Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on November 23, 2025, 12:26:43 pm
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I've traced 120hz hum down to the reverb section of a ReVibe Mod that seems to be coming from the B+ C node, 30uf cap. I think it may be in the cap because other components in the circuit pick up the hum at that point.
I hooked up my scope's probe to the terminals on that cap, and set the scope for 100mv/div with AC coupling.
Although I sporadically got an image of a sine wave of around 120hz, I did get voltage on the scope that reads about 750mVrms.
Is this methodology valid, and is this indicative of a bad cap?
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Although I sporadically got an image of a sine wave of around 120hz, I did get voltage on the scope that reads about 750mVrms.
the voltage constant or sporadic also??
you can also use a Meter, set it to VAC, start at the 1st filter cap and work your way "in" to the last node.
typical would be something like;
2-4VAC at 1st tap, 100mV at 2nd tap, 50mV, 20mV.....
each tap should "kill" the AC more n more.
If.. the VAC "spikes up" at a tap, that node should be suspect.
a quick test, gator-clip a bigger 50-100uF cap in parallel with your 30uF (MAKE SURE voltage rating is equal or greater than what's there)
you should "hear" a decrease in background hum
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Interesting results:
ESR meter shows all 5 caps, Nodes A-E, have proper readings.
I don't have a 50-100uf cap to test, just a 20uf.
Node A: This Node does not connect to any points in the circuit. It just receives the power off the full wave rectifier, and passes the signal along to Node B, and on.
1.81 vRms--STEADY
120Hz Big sine wave skewed at top
Node B:
1.01 vRms-STEADY
120Hz large sine wave-pretty smooth-not as big as in Node A
Node C:
638-655 vRms- fairly steady
0 Hz--sometimes tiny sine wave appears-scope reads 0 Hz
Node D:
395 vRms-STEADY
0 Hz-no sine wave
Node E:
243 vRms-STEADY
0 Hz-no sine wave
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Node E:243 vRms-STEADY0 Hz-no sine wave
most scopes should display 243mV, most nodes that far down the chain should be ~~~20-40mVrms
time to repeat with a good DVM
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I don't have a 50-100uf cap to test, just a 20uf.
20uF will work fine for a test.
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Sorry, nodes C, D, and E are in mV.
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figured they were, my scope will display down to ~~~ 20mV so your scope might be sketchy, that's why I recommended a redo with a meter.
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I use a volt meter in the way Shooter described.
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I tried checking the VAC with my DMM on the filter caps, and got screwy numbers. I don't have a DVM. Maybe that's the issue.
The DMM has an AUTO and a Manual function for voltages. I tried both methods and got these numbers that don't make any sense.
24.18, 23.65, 21.38, 16.47 and 12.24 for A-E respectively.
The DMM's manual shows the following AC ranges: 400mv, 4V, 40V, 400V, and 750V.
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I guess my DMM isn't sensitive enough.
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i use DVM n DMM interchangeably
are those numbers millivolts, volts, kilovolts...?
on the 400mV scale that is the MAX reading for that setting, guessing IF...the volts are stable it should be able to see 40-50mVac fine.
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This seems crazy, but the DMM is in Manual mode and these numbers are all in Volts.
Its in the Manual 40V range since lower range gave me OL.
Doesn't make any sense. Now these numbers probably are not true VRMS. My scope's numbers were in VRMS.
Maybe that's the explanation. But even if you multiply them by .7, they're still much higher than you predicted Shooter.
The DC voltages on the filter caps are 339, 337, 320, 288, and 262 respectively, and are right on with the schematic.
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This seems crazy,
:laugh:
what I thought with your scope readings, hence meter, which still yields "crazy" so I suspect you either have equipment issues or ya got a gremlin that's all excited about being chased!!!
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I couldn't have said it better myself!
I think I've probably wasted enough of everyone's time on this. Thanks to all for your input. As always.
Happy Thanksgiving!
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This was my first project. The circuit is designed for a full wave rectifier with a pair of diodes on each leg of the transformer's secondary going to the filter caps. The transformer has a Center Tap.
Not knowing at the time the difference between a full wave rectifier and a bridge full wave rectifier, I bought a 3N259 bridge rectifier. In the process of wiring it in, I learned that I should not connect the ground lead of the rectifier into the circuit because it would result in too much voltage with the CT also being used. The CT is wired to the first filter cap. So I was told to cut that lead off so I wouldn't screw up. So I did.
And that's the way this has been wired. The voltages at the filter caps are spot on. Its worked fine, but for this 120hz hum.
