Reverb Hum

[wsscott]

Two years ago I built a "Revibe" type of unit, with lots of help from Sluckey, Shooter, and others, based upon a Magnatone 10A circuit because I wanted the true Vibrato of the Mag.  So it has just Reverb and Vibrato, and then it connects to the amp for sound, etc.  Its a tube driven reverb.

Anyway, its had some hum issues that aren't bad, but I thought I would try to resolve it just to clean the sound up some. So I started working on it today.

One thing I did in trying to diagnose where the hum might be coming from was to disconnect the RCA cable going from the circuit and into the spring reverb tank.  I got a really loud screeching sound when I did that.  Is that normal?  Is it feedback or something?  I did the same thing with the output cable but didn't have any issue doing that.

Then I thought I would take out the tube powering the Reverb Driver and see what happens.  Well the hum is gone.  But of course there's no signal from the input going through that part of the circuit and so no sound output when my guitar is played through it.  There is just very small amount of normal hum that is carried through to the amp even when the tube is removed, and it completely disappears when I unplug the output cable to the amp.  So the natural sound of the unit is getting through to its output jack and onto the amp.

I put that tube back in and removed the Reverb Recovery tube, and the effect is the same.

So it seems to me that the hum is coming from the Reverb part of the circuit.  I thought it might be a ground loop of some kind, and have reduced the number of grounding points, but doesn't seem to make any difference.

Thoughts?  Thanks.

[sluckey]

Schematic?

[wsscott]

Sure--Here you go.

[jim]

 You could remove the tank from the cab leaving it connected to see if the hum changes.  It might be the proximity to the power transformer.  Jim

[wsscott]

Tried that Jim, but no change.

[sluckey]

Schematic looks solid. I'm guessing layout or possibly ground loop.

[wsscott]

Sluckey, could you give me your thoughts on these points:

In order to try and eliminate possibility of ground loop, I changed the grounding on the following:

1.  I removed the wires from the ground lug on the Reverb RCA Input and Output jacks which had been connected to a grounding lug on the chassis, and just let the jacks ground to the chassis with their housing.  OK?

2.  I originally had a wire going from Pin 3 GND Lug on the Reverb Intensity pot to the Middle lug on the Reverb/Vibrato STEREO Footswitch, and also another jumper wire going from Pin 3 to the GND Buss.  I removed the wire on Pin 3 going to the Footswitch and left it disconnected so that it is being grounded just by the jacks connection to the chassis.  OK?

3.  Why would I get a loud screeching sound when I disconnected the RCA cable at the Reverb Input jack on the chassis?  I tested this before I started making any changes.  I feel like when I've disconnected it previously in an effort to locate the cause of the hum, it didn't make any sound at all and the hum stopped.

Thanks.

[sluckey]

Wow! The original thread on this project has over 700 replies. Not gonna relive that!   

The only suggestion I have is measure the hum level at the output jack. Then disconnect the power cord green wire from the chassis. Does the hum level decrease?

[wsscott]

How do I measure the hum level at the output jack?  I know how to measure the output level at the jack using my oscilloscope, but don't know how to do hum?  Do I switch it from measuring AC to DC?

[sluckey]

Hum is just a signal. If you can hear it, your scope can see it. You can always just use your ears to tell if the hum decreases, but your scope can put numbers on the levels.

[wsscott]

Yea, I just realized that after sending my Reply.  Sorry.

[wsscott]

So with the Reverb Tank connected, and Revibe not connected to Amp, I get the following results:

1.  NO Signal at Output Jack of Revibe--0hz
2.  V-6 (Reverb Driver), Pin2---160-200HZ; 18mVrms; 60mVpp;  NO change in readings with changes made at Main Input Volume, Output Volume, or Reverb Intensity pots.
3.  V-7 (Reverb Recovery), Pin 2---NO Signal--0hz; 0mVrms; 0mVpp
4.  Reverb Input Jack at Signal lead---Rapid Fluctuations in Signal--20khz; 15mVrms; 50mVpp--Main Input Volume control affects signal and can bring it down to 0hz
5.  Reverb Output Jack at Signal lead---No Signal--0Hz; 0mVrms; 0mVpp.

