Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on March 24, 2025, 01:25:11 pm
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Several years ago I built a mod of a Mag10A's pre-amp circuit to be used as a "ReVibe" type effects unit. Sluckey, with others, helped greatly with the schematic's mods and the build. This was my first build from scratch so there was a lot of learning for me to do.
Anyway, it works great except it's always has had a Hum when the Reverb is turned on that I can't get rid of. When the Reverb is Off, it's perfectly quiet. I feel like it's a ground loop, but I can't figure out how to fix it.
Attached is a copy of the schematic for this part of the circuit.
It is wired as follows:
The RCA Input and Output jacks are attached to the chassis and isolation washers are used. I hoped that might help, but apparently not.
The Neg. side of the 1uf cap is wired directly to the Tip tab on the RCA Input Jack, and the Shield tab on that Jack is wired directly to the GND Chassis BUSS. The Input on the tank's RCA jack connects with a RCA cable to this input jack.
Then the Output Tip tab on the RCA Output Jack connects to the Grid on the Recovery Tube through shielded cable whose shield is grounded only on 1 end at the Shield tab of the RCA Output jack. There is a 47K ohm resistor connecting the Tip tab with the Shield tab on that RCA Output jack. The Shield tab on that jack is then wired directly to the GND Chassis BUSS. The Output on the tank connects with a RCA cable to this Output jack. The Shields on both of these RCA jacks are in continuity with the Chassis. So its pretty simple.
Any thoughts on what's causing my HUM? Thanks.
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Which way around is your reverb pan mounted? (If the output transducer coil is sitting next to your amp’s PT, it will hum more)
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The Output is at the opposite end from the transformer. So its not close.
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does it hum as a "stand-alone unit"
my ReVibe would hum "in circuit" but standalone it was fine, I had to "float ground" on either the amp or the revibe
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It has no Output tubes, so its output just goes from it's output jack and then through a guitar cable to the input on my amp. It's perfectly quiet just sitting there as long as the amp isn't turned on, or if the amp is on and the signal is bypassing the Reverb part of the circuit.
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Disconnect the power cord green wire from chassis. Any better?
Show us the entire schematic.
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No difference if mains ground is disconnected.
Attached is entire schematic.
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move V1's "E" tap to "D" for testing
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You have several threads that deal with this same issue. In one thread you said you had solved the hum. Read this thread and see if there are any clues.
https://el34world.com/Forum/index.php?topic=25746.msg279595#msg279595
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Shooter-Do you mean swap D & E and V1's Pins 1 and 6?
SEL 49 I got the hum much lower than it had been and thought I'd just live with it. I can lower the Output Level and keep the Reverb Intensity low, and it's certainly acceptable at those levels, but I'd just like to solve this itch that I've been scratching for a long time.
Don't you guys spend a lot of time on this. As SEL 49 said, its been discussed a lot.
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Use Tap D for V1, NOT tap E
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No difference with D node on both plates of V1.
Shooter, was your ReVibe tube driven or transformer driven?
When you say "in circuit" and "standalone", what do you mean exactly?
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mine was the one Doug has posted with the 6V6
by stand-alone, I used it like a pedal, guitar>Revibe>amp
EDIT: so I had 2 separate PS's, one in the Revibe, one in the amp. Once I "lifted" ground from one unit the hum went to an "acceptable" level
I my world that means I can hear it with no signal, but once I roll the guitar volume past ~1 it's no longer an "issue"
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Shooter--Did you lift the green Mains safety ground wire, or some other ground? Wouldn't that be dangerous not to have the PS grounded to the chassis?
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used a 3 prone to 2 prone adaptor on the ReVibe unit.
when you work on 120,000 VDC for a living, "Dangerous" is sorta relative :icon_biggrin:
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Did you use 5 B+ filter caps in the power supply, like in the schematic?
I think, if you haven't done this, you need to try isolated RCA jacks for the reverb tank cables.
And make sure the verb tank is only at 1 point/1 RCA jack in the tanks chassis/housing, not at both RCA jacks.
So looking at the schematic;
V6 1M grid R and 270 K R and the RCA jack ground for the verb tanks input, get grounded with the B node filter cap.
Then for V7-A, the 47K grid R, 2K7 K R and the RCA jack ground for the verb tanks output, get grounded with the E node filter cap.
And the verb ft sw ground, V7-B's 1M verb pot on the grid, 2K7 K R, 100K output pot and the output jack, get grounded with the C node filter cap.
I'd try that and see if it helps.
And you still probably have to lift the chassis ground from the chassis in the preamp/verb/vibrato unit.
It will still have a earth/chassis ground the through the guitar cable and chassis mounted 1/4" jacks.
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used a 3 prone to 2 prone adaptor on the ReVibe unit.
Try it and see if it kills the humm?
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The problem might be that with the dry signal path going from V6 A/B's K's to V7 B's K there's only 15K between them separating/isolating them from each others ground. :dontknow:
Merlin might have a way to fix this humm problem?
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I did use a 3 prong to 2 prong converter with no impact.
I'm using isolation washers on the RCA jacks so the jacks themselves are isolated from the chassis.
I used all 5 B+ caps as shown in the schematic.
Not sure what you mean by this phrase: "And make sure the verb tank is only at 1 point/1 RCA jack in the tanks chassis/housing, not at both RCA jacks.
I feel like the Output Level pot is where the hum appears. Adjusting the reverb Intensity pot minimally affects the hum, so that's why I think it's coming after that point in the circuit. Maybe the Output Level needs to be set very low. If the Input test signal is 1kHz at 150mVrms, should the Output level of that signal that goes from the Output Jack to the amp also be at that level?
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If the Input test signal is 1kHz at 150mVrms, should the Output level of that signal that goes from the Output Jack to the amp also be at that level?
while signal amplitude may cause distortion by OD'ing the amp, it typically doesn't "induce hum"
maybe it's scope time to figure out hum Frequency, where it "shows itself" along the signal path...
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I don't want to beat this horse too much longer, but here's what I've determined.
The guitar is plugged into the reverb/vibrato headunit's Input Jack, and the headunit's Output Jack is plugged into the Input Jack of the amp. Standard guitar cables are being used.
The Output Level pot causes the hum to increase or decrease when the Output Level pot is increased or decreased regardless of whether the Reverb is On or Off.
The Reverb Intensity pot only has a very minor affect on hum, but only when the Reverb is ON.
If the Reverb is Off, increasing or decreasing the Reverb Intensity level have no effect on the hum.
My conclusion is that the Reverb circuit is fine. The hum is in the Output Level adjustment. My "guess" is that it is increasing the gain of the Output signal going to the Input Jack on the Amp, and maybe its just "line-level" noise that's being magnified.
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I'm using isolation washers on the RCA jacks so the jacks themselves are isolated from the chassis.
I used all 5 B+ caps as shown in the schematic.
Ok, these are both good.
But how did you ground the circuit grounds to the filter caps grounds?
In the verb circuit, did you ground the grounds like I wrote out?
Not sure what you mean by this phrase: "And make sure the verb tank is only at 1 point/1 RCA jack in the tanks chassis/housing, not at both RCA jacks.
Take the tank out and look inside at the RCA jacks. Only 1 of those tank jacks should be grounded to the tanks chassis/housing/frame. If both are grounded to the chassis/housing/frame it's a ground loop. Disconnect 1 of the tanks chassis/housing/frame grounds if that's the case. That will break the ground loop.
I feel like the Output Level pot is where the hum appears. Adjusting the reverb Intensity pot minimally affects the hum, so that's why I think it's coming after that point in the circuit. Maybe the Output Level needs to be set very low.
The output level needs to be set loud enough to make up for any losses in the signal from the vibrato and reverb. So unity gain or a maybe little more, if it can supply that?
If the Input test signal is 1kHz at 150mVrms, should the Output level of that signal that goes from the Output Jack to the amp also be at that level?
That would be unity gain, not less gain/loss, not more gain/amplified/boost.
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I'd also try this.