So my final thought on this: Is that the cause for this 120hz hum? Should I just remove that bridge rectifier and build a full wave with 4 diodes instead, and leave the CT connected as is to the first filter cap?
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some lite reading;
take any good working tube thing you have, amp, PA.... and repeat your test of the taps with both your scope n meter. POST both AC and DC volts.
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Well I'm totally baffled. The voltages on the tube are spot on. I've changed tubes, and no change in the hum. The hum appears only when the 12AX7's pin 6 plate is connected to the 270K resistor coming off the Node C filter cap. There's no 120hz signal from node C going into the 270K, and there's no 120hz signal after the 270K if it is NOT connected to Pin 6, plate. There is only 120hz hum when the backside of the 270K resistor is connected to Pin 6. The resistor reads within specs.
Will there be a Thanksgiving Miracle this year??
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:laugh:
..."The ways of the Lord are mysterious..."
guessing you rolled a different tube to eliminate the tube
try temp'n in something between 500pF - .01uF cap across the plate R for that tube (Cap in parallel with resistor)
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Yes, the tube was the first thing I tried.
I'll give the other option a try. Thanks for the input.
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I put a .01uf in parallel across the 270K plate resistor.
It shows over 340VDC with DMM and 1.5 VRMS with AC Coupling on scope, and 0Hz for the signal that was 120hz.
So that's good but the 340VDC is way too high. It was, and should be, around 204 or so coming out of the resistor and into the plate. Node C voltage into the resistor should be around 320VDC and it also went up to around 375VDC.
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So that's good but the 340VDC is way too high. It was, and should be, around 204 or so coming out of the resistor and into the plate. Node C voltage into the resistor should be around 320VDC and it also went up to around 375VDC.
was "normal" before you added the cap???
IF SO....
measure VDC across the cathode R for the tube
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I decided to solder the cap across the resistor since the numbers were so screwy.
Interesting after I did that and turn on the amp, all of the Nodes were way high. Then after about a minute they all came back down to where they're supposed to be.
As far as adding the cap, it made no difference. Still have the 120hz signal. I think the prior reading was because I didn't have good connections. Also the VDC across the cathode resistor is 1.8VDC where it has always been.
So adding the cap made no difference and didn't drop the voltages or the Hum.
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For some reason the filaments on all the tubes have gotten really slow to warm up. The power light comes on and measures fine. Probably from screwing around with all this stuff. When the tubes aren't lit, then no current is being drawn, and so the voltages on the filter caps and their connections are high--at least I think that's why they're high. When lit, the voltages are normal. Hopefully this quits acting up.
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so the voltages on the filter caps and their connections are high
yep, I use the "high" voltage to spec the filter caps, I've even noted on the schematic the "high-time" :icon_biggrin:
the last amp was almost 30 seconds to drop down to normal operating VDC's
30 seconds at 400vdc across a cap rated at 350vdc might make a messy
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Shooter-when talking about VAC on the filter caps you said above:
"typical would be something like;
2-4VAC at 1st tap, 100mV at 2nd tap, 50mV, 20mV.....
each tap should "kill" the AC more n more.
Are these voltages RMS or PP?
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I measure n post in RMS, but the "filtering" should follow the same VAC decrease PP or rms. doesn't really matter which you use, just DON'T mix the metaphors!!!
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I ask because my nodes from A-E, in RMS, read, with my scope in AC coupling mode, 784mv, 715mv, 569mv, 329mv, and 207mv. Both Nodes A and B have 120hz signal showing on them, and my scope doesn't/can't read any AC on the other nodes. These seem much higher than you suggested they should be, and the B Node feeds the Reverb Driver circuit, which ultimately shows up in the C Node on the last 12AX7 where the Reverb Intensity pot comes into play.
I was thinking maybe the 120hz hum is actually coming from the B Node, 40uf filter cap, and being passed on through the circuit and amplified at the C Node plate on the last tube before going to the Output jack.
The cap seems ok, no visual leakage, and no ESR issues. So that's what's got me still puzzled.
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for testing you could probably move the "reverb tap" downstream, VDC will be less downstream but there should be enough current for a couple extra AX7's to "make it work" n listen for the hum.
you're most likely chase'n a gremlin though, my sense is you have something mechanically poor, (sockets, solder, ground points...), or cross-coupling in the wires picking up the 120hz
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Yea, I never thought tracing something down like this would be so hard. Thanks for your help.
