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PostPosted: Sat Feb 11, 2017 3:02 pm 
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I built up a tweed royal over the new year; great little amp, and have been having a lot of fun with it. First amp build for me, and I had to troubleshoot some general hum once first built (learnt about the importance of heater wire routing too late!).

I've made the mod suggested by Morgan:
"My amp doesn't have an increased line noise hum when I switch off the cut control. Looking at the wiring diagram, I noticed that the 25µf cap that hangs off the cut control is grounded to the main filter cap area. Out of habit, when I built my amp, I grounded that cap to the pre amp side of things - it is grounded to pin 1 of the second volume control, the same spot where the .0047µf tone cap is grounded to the second volume control. It's purely an empirical observation, but maybe I stumbled onto something. Try it."

That mod helped a lot (thanks Morgan!), and i rerouted (bent) the heater wiring and also put some copper foil on the inside of the cabinet to shield the interior of the chassis. In full Champ [SE, tone off (CCW), Cut full (CW)] or Deluxe modes [PP, tone on, cut off (full CCW)], it's pretty quiet now, and I'm happy with it (the SE mode still has more hum than the PP mode, but I can live with it; I'd probably need to completely totally redo heater wiring and use some shielded cable as suggested by Morgan elsewhere in these forums to clean it up any more).

I do have an interesting hum "in-between" settings while in PP mode though. If I have the cut control anywhere from about 3.30 to 5 o'clock (full CW), I get variable (in volume) hum when the tone control sweeps from about 11 o'clock to about 4 o'clock. The hum is loudest at about 1 o'clock, gets quieter as you move toward 3, then abruptly get louder between 3 and 4 o'clock before disappearing immediately after 4 o'clock. This happens without any cables plugged into the inputs, and is present when volume knobs are at zero (full CCW). The hum seems to mostly go away once either volume knob is cranked past 3 o'clock. If the cut control isn't in the range of 3.30 to full CW, the tone knob works just fine without causing any changing hum.

The other really weird (to me) thing about this, is when I have the BYOC 27v boost in the signal chain anywhere in front of the amp, on or off (but louder when on), the variable hum as described above is accompanied by a soft high pitched whine that changes in pitch with the level of hum (lower pitch with higher hum). The whine volume is modulated by either volume control (inaudible when vol off, increasing volume till about 3'o'clock after which it disappears altogether. There is no whine from the 27v boost at all when the there is no hum already on the TR; the pedal works just fine on my other amps with no whine at all.

Do these symptoms mean anything to anyone? Happy to take some gut shots if it will help. I'm guessing there is something wrong with the cut control and negative feedback once past 3.30 o'clock, but don't know where to start troubleshooting. Furthermore, I don't know if this problem was present before I made Morgan's mod with the 25µf cap grounding.

Any suggestions would be greatly appreciated.

Thanks guys!


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PostPosted: Sat Feb 11, 2017 3:38 pm 
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Interesting symptoms.

Since you're characterizing the cut control noise as a hum and the boost noise as a whine, I'm guessing the cut control noise is lower in pitch. Is it close to an A note (440 hz)? That would be power supply noise.

Is the boost the only pedal in the chain? What power supply are you using? Can you get any other pedals to do that? That sounds like it could be switching power supply noise.

It's just a guess but it sounds like possible layout issues. Any long wire that runs to a preamp tube grid (pins 2 and 7) can basically act as an antenna. I believe the TR has these wires on the volume pots. Those are good places for shielded wire. Ground the shield only at the volume control side to lug 3. A lot of people also like to move the 68k input resistors from the board right to the input jacks. Then twist the resistor leads together and run shielded wire right to the first preamp tube grid. There are diagrams online.

Photos might help identify potential problems.

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PostPosted: Sat Feb 11, 2017 5:14 pm 
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Hi Morgan,
Yep, the hum is invariably close to a B flat. The whine changes in pitch as the tone knob sweeps from 11 to ~3.
The boost will cause the whine regardless of whether it's the only pedal in the chain, or part of a much longer chain. I've had it set up on it's own wall-wart power (rated to 200mA, no daisy chaining), as well as an MXR Isobrick. It behaves the same regardless of how I'm powering it (and weirdly causes it even when it's switched off, but must be plugged in to power source). I have a bunch of other boosts (BYOC mosfet, Germanium, confidence), as well as other assorted overdrives, reverbs and EQ pedals, and no whine with any of them. It took me a while to actually isolate the 27v boost as the cause. I've even tried putting a ground lift (Radial Big shot ABY) in the chain between the 27v boost and amp, and it makes no difference to the whine.
Yeah, definitely a strange one. It seems odd to me that the hum is only in very specific circumstances; like I mentioned in OP, in full Champ or Deluxe mode, it's just not an issue all. Is there any chance it's a faulty Cut pot? When I was removing and resoldering the 25uf cap for the grounding fix, I noticed a small amount of smoke come out of the back of the Cut pot. The smoke must have come from something, but I have no idea of the inner workings of these SPDT pots (they also seem impossible to find online for a replacement).

