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new 5f2a-ish build and there's a lot of buzz...

Discussion in 'Shock Brother's DIY Amps' started by goonie, Jan 4, 2021.

  1. goonie

    goonie Friend of Leo's

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    My Pro Junior to 5f2a-ish conversion project has come together quickly over the past week (being on annual leave helped there). I started her up tonight and plugged in, and it's a buzzy beast. As I haven't used a kit and it's different to a 'normal' 5f2a in a number of ways (i.e. SS rectifier, tag strips instead of board, choke, one input), the layout is also different but I have tried to follow convention where possible.

    Actually it sounds great, if you can ignore the buzz - which isn't really possible as it's a rather loud buzz which gets louder as volume increases. With volume on zero, the buzz is not there and the amp is fairly quiet, just a very mild hum - at zero, it's quieter than the Pro Junior was!

    I've been reading @robrob 's troubleshooting guide and he refers to an example where he tracked down a buzz: "Since the volume and tone controls affected the noise I knew it was entering the amp before the control--somewhere between the guitar and the volume pot".

    So I guess that is where I should focus too -- before the volume control.

    I'm aware that noise related to grounding issues is common. In my case though, with the buzz being volume dependent, can I rule out bad grounding or poor grounds design as the cause?
     
  2. cometazzi

    cometazzi Tele-Holic Silver Supporter

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    Is the buzz the same if you:

    1) have nothing plugged into the amp

    2) have just a cord plugged into the amp

    3) have a cord plugged into the amp, and a guitar on the other end

    This applies also if you have an FX loop installed. If so, try the above, as well as just putting a short patch cable from the send to the return. I've gotten loud buzzes before in SS amps with having my wiring backwards or poorly soldered on jacks, and also dirty or defective switching jacks on inputs and FX loops.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The description of *buzz* makes me wonder where the artificial HT CT is grounded.

    Post some pictures so we can get some eyes on this variant.;)
     
  4. timfred

    timfred Tele-Meister

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    1. Count your ground paths for the input jack and first triode grid leak resistor (including the jack itself, if it's not isolated from the chassis.). Should be exactly one with a clean (unshared) current path back to mains earth. Also, is the input jack connection to first triode grid shielded? If not, try other routings. After checking that....
    2. 50 or 100 Hz Hum? If 50 Hz - probably heater dress around the preamp tube. If 100Hz - probably something in the power supply not providing enough filtering to the first triode.
     
  5. goonie

    goonie Friend of Leo's

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    I was trying to avoid that because you're all going to laugh at my ineptitude:eek:

    Seriously though, will do, but right now it's a bit of a mess as the very first time I started it up it actually squealed like a pig. That was due to both pots being faulty (good lesson there - test them first!). I didn't have any more 1M pots, but had a 250K on hand so I temporarily wired it up 'Champ-style' with the 250K as volume. That fixed the banshee and I got to play some sweet, albeit buzzy, music.

    So I'll get some new pots today, wire it up with vol and tone and then post pics when it looks half decent :)

    Re grounding, am I correct in thinking that grounding is OK if the buzz is not there at zero volume?

    I have the 'power section' filter caps grounded to the PT bolt. Almost everything else, including input jack, is buss grounded to a separate 'preamp section' bolt in the chassis. The only exceptions are:

    - heater CT has its own ground, right next to the 12ax7. This was to avoid having to run a long wire back to the PT bolt. In Uncle Doug's video where he converts a Champion 600 (PCB amp) to point-to-point, he also grounds the CT resistors at a convenient spot near the 12ax7. Though if it will help, I can run a wire back to the main PT bolt ground.

    - speaker jack is grounded, again to avoid a long wire back to the central 'preamp' ground.
     
  6. goonie

    goonie Friend of Leo's

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    No FX loop for me, I just plug in :)

    Yes, the buzz is the same in all the three scenarios above.
     
  7. goonie

    goonie Friend of Leo's

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    Thanks for the ideas!

    I'm not yet able to differentiate between 50Hz and 100Hz hum, will have to learn about that.

    So you're saying the input jack and first triode grid leak resistor should have their own dedicated ground, separate to all others?
     
  8. timfred

    timfred Tele-Meister

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    That’s best practice. Worst is to have a ground loop where the grid leak resistor is grounded at the tube socket and the input jack is also chassis-grounded. Pick one.
     
  9. goonie

    goonie Friend of Leo's

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    Oops I forgot another grounding point - the SS rectifier is on a tag strip with its own ground too. So that's in all - SS rectifier, PT bolt, main 'preamp' bolt, virtual heater centre tap, and speaker jack. I should definitely reduce that. The problem is that because I'm using the Pro Junior's chassis and re-using the existing locations for inputs, knobs, valves, transformers, speaker jack etc, implementing a logical grounding system is a bit tricky. For example in the image below the speaker jack is bottom left, and the central preamp ground is 7-8 inches away near top right.