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I was wondering if there would be any benefit in lowering the plate voltage provided by Node E on V7, Pin 1 Plate, down from the current 136VDC? The specs call for about 122VDC.
If so, what should I do? Increase the dropping resistor between Nodes D and E, or increase the plate resistor at V7 and maybe V6 too? If so, any suggestions on the size resistor? Its currently a 10K dropping resistor connecting to a 1uf cap for Node E.
That Node E also serves the V1-Pin 6 Plate. This half of the tube is receiving the signal from the Input jack. The specs call for 72VDC, but it reads 161VDC. The signal is clean when the Volume Pot is below half way, but gets dirty soon. So maybe cooling that input would give more clean headroom out of the first preamp tube.
Node E at the filter cap is 261VDC, and the specs show it should be 212VDC.
So all of the Node E voltages on the plates are higher than spec.
All of the other VDC Nodes are in line with the specs of the schematic.
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Well I'm giving up on the last thoughts I had.
I feel like its related to the reverb Wet signal being amplified after it goes through the Reverb Intensity pot and connects to the Pin 7 grid.
The Dry Signal bypasses the reverb tank entirely and goes into the Cathode pin 8 of V7 and then through the Output Level pot and to the Output jack. There is virtually no hum when the signal goes through the circuit this way.
If the Wet Signal is used, by either turning on the Reverb foot pedal or disconnecting the foot pedal, the hum appears. Its level increases with turning the Intensity Pot. This pot is controlling the amount of signal going to the Grid vs. going to ground.
So I need to figure out some way to deal with that. An older schematic on which this circuit is based, has the foot switch connecting between the .047 coupling cap and the input on the Reverb Intensity pot. The main difference is that the Ground for the foot switch is before the pot and thus before the grid, vs. in my build the foot switch's Ground is on the output lug of the pot and thus after the grid.
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I'm pretty sure I know what circuit you are talking about, but I bet no one else does. Please post your as is schematic and layout so everyone can understand better what you are experiencing.
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Sluckey-sure happy to post your schematic. I don't have a current Layout. The components are still the same. I finally traced the hum to a 120hz signal which comes off Node B, and then onto Node C but not as strong, and then not detectable by my scope on Nodes D or E.
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figured that was what you were working on.
Is the hum there with no input, or only when you "add signal"
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Even with no input.
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I checked it again and I've got a 120hz signal at the Node B filter cap, and that signal also shows up on V4, Pins 1 and 6 plates which are the Vibrato Oscillator/Driver circuit, and V6, Pins 1 & 6 plates which are the Reverb Driver circuit.
Removing the RCA cable at the Reverb Input jack kills the hum.
It seems that it must be a bad Node B, 40uf, filter cap, since the signal seems to be originating from there, at the cap, and goes on out from that point through the wiring directly to V4 and V6. Nothing is between the cap and the connection on those plates.
So I'll give that a try.
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Removing the RCA cable at the Reverb Input jack kills the hum.
before you head for the cap;
remove the Ground-part of the cable but leave signal part connected, 120hz??
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The jack is isolated. Do you want me just to take the jack’s ground lug out of the circuit by un-wiring it?
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Interesting, although I removed the ground wire connection on the RCA Reverb Input jack, I still have reverb and I can control it on/off with the foot pedal. How is that?
As to the hum, with the Reverb On, the hum is louder than when it is turned OFF, but probably not as loud as before when the jack was still wired to the buss ground.
The Reverb Intensity pot increases the loudness of the hum, as well as the amount of Reverb.
I haven't checked the scope for 120hz signal readings, but I assume its still there in the same places as before, since that's the hum I'm hearing--I think.
So I would say its better, but I would like it to be quieter--if that's achievable with this circuit.
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https://el34world.com/Forum/index.php?topic=32980.msg368863#new
both of you are probably chasing the same gremlin but from different mothers :icon_biggrin:
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So I checked the continuity of the jacks on the tank.
The Reverb Input jack is isolated from the chassis, and only the Input jack's Tip is connected to the jack. The ground wire is now disconnected from the Input jack that is isolated on the chassis.
With both Input and Output plugs connected to their respective jacks on the chassis and on the tank, there is continuity between the sleeves of both jacks and also continuity between both sleeves and chassis.
So even though the Input jack on the chassis is not grounded, the cable that connects from it to the tank is being grounded by the Output Jack which is grounded to chassis.
So the grounds on the jacks on the tank must share a common ground. Either the tank's shell or their wiring. Maybe these jacks are not insulated from the tank chassis, and that's how it's happening.
