds NFB. For that I put in 50k pot with an 18k resistor in series. That allows me to adjust from 18k to 68k NFB.

    This amp has been a lot of fun to experiment and learn with. Can't wait to start my next project.
     
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