If you reverb ft sw is not isolated from the chassis, could be a ground loop.
Try leaving it wired up, but remove it from the chassis. Wrap it up with some tape so you don't short something out.
The ground for that verb ft sw should be grounded with the output pot's ground, V7 B's 2K7 K R, 1M reverb pot, the output jack and C node filter cap. If you were using just that jacks connection to the chassis as ground, you'll need to isolate it from the chassis and then run a wire from that jacks ground lug over to the C node's filter caps ground wire.
And the output jack should be isolated from the chassis too.
These 2 jacks and the output circuits grounding might be why the humm gets louder turning up the output pot.
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And can you please post some pics so we can see how it's laid out and how you wired it up?
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Willabe-The footswitch jacks are also using isolation washer to isolate the Reverb and Vibrato from the chassis.
As to the tank, I had already unsoldered the connection on its Input jack to isolate it and that made no difference.
I'm attaching my Layout. It's ugly, but it is the way the amp's built. I separated the Grounds for the filter caps from the Grounds for the preamp that are located on the circuit board, and used the Ground BUSS to ground everything. So I think its grounded as you suggest, but I may be wrong.
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As to the tank, I had already unsoldered the connection on its Input jack to isolate it and that made no difference.
So it had both RCA jacks grounded to the housing?
And now only 1 RCA jack is grounded to the housing?
Go back and read what I added to my last post, about the output jack grounding.
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You said you used 5 B+ filter caps like the schematic. I only see 4 filter caps.
And that is not grounded anything like I wrote out.
You ground all the grounds from a tube section with filter cap's ground lead that feeds that tube section.
Are all the green wires ground wires? You should only use 1 color for all the ground wires and only that color for the ground wires. Then you can trace the ground scheme.
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The E node 1uf cap is on the upper right hand side of the board, to the left of the large 470 ohm resistor on the far right end of the board.
You are correct on how I did the grounding.
The Layout is not consistent with colors using DIY Layout Creator. Remember this is the first time I ever did anything like this, so it's not pretty, but the unit seems to work pretty well. I created the Layout from the Schematic with no one assisting on how to create the Layout. Sluckey was great with the Schematic, but he said I needed to learn how to do a Layout by myself. And that was a good way for me to learn.
I did some adjustments to the Output Level pot, and with a guitar signal input I can get proper output volume with extremely low hum right at about Level 1, or 7 o'clock. The pot is just barely turned on. The more Output, the louder the hum. I also inserted a 150mVrms signal and with the Output Level in this position, the Output jack reads about 27mVrms. A much lower amplitude but works.
So maybe I should just live with it. It's not like I've got to push the amp's volume way up. But there's still that itch!!
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I did some adjustments to the Output Level pot, and with a guitar signal input I can get proper output volume with extremely low hum right at about Level 1, or 7 o'clock. The pot is just barely turned on.
There is an onboard 1M resistor connected to the output pot. Change that to 2.2M or 3.3M and you'll be able to turn the knob up higher. This won't change your hum issue but it may just feel good being able to turn the knob higher.
Alternately, replace the output pot with a 25K-A pot for the same results.
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...with a guitar signal input I can get proper output volume with extremely low hum right at about Level 1, or 7 o'clock.
This indicates that you don't really have a hum issue.
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I also inserted a 150mVrms signal and with the Output Level in this position, the Output jack reads about 27mVrms. A much lower amplitude but works.
Lower than unity gain.
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Thanks fellas.
SEL 49-are you saying there's no issue of hum? Sorry but I hear hum, so I don't understand your comment.
Willabe-yes lower than unity gain. But is that normal, or is it just a meaningless fact?
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SEL 49-are you saying there's no issue of hum?
I didn't say that. I said, "This indicates that you don't really have a hum issue." That was in response to your remark, "proper output volume with extremely low hum". Extremely low hum is a non-issue in my mind. Don't you agree?
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Sure, I agree with that.
Its not that the hum isn't there, it does exist, its just that its controllable.
So I guess the "salve for my itch" is the conclusion that the circuit is fine, the build is fine, and that's just the way it is.
Like some of the mechanics at car dealers say about some MB's and BMW's, "They all do that"!! HAHa!
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Noting that you report that the hum occurs when the reverb is activated (but isn't present on the dry signal) it seems that the problem is grounding of the signal returns for the (highly noise-sensitive) reverb recovery stage is getting muddled/overwhelmed by higher current returns. As this is a stand alone effects unit that is intended to be plugged in in front of a guitar amp, you might want to consider regrouping the ground returns and installing a signal ground hum-loop blocker between the ground buss and the chassis of this stand alone unit. This should significantly quieten/dampen loop hum.
Here's an example of how I wire 6G15 ground returns to eliminate loop hum - I can vouch that this yields a quiet noise floor. The same principle of galactic grounding around filter cap nodes with a daisy-chained buss to a hum-loop blocker can be applied to your unit - but it may require a bit of re-work.
Or maybe you're happy to live with it
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Tubeswell-thanks for the lead. I've seen that sort of wiring before. A couple of years ago I got a Pyle PHE 400, but it really didn't make much difference. Originally I built this to use with a Fender Champ kit that I built, and then later I build a mod of the Magnatone Twilighter. When I built the Maggie I install a AUX power socket for plugging this headunit into to hopefully solve the grounding/hum problem. And that's how I use it now, but still the issue. I've certainly improved the hum issue since I built it, but it's just this nagging bit that hangs on. I'm not one to give up on stuff, so that's why it's more of a matter of principle now. I'm not a perfectionist, but I do think logically and analytically, so that's why this dog won't leave this bone alone.
I've wondered if that last section of the circuit where the resistor and pot come off the plate and go to the Output jack is where it all lies.
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Due to a need to have 3-prong safety earth mains cords on all electrical gear of this type, stand alone units are prone to hum loop if you don't get very fastidious with the ground return scheme.
The way you group the ground returns is important. The place you choose to attach the hum-loop blocker to the chassis is important. Isolation of all jack ground returns from the chassis (and having separate return wires to the relevant daisy chain location on the ground buss) is important.
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Willabe-yes lower than unity gain. But is that normal, or is it just a meaningless fact?
No, it's not normal.
Your knocking down, way down, the output from your guitar PUP's.
Lots of guitar pedals have an output volume control so you can adjust the output with the FX turned off to match the output of the FX on, so their both the same.
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I did some adjustments to the Output Level pot, and with a guitar signal input I can get proper output volume with extremely low hum right at about Level 1, or 7 o'clock. The pot is just barely turned on. The more Output, the louder the hum. I also inserted a 150mVrms signal and with the Output Level in this position, the Output jack reads about 27mVrms.
That's just 1/6 of the input.
What ever your PUP's put out the output of the pre/vib/verb unit will be only 1/6 of what the PUP's put out at that setting.
A much lower amplitude but works.
Just barely.
You should not be hand cuffed like that because you have to set the output volume so low to keep the humm down. That is not normal for any build.
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Willabe-In your Reply #26 you said "You ground all the grounds from a tube section with filter cap's ground lead that feeds that tube section.
Are you saying that "You should ground ..." or that I in fact "grounded all the grounds...". Are you saying the way I should ground the grounds from a tube section is to ground it directly with that tube's Filter Node cap?
In my case, all the filter caps' Neg/GND leads are wired together and then 1 wire connects them to the GND BUSS. Also, for what its worth, the two 100 ohm resistors which connect to the Pilot Light and the 6.3V filament leads for the artificial center tap, connect to the Node A Filter cap, which along with the other filter caps attach to that same GND BUSS.
If I recall correctly, the rectifier I purchased was a full wave bridge rectifier, and I was told not to wire its GND lead because it would produce too much voltage and that I should cut that leg of the rectifier off. So I did.
Would there have been any need to use the 100 ohm resistors for an artificial center tap?
I used a Hammond 269JX that of course has a center tap for the Secondary power supply. I connected that wire with the Primary's Green Mains ground lead directly to the chassis. Its 6.3V filament leads do not have a center tap.
