hum issue RESOLVED

[tubenit]

Guys,

Despite this issue, this is one of the sweetest sounding and transparent touch sensitive amps that I've played.

I think this is the first amp that had hum in yrs that I couldn't resolve.

I had quadruple checked the grounds and reflowed solder joints multiple times.
I'm using shielded wiring on long runs.
I tried tying the heater 100R resistors to the power tube cathode. No change.
I've replaced a cap that I thought might be a problem. No change.
I replaced the reverb 1M pot.

Here are the symptoms;

I can remove the reverb tube and disconnect the reverb tank and the reverb pot still makes the amp have less hum when the reverb is full on. Dialed to 10.

IF I crank all the pots to about 7-8, the amp will start to vibrate and have a slight oscillation. When I say vibrate, I mean with my hand on the chassis or cab ........all of it physically vibrates & there is a slight oscillation and hum.

I can bypass the OD stage and it still does the same thing on the clean channel.

I've tried replacing all the tubes with others and no difference. Another speaker makes no difference.

At volumes that I normally play the amp, the amp hum is noticeable but only slight. Turning up the volumes on the clean and master volume increase the hum/vibration.

I've noticed turning the treble down on the clean tone knob lessens the hum. Increasing the treble on the OD tone lessens the hum.

I'm stumped.   What would you guys suggest trying at this point?

Could the OT wires being reversed do this when there is no NFB?

With respect, Tubenit

[FYL]

I'd check the PT. What you describe could be a defective assy, with shorted lams, and/or a poor mechanical alignment.

[tubesornothing]

When grounding problems get tough, here is what I do:

(1) is it really hum?  or is it a noise?  If its hum, is it 60Hz or 120Hz ?  This will help to narrow it down.

(2) Try connecting a DC power supply or a bunch of batteries to the heaters - does it go away?  This will help to narrow it down.

(3) I have had huge problems with hum getting into the sensitive reverb grounds. Make sure the cathode ground for the reverb retrun is well away from other noiser grounds.  I usually have the preamp chassis ground seperate from the PI/power stage chassis ground.

[HotBluePlates]

As FYL said, hum with physical vibration is usually a short or manufacturing defect in a PT or choke.

Assuming you can confirm/deny that cause first, does the hum stop when any 1 tube is pulled in the signal chain?

[plexi50]

Pull the rectifier tube and flip everything on. Does the chassis still dance? Maybe the PT is vibrating only when under a load

[stingray_65]

I had a BUZZY Pt shake V1 and  the whole thing hummed. Tube wasn't microphonic in the classic sense, but shaking it 60 x's a second it was a bit noisy.

Try loosening the PT screws, I'm sure you got that thing screwed on there good and tight. Loosening the screws will not allow as much vibration to be passed into the chassis.

Don't think like I did and rubber grommets are the cure. a sick PT only gets worse till it dies, promise you that!

I opened up the mounting holes to accept a grommet in each corner which worked till the PT died (few hour later) and had BIG holes where I wanted to mount the replacement

BTW, was 60 hz. to tell if it's 60 or 120 (I can't tell without comparison) I plug a cable into the input and thumb the exposed tip. thats always a 60 hz hum.

good luck Jeff, You'll beat this and we all might learn somethin.

Ray

[tubenit]

Guys, I really do appreciate the responses!  Thanks!  I can't remember for sure, but I think when I put my hand on the PT and the OT that it actually felt like the OT was vibrating more strongly. I'll try that again.  And I'll try it without the rectifier tube in and report back on that on both of those.

I like the idea of loosening the PT nuts slightly to see what happens. I'll definitely give that a shot. And yeah, I have them cranked down pretty tight.

The thing is that PT and OT aren't that old.    

With respect, Tubenit

[tubenit]

Wow!  I didn't remember this correctly, so I apologize for taking us down a rabbit trail.

The amp chassis does NOT vibrate using a remote cab & speaker. But the remote cab and speaker vibrates but not nearly as volatile. In other words, I can NOT feel anything vibrating on the amp chassis using a remote speaker.

