Squealing in 5e3 with channels bridged and all knobs turned way up

Discussion in 'Shock Brother's DIY Amps' started by doof, May 14, 2021.

  1. doof

    doof Tele-Holic

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    EDIT: I posted some gut pics further below in the thread.

    Hi everyone,
    I built my first amp last year, a 5e3 (no kit, just sourced all parts myself), and just looking to get some opinions as to whether this is an actual problem or something I shouldn't worry about.

    So, with Normal 1 and Bright 2 bridged, and my guitar plugged into Bright 1, both volumes up around 9 or 10, everything sounds great as long as I keep the tone knob under about 5. Once I push the tone past 5 or 6, I get a very high pitched squealing. I'm wondering if this is due to the increased gain of bridging the channels, and therefore something to be expected since I'm using the amp outside of it's designed use?

    If I do not bridge the channels, there's no squealing whatsoever no matter the volume/tone settings (this works perfectly on both channels).

    I got out a chopstick and started pushing wires around thinking it might be a lead dress issue. When I moved one of the wires from the phase inverter, the squealing quickly got much quieter and I thought I was on to something, BUT, after some trial and error I found out simply placing my hand in that position had the same effect, even though I was not touching any wires. Just to be clear, I place my hand over the back of the chassis , around the middle, and I'm not touching or contacting anything, just hover my hand in the air, and this really attenuates the squealing a lot.

    Now, having the tone around 4 is plenty bright enough for me, so I can't see myself trying to play with the settings that cause the squeal, but it just bugs me that there might be some underlying problem in there. I'd love to hear from the members here who are more experienced than I am.

    Some details:
    • Mostly stock 5e3 circuit following RobRob's layout, but I included his PPIMV mod (deleted Normal #2 input and placed the PPIMV pot there instead, and I used shielded cable grounded on only one end).
    • Standby Switch in the "ground" hole on the chassis.
    • Used a Hammond 1760H OT and have 4,8,16 ohm outputs wired to a DPDT On-On-On switch.
    • I'm using a new EHX 12AY7 in V1, old-but-good RCA 12AX7, old-but-good matched Magnavox 6V6s, and an old RCA 5Y3GT. I have done a lot of tube swapping and the problem persists no matter what tubes I have installed.
    • All voltages test within 10% of typical 5e3 voltage charts I see online
    Anybody have any ideas or input? Please let me know if I've left out any relevant details. Thanks everyone!
     
    Last edited: May 15, 2021
  2. Cantbreak100guy

    Cantbreak100guy Tele-Holic

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    I had the same issue. It would squeal after I turned the bright volume up around 9 or 10. It stopped when I got a better cable for the jumper (Mogami cable with Eminence pancake plug).
    I'm not sure if that solved it permanently, so I'll be watching this thread to pick up any ideas from the members here.
     
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  3. doof

    doof Tele-Holic

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    I have tried a few different cables for the jumper, but admittedly all cheapies. I've got some shielded Rapco cable left over from a pedal board build a few years back, I'll see if I can't get around to making another cable this weekend. Thanks for throwing in your $0.02!
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Did you install a negative feedback mod like Rob's?

    Does your OT primary blue wire connect to V3 pin3?
    Does your OT secondary black wire connect to ground?

    Do you have control grid stoppers on V2 or the power tubes?

    Do you have screen resistors on the power tubes?

    Post some pictures of the guts.
     
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  5. doof

    doof Tele-Holic

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    nope, no negative feedback circuit is installed.

    OT Primary Blue wire is connected to pin 3 of V3, and OT Secondary Black wire is connected to the sleeve lug of the output jack, and that has continuity to the chassis.

    I've got 1.5k grid stoppers between pins 5 and 6 on both power tubes, and it's my understanding from Rob's site that the pre phase inverter master volume acts as adjustable grid stopper/grid leak resistors.

    I do not have any screen resistors on the 6V6s.

    If I turn the master volume down while the squealing is happening, I have to turn it down to where the amp just doesn't sound good anymore before the squealing stops.

    I'll take some gut pics when I get home from work. Thanks for your time!
     