Hope this is helpful. I do have an entry level digital table oscilloscope if that would be useful.
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So I did some more tests.
The Hum is there whether there is any input signal or not, and is lower when the reverb is turned off.
If I disconnect the Reverb TANK completely from the circuit by disconnecting the RCA cables from the RCA Jacks on the chassis, the result is the same as above.
So the Tank has no impact on the HUM.
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Also, with the tank removed, and the guitar plugged in, guitar sound (as well as the HUM) is heard at amp regardless of whether the Reverb is switch on or off. Just no reverb effect.
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Willabe-In your Reply #26 you said "You ground all the grounds from a tube section with filter cap's ground lead that feeds that tube section.
Are you saying the way I should ground the grounds from a tube section is to ground it directly with that tube's Filter Node cap?
Yes, this ^^^^^^^^^^^^
Each B+ filter cap node feeds (supplies B+ to) a tube/tubes in a section of the build. The grounds for that tube/tubes get grounded with that B+ filter caps ground lead.
This forms a ground star. Then you daisy chain all the B+ filter cap ground stars together. Then you ground the daisy chain to the chassis at only 1 point.
Read this from Merlin on grounding, go down to multiple star grounding;
https://www.valvewizard.co.uk/Grounding.pdf (https://www.valvewizard.co.uk/Grounding.pdf)
In my case, all the filter caps' Neg/GND leads are wired together and then 1 wire connects them to the GND BUSS.
That's not the way you should run your grounds. You said earlier that you "separated all the filter caps from the preamp". You don't do that. You probably were thinking of when the main B+ filter cap that feeds the OT CT (feeds the power tubes) is grounded separate from the preamp chassis ground. That's because the power tubes draw a lot of current and the ground on that filter cap if grounded with the low current draw preamp tubes will modulate the preamp tubes, causing humm. But this build has no power tubes.
And you wired up 1 of the ground busses (you have 2 busses) in the worst place. The most current in that build is at the 1st B+ filter caps ground lead with the PT's CT. So it's the noisiest ground in any build.
You did the best thing to run the PT's B+ CT directly to that 1st filter caps ground lead. But then you shouldn't grounded 1 of the ground busses with it. You should have run the buss and filter caps in separate ground stars. But at least connect that buss to 1 of the other nodes
Also, for what its worth, the two 100 ohm resistors which connect to the Pilot Light and the 6.3V filament leads for the artificial center tap, connect to the Node A Filter cap, which along with the other filter caps attach to that same GND BUSS.
The 2 xs 100ohm R's grounded there is probably ok.
Would there have been any need to use the 100 ohm resistors for an artificial center tap?
Yes, that PT's secondary heater supply has no CT.
I used a Hammond 269JX that of course has a center tap for the Secondary power supply. I connected that wire with the Primary's Green Mains ground lead directly to the chassis. Its 6.3V filament leads do not have a center tap.
I looked at the spec shet for that PT, I see no "Primary's Green Mains ground lead."
Look at how Merlin shows how to wire up a ground buss, bottom example b.
2nd Merlin drawing shows how to wire up a tubes grounds with the B+ filter cap that feeds it B+ dcv.
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Your PT spec sheet link;
EDB269JX.dcd - 269JX.pdf (https://www.hammfg.com/files/parts/pdf/269JX.pdf?v=1697661948)
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The CT for the PT's Secondary is grounded to the chassis at the same point as the PT's Green GND wire.
The 100 ohm resistors creating an artificial CT for the 6.3VAC filament leads is connected to the Node A filter cap, which is connected to Nodes B and C, which are connected to the GND BUSS. Node D is connected directly to the GND BUSS, but not to the other caps. Node E is connected to the GND connections on the Circuit Board which is only connected to the GND BUSS at 1 point at the far right end of the amp where the BUSS connects to the chassis.
Looking at the pots from the rear, with their tabs L to R being 1, 2, and 3, Tab 1 for the Volume, Bass, Output Level, Vibrato Intensity and Speed are all connected to the GND BUSS.
The Switching Input and Output jacks are also grounded to the GND BUSS.
I hope I'm not making this too confusing.
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The CT for the PT's Secondary is grounded to the chassis at the same point as the PT's Green GND wire.
The 100 ohm resistors creating an artificial CT for the 6.3VAC filament leads is connected to the Node A filter cap, which is connected to Nodes B and C, which are connected to the GND BUSS. Node D is connected directly to the GND BUSS, but not to the other caps. Node E is connected to the GND connections on the Circuit Board which is only connected to the GND BUSS at 1 point at the far right end of the amp where the BUSS connects to the chassis.
Looking at the pots from the rear, with their tabs L to R being 1, 2, and 3, Tab 1 for the Volume, Bass, Output Level, Vibrato Intensity and Speed are all connected to the GND BUSS.
The Switching Input and Output jacks are also grounded to the GND BUSS.
I hope I'm not making this too confusing.
Your grounding is bad. That's why you're getting hum
You should use the grounding scheme I posted above with the hum-loop blocker. It is based on Merlin's daisy-chained galactic ground return scheme
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Tubeswell-Thanks. Looks like I'm going to have a busy afternoon!
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The loop blocker should be the only connection between the signal ground return buss and the chassis
All other ground returns (including high-current retruns) should go to the buss BUT the high current returns go to the opposite end of the buss that the low current returns go to, and the loop blocker is at the low-current return end of the buss.
At various points along the buss, there will be clusters of returns that go with respective filter cap ground for the filter cap that is supplying those components. These clustered returns need to be daisy-chained in sequence with the order that the power supply dropping resistors/filter cap nodes are set out
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Yea, I think at least part of my problem is that Node E is carrying 258VDC and it is grounded on the low voltage ground buss on the circuit board, which then connects at only 1 point to the GND BUSS. All the other nodes connect to that GND Buss at the opposite end of the BUSS. So Node E is at the far right end and is connected to the GND BUSS with all the other grounds for the pre-amp section.
Seems like this might be fixed/improved if I disconnected the Node E cap from the GND Buss at that point and somehow get it connected all the way down to the other Nodes. I hate to have a long wire running the length of the chassis, but I don't see any other way to do that. Does this make sense to do?
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How to daisy-chain ground returns in an amp
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Yea, I think at least part of my problem is that Node E is carrying 258VDC and it is grounded on the low voltage ground buss on the circuit board, which then connects at only 1 point to the GND BUSS.......... Does this make sense to do?
No.
And it's not the voltage, it's the current.
You have 2 busses, that's not good either. And the ground connections to the busses are supposed to be laid out, gathered together in order, organized with the other grounds from that same circuit. Not randomly connected.
You don't just daisy chain all the B+ filter caps together then connect the buss ground to the filter caps daisy chain.
Look at how Merlin did the ground buss in his pictures that I posted.
And read his grounding link.
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Gotcha.
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Okay, so I've read the article by Merlin, and that is helpful.
I've gone back to my Layout and the Circuit Board, and identified the Filter cap Nodes for the connections on the circuit board's "Buss" grid. So I will rewire them so that they are connected to their appropriate Filter caps.
I will also remove the daisy chain wiring between the filter caps, and have them all go directly to a single star point, which I will connect to the Buss Bar at its far left end.
Then there are the connections to the Buss Bar from Tab 1 on some of the pots, the Input and Output jacks, the Vibrato footswitch, and the Reverb Input and Output jacks. Should all of these connections be removed from the Buss Bar, and wired to another star point, maybe using a terminal strip, and then wire that directly to the Buss Bar? Or should their Grounds also go back to the Filter Nodes that they relate to?
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Grounds go back to the Filter Nodes that they relate to
^This^
If you’re doing a signal star ground it’s got to have the loop blocker
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I guess I was planning on doing what is called the multiple star grounding in Section 15.8 of the article. I guess under that method, I could leave the 5 filter caps still daisy chained together, but run a single wire to each filter cap node from the group of ground wires coming off the signal circuit associated with each node. Those 5 nodes are daisy chained and connect to the Ground BUSS at only one point. And I wouldn't need to do the loop blocker. Does this sound correct?