The reverb pot turned all the way up is quieter. Turned down is noisier.

The preamp tube socket area seems to be VERY microphonic.  I tried a 12AU7, 12AT7, 5751 and 12AX7 and all of them are microphonic in that position. In fact, the heater wires going to that tube are microphonic as well as everything on the chassis around that tube.

Having said that with NO tube in that socket, nothing there is microphonic.

The vibration is from oscillation and I don't think it's from the PT or OT.

Even with no NFB, could the OT wires being reversed make a difference?  I'm somewhat grasping for other things to try.

With respect, Tubenit

[phsyconoodler]

A simple way to find out if reversing the OT wires would do anything is to remove the power tubes and use your DVOM and measure the resistance from the OT center tap to the plate of each tube.If there as a big difference then yes,reversing the plates might help.If it's the same it probably won't do anything.

  Also measure for DC voltage backing up the input grid.You may have a bad coupling cap if that's the case.I had an issue similar to this and I isolated it by tacking in a .1uf cap in series with the input grid(pin 2 or 7,whichever you use).That stopped it completely.It did not hurt the sound of the amp in the least to have that cap in there either.

[tubesornothing]

OK, here is a shot in the dark, but it has caused me problems twice in the past making tubes extremely microphonic.  Input jack wired incorrectly or one of the input jacks not shorting properly (this has even happened on new jacks).  Totally makes the whole amp microphonic.  Tapping pots, wires, tubes, sockets, resistors, can totally hear it through the amp.

[Dave]

Hey Tubenit,

I caught some grief over this method a while back. I think it was HBP that didn't think it stood up to logic, but I insist that I have had excellent luck with it.

Get yourself a test lead with alligator clips on both ends. Turn your amp on and set it to whatever setting gets you the most hum.

Connect one end of the test lead to someplace on the chassis where there is nothing else grounded. Systematically connect the other end to the grounded end of everything that is grounded.

If you connect to something that causes a noticeable difference in the hum, disconnect that ground and find a different place to ground that component.

This is how I have successfully combatted ground loop hum in many amps including commercial amps as well as scratch builds.

Dave

[rzenc]

Happened to me once. Have you tried to tight the contacts on the offending socket?

Hope this helps.

Best Regards,
Rzenc

[tubenit]

Guys,  thank you for more ideas.  I will try eliminating more stuff based on some of the responses.

The shortening on the input jack is working just fine & the input jack has worked for quite some time thru various revisions of this amp chassis. I don't think that's it.

The socket is making good contact with the chassis and the bolts are on tight.

I'll try physconoodler's and Dave's suggestion within the next couple of days and report back.

Thanks, Tubenit

[jojokeo]

Jeff,
I would think you know about placements for things already but w/ oscillations in a high gain amp, problems go up exponentially and these are a result. High gain amps cause me to go the extra mile to keep signal ground and chassis grounds seperate for as long as possible. I put my filter caps as near as possible to the tubes they supply and their associated cathode's ground also. All grid wiring has to be kept real short and grid stop resistors mounted on the tube's sockets. Etc, etc, etc... All of these little kinds of things help and putting every component on a tag board isn't the best method w/ high gain amps. I know you've aready built the amp but I'm throwing stuff out there for general info. purposes. (I feel like I'm giving the instructor info he already knows - sorry) Many have already offered you great advice, I'm just chiming in trying to help or give moral support.

It could really help to see how and what the chassis looks like as for parts placements, wiring and grounding.

The OT secondary and the main filter cap areas are the two highest current areas to beware of avoiding w/ anything carrying a small signal and/or it's ground which is ripe for inducing oscillations & noise.
Your early description definitely sounds like you're dealing w/ oscillations. They aren't always heard and can be easily out of our hearing range. They get much worse usually when turning up the gain/vol dials as you've said when you feel/notice it and not always when on "10". They can find their very own sweet spot on "8" or another setting. If your amp is having any lack of sustain in the slightest, this is another dead giveaway for this problem. I wish I could help you further but I would only be guessing at things at this point. If you do find this specific vol thing out, I have an easy little trick to try that's worked for me once. Good luck my friend!