    Image-4_June2106_PG_CLM_Ask-Amp-Man_Pro-Jr-rear_WEB.jpg

    So would it be better to ground the speaker jack itself or run a wire back to the main ground? Guess I can try it both ways.

    I plugged a guitar in again this morning and it's actually quieter than I thought - must have seemed buzzier late at night! Using a humbuckered guitar it's even better. But definitely needs improving.

    As I stated earlier though, the buzz is non-existent with volume at zero and then gets worse as volume increases - doesn't this suggest that grounding my not be the culprit, as a ground noise would be constant and steady?
     
  10. goonie

    goonie Friend of Leo's

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    Just thought of another possible source of noise. I took Aiken's advice and soldered the grid stopper direct to the 12ax7, however I have read elsewhere that it should be soldered to the input jack. Given that there are two differing schools of thought, it may not matter too much.
     
  11. chas.wahl

    chas.wahl Tele-Meister

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    Pretty simple: Your low E string on a 6-string guitar is 82 Hz, so if the buzz is higher, it's 100 Hz, and if lower, then 50 Hz. The G# on your guitar (4th fret, low E string) is about 104 Hz. Also, if you have a piano or keyboard, then the G# that's a bit less than 2-1/2 octaves below middle C is about 52 Hz, while 104 Hz is the G# an octave above that.

    That's the first thing to check, but you might also consider elevating the voltage of the heater ground to about 19 V, by connecting the center tap of the heater circuit to the #8 pin (cathode) of your 6V6 (see Merlin Blencowe's article on "Heaters"). You should verify that your heater circuit is truly balanced, by measuring the AC voltages between each leg of the heater circuit and the center tap. You haven't said whether you are using a center tap supplied with the power transformer, or using a "virtual center tap" with one (equivalent value) resistor from each heater leg to ground. If the latter is the case, then you should check the resistor values you're using to see they're equal, or better yet, use Merlin Blencowe's "Humdinger" (in the same article) to get variable balancing of the heater circuit.
     
    Last edited: Jan 5, 2021
  12. goonie

    goonie Friend of Leo's

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    Thanks @chas.wahl
    I am indeed using a virtual heater centre tap as the Pro Jr PT I'm using has no heater CT. The 100K resistors are within 1% of each other. That humdinger looks like a great idea.

    I think I've been a bit careless with the heater wire twisting and lead dress. Was rushing to get the amp done. Now I have actually built a working amp, and had the satisfaction of playing it, I'm going to go back and redo anything that I'm not proud of.

    I'm also going to simplify the grounding to a 'star' with all power and preamp section grounds leading back to one point as per @King Fan 's recent layout, rather than my current layout of two separate main ground points.
     
  13. dougsta

    dougsta Tele-Meister

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    100 Ohm resistors are commonly used for creating a fake CT. You won't be able to hear the hum from the lead dress over the buzz if you don't have the 2 100 ohm resistors installed.
     
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  14. chas.wahl

    chas.wahl Tele-Meister

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    Yeah, I've never heard of anyone providing a virtual center tap for heaters with 100k resistors; 100 ohm is what's typically used.

    I think that Merlin Blencowe probably quantifies the dissipation through the resistors. It's not really high, so if using the Humdinger approach (with a 250 or 500 ohm pot), I think that a quarter-watter works fine.
     
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  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm glad you're going to try that, but IME it actually makes a really minimal difference in these small amps when compared to a well-done split bus like the one Rob shows for his 5F1 and 5E3. If it's me, I'm gonna try to localize the buzz and attack the most likely suspects first. Pics could really help -- I get way confused with just words describing what goes where. Hang in there, you'll get this.
     
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  16. Nickfl

    Nickfl Friend of Leo's

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    Who says this? It's always better to have it right on the socket pin. Some people put them elsewhere for convenience, or "neatness" or because they think it'll put stress on connections if it's hanging off the socket (it won't), but I've never seen anyone argue that putting the grid stopper far from the tube socket is better for noise.
     
    Last edited: Jan 5, 2021
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  17. goonie

    goonie Friend of Leo's

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    Thanks @Nickfl I'll leave it right where it is!
     
  18. goonie

    goonie Friend of Leo's

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    Pics are coming! Just got a little tidying up to do :D
     
  19. goonie

    goonie Friend of Leo's

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    Great pick up thanks @dougsta
    Classic rookie mistake confusing 100 with 100K!
    Looks like I'm off to the electronics supply store again... Since I'm fixing it up I may as well install the humdinger, if I can get a suitable trimpot.
     
  20. goonie

    goonie Friend of Leo's

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    Well it's amazing what a difference 99,900 ohms makes!

    Sounding a lot quieter now. This forum is an incredible resource, so many knowledgeable and generous people. Thank you all!

    Just got to re-install volume and tone knobs and it's pretty much done. Would still like to make it a bit more hum-free but think I'll just play the $%#@ thing for a while.
     
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