If so, how about the other grounds that I mentioned that are currently grounded to the BUSS, ie. as mentioned below, the pots, jacks, foot switches? Do they need to also connect to the stars for each of the signal nodes described above, or can they stay where they are?
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I will also remove the daisy chain wiring between the filter caps, and have them all go directly to a single star point, which I will connect to the Buss Bar at its far left end.
I wouldn't do that. I would do it like Merlin and K. O'Connor and do a wired galactic ground, ie, multiple ground stars daisy chained together and grounded to the chassis at only 1 point.
Bring all the grounds together for all the tube sections being fed by a filter cap node. Then run a wire from that filter caps ground lead to those gathered grounds. That's a ground star. Then you run a wire (acting as a buss) from that ground star to the next ground star. And somewhere along that chain you run a wire for the chain to the chassis for the chassis ground.
You could have put each filter cap right next to it's star ground on the buss, look at Merlin's multi star buss drawing. It's really very easy to do.
And the chained/wired buss on the board that goes to the other buss, those board grounds need to go to the tube ground star they belong too.
And all the isolated jacks, including the ft sw jacks need to have their ground wire go to the ground star they belong too.
You never answered me if the verb ft sw jack is isolated or grounded to the chassis where it's mounted?
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And I wouldn't need to do the loop blocker. Does this sound correct?
You still need that loop blocker. Because it's 1 unit plugged into a 2nd unit. They both have chassis grounds with a 3 prong power cord. And the guitar cord plugging the preamp/vib/verb to the amp it's ground makes the loop complete.
Now if you get your grounding right, then use a 2 prong adapter, you might not need the loop blocker.
If so, how about the other grounds that I mentioned that are currently grounded to the BUSS, ie. as mentioned below, the pots, jacks, foot switches? Do they need to also connect to the stars for each of the signal nodes described above, or can they stay where they are?
YES. They ALL need to be moved to their ground star.
Tubeswell's grounding worked for him, and it will work for you if you do it as he did it.
I just don't like a single star ground. Just different thinking. That's a lot of wires running across the chassis to 1 ground star point.
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Thanks.
Sorry, yes the foot switches are isolated too using isolation washers. The Reverb footswitch is grounded to the Reverb Intensity Pot, Tab 1, the far left tab on that pot looking from behind. That pot is grounded via its connection to the chassis.
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The reason for keeping the signal ground in a separate wire (as Willabe notes) is to only reference this to the chassis at 1 point (via the loop blocker).
We do this at the input end of the signal ground buss because that is the most noise-sensitive gain stage, and we want the filter cap for this stage to be riding as close as possible to the chassis ground reference point so that any micro voltage changes that might be in the buss which emanate from higher-current parts of the ground return (or even from radio wave interference) result in the least amount of interference with the most noise-sensitive stages in the circuit. (Think of the signal ground return as carrying all the return currents - and changes in current).
If you have multiple chassis contact points, you create multiple loops which exponentially increase loop current feedback/hum. In a standard guitar amp, this usually isn't much to complain about, but because this is a separately powered effects unit in the signal chain, which has to have it's own mains safety ground chassis connection, you have to work harder to eliminate sources of ground loop feedback.
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Noting that you report that the hum occurs when the reverb is activated (but isn't present on the dry signal) it seems that the problem is grounding of the signal returns for the (highly noise-sensitive) reverb recovery stage is getting muddled/overwhelmed by higher current returns. As this is a stand alone effects unit that is intended to be plugged in in front of a guitar amp, you might want to consider regrouping the ground returns and installing a signal ground hum-loop blocker between the ground buss and the chassis of this stand alone unit. This should significantly quieten/dampen loop hum.
Here's an example of how I wire 6G15 ground returns to eliminate loop hum - I can vouch that this yields a quiet noise floor. The same principle of galactic grounding around filter cap nodes with a daisy-chained buss to a hum-loop blocker can be applied to your unit - but it may require a bit of re-work.
Or maybe you're happy to live with it
Tubeswell, got any gut shots of the reverb unit with that wiring? Really curious how you solder up that many ground wires to a single point....
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I was going to start working on the grounding changes today, but wanted to clarify something.
If you look at my Layout, what I understand I should do is to disconnect all of the components attached to the "ground bar" at the top edge of the circuit board, and wire them to individual "star points", which would then be wired to their corresponding filter cap power source. These components that I would be disconnecting are associated with Nodes B, C, D, and E. Are these the only "signal grounds" that I need to deal with?
Or do I only need to deal with the grounds that are shown in the Reverb Driver/Recovery section of the Schematic starting with the Ground at the Tank's Input jack and going through the Ground on the V7B 2.7K Cathode resistor?
Since the unit has no hum when the Vibrato is ON, or when the Vibrato and Reverb are OFF, it seems like only the Grounds in just this section need some changes.
Is there a way I can use my Oscilloscope to identify where the hum originates?
Thanks for your patience.
There are many more grounds shown on the Schematic, but I'm assuming that this is what is being suggested I do.
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I connected my scope to the output jack, with no input signal, AC coupling. It showed a slightly fuzzy sine wave of about 10hz with about 16mVRMS. The scope was set to 50mV/div which is the smallest it will go.
I was expecting a 60hz signal.
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Is there a way I can use my Oscilloscope to identify where the hum originates?
yes n no
If the source is a "stage" along the signal path, ya; signal good, signal good, oh, that's ugly...
If the "source" is the PS, either the B+ side or ground side, then NO, it will be "riding" on that "line" everywhere you look.
the important part you should "see"; What is the frequency and amplitude, and which "line" is it most prevalent... ground buss, B+ buss
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It showed a slightly fuzzy sine wave of about 10hz with about 16mVRMS.
the frequency is within the "tremolo range"
monitor the 10Hz, turn trem on/off... any change??
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I just realized I didn't have the foot switches plugged in. The tank is not connected. Now the foot switches are connected.
When I turn On the reverb I get a 120hz 6mVRMS sine wave. When I turn it off, I get a straight line. I connected the tank and get the same readings.
When I turn On the vibrato I get a much larger signal, that moves. So I think that's normal. Its 4hz at 101mV.
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I turn On the reverb I get a 120hz 6mVRMS sine wave.
When I turn it off, I get a straight line.
now move scope to the REV driver (before tank) and repeat.
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Do I connect the probe at the Reverb Input Jack or on the V6's 1uf coupling cap negative side, or where? Thanks.
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I connected the tank, and then connected the probe to the Neg. side of the 1uf cap that comes off the V6 plates that are connected together. With the reverb ON the signal is fuzzy and jumping all around but it looks like it's closer to 60hz or less, but those numbers are continually changing.
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I'm probably not doing something correctly with the scope.
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VERIFY your scope probe is rated 350VDC or more.
IF so;
probe each of the PS TAPS B-D measuring VAC (do this with BOTH trem n VERB OFF) record values
Repeat with VERB ON
post
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Shooter--My probe is rated up to 200VDC in 1X mode and up to 600VDC in 10X mode. So I'll set the probe and the scope to 10X.
I'll set the scope to AC coupling.
Then connect the GND lead to chassis.
Then use the probe to check AC voltage at the + side of the Nodal caps, B, C & D. Correct? When you say "PS Taps" I assume that's what you mean? I'm not experienced with the scope at all, so that's why I'm asking all these questions.
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..... what I understand I should do is to disconnect all of the components attached to the "ground bar" at the top edge of the circuit board, and wire them to individual "star points", which would then be wired to their corresponding filter cap power source. These components that I would be disconnecting are associated with Nodes B, C, D, and E. Are these the only "signal grounds" that I need to deal with?
Yes, disconnect all the components attached to the (wired) ground buss at the top of the board.
And disconnect all the components attached to the ground buss above all the pots.