[DummyLoad]

tubenit, what does the filament chain routing look like?  e.g. v1 to v2 to v3, etc... 

[sluckey]

Did pulling V1 kill the hum?

Here are a couple easy things to try...

Pull the OD tube and the reverb tube and leave the OD switch off. Put a ground clip lead directly on the top end of the .01 cap on the board. It's labeled 'E2'. Does this kill the hum? If not, the hum is from the PA and/or PS. If it does kill the hum, move the ground clip closer to the input jack, first grounding the tip of the FX send jack, then the top side of the 1MA volume pot (the one after V1B), then the grid of V1B, then the top side of the 1MA volume pot (the one after V1A), then the grid of V1A.

Bridge a 20 to 40µF cap across each of your filter caps (one at a time).

If the above doesn't reveal where the hum is coming from, try powering the filaments with a 6V lantern battery.

[tubenit]

The filament chain looks like the layout I posted (if I am understanding your question correctly).

The layout could be a factor?  In a small combo amp, I have tended to place the reverb on the far end prior to the preamp tube. It's always worked for me in the past. This is in a pretty small tweed combo type chassis. The amp has been rebuilt several times since but the photo of the original is here:

http://s28.photobucket.com/albums/c216/tubenit/?action=view&current=chassisexteriorpotside.jpg

The amp has incredible sustain. Typically I play it with the volume and each of the OD controls at around "5". It has minimal hum at that level. No one has ever commented about the hum, but me. The band and the sound board guy haven't said a word.  I'd say the hum noise is different but around the same level of hiss that I had on the vintage Princeton Reverb I had. It's just that I am used to very quiet amps at idle. I played the amp for a number of months before ever turning it up and hearing the oscillation.  

soundclips here:   http://www.soundclick.com/player/single_player.cfm?songid=9534655&q=hi&newref=1
http://www.soundclick.com/player/single_player.cfm?songid=9575220&q=hi&newref=1

However, IF I dial the volume to 10" and the OD controls to about "8" then it has the oscillation/vibration issue. At when that kicks in, it's obnoxiously loud.     I'll never play it that loud. I just want an amp as quiet as possible.

Sluckey, I will walk thru your suggestions step by step as soon as possible and report back.

Pulling V1 totally kills the hum.

THANKS guys!  All of you are incredibly generous with your information and help. This is a fabulous forum to be a part of.

With respect, Tubenit

[tubenit]

Sluckey,

Once again, your knowledge and generousity has helped me out!  While I have had great help from dozens of guys on the forum ........ I think you still hold the lead in helping me narrow down the issues & resolve them.

I did pull the reverb and 5879 OD tubes. It was still noisey before grounding anything out.

I marked on the schematic where the issue is. It's between the V1a section and the volume pot. It is like whisper quiet every place else like I would want it to be.  

Grounding V1-2 grid did nothing to eliminate the noise. But grounding the volume wiper does eliminate noise.

Maybe a bad cap? or a bad pot?  or a bad solder joint?  I can easily rebuild that entire section. I paralleled a .01 & .001 cap into the volume/tone section.

With respect, Tubenit

[tubenit]

OK, I thought to try something else and narrowed it down even more. I repeated this several times to be sure.

I'm thinking one of the silver mica caps is bad/leaky?  Or maybe a bad solder joint or something? Perhaps the tone
pot is bad?

I clipped off the switchable 390p & the noise is still there despite grounding that pot lug where the other silver mica
attachs.

Tubenit

[sluckey]

I clipped off the switchable 390p & the noise is still there despite grounding that pot lug where the other silver mica
attachs.

I'd suspect wiring, layout, or connections before suspecting the silver mica. I would still try bridging that filter cap for node E. I'd love to see some gut shots showing how the control panel connects to the rest of the amp and where the pots/switches are physically located.