When connect to Main Amp:
1.  Revibe's Main Input Volume, Treble and Bass have NO effect on Hum.
2.  Reverb Intensity pot changes will increase and decrease volume level of hum
3.  Output Volume pot changes DRAMATICALLY increase and decrease volume level of hum.

[sluckey]

Well, you checked a lot of stuff, but you ignored the only suggestion I gave you.   ARGH!   

[wsscott]

Sluckey I assume the suggestion you're referring to is disconnecting the Green ground wire from the power cord to the chassis.  Its wiring point to the chassis is under the board, and I didn't want to have to take that out, so instead I used a 2 prong adapter that doesn't have a ground going into the receptacle.  So I tried that, and the hum is still there and reacts the same to changes in Reverb Intensity and Output Volume.  No change.  If the adapter is not the proper way to do this type of test, then say so and I'll have to take the board out.

[wsscott]

I got side-tracked on the Hum issue, because after I tried to clean-up the grounding, then I couldn't get either the Reverb or the Vibrato to work.  I finally got the Reverb working, but now the Vibrato only works when I jumper the Vibrato pin in the Stereo jack to Ground.  I've got the Fender dual foot switch with a stereo plug, and it had been working fine, but now it only works with the Reverb.  Doesn't work with the Vibrato.  And I checked the footswitch to make sure both switches work, and they do, and I replaced the Stereo jack.  Its clearly a grounding issue but I can't find it.  Will keep looking.   

[wsscott]

Well I'm stumped.  Its wired up just like it was before, with a Stereo Jack Footswitch input that was working before for vibrato and reverb, and now only the reverb will respond to the footswitch.  In fact, sometimes I have to remove the plug and plug it back in again for the reverb to respond. Then I can turn the reverb on and off. If the plug is not in the jack the the reverb is always on, but the vibrato won't work unless I jumper its pin on the jack with the chassis, but it still doesn't respond to the switch.  Any ideas?

[wsscott]

I plugged in another "single" switch foot pedal and it controls the vibrato circuit fine, but of course doesn't control the reverb.  So I guess its the switch for some reason.

[wsscott]

Fixed.  Broken lead to the Tip inside the jack.

[wsscott]

I cleaned up a lot of the noise by adjusting the placement of the B+ Node power wire connecting to the 5K, 10W resistor and the plates of the 12AU7 Reverb Driver tube.  The A, B, C and D Node caps are connected on a separate BUSS to Chassis GND.  The E Node cap is connected to the MAIN GND BUSS, which connects to the Chassis.  The only other direct Grounds to Chassis are the Dual Foot Switch Jack and the Input and Output Reverb RCA jacks, since they are not isolated and are attached to the Chassis.  I do have the SHIELD lug on the Foot Switch Jack also wired to Pin 3 on the Reverb pot, which also connects Pin 3 to the MAIN GND BUSS.  The ground lugs for the RCA jacks also are wired to a lug screwed to the Chassis.  The PT ground wire is separately wired directly to the chassis.  Any thoughts on changes to any of this?  Thanks.

[tubeswell]

Do a search on ‘hum loop blocker’

[wsscott]

Thanks. I think I may have read about that on Rob Robinette's site.  I'll check it out again.  About a year ago I got a Pyle PHE-400 Hum Eliminator, and it did reduce the hum, but for some reason it also lowered the volume and the high frequencies when the guitar is played through the amp.  Don't know why.  I hooked up a signal generator and put my scope on the Pyle's Input and Output jacks, and the reading is the same on both, 1KHZ at 100 mVRMS, and a clean signal.  The guitar plugs into the revibe's Input as usual, and then I connect its Output to the Pyle's Input, and then the Pyle's Output to the guitar amp.  I communicated with Pyle, and they don't know why I would be experiencing the change in the Output signal.

[tubeswell]

Pyle PHE-400 Hum Eliminator

Not what I meant.

[wsscott]

Yes, now I know what you meant.  In fact I actually have that in my files and looked at it yesterday too.  My grounding is similar to that.  After the work I did recently, and moved the wiring for the B+, the hum is really minimal when the Reverb is off.  Its more pronounced when the Reverb is turned on, but much better than before.  Maybe tube reverb circuits are a little noisier than just the Dry signal?