Then wire ALL the components grounds to their own individual ground star on the buss either above the pots, or on the board. Just 1 buss, not 2. Gather the stars ground leads close together on the buss, right next to each other, touching. Look at the drawing attached, 15.10 b. All grounds in the star brought very close together then buss wire separating that star from the next ground star.
Then disconnect the ground's on the filter caps from each other. They should not be daisy chained together. That would cause a ground loop together with the ground buss.
Then run a single wire from each ground star over to it's own corresponding filter cap ground lead. Should be 4 wires total, nodes B,C,D,E.
Leave node A as it is, with the PT's CT going right to node A's caps ground lead. Then run a single wire from node A's ground lead/PT CT over to the very end of the ground buss closest to the filter caps. Make sure there's some distance (wire) between that ground connection to the buss and the ground star closest to it. Do not connect it to any ground star.
Leave the ground buss connected to the chassis at the input jack. That will be the only chassis ground. (Except for the power cords 3rd safety wire ground. That stays grounded at the other end of the chassis.)
I'd try it like this and use a 3 prong to 2 prong adapter and see it the humm is gone. If not then I'd add the hum loop blocker tubeswell showed.
Or, just go ahead and put the hum loop blocker in now. :dontknow:
Or do I only need to deal with the grounds that are shown in the Reverb Driver/Recovery section .... Since the unit has no hum when the Vibrato is ON, or when the Vibrato and Reverb are OFF, it seems like only the Grounds in just this section need some changes.
Ok, but how will you do that? The grounds are all tangled up with 2 busses and the daisy chained B+ filter caps. You'd have to figure how to untangle the verb section and add it back in to the ground buss somehow.
I'd just redo the whole thing. To me it will be easier.
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VERIFY your scope probe is rated 350VDC or more.
IF so;
probe each of the PS TAPS B-D measuring VAC (do this with BOTH trem n VERB OFF) record values
Repeat with VERB ON
post
He has an E node too.
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Willabe-Thanks for your detailed explanation, and the time it took for you.
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oops, E also
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Shooter-When you have a chance could you answer my question about the scope setup in #70 above? Thanks.
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:laugh:
when I "liked" it that was affirmation, add TAP E, it was early
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Thanks.
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Shooter, all the Nodes read 2.04Vrms when reverb is On and Off.
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60hz, 120hz, some other frequency?
sorry didn't specify that earlier.
once you have that;
re-check with your DVM, AC volts(milli-volts)
THEN;
move the NEG probe to each "ground" point for every tube, you can leave the RED + lead on any TAP, what i'm looking for is a "bump" in the otherwise "constant" readings
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It was showing 0Hz on all of them. What should the scope’s settings be? Mine doesn’t go below 50mv/div.
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move on to the DVM test for now.
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Ok, for the DVM test, I connected the red lead to Node D+, set the meter to VDC not VAC, and then used the black lead to measure each connection at the Ground BUSS. They all read the same 282VDC.
The Cathodes of the tubes get to the ground BUSS through some component, ie. their cathode resistors. None go directly from their cathode to the BUSS.
Everything is grounded at the BUSS, except for maybe a pot that is grounded to the chassis through its direct connection to the chassis, and the Green Mains ground which has its own separate ground to chassis.
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Everything is grounded at the BUSS, except for maybe a pot that is grounded to the chassis through its direct connection to the chassis, .....
Which pot?
If it's for the verb output, that could be the problem.
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They all read the same 282VDC.
DVM, AC volts(milli-volts)
testing a ground noise problem is "finding" a point where "resistance" is higher than the rest of the grounds
ways to do that;
ohming chassis to each stages ground point, sketchy if you're trying to measure milli-ohms with a typical meter
measuring the "AC" creeping along the ground path. also sketchy with a typical meter but yields better results than measuring ohms.
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Wallabe-The Output Level pot has its Tab 1, far left tab looking at rear of pot, connected to the BUSS. Of course the pot is screwed to the chassis. Would that be the cause of the HUM? How else would that tab go to Ground as shown in the Schematic?
The Reverb Intensity pot has its Tab 1 connected to the Reverb F/S Jack's ground tab, and that Jack is isolated from the chassis.
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Shooter, when I had the DM in AC millivolts setting I was getting OL, so that's why I switched over to VDC thinking that's what you meant since I couldn't get a reading otherwise.
Looks like I'm running up on a dead end with the equipment I've got. I don't think my scope is sensitive enough for the job either.
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Wallabe-The Output Level pot has its Tab 1, far left tab looking at rear of pot, connected to the BUSS. Of course the pot is screwed to the chassis. Would that be the cause of the HUM?
Wallabe? :laugh:
All 3 solder tabs on a pot are isolated from the pots metal back cover. That cover is just for shielding of the pots internal resistive track.
So no that wont cause humm, it helps stops any humm.
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WILLABE!! Yes, that's what I thought. I did dis-connect it though just to see if the hum was affected and it wasn't.
I got a Spectrum Analyzer App to check the frequency and it 120hz. There's about a 4db difference between the levels when the reverb is Off vs. On.
At least now I can tell if its 120 or 60hz!!
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Willabe-I'm going to work on your suggested re-wiring this weekend and need some clarification.
Node B has 3 components; C has 5; D has 1; and E has 3. So that's 12 wires to connect to 4 filter caps. That's a lot of wires to be added to the chassis, and I'm trying to figure out the neatest way to do it.
I've thought of adding a terminal strip to an isolated mounting screw at the center, top edge, of the circuit board. I would use 4 of its terminals. Then 1 wire from each of those 4 filter caps would be connected to each terminal. So those 4 wires would be coming from left to the center of the chassis where the terminal board is. Easy.
Now the hard part is getting the wires neatly from the 4 sets (B,C,D, & E) of components to the terminal strip. Do I have to run separate wires from each component to its corresponding terminal, or can each group of components be soldered together and then 1 wire run from each group to the terminal? If they can be soldered together, can that be on the Ground BUSS above the circuit board? I can't just have them dangling. Or is there some better way to do it?
Then Filter Cap A remains an issue. The CT for the PT Secondary is grounded and mounted on the same screw as the Mains Safety Ground wire. It is located under the circuit board. It will be a real PIA to take out the circuit board to get access to that CT wire. (I didn't realize at the time I installed it that it would be covered by the board). Can I just wire that cap directly to the Ground BUSS wire?
Finally, the C Node has a ground from Tab 1 on the 1M Intensity Pot and a ground from the Reverb F/S, as well as 3 other ground connections. That F/S is isolated from the chassis. Can the F/S ground lug be wired to Tab 1 on the Pot, and then a wire run from Tab 1 to the other components for that C Node?
Hopefully this will be all I need. Thanks again.
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This is all still pretty much the same question. Answer;
Use a ground buss with multiple ground stars on it.
We have shown you several pictures how to do this from Merlin. (See below at the bottom.)
Node B has 3 components; C has 5; D has 1; and E has 3. So that's 12 wires to connect to 4 filter caps. That's a lot of wires to be added to the chassis, and I'm trying to figure out the neatest way to do it.
I've thought of adding a terminal strip to an isolated mounting screw at the center, top edge, of the circuit board. I would use 4 of its terminals. Then 1 wire from each of those 4 filter caps would be connected to each terminal. So those 4 wires would be coming from left to the center of the chassis where the terminal board is. Easy.
No, not easy. Use a ground buss, not a terminal strip.
Looking at the schematic drawing in reply #6, your #'s are way off, low.
Do I have to run separate wires from each component to its corresponding terminal, or can each group of components be soldered together and then 1 wire run from each group to the terminal? If they can be soldered together, can that be on the Ground BUSS above the circuit board? I can't just have them dangling. Or is there some better way to do it?
I'd use the ground buss on the back edge of the board. The buss stars cuts down on wires and their length.
Then Filter Cap A remains an issue. The CT for the PT Secondary is grounded and mounted on the same screw as the Mains Safety Ground wire. It is located under the circuit board. It will be a real PIA to take out the circuit board to get access to that CT wire. (I didn't realize at the time I installed it that it would be covered by the board). Can I just wire that cap directly to the Ground BUSS wire?