[tubenit]

Here's what I'll do. I am 1st going to disconnect the tone stack from the volume pot and see what happens? 

Yeah, after thinking about it, I don't think it's the 390p cap since grounding the volume wiper eliminates noise and signal would be going thru that cap.

Then I'll clip another filter cap across node E and report back. I've never had a filter cap go bad before, so that will be a first.

Thanks again for the help! It sure is appreciated!

Best regards, Tubenit

[sluckey]

The cap may not actually be bad. But 10uF may just not be enough. Bridging with a 20-40uF will let you know either way. And the filter cap may not be an issue at all. Just want to eliminate it from the suspect list.

[tubenit]

OK, I removed the tone pot from the volume pot. Noise is still there.

And I did jumper a 47uf/500 across node E that feeds both plates on V1.  Noise is still there. I did the same thing on node D & the noise is still there.  The plate voltages on V1-1 & V1-6 look good to me.

I went ahead and replaced V1-1 resistor & cap. Noise is still there.

I tried jumpering from the non grounded lug to the wiper of the volume pot. Noise is still there.

Grounding V1-7 eliminates noise. Grounding V1-2 has the noise still there.

I am really puzzled with this one. Even with the 5879/OD & reverb tube out, the noise is very obnoxious when volume and master volume are on full.

Now what??!!   Is it possible to just have a bad tube socket? I don't see any signs ever of arching. The socket looks good.
 

With respect, Tubenit

[phsyconoodler]

Put a .1uf cap in series with  pin 7.In other words,put a cap in line right on pin 7.If it stops the noise then you do indeed have a leaky cap.

[sluckey]

Put a .1uf cap in series with  pin 7.In other words,put a cap in line right on pin 7.If it stops the noise then you do indeed have a leaky cap.

What? Don't do that. Pin 7 must have a dc resistance path to ground (more correctly to the cathode). If you break that dc path with a cap, the tube bias will assume some unknown state and fail to control the tube properly.

[phsyconoodler]

Really? The grid has AC.The cap will block the DC to the pot or input.I've done this before and it worked.Then I found the leaky cap and replaced it.
  Actually it was the input grid,not the other side.

[tubenit]

At this point, I am going to replace the volume pot and if that doesn't solve it, then I'll redo the heater wires to and past that socket. If that doesn't work, then I'll replace the socket.

Not sure what else to try. The problem seems to be from that V1a gain stage going into the V1b gain stage if I am understanding what I am doing correctly.

I did replace the coupling cap from V1-1 to the volume pot using a substitution box, but it made no difference at all.

With respect, Tubenit

[Dave]

Did you do the alligator clip dance? I'm telling you, it has helped me many times.

Dave

[tubenit]

Dave,

I actually did the alligator clip dance with the reverb grounding and the heater center tap prior to you mentioning it.

Having said that, the issue seems to be dramatically connected to the V1a area going into V1b. So there isn't much grounding there to sort out. Two cathode resistor/caps and the ground on the volume pot (the tone stack is removed and it's still doing it).  I will try your suggestion with those however and report back.

Thanks for the input. This really seems to be an odd problem but I am resolved that I will get it corrected.  

With respect, Tubenit

[phsyconoodler]

I had the same problem with a 5E3 and I found it by using a cap in series with the input.Iy=t did not make the tube runaway and it stayed biased,but the noise and microphonics were gone.Like sluckey says,don't try it on pin 7,but it worked for me on the input grid.

[plexi50]

Are you sure there is no tiny bit of solder on the preamp socket that may be touching another pin? Or even chassis? Maybe a socket is bad and one of the pins is broke inside the socket making an intermitant connection. It really sounds ground related. Cant wait to see what it turns out to be

[tubenit]

Looking at V1a ............  I have replaced:

- the plate resistor and cap
- the cathode resistor and cap
- the tube socket  &  I tried another tube there also
- the volume pot

& I rewired the heater wiring and reversed the green and white incase the 5879 heater wiring to the 12AX7 wiring would be creating a hum.

I completed disconnected everything relating to the reverb.