[tubeswell]

Depends on the source of the hum. Reverb recovery stages are sensitive to noise because they are amplifying the pan’s output transducer (which  typically has a smaller output signal than a single coil guitar pickup). You can ‘cure’ that a number of ways e.g. with a large cathode bypass capacitance, or pan send/return cables with good quality shielding, or clipping some ferrite beads over the cables, or using a grid stopper right on the pin of the recovery stage, or orientation of the pan’s output transducer further away from sources of high EM flux density (like the PT), or using the correct horizontal or vertical pan mounting (depending on the type of transducers installed).


But if the hum is from a ground loop or some other source, you have to focus on eliminating that.

[wsscott]

I don't have any shielded cable going from the Driver or Recovery tubes to their Input or Output RCA jacks for the Reverb.  Should I use shielded cable for the wiring going from the Reverb Output jack to the Grid (Pin 2) on the Recovery tube (12AX7)--or anywhere else for that matter?

I've got a 2.7K on that same tube's Pin 3 Cathode to GND.  Is that big enough?

There's also a 47K grid stopper on that tube's Pin 2.

The tank's Output jack is at the opposite end of the cabinet from the PT, but the jack itself on the chassis just a couple of inches from the PT.  Maybe an issue?  Unfortunately I don't have any place to move it to.  Really tight build.

Thanks.

[wsscott]

I was watching a video on Active vs. Passive DI boxes, and it commented on the difference in impedances between the 2 types of boxes.  Apparently the Passive type boxes using isolation transformers have a much lower impedance output, and as a result the output has a muddier tone with less highs, and lower volume level.  So that's probably the reason for the symptoms I have with the PHE-400.  Is there a way to measure the impedance of a guitar output with a scope or some other device?

[tubeswell]

The tank's Output jack is at the opposite end of the cabinet from the PT, but the jack itself on the chassis just a couple of inches from the PT.  Maybe an issue?  Unfortunately I don't have any place to move it to.  Really tight build.

Yes could be. While the hum would be worse if it was the pan's output transducer close to the PT (because of interaction with the transducer's own flux density field), any signal wire is effectively an inductor, and so the signal wire connected to the reverb recovery stage's grid  could be picking up some unwanted EMF coupling from the PTs flux density. (Is the hum a constant 'buzz'?)

[zircontweezer]

FWIW I didn't like the noise from having the recovery grid extending all the way to the footswitch and put an opto isolator in the amp instead across the recovery grid. It still bleeds a small amount of reverb as I didn't do the input of the pan as well but it's a lot better noise wise imho. I also went down a rabbit hole with the grounding of the pan and ended up isolating both send and return and put a separate ground wire zipped to the RCA cable. That was on my home brew DR.

[sluckey]

FWIW I didn't like the noise from having the recovery grid extending all the way to the footswitch and put an opto isolator in the amp instead across the recovery grid.

Fender addressed that issue with the reissue and custom shop amps. The footswitch now connects to the reverb pot where the signal is much bigger.

[wsscott]

Tubeswell in response to your question  (Is the hum a constant 'buzz'?), no I don't have any buzz, just a low pitch hum.  With reverb off I consider it probably normal for a tube amp, at least the 7-8 that I've been exposed to, it just gets louder when the Reverb is turned on and its loudness, not pitch, increases or decreases with the change in the Reverb Intensity or the Output Volume pot.  There was a buzz until I moved the wire I commented on earlier.  Anyway, maybe its just that the signal develops more gain after going through the Reverb circuit, and so the normal hum of the amp just is getting amplified a bit. I'll check it out on my Gretsch/Valco 1966 Variety Plus amp that I've got, and see if the Gretsch does the same thing.

[wsscott]

I checked it on the Gretsch, and the Gretsch amp has some background hiss and hum as you might expect for an amp this old, with its original tubes and caps.  The level of the hum is about the same as the Revibe.  The difference is that the volume level/intensity of the hum doesn't change, ie. increase, when the Reverb is switched on.  It stays at the same level.  Whereas mine increases when the reverb is switched in.  The Gretsch's reverb is powered with an output transformer, not tube driven as the Revibe.

[wsscott]

Thanks for everyone's input.  I think I'll leave well enough alone at this point and just use the Output Level pot to control the level when using the Reverb effect, when and if needed.