I'll take some photos tomorrow and post then; I have to head out for the night right now. Thanks Morgan for the thoughts and reply!


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PostPosted: Sun Feb 12, 2017 9:40 am 
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Here's some photos of the build. Happy to take more if helpful.


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PostPosted: Sun Feb 12, 2017 9:42 am 
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and some more...


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PostPosted: Sun Feb 12, 2017 9:48 am 
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last 3...


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PostPosted: Sun Feb 12, 2017 12:06 pm 
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You have several turrets that appear to be unsoldered from the top side--see cropped image below. Are they soldered on the bottom of the turret instead?

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PostPosted: Sun Feb 12, 2017 12:24 pm 
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Hi DVM,
Yeah, the solder is underneath where the wires come in from the bottom. I can confirm those resisters are firmly soldered in their turrets. Would it be worth also dropping some solder in the top?


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PostPosted: Sun Feb 12, 2017 12:39 pm 
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swervinmervin wrote:
Yeah, the solder is underneath where the wires come in from the bottom. I can confirm those resisters are firmly soldered in their turrets. Would it be worth also dropping some solder in the top?

No, not necessary, as long as the soldering on the bottom is solid.

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PostPosted: Tue Feb 14, 2017 12:11 pm 
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There are a few layout issues you can take care of before you start adding shielded wires. Compare your wiring on the input side of the amp chassis compared to an old tweed amp.

Notice how your wiring on the input jacks and 12AX7 tubes has large loops that come within and inch of each other:

Image

On an old school tweed deluxe, notice how the wiring on the pots, jacks, and tubes is flat against the chassis:

Image

Image

I suggest re-doing the wiring to make it more like the tried and true 50's method. You might not have to even resolder anything; you might just be able to bend the wiring to get it out of the way. Get that wiring flat against the chassis and separate each wire as much as you can. The way it is now, you have low strength signal-bearing wire right next to amplified, noisy wiring coming off the pre amp tubes. That could be causing some of the issues.

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PostPosted: Tue Feb 14, 2017 12:13 pm 
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If you do end up wanting to move the input resistors off the board, here is how you do it:

Image

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PostPosted: Wed Feb 15, 2017 6:40 am 
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Thanks so much Morgan. I'll give that a try and see how it goes. Great idea. I'm the first to admit that my lead dress is terrible, but that's the point of building your first amp; to learn about these things! Even with all the internet reading I've done to date, I hadn't thought of simply getting these low-current wires tucked into the chassis. Could be a very easy fix. I'll let you know how it goes. Thanks again for your thoughts!


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PostPosted: Fri Feb 24, 2017 10:59 pm 
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I wanted to follow up on this, as I seemed to have mostly fixed it. Been busy with work and life etc, and only just got time to sit down and put some effort into this. As Morgan suggested, I think most of the problem was lead dress, but not in the expected spot. I did my best push all the wires against the chassis, as suggested by Morgan, but that made no difference. Out of desperation, more than anything else, I did a bit of chop-sticking with the amp on, and noticed if I moved the orange or white wires going into the SE/PP selector switch, it would alter the volume of the hum and whine. So I desoldered them, made a solid solder joint in the orange wires rather than the cap, and rerouted them as far away as I could from the heater wire heading out from the lamp to the tubes, and lo and behold, the hum that fluctuates with the tone knob when the cut dial is full CW has gone! The whine is still there, but to a lesser extent (I think; it's hard to tell cos it fluctuates in volume with the volume knob) when I have the 27v boost connected, but I think I just will stick to my BYOC germanium and mosfet boosts with this amp, and then i won't have to worry about it. Pics of rerouted wires below. Question for everyone: I'm assuming the hum was originating from the heater wire coming from the lamp; what is the best practice for routing that heater wire? Should it be run along the bottom of the chassis as much as possible? It seems people tend to worry mostly about the routing of the heater wire around the preamp tubes, as that's where the unamplified signal is. I have no idea what the orange and white wires on the SE/PP switch are carrying, so I can only speculate how the hum from those heater wires ended up in the signal. Any ideas?


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PostPosted: Sat Feb 25, 2017 1:12 pm 
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Hmmm, sounds like the hum may have been due to the proximity at that rotary switch of the high voltage output from the rectifier and the wires to the 6V6 cathodes? Though the rectifier output is DC, it still has a lot of residual AC "ripple" in it at that point.

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