Disconect the A node filter cap ground from the chassis ground. You should be able to do that from the circuit board and not from the chassis below. And the power cord safety green wire should be bolted to the chassis by itself, nothing else should be grounded with it.
Finally, the C Node has a ground from Tab 1 on the 1M Intensity Pot and a ground from the Reverb F/S, as well as 3 other ground connections. That F/S is isolated from the chassis. Can the F/S ground lug be wired to Tab 1 on the Pot, and then a wire run from Tab 1 to the other components for that C Node?
Yes, that should work.
Yes, disconnect all the components attached to the (wired) ground buss at the top of the board.
And disconnect all the components attached to the ground buss above all the pots.
Then wire ALL the components grounds to their own individual ground star on the buss either above the pots, or on the board. Just 1 buss, not 2. Gather the stars ground leads close together on the buss, right next to each other, touching. Look at the drawing attached, 15.10 b. All grounds in the star brought very close together then buss wire separating that star from the next ground star.
Then disconnect the ground's on the filter caps from each other. They should not be daisy chained together. That would cause a ground loop together with the ground buss.
Then run a single wire from each ground star over to it's own corresponding filter cap ground lead. Should be 4 wires total, nodes B,C,D,E.
Leave node A as it is, with the PT's CT going right to node A's caps ground lead. Then run a single wire from node A's ground lead/PT CT over to the very end of the ground buss closest to the filter caps. Make sure there's some distance (wire) between that ground connection to the buss and the ground star closest to it. Do not connect it to any ground star.
Leave the ground buss connected to the chassis at the input jack. That will be the only chassis ground. (Except for the power cords 3rd safety wire ground. That stays grounded at the other end of the chassis.)
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When you do a layout from scratch, you make it with the grounding in mind. The grounding should not be an after thought.
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Thanks for the clarification. Helpful.
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Willabe-I started out with re-wiring the Green Power Ground and the R/Yellow Center tap which were previously sharing the same connection to chassis. Now, the Green Power Ground is connected by itself to a lug on the chassis. Then as you suggested, I wired the R/Y Center tap to the Node A Filter Cap's Ground lead, and then connected that lead to the Ground BUSS at the end by the other filter caps.
I previously had connected the ground leads of the two 100 ohm resistors for the filament 6.3VAC wiring artificial center tap to the A Cap's ground lead. Where should those ground leads for the resistors be connected now?
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You can put the ground leads of those two 100 ohm resistors about a 1" or 2" further away/apart from the R/Y Center tap to the Node A Filter Cap's Ground lead on the ground buss. Just a little distance away so the caps charging current can cycle without disturbing anything else.
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Willabe-I'm finishing up the ground stars. I need some clarification on your comments at #71 above. I understand to disconnect the daisy chain between the filter caps. Am I correct that the ground stars don't connect to the Ground Buss but they connect only to the respective Filter Caps' ground leads, and that only the Filter caps' ground leads connect to the BUSS?
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Am I correct that the ground stars don't connect to the Ground Buss but they connect only to the respective Filter Caps' ground leads, and that only the Filter caps' ground leads connect to the BUSS?
No. You have it backwards.
What do you see in the ground buss drawing from Merlin?
You use a buss because it runs the length of the chassis, passing by ALL the different circuits and their ground leads. You form the ground stars on the buss.
You gather together ALL the ground leads from a single B+ nodes circuit that it feeds. They go to the buss, gathered tightly together. Then you run a wire from the B+ nodes filter caps ground lead over to the buss ground star for that nodes cap.
I've written this out several times times for you, it's not hard to do or understand.
Look at the drawing below, the bottom example, b. Wire it just like that. Just imagine that the filter cap is further away, but it's the same thing, just a longer filter cap ground lead wire.
There's 2 separate ground stars in that drawing in the center, see the 2 blue filter caps in the center? Their both on the buss.
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Here this drawing from Merlin, that tubeswell posted way back, might be clearer for you. It even has ground stars labeled on it. Says local star.
The ground stars are connected by what's labeled as daisy chain, but a buss is pretty much the same thing, both do the same thing.
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I need some clarification on your comments at #71 above. I understand to disconnect the daisy chain between the filter caps. Am I correct that the ground stars don't connect to the Ground Buss but they connect only to the respective Filter Caps' ground leads, and that only the Filter caps' ground leads connect to the BUSS?
It seems pretty clear;
Yes, disconnect all the components attached to the (wired) ground buss at the top of the board.
And disconnect all the components attached to the ground buss above all the pots.
Then wire ALL the components grounds to their own individual ground star on the buss either above the pots, or on the board. Just 1 buss, not 2. Gather the stars ground leads close together on the buss, right next to each other, touching. Look at the drawing attached, 15.10 b. All grounds in the star brought very close together then buss wire separating that star from the next ground star.
Then disconnect the ground's on the filter caps from each other. They should not be daisy chained together. That would cause a ground loop together with the ground buss.
Then run a single wire from each ground star over to it's own corresponding filter cap ground lead. Should be 4 wires total, nodes B,C,D,E.
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It seemed to me, apparently incorrectly however, that if you connect Filter Cap ground node A, to the Node A star ground which is attached to the BUSS, that when all the Filter caps get connected on their separate stars which are all wired on the same BUSS, that the grounds would all be picking up all of the nodes of the star points since everything is connected to the same BUSS. And that's what I assumed was causing the ground loops. Sort of like if you have 4 shots of liquor: bourbon, gin, vodka, and scotch, in separate shot glasses (the star ground points), and then pour them all into a big glass of tequila (the BUSS). You've then got everything blended together, and it will be bad (ie. Ground loop).
But I'll stop thinking about the "logical" theory of this methodology, and just do it. Maybe after I have a bourbon--by itself!! Ha Ha. Thanks.
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It seemed to me, apparently incorrectly however, that if you connect Filter Cap ground node A, to the Node A star ground which is attached to the BUSS, that when all the Filter caps get connected on their separate stars which are all wired on the same BUSS, that the grounds would all be picking up all of the nodes of the star points since everything is connected to the same BUSS. And that's what I assumed was causing the ground loops.
How is that any different then how you have it wired now? All the grounds are connected.
All the component grounds, including the pots go to 1 or the other ground buss and the daisy chained filter caps all go to the upper ground buss, so do all the jacks and ft sw.
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...... that the grounds would all be picking up all of the nodes of the star points since everything is connected to the same BUSS. And that's what I assumed was causing the ground loops.
If you ground both ends of the shield on a shielded cable, that's a ground loop.
Signal runs down the shield, to the chassis, through the chassis over to the other end of the shield, runs back down the shield, to the chassis, through the chassis, over to......
And round and round it goes. That's a ground loop.
Same thing happens if you ground both ends of the ground buss to the chassis. It makes a ground loop.
Signal runs down the buss, to the chassis, through the chassis over to the other end of the buss, runs back down the buss, to the chassis, through the chassis, over to......
Round and round it goes.
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Good explanation.
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Finished it up, and the hum seems much better. It's still there but much improved.
However, now I've got a problem with the 6.3VAC filaments not heating up.
The amp had been running fine, and I let it run for awhile and turned it on and off several times with no issues.
Then I turned it on to check my guitar's sound, and now I'm not getting any sound. The filaments are not heating up. The pilot light is on, and the filaments get their power from there. There is continuity from the pilot light wiring to all of the filaments.
The filament voltage seems high however at 6.9VAC. The only thing I noticed is that the wiring on the 2 tabs on the pilot light socket appeared to be touching, ie. 1 wire on one tab was touching the other tab. So maybe a short. I was adjusting the pilot light bulb holder this weekend and maybe it got into a position that caused a short. Could that have damaged the transformer's filament windings? But could the pilot light still illuminate if there was a short/damage in the secondary filament supply?
The caps are all getting proper voltages, so I think the Secondary winding is okay.
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The filament voltage seems high however at 6.9VAC.
You have heater acv, so the PT's heater wind should be good.