So would that create a ground loop hum?
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would that create a ground loop hum?
yep
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I un-soldered the reverb tank's Input Jack grounding tab so that now the tank's Input Jack is Ungrounded and its Output Jack is grounded.
The amplitude of the hum is less, but the 120hz hum is still there.
The hum remains even if I disconnect the tank, and even if I remove all of the tubes (testing for hum 1 by 1) except for the last tube, V7, which handles the Reverb Recovery and the final Output.
I've tap tested all of the tubes, components and solder connections and only the V4 tube that is for the Vibrato Oscillator/Driver circuit is a little noisy. It's just you can hear the tapping on this tube, where the other tubes don't pass the sound through the circuit to the Output. But it's not a hum or buzz or hiss, just sound of the glass being tapped.
V4 is getting its voltage from Node B, which is where the 120hz signal at 611mVrms/145mVPP seems to originate. Node B also sends voltage to V6 for the Reverb Driver.
So I think the next thing I'll try is replacing the Node B Filter Cap.
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I think the next thing I'll try is replacing the Node B Filter Cap.
when you do;
relocate that caps ground to it's own dedicate chassis point, so don't solder it to the rest of the caps ground point.
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I am using a ground buss to which all grounds are connected at various points along the buss. It's a 14 gauge copper wire that I coated with solder. It's only grounded to the chassis at the Input Jack after going through a Hum-Loop-Block circuit that I added. It uses a 10ohm resistor and 2 diodes and a cap. So I think that isolates the grounds.
The Node A Reservoir cap, and the Nodes B-E Filter caps each connect to that buss at separate locations, and each of the grounds in the circuit come back to the buss where the Node providing voltage to their part of the circuit is attached. So it's sort of like a Star Ground.
So should I keep the Node B cap's ground as is (ie. connected to the ground buss), or move it directly to its own point on the chassis, or do something else with it?
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I decided I would check each of the pots for DC leakage and found something interesting.
Only the Vibrato Intensity pot showed any DC voltage on its lugs. It shows 1.2VDC that increases as the pot is increased and gets up to about 7VDC. I thought there should not be any DC on any of the pots.
The voltage at the .068uf coupling cap connecting the V4's, Node B (where the 120hz signal seems to originate) plate to the pot, reads 1.2VDC. It reads about 120VDC going into that cap.
So could this cap be bad?
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It shows 1.2VDC that increases as the pot is increased and gets up to about 7VDC.
we might have a winner!!
I would replace it, .068 isn't a "magic number" anything from .01 to .1 will work for testing
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Only the Vibrato Intensity pot showed any DC voltage on its lugs. It shows 1.2VDC that increases as the pot is increased and gets up to about 7VDC. I thought there should not be any DC on any of the pots.
The voltage at the .068uf coupling cap connecting the V4's, Node B (where the 120hz signal seems to originate) plate to the pot, reads 1.2VDC. It reads about 120VDC going into that cap.
So could this cap be bad?
You're chasing rabbits and you found nothing. If you look at the schematic and do a little bit of ohms law math, you'll see that there should be 7.6V on that pot when it is turned max CW. Since the .068 cap is tied to the pot, it will have the same voltage as the pot. Nothing wrong here.
The only way to accurately check for coupling cap leakage is to disconnect the downstream side of the cap and check for dcv on the dangling end of the cap.
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I disconnected the downside lead of the cap at the top lug of the pot and the VDC was jumping all around with no steady reading. I used the foot switch to see if that would affect it but it didn't. So I don't see how I can test that cap as suggested. But with that lead disconnected at the pot, the center lug does read 7VDC full CW. So SEL's math seems correct, so I assume the cap is good, and I'll have to try replacing the Node B filter cap.
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and the VDC was jumping all around
assuming that was measured at disconnected end??
IF so;
probably a good indication your meter ain't up to the task, or the jumpin all around is voltage so small it's insignificant.
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Shooter, yes I disconnected the cap from the circuit on the downside and measured the voltage there. My meter was set for auto-ranging. Maybe the jumping is because it’s in the oscillator circuit.
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Are y'all even looking at the schematic? What does the note in the gray call out box say?
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:l2:
I looked to see where I went wrong on the VDC readings, doesn't that count?? :icon_biggrin:
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Having strapped another cap across the Node B cap with no effect, I'm doubtful changing the cap will make a difference.
But before I do change the cap, I'd like your comments about how the reservoir and filter caps have been wired/grounded by me.