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  6. Commodore 64

    Commodore 64 Friend of Leo's

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    You just have too much gain in all frequencies with everything dimed. You could try snubbers across the plate resistors. Cascading gain like that, your lead dress has to be perfect. Even then you may get the squealies. You are cascading 3 fully bypassed, and similarly biased triodes with no attenuation. I think that can sound very harsh. Especially if you have small filter caps, saggy rectifier, great big coupling caps (.1), you just have a great big mess of clipping what you just clipped.. If you LOVE that sound, that's tots cool. Then you just need to find the best lead dress orientation, lengths, shielding combination you can to beat them squealies.

    Try a split plate load resistor on V1A. Replace your 100k plate load with 2, 56k in series and tap your signal off the center. That'll dump some gain, whilst keeping the tube at the same operating point.

    Try lifting the bypass cap on V3 (dumping gain). Or try partial bypass (0.68 Uf or 1Uf or something)

    Or put a 512k attenuation resistor between V1A or and B or V1B and V2A. But then you are attempting to layer gain with symmetric clipping and, I think there's much better dragons to chase. Layering symmetrically clipped stages doesn't tickle my toes (or ears). But I do dig the cascaded, asymmetrically-clipped, bias stages of the typical Marshall.

    Try separating the cathodes of V1A and V1B and biasing them differently. You could go Marshall hot into cold, where V2A is biased cold, with no bypass. Might want to try an 12AX7 in V1 if you do this, just see which you like better.

    Put a great big grid stopper on your cathodyne if you haven't already.
     
    Last edited: May 14, 2021
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  7. doof

    doof Tele-Holic

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    I think you answered the main question I had, so it's reasonable to assume that this is simply how this circuit might respond with everything turned up that high. Thanks for writing all that out! I will get to work on your suggestions this weekend, thanks very much!
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Maybe for your particular 5E3 circuit it may be reasonable but ime it is not reasonable to come to this conclusion for the 5E3 circuit in general. Plenty 5E3 do not have these issues. It seems there is some oscillation when all the knobs are turned up. Why that is happening has not been determined yet.
     
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  9. Commodore 64

    Commodore 64 Friend of Leo's

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    Do the split load thing. If that solves it, you have too much gain for your layout. You can then decide what you wanna do. Going back to full gain is just a matter of moving that one wire to the end of the 56k series resistors. Super simple. And while you are tinkering, don't worry about twisting/hooking mechanically attaching the wires. Just solder them there, so you can easily unsolder them and swap stuff.

    I've chased this one a bit. At one point I thought it was my MOSFET source followers, but I resolved the squealies with lead dress, and in some cases, split plate load resistors to dump gain. Here's a table I made back when I was pulling my hair out.

    [​IMG]
     
    Last edited: May 14, 2021
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  10. Paul G.

    Paul G. Friend of Leo's

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    "Doctah! Doctah! It hurts when I do this!"
    "Don't do that."
     
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  11. doof

    doof Tele-Holic

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    Ok, here are some pictures. Be gentle, it was my first amp.
     

    Attached Files:

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  12. doof

    doof Tele-Holic

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    Ok, well there we go, definitely reason to do some more troubleshooting. Thanks for letting me know this isn't a common occurrence.
     
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  13. 2L man

    2L man Tele-Holic

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    For a first amp your work is very good!!!

    There is at least one ground loop when you have soldered capacitor to potentiometer case when you obviously also made pre amp return current flow thru chassis because you don't have solid ground bus on circuit board.

    Return current finds "easiest" path to power supply CT / negative / neutral but when there is audio signal in it it can get mixed badly. I can't say does this cause your problem but it won't have any advantage.

    If you don't find easy fix it is not difficult to modify amp. Connect board ground buses together to a single solid bus. Detach all circuits from chassis except on input jack and connect them to this ground bus.

    Or do Star grounding. Detach all amp circuits which you have soldered to chassis and wire them direct to power supply negative. Doing either chassis is not part of active amplifier circuit anymore.

    Mains cable Earth wire must be connected to chassis because it makes amp safer. Also one input jack ground needs to have connection with chassis because amplifier input is most sensitive to electromagnetic noise. This connection also "nulls" potential between guitar and earth globe thru guitar cable, chassis and mains cable earth wire.
     