If a PT secondary has no load on it it's acv will be high.
Being very careful, put the amp in a room where you can turn out the lights, black out drapes over the windows, so it's totally dark. Turn the amp on, then black out the room, look to see if every tubes heater is lighting up. Some tubes heaters light might be just barely visible, look closely.
Also check the 2 @ 100 ohm faux CT R's. Might have burned up.
This should tell you what's wrong.
When you measure heater acv, use alligator clips on your meter leads, clip 1 lead on 1 tube socket heater pin and clip the other lead to the other tube socket heater pin. Do not clip 1 meter lead to a socket pin and the other meter lead to ground.
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Ok, looks like it's either with the Reverb F/S Jack, or the switched style Output Jack, or the Output cable. Whewww!
Hopefully a much easier fix.
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Well???
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I haven't had a chance to get to it yet. I want to re-solder the wiring at the pilot light to make sure there are no dry joints. That was what I re-positioned in doing the ground wiring, and since at one point I didn't have any voltage at the tube heaters but had continuity between them and the pilot light, I thought best to start there.
The reason I mentioned the jacks was thinking it was a sound output issue, and of course the jacks wouldn't prevent voltage getting to the filaments.
I'll keep you posted. Thanks.
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Interesting development. I was reviewing the circuit on the Reverb Intensity pot and found an issue. Currently the Reverb jack is, and has been, isolated from the chassis, and its Ground point is only connected to the #1 far left tab on the Reverb Intensity pot. That tab is not connected to any ground point. The schematic shows that it should be, but it's not. I had previously disconnected its ground, but it still functioned, mysteriously. The pot is not isolated from the chassis, and there is continuity between the chassis and back of the pot. If no plug is plugged into the Reverb jack, there is no continuity between the jack's ground point and the chassis or the BUSS--as it should be. BUT, if I plug in the plug that connects to the footswitch, there is continuity between the jack's ground lug and chassis. There shouldn't be any continuity. If I take just an un-wired 1/4" plug, and plug it into the reverb footswitch jack, there is NO continuity between the jack's ground lug and the chassis or the BUSS, as I think it should be. The footswitch is a dual switch for Reverb and Vibrato. It has a stereo 1/4" plug. Since I'm using 2 separate isolated, non-switched, 1/4" jacks, I have a stereo to mono adapter that connects the stereo plug on the footswitch, to the mono adapter, which then plugs into the jacks.
Seems to me that somehow, the mono reverb plug on the adapter is grounding out somehow, even if the adapter is not plugged into the stereo footswitch plug.
If I'm correct then this grounding is probably also affecting the Hum. So I need to deal with this plug. Thoughts.
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Interesting development. ......
If I'm correct then this grounding is probably also affecting the Hum. So I need to deal with this plug. Thoughts.
Before we go chasing rabbits, you last said the build was not working at all, no output.
Is it now working?
If it's working, is the humm gone, mostly gone?
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Yes, it started working. I think it was a cold solder joint at the pilot light. I re-soldered that connection just to be sure.
The hum is certainly improved. When I was trying to fix the above problem when the heaters weren't getting power, I was plugging and unplugging jacks, and noticed that I had gotten the power to the filaments back and it was making sound, but the reverb jack/plug was acting erratically and there was some feedback.
So I think we're dealing with 2 different issues. The power to the filaments, which I think is resolved. And now the reverb jack issue I have described.
I'm thinking that maybe if I fix the plug that is shorting to ground somehow internally, then I can properly wire the #1 lug on the Intensity pot to the ground BUSS. By fixing the plug, that will eliminate the Intensity pot grounding to the chassis, which may be causing a ground loop. And by wiring #1 lug on that pot like is shown on the schematic to the BUSS it won't have 2 ground points to create a ground loop.
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and its Ground point is only connected to the #1 far left tab on the Reverb Intensity pot. That tab is not connected to any ground point. The schematic shows that it should be, but it's not.
Hook up that ground connection to the buss. Do this 1st.
I had previously disconnected its ground, but it still functioned, mysteriously.
See below for why it still worked.
If I take just an un-wired 1/4" plug, and plug it into the reverb footswitch jack, there is NO continuity between the jack's ground lug and the chassis or the BUSS,
This is a big clue. ^^^^^^^^^^^^^^^^^
The footswitch is a dual switch for Reverb and Vibrato. It has a stereo 1/4" plug. Since I'm using 2 separate isolated, non-switched, 1/4" jacks, I have a stereo to mono adapter that connects the stereo plug on the footswitch, to the mono adapter, which then plugs into the jacks.
The stereo to mono adapter is isolating the tips/hot but sharing the ground. So your picking up the vibrato ft sw jacks ground.
If I'm correct then this grounding is probably also affecting the Hum.
No it's not affecting the humm.
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You said the chassis verb jack is a stereo TRS, yes? That's why you need a stereo to mono adapter to use with the verb/vib ft sw. Which has 2 separate jacks.
Right now on the chassis stereo TRS isolated ft sw jack, do you have any wires hooked to it's ground?
You said you have a wire from that ft sw jacks ground lug going to the far left verb pots lug. (Yes I know it's not wired to the buss.)
But did you also hook up any other wire to that verb/vib ft sw ground lug? Like maybe you ran a wire from the verb/vib ft sw ground lug over to the B node filter cap that feeds the vib osc/driver?
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If you stay here for a little while I think we can finish this.
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I connected the wire from the #1 lug on the Intensity pot to the Ground BUSS, and I would say the HUM is fixed!
In my post #88 I measured the noise difference between Reverb On/OFF at about 4 db. Now it's down to a difference of 1.2db. Really very quiet.
Also, I get no increase in Hum when I change Volume, Reverb Intensity, or Output Level. Before I was getting louder hum when I increased the Output Level. So that's great. I think the Output Level can now be adjusted to get unity gain with the input signal and no increase in noise.
I think the little bit of hum that's left is normal and expected with a tube amp.
So I'm very happy!
As to your question, the chassis reverb jack is Mono, but the plug on the cable from the dual Reverb/Vibrato F/S to the Reverb and Vibrato mono jacks is Stereo. So that's why I needed the adapter.
So I think it's time for a drink!! Thanks everyone for your help and patience. The re-wiring of the grounds to match up with the Filter Cap Nodes really seemed to be the trick. Thanks again.
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I connected the wire from the #1 lug on the Intensity pot to the Ground BUSS, and I would say the HUM is fixed!
:blob8: :happy1:
Now put a 3 prong to 2 prong adapter on the verb/vib units power cord plug.
Any better? Does it lower the last little bit of humm any?
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As to your question, the chassis reverb jack is Mono, but the plug on the cable from the dual Reverb/Vibrato F/S to the Reverb and Vibrato mono jacks is Stereo. So that's why I needed the adapter.
Ok, my mistake, got it backwards, there's 2 chassis mono ft sw jacks. The ft sw cable has stereo TRS jack on it's cable end.
The footswitch is a dual switch for Reverb and Vibrato. It has a stereo 1/4" plug. Since I'm using 2 separate isolated, non-switched, 1/4" jacks, I have a stereo to mono adapter that connects the stereo plug on the footswitch, to the mono adapter, which then plugs into the jacks.
The stereo to mono adapter is isolating the tips/hot but sharing the ground. So your picking up the vibrato ft sw jacks ground.
That's where it was picking up the ground for the verb intensity pot. But it didn't like it and hummed.
When you wired the verb pot ground, which also has the ft sw ground wire connected to it, to the buss at the C node, it liked it, and most/a lot of the last remaining humm went away. This was after all the other re-grounding making a bussed multi star ground system, had already killed a great deal of the humm.
Bussed multi star ground, nice.
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2 last humm level tests;
Because that ft sw jack has a shared ground on it's stereo to mono adapter, try this;
1. With the reverb and output volume where you normally will use it, turn off the vibrato with the ft sw, listen for humm, now turn on the vibrato with the ft sw, listen for humm, any difference?