The CT from the PT is soldered to an isolated turret, which then connects via a 1" jumper wire (the CT wire was too short) to another isolated turret, where the Negative lead on the Node A reservoir cap is connected.
From that point a jumper wire is soldered to the ground buss that is only connected at the opposite end to the chassis through a Hum Loop Blocker circuit.
Each of the remaining 4 filter caps, Nodes B-E, have their Negative leads connected to separate points along the ground buss wire. Grounds from components are then wired to the grounding point of the ground buss where their Filter cap Node is wired. None of the filter caps are connected directly to one another, and none of them are connected to the Negative lead of the Node A reservoir cap. Their only "connection" with one another is through the ground buss.
Any helpful comments?
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through a Hum Loop Blocker circuit.
been moving snow n wood all day so I mighta missed it in the schematic we're all using??
From that point a jumper wire is soldered to the ground buss that is only connected at the opposite end to the chassis
maybe for testing you remove the "jumper wire" the "loop-blocker" and just ground that point to the chassis
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Shooter-I just added the Hum Loop Blocker recently. So it's not in the schematic. It improved some other issues, but not the 120hz hum I'm trying to fix.
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So you're saying if only V7 is connected, the hum is present? And increasing the reverb intensity increases the hum?
If you ground the grid of V7A does the hum stop?
Perhaps you can put a jumper from the ground point of V7A and connect it to different parts of the bus wire to see if that makes a difference in the amount of hum?
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Thanks. I’ll give both a try.
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AlNewman-I connected a jumper from V7A, pin 2, Grid to the chassis and the hum was much louder.
However, when I connected the jumper to the isolated ground buss, it is much quieter than when no jumper is connected. So that is a definite improvement. Maybe a level I can accept. It is much better.
So the way that grid is currently wired, it connects via an insulated cable to the pin of the Isolated RCA Reverb Output Jack, and it's shield is only connected to the ground tab of that jack, which then goes to the isolated ground buss for its ground.
There is a 47K resistor between that jack's tip and ground tab. That ground tab is sharing the ground for the Reverb Output Jack and the resistor.
So, given this info, what do you suggest I do. Is that 47k resistor part of the problem?
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Is that 47k resistor part of the problem?
IF.. it's the one circled;
It's typically soldered to the sockets grid-pin at one end, the ground Buss at the other.
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Shooter, that's the resistor I'm talking about.
I realized that when I connect a jumper from the grid to the ground buss, it cuts off the reverb, so that's why its quieter.
I should add that the Reverb Tank's Output Jack is grounded to the tank's chassis. Its Input Jack is isolated from the tank's chassis. Both of the jacks are isolated from the amp's chassis using isolation washers.
So the tank's Output Jack is grounded to the tank's chassis, and its also grounded to the isolated ground buss. Maybe that's part of the problem. But it needs that connection the buss ground for the Reverb to function.
The resistor is connecting at the RCA Reverb Output Jack's pin, and also I've got the shielded cable with its isolated lead connected to that point and running to the grid pin on the tube socket. The shield for that cable is connected to the same jack's ground lug which is connected to the isolated ground buss.
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it's shield is only connected to the ground tab of that jack, which then goes to the isolated ground buss for its ground.
temp the ground wire to the chassis n off the isolated ground buss, better, worse?
FWIW I flunked story problems in math, but the ones with pictures I did ok in :icon_biggrin:
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FWIW I flunked story problems in math, but the ones with pictures I did ok in :icon_biggrin:
As the saying goes, a picture's worth 1000 words.
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I've tried sending a photo, and its only 1.6megs, but having an issue.
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No luck. I've tried 4 times but it won't upload. Don't know why. Sorry.
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Shooter, the hum is lowest when the Output Jack is grounded to the ground buss.
There's really no difference in the level of hum when I disconnect the shield's ground totally. So I don't think shielding the cable going from the Reverb Tank's Output Jack to the grid makes any difference. Surprising. It has the same level of hum regardless of whether it's grounded or not.
So when the Reverb is Off the hum is barely there. When it is on, the hum is a bit louder and increases some with the Intensity level of the Reverb pot. It is at a low level though, so maybe I'm just being too picky.