NOTHING has changed!  

Sluckey or someone ........ can you confirm if I have a problem with V1a with where the alligator clipped spots to ground are?

Am I looking in the right area?  Is V1a into V1b the issue?

Thanks, Tubenit

[tubeswell]

Out of curiosity, where are your ground return paths grounded tubenit? (Sorry if I missed this in an earlier post)

[tubenit]

I used Hoffman's grounding scheme like I do in all my amps. Preamp filter caps and pots and preamp grounds all on a buss wire. It worked in the Tweed Overdrive just fine which is even higher gain.

I did try Dave's alligator clip dance but it made no difference.

With respect, Tubenit

[tubesornothing]

Do me a quick favor:  on v1a, unsolder the 33k and ground the input grid.  I realize you have already tried grounding the grid, but unsolder that 33k - just a hunch.

[VMS]

Have you tried feeding the V3b from node D or C?

Not sure if this is a problem, but usually one PS node feeds no more than two stages, in your case node E would feed V1a and V1b.

[plexi50]

Do me a quick favor:  on v1a, unsolder the 33k and ground the input grid.  I realize you have already tried grounding the grid, but unsolder that 33k - just a hunch.

Thats all i can think of thats left to question. That input jack might be the demon

[tubenit]

Unsoldering the 33k from the input grid still creates alot of noise.  

With the 33k unsoldered off of V1-2 and V1-2 grounded, it is still noisey.

With the 33k unsoldered off of V1-2 and V1-7 grounded, it is quiet.

I did try powering V1 off a different node last night. No change ........ still noisey.


Man, I am at a real loss on this one?    

Am I correct thinking the problem is between V1a & V1b?  Have I accurately isolated it to that position?

PLEASE offer some more ideas if you have any.  I'm really stuck on this one.

I still have ALL the reverb unsoldered from the circuit & the tone stack disconnected. That actually seems to have made things worse. The hum is worse with all that out of the circuit.

With respect, Tubenit

[sluckey]

Jeff,  you're doing all the right stuff. At this point we need to see some hi rez pics. And make a sound clip of what you're hearing.

What are the voltage readings for pins 1, 2, 3, 6, 7, and 8? And take some resistance readings for pins 2, 3, 7, and 8. All resistance readings to ground (HT CT) and other probe directly on the socket pins.

[VMS]

Could the reverb pot be too close to the input jack or the volume?


On a side note:
Which way the 47pF cap is on the amp? Layout shows it parallel to plate resistor and in the schem it is between cathode and plate.

[tubenit]

The amp looks a MESS because I have the reverb and tone pot unsoldered.  And the amp has been thru numerous incarnations.  So, PLEASE cut me some slack on how messy it looks. It looked neat in the first build. In case you're wondering the grounds to the cathodes of V1 are good. I did run/swipe  a wire under the tagboard to make sure there wasn't something there and it did OK.
 

The 5879 tube is to the left of the preamp tube in the photos. But please remember, the problem exists with the OD bypassed and the 5879 tube removed.


ALL of the reverb is disconnected.  I completely removed it from the circuit. The 47p cap was removed earlier yesterday.

I checked all resistor values and they are correct.

V1-1 193v
V1-3  1.6v

V1-6  188v
V1-8  1.6v

resistance

V1-2  39k
V1-3   1.5k
V1-7    varies with volume pot  to about 1M
V1-8   1.48k


I don't get a reading of anything on V1-2 or V1-7 but I am not sure how to measure for that?

With respect, Tubenit

[sluckey]

Tried the lantern battery to power the filaments yet?

[phsyconoodler]

On V1,pin 2 you will get an AC voltage reading from your guitar.You need either a guitar plugged in and being played or a signal generator to produce the AC signal.
  See what I think is that your microphonic noises are from DC backing up to the input jack.
A few years ago I had this issue on a Tweed Deluxe.I would get this microphonic ringing and noise even if I touched the input jack.Someone here on the forum suggested I put a .1uf cap in line with the input signal,effectively blocking DC voltage from reversing up to the input jack.
  It worked like a charm.That's why I suggested it to you.
 I understand Sluckeys concern about having the tube find a bias point,as he says,'unknown' and running wild.Well it didn't and this is simply a diagnostic aid to isolate your problem,not a permanent thing.
   For the time it takes,it may be an idea.Lord knows you've tried everything else.