2. How about with the ft sw unplugged, any difference in humm?
And do both the verb and vibrato work with the ft sw unplugged?
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I wired that Reverb Ground to the Node E connection and not Node C. I guess I could change that wiring if you think it might make a difference.
Using the 3 prong to 2 prong converter for the power plug, I really can't notice any difference. If there is any, its really really a small amount of decrease.
The answer is No to both of your questions. The hum is very minor but not affected by those changes.
Yes, with both Reverb and Vibrato unplugged, both are running.
The Output jack I'm using is a switched style, but I'm not wiring in the switch leg. So the middle leg of the jack is not wired, just the 2 outer legs go to input source and ground BUSS.
Should I wire the middle one into the circuit with the left side leg, or just leave it be? Should I just use a regular 2 leg jack for the Output jack? And finally should I isolate that jack from the chassis?
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I just checked and the main Output jack that connects to the amp is already isolated from the chassis.
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I wired that Reverb Ground to the Node E connection and not Node C. I guess I could change that wiring if you think it might make a difference.
Yes, I would, that was the whole point of all this. Just un-solder it, and use a gator clip cable to try the C node ground star. It might be a little better, if not put it back where it was. E node verb recovery triode is directly before and drives the C node output triode.
It seems that the verb intensity pot liked being grounded to the buss E node better than sharing the vibrato B node ground star through the stereo to mono adapter ground.
The Output jack I'm using is a switched style, but I'm not wiring in the switch leg. So the middle leg of the jack is not wired, just the 2 outer legs go to input source and ground BUSS.
I think the vib jack's switch grounds the oscillator which turns it on so the vib works without the ft ws. Take a short gator clip cable and with the ft sw unplugged short the vib ft sw tip to ground.
Does the vib work now?
If so wire up the jacks switch if you want the vib to work without the ft sw plugged in.
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Sorry, you misunderstood. The Vibrato works fine. I was talking about the output jack from the revive to the amp, not the vibrato output jack. I was wondering if I should wire the main output jack so it is functioning as a switched jack, or change it out for a normal jack, ie. not a switching jack.
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I don't know why Sluckey drew that output jack with a tip switch.
I can't think of a reason you would want to kill the output if nothings plugged in? Nothing would burn up with nothing plugged in.
You can use that jack, but I'd leave it, just don't wire up that switch, you don't need it.
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Thanks for the confirmation.
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Also, I get no increase in Hum when I change Volume, Reverb Intensity, or Output Level. Before I was getting louder hum when I increased the Output Level. So that's great. I think the Output Level can now be adjusted to get unity gain with the input signal and no increase in noise.
After you get that last ground wire from the verb intensity pot to the right node, try below;
Test it for unity gain, and see if there's any noticable humm. You couldn't do that before, at unity gain it had too much humm to run at that setting. I know you said you think you could now, but lets see for sure.
Then see if you can push it past unity gain some, any humm increase. Sometimes guys will use a unit like this for a little boost to push the amps front end a little. Comes in handy for some things.
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I think the little bit of hum that's left is normal and expected with a tube amp.
Are you testing with Hummbuckers or single coil PUP's?
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I think the little bit of hum that's left is normal and expected with a tube amp.
Are you testing with Hummbuckers or single coil PUP's?
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I'm using single coils and also a 1khz signal injector. I didn't notice any real difference in hum level changing the ground on the Reverb Intensity from Node E to Node C. The hum with the Reverb On is still very minor, and it barely changes at all with increase in Output Volume or Reverb Intensity or Master Input Volume. I am also able to get unity gain now of a 1khz, 150 Vrms, signal where before I couldn't take it past about 26 Vrms due to the hum. So this is great. I can take the Output Volume all the way up and exceed unity by just a little bit, but then I can increase the Master Input Volume and push it further, but the scope shows the signal getting some distortion at that point. So all of this is great. I don't think there's anything left to do.
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The hum with the Reverb On is still very minor, and it barely changes at all with increase in Output Volume or Reverb Intensity or Master Input Volume. I am also able to get unity gain now of a 1khz, 150 Vrms, signal where before I couldn't take it past about 26 Vrms due to the hum. So this is great. I can take the Output Volume all the way up and exceed unity by just a little bit, but then I can increase the Master Input Volume and push it further, but the scope shows the signal getting some distortion at that point. So all of this is great. I don't think there's anything left to do.
Agree, that's great, very happy for you. You did a good job! :icon_biggrin:
How does the Maggie pitch shift vibrato sound? I love my Maggie's vibrato.
I didn't notice any real difference in hum level changing the ground on the Reverb Intensity from Node E to Node C.
That was worth trying, now you know.
I'm using single coils....
That's great, even with single coils it's very quite. Might even loose the last of that humm with Humbuckers.
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Yea, the Varistors in the Maggies for the true pitch shifting Vibrato are really cool. Love them. This mod was based on the M10A circuit. I have a 1964 Gretsch Country Gentleman that I got in high school, so its humbuckers really give a nice sound too.
I haven't seen Sluckey on the Forum recently. Is he still active, or .... ?
Thanks again for all your help.
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I haven't seen Sluckey on the Forum recently. Is he still active, or .... ?
No, not here;
https://el34world.com/Forum/index.php?topic=31909.msg351724#msg351724
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Yea, the Varistors in the Maggies for the true pitch shifting Vibrato are really cool. Love them. This mod was based on the M10A circuit.
Probably sounds best driving the varistor's harder by turning up the input volume and using the output volume for just enough output to get unity sound. The more you push those varistor's the more intense the vibrato should be.
Should be able to get a much fuller vibrato now. And that was part of the problem with having to turn down the output control to keep the humm tolerable.
I have a 1964 Gretsch Country Gentleman that I got in high school, so its humbuckers really give a nice sound too.
I'm sure it sounds great!
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Yea, the Varistors in the Maggies for the true pitch shifting Vibrato are really cool. Love them. This mod was based on the M10A circuit.
Probably sounds best driving the varistor's harder by turning up the input volume and using the output volume for just enough output to get unity sound. The more you push those varistor's the more intense the vibrato should be.
Should be able to get a much fuller, richer, stronger vibrato now. And that was part of the problem with having to turn down the output control to keep the humm tolerable.
I have a 1964 Gretsch Country Gentleman that I got in high school, so its humbuckers really give a nice sound too.
I'm sure it sounds great!
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Yes, I love the CG. Just wish I could play a lot better. I'm better with rhythm than lead. I use round wound strings most of the time on the CG, and flat wounds on the Warmoth Strat I built. The flats give it a much more mellow tone.
As to the vibrato, yea the varistors really give it a different sound than tremolo. I built a mod of the new Magnatone Twilighter amp, which has both tremolo and vibrato, but I decided just to build it with tremolo since I had this unit for Vibrato. The tech at Magnatone share their schematic with me which hadn't circulated on the internet a couple of years ago. What I've found that's really neat is to have both the Vibrato and the Tremolo on together. You really get a great effect, a lot like an Echo effect.
Anyway, nice to have this much improved and pretty damn quiet. The biggest change is how the Output Level is now much more useable. I don't know how the grounding was affecting the Output Level pot's "sensitivity", and maybe you know what was happening, but it now seems to have the range of a 100K pot vs. what it was like before.
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Is the vibrato stronger now that you can turn it up?
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I never had a problem with the Vibrato. Always very quiet, no hum. So it's great. The Reverb can go all the way without being an issue. The hum might increase a very minor amount but that's only when its Intensity gets 90-95% fully turned up. Its pretty consistently quiet all the way up.
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I got the "Revibe" reinstalled into its cabinet, and hooked it up to my amp, and it's incredible! It's even quieter now than when the chassis was out of the cabinet and on the work bench when I was testing it. The "hum", and I don't even think it would be appropriate to use that word anymore, is almost totally gone. I would say I should use the Italian word "Perfetto"! to describe it.
This has been a long journey over the years! A Long and Winding Road. But finally got there! And I learned so much along the way. Thanks so much for everyone's help. Best.