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so maybe I'm just being too picky.
the sub-circuit you chased the gremlin to is the most sensitive of the whole circuit, not a reverb guy so just guessing the recovery stage will have a signal feeding the grid in the small mV range. any "noise" in that area approaching signal level is gonna be a "problem"
my "too picky test";
put a signal into the box, if I can make the background "noise" go away with "volume/gain" levels at 1 or 2 on the dial will "pass"
If it's still annoying at 3-4, "failed" then i'm hunting or scaping the sub-circuit or whole box :icon_biggrin:
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Yea, I don't understand how I hear 120 hz hum on my amp, but don't see a signal for it at the Output jack of the "Revibe". It would seem that if I can hear it coming through the amp that the Revibe is connected to, and that amp on its own doesn't have any hum, that I should see its signal at the Revibe's Output that sends the signal on to the amp.
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I should see its signal at the Revibe's Output that sends the signal on to the amp.
by "signal" you mean 120hz hum?????
if that's the case "no 120hz at amp input", look at amp speaker n see if it's there
does either the Revibe or the amp "float ground" by using a 2 prone adaptor?
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I've tried the 2-prong adapter for the plug, and the 120hz hum is still there.
What I was trying to say is that when there is no Input signal going into the Revibe, and the Revibe is turned on, and I check the Output Jack on the Revibe that goes to the Input on the amp, I can't read any 120hz or other signal at that point, but I can hear the hum when the amp is turned on. If I turn off the amp, then all is quiet.
It gets louder when I turn the Output Level pot CW.
I guess maybe the 120hz signal is so weak that its not audible, or measurable, until its amplified by the guitar amp.
I think the best practical solution at this point is just to keep the Output Level setting on the Revibe low.
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the 120hz signal is so weak that its not audible, or measurable, until its amplified by the guitar amp.
as a field engineer; "IF I can't measure it, it doesn't exist" :icon_biggrin:
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Probably a good philosophy to have!
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I thought maybe 2026 would be the year I solved this issue, but not so.
I decided to give it one more try, and here's what I did with no success:
I previously put in another 12AX7 for V7, and no change.
I jumped in parallel another 50uf/400V cap across the existing 30uf Node C cap. No change.
With reference to the partial schematic shown in Post 59 above:
I disconnected the grid input lead at Pin 7 on V7 coming from the Reverb Intensity pot. No change. Reinstalled.
I disconnected the cathode input lead at Pin 8 on V7 carrying the dry signal from the cathodes of V6. No change. Reinstalled.
So everything going into V7B has been tested. The tube is good, the Node C power supply is good, the wet input signal is good, and the dry input signal is good.
So I'm left with the .047uf cap, the 1M resistor, the 100KA pot, and the output jack. All of which test good.
So I'm finished on this one. Maybe one day I'll build a new one using the same schematic, but with a better layout and better focus on grounding and lead dress. This was my first build from scratch, so I'm still very pleased with the result.
Thanks again for your help and patience with me.
Happy 2026!
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Just to prove I'm not hearing things or imagining the hum, I've attached a screenshot of my scope's image of the "Hum".
The setup is as follows:
No input going into the Revibe unit.
Output jack from Revibe has a 1/4" plug, with scope probes attached to tip and sleeve connection points.
Scope is set to 10ms/50mV. 50mV is the most sensitive my scope will go.
Output Volume pot is FULL CW. Anything less and my scope can't detect it.
Input Volume adjustment has no effect on the image or the hum.
As you can see it's a 120hz sine wave, of 9mVrms or 32mVPP.
So since there's no Input signal from an external source, its being generated internally, and when plugged into an amplifier, the amplitude of the 120hz signal is greatly increased by the gain of the amplifier and is audible.
So at least I can see the hum now. Still no solution, but I just wanted to let anyone who's interested see what's going on.
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since you can see it, it IS an issue :icon_biggrin:
scope probes attached to tip and sleeve connection points.
if you get bored;
move the ground probe of the scope to various grounds within your box
so start at the input jack ground, or the PS mains ground to chassis, recheck 120hz amplitude
every ground point you use should produce about the same amplitude 120hz at the scope, I'm only interested in deviations in amplitude <> 25% of your "test" amplitude.
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I'll let you know what I find out. I need to borrow a buddy's more sensitive scope that will hold the readings better. Mine is pushing the limit.
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As you can see it's a 120hz sine wave, of 9mVrms or 32mVPP.
or not, the 9mVrms = 32mVpp, techs like rms, musicians get excited when the salesman quotes Power in pp
stick with 9, is it "stable" on your scope?
add a 480hz, 50mVrms signal to the input
what you want to measure is the ratio.... 47mVrms(480hz): 9mVrms(120hz..."noise")
since 480 is 4X 120, they should both be visible on the scope.
I'm not sure what a "good number" is for S/N in a preamp, maybe one of the sharper knives??