    Last edited: May 15, 2021
  14. Commodore 64

    Commodore 64 Friend of Leo's

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    Dude, that amp looks beautiful. Nice job. Other thoughts:

    1. Vacuum out the stray bits of chassis grindings.
    2. Put some hours on the amp. More than one person has reported that as components "break in" some of the gremlins get better.
     
  15. 2L man

    2L man Tele-Holic

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    Guitar might be what partially cause it so turn its volume down and test if it quits when you detach cable from input?
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Nice looking build.
    Have you tried the amp in different locations? Sometimes lights or wall wiring, etc. can interfere.
    Does the amp have a Faraday cage shielded back plate? Your comment of reduced squeal when a hand is near the back makes me ask.

    The MV wiring does not have shielding wire close to the pot. Those wires are also close to the ground wire from the ground bus. Ground wires are not necessarily quiescent. As @2L man pointed out the ground scheme is not optimal. There may be consequences as 2L man noted.

    It looks like the two 68K control grid resistors of the normal channel have been paralleled? I can't make out the color codes of most of the resistors. Have any resistor or cap values been changed?

    The 500pF cap between the tone and volume pots is hanging out in the air like an antenna. Try to get that tucked up to the chassis.
     
    Last edited: May 15, 2021
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  17. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Very clean wiring. There may be a little (very little) cleanup on the wiring around the pots. Shorten the wire runs if you can.

    I had problems with mine... turns out it was the EHX 12AY7. I had a few others in a box, some old some new, some more EHX branded... I’ll put it this way: I will never use EHX tubes ever again (I’ve got EHX 6V6s that are problematic as well).

    Anyway, it was V1 in my case.

    Rob’s MV mod is simply subbing the 1 meg on board resistor for a 1 meg pot, correct?

    If swapping V1 doesn’t resolve it, you might try shielded wire on the pot you’ve got in the input hole (if it isn’t already shielded wire)... that’s the MV pot?
     
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  18. monkeybanana

    monkeybanana Tele-Holic

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    I had a similar issue. I wire the 68k resistors off the board like the blackface circuits because my board was leaking. When plugged into the bright channel if I dimed all the knobs including the normal channel I would get a weird noise. By moving the wires off the 68k resistors. The noise went away. I see why people use shielded cables there. But my noise went away.
     
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  19. Jared Purdy

    Jared Purdy Friend of Leo's

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    Your post got me curious, as a I have Fender 57' Custom Deluxe. I can't remember the last time I turned it up to that volume. It's not my normal noodllng volume, which more often than not, borders on pretty clean. None the less, I decided to give your settings a whirl. Holy sh*t!! Loud! Ya, Neil Young and Crazy Horse tones were dripping from that amp.

    I used my 2019 60th Anniversary Gibson R9, as I knew that would push it to a level of tones and volume that a Strat simply cannot get to. I jumpered the cables as per what you did, set both volume controls to 10, and the tone control to 9. If I wasn't more concerned with my neighbours, I would have just sat there for hours noodling away on blues chords, and listening to the incredible sustain. My word. For a few precious moments, I sounded like some of the musicians who's albums I've got from the 60's and early 70's. I wish.

    I decided to try it with out jumpering the inputs, and it was brighter, a bit more shrill, but nothing that couldn't be adjusted with the tone controls on the LP. Back to your issue: I experienced no high pitched squealing, as you experienced. I hope you find the solution to the issue.
     
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  20. monkeybanana

    monkeybanana Tele-Holic

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    Here is another possible reason to dime the controls: Keep in mind this is a clone. When I am playing at home I turn up my volume and tone pretty high up to get the sound I like. I then use the other channel’s volume to lower the actual volume and it works backwards. 12 on the volume of the channel I am not using is the quietest and as I turn counterclockwise the actual volume increases.


    I do this even with a band because the amp is LOUD but I really like the tone.


    I also use the volume knob on my tele a lot with this amp. Lots of cool sounds.

    I brought a Princeton Reverb clone to play with the band last week as we play some surf but I missed the Deluxe. I have too many amps but I want to try a Harvard or some brown amps next. Too many amps!
     
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