[tubenit]

Phsyco,

I'll give it a shot also. Couldn't hurt to try it for a few seconds and maybe that'll work?

With respect, Tubenit

[tubenit]

Tried the lantern battery to power the filaments yet?

I have not tried that yet, but I can. I'll have to go purchase a battery though. Not sure when I'll have time to do that, but I'll attempt it. It may be this wkend before I have time to go by a store and purchase one.  I take it that I need to power all the filaments or just the preamp filaments when I try this?

Q1)  With the 33k input resistor removed and V1-2 grounded, it is still noisey.
        Does this effectively remove the input jack as a concern?  I have an
        extra switchcraft jack that I can easily install if needed.

Q2)   The on/off switch seems to be more noisey clicking on and off. Any chance that
         could induce hum that showed up more on the V1a 1st gain stage?


Q3)   The other thing I pondered is I am wondering if the resistor between node
         D & node E could either be defective OR that a solder joint on that B+
         rail might be bad.  I think I'll replace it & resolder.  Could that be a
         concern? 

Q4)   There is some very slight hum from the speaker grounding all the points that
         Sluckey stated to try. Grounding those makes the amp either super whisper
         quiet or on V2-7 ........ it sounds "normal" to how my amps typically sound at
         idle. Grounding V2-2, it is VERY loud when I turn the vol & master vol to 10.
         That is what I should be hearing correct?  The slight hum is slightly louder
         as you move towards V1-a but it sounds like I would expect the amp to.



The other thing I haven't tried is replacing the shielded wire from the volume pot wiper to the V2-7 grid.  It did not test shorted internally or shorted to ground. Having said that, it seems like I had a shielded wire test OK once and prove to be a problem.  So I'll replace that and see if anything else happens.

I know I am somewhat desperately grasping at straws here, but I am at a loss to logically explain why the volume of hum is so loud at the V1a to V1b area?

The hum with V1-2 shorted to ground is probably 3-4 times the hum with V1-7 shorted to ground.  The hum with V1-7 shorted to ground is what I typically hear in my amp builds at idle.

With respect, Tubenit

[tubesornothing]

Q1 - yes this removes the input jack and resistors from the equation.

It is something around V1A.  Here are some random thoughts:

- bad socket
- cracked component (like a resistor)
- cathode ground point is too close to a power stage ground point and hum is getting in that way
- bad tube

[tubenit]

I replaced the socket with a brand new one.

Replaced the resistors & caps

Tried 5 different tubes of various gain level

Using the same paralleled terminal layout and power supply hookup as I have in other amps successfully & even in this amp in previous incarnations of it.

Still makes the noise between V1a & V1b

Thanks for the ideas & please share any additional things to consider. Your post helps confirm that I'm at least considering some of the right areas to look at.

With respect, Tubenit

[plexi50]

Wow! Im starting to think that Casper may not be a friendly Ghost. Boy when you find this problem you will never forget it
Neither will i   Have you ruled out any bad wires broke internaly under the insulation?

[tubesornothing]

Sorry about that - didn't mean to reiterate things you have already tried.

I had a reverb return noise that caused me lots of trouble.  HBP helped me to track it down to the cathode ground of the reverb return.  I am wondering if you might be having the same problem but with V1A.

Perhaps - just as a something to do until a good idea comes along - detach the V1A cathode ground, and temporarily jumper it to other parts of the amp. See if the noise changes.

Irf not that, since you have already tried everything on the grid of V1A - all thats left is noise getting into the plate of V1A...

[softwarejanitor]

Does V1 have an aluminum shield over the tube like a lot of old Fenders do on the preamp tubes?  If not it might be worth adding one to see if that helps.