hum issue RESOLVED

[tubeswell]

So when you pull the V1 tube, the hum goes away?

[tubenit]

Yes, when V1 is pulled there is NO noise.

Shield over the tube makes no difference.

I  tried the .1 cap ...... no difference.

I replaced input jack ....... no difference.

I replaced the wire from volume wiper to V1-7 ....... no difference.

I removed the chassis from the cabinet  ........ no difference.

I measured the heater wire voltages up and down the amp.  They are spot on 6.3 vac.

I am truly baffled. 

IF this is accurately between V1a and V1b ............... there just isn't anything left to replace or change.

Don't have the lantern battery but I am willing to try that even though I dubious it will make a difference.

Could the problem be someplace else but is more pronounced in the V1a to V1b area ??????????

with respect, Tubenit

[tubesornothing]

Jeff,

My theory is that the hum is getting into V1A - you have already proven that it is not through the grid.  So I am thinking it is getting through the plate or the cathode.

I suggested earlier to detach the cathode ground and put it on an alligator clip to other places.  If not that, then the noise is getting in through the plate somehow.

[tubenit]

I suggested earlier to detach the cathode ground and put it on an alligator clip to other places.

That was done yesterday. And I used a cap substitution box for the V1-1 coupling cap going thru a handful of values and then soldered in a brand new Orange Drop. And both the plate and cathode resistor were replaced also along with the volume pot etc...

with respect, Tubenit

[Ritchie200]

Jeff,

Are all your pots nice and tight, bare metal to bare metal onto the chassis?  Input ground to your pot ground?  Sorry, I could not see detail on you Photobucket page.

Jim

[tubenit]

Yes all pots and input jack are tight. And I quadruple checked the ground with a voltmeter.

Tubenit

[tubenit]

I may not have a chance til wkend to try this, but I will try these bypass ideas next to try and isolate the area definitively.  AFter doing so, I'll report back again.

Can a bad on/off switch cause a severe hum?  Keep in mind the hum only becomes pronounced when the volume and master volume are at above 5 or 6. The amp has no noticeable hum with the vol and mstr vol. at zero. It's completely quiet.

Tubenit

[sluckey]

How far are you willing to go with this? I'd like to see you abandon that ground buss on the back of the pots. Think about it. Every pot provides a potential ground loop thru the body/nut. This may or may not solve your problem but it sure won't hurt. And as a bonus, if you ever need to change a pot it will be much easier.

This Ceriatone buss would be very easy to do on that amp and would look much cleaner. And the buss gets connected to chassis at only one point--- right near the input jack. Just connect the #14 buss to your board grounds and run short wires from the pot grounds to the buss. (Notice that the power tubes cathode resistor/cap are not connected to this buss in this Ceriatone scheme.)



I prefer the buss scheme on my November or Lightning amps, but that would be much harder to do at this point than the Ceriatone buss.



I know this is a lot of work and I understand if you don't want to try it. We're kinda reaching for solutions at this point though. I'd try the lantern battery first, though I'm not very optimistic. Could be a reason to get a good lantern style flashlight though.

[plexi50]

I have to wonder if the PT is some how damaged and the heater winding may be picking up some osscillation from the PT. This is a very cool thread mystery. But i understand very stressful. The battery test will answer the heater question

[Fresh_Start]

For whatever it's worth, I've used a buss similar to the Ceriatone setup on 3 Blackface Fender clones.  It has worked very well for me.

I'm grabbing at straws, but these two comments caught my eye:

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.

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

I know that you tried a 47uf cap across the 10uf filter cap at node "E".  Did you try an outright replacement of that filter cap with a 47uf?  How about a 1/100th poly cap in parallel across that last filter cap?  How about a 15K isolation resistor instead of that little 1K?  1K with a 10uf cap is not much filtering in my limited experience.

Node "E" feeds V1a & V1b, but it also feeds the reverb recovery triode (V3b).  Would it be difficult to try feeding V3b from node "C"?

Like I said, this is just grasping at straws but it seems like you've tried everything imaginable except re-working the power supply.

Keep at it Jeff!

Chip

[tubeswell]

Have you tried dumping the V1 plate(s) to ground with something like a .1uF high-voltage (film) cap (to see if that kills the hum)?

[tubenit]

One of the things I've mentioned that may not stand out in what I have said is how quiet the amp is at the normal level that I play it. The normal level is how loud it is when I play it with the band. Vol, and OD controls are on around "5". No complaints by anyone. About the same noise as the PR I had owned but a different noise.   When I dial those same controls ....... or the clean master volume to "7".  The noise is probably 7 or 8 times louder (or more)  then the volume I usually use and the amp chassis actually vibrates physically.

This amp (despite the hum) is one of the most transparent, touch sensitive and expressive amps I've built at "normal" volumes.


I've got a reasonable hunch of what it might be!  I think?  

If you count original builds and incarnations, I've built maybe 20+ different amps. ALL of them have been either super quiet or very reasonably quiet.

I typically use a paralleled terminal or tagboard approach with B+ running along the chassis UNDER the plate, grid and cathode wires.  

However, the most recent of  one (of maybe 4 or 5) incarnations was Geezer's HoSo56 with 6V6's. When I changed the position of the 5879 tube to the Tweed OD Lite, I did not have enough wire length to run the B+ wire under the plate,grid, & cathode wires so I ran it above them and somewhat close to them.

I'm betting a nickel that's the issue! It was staring at my all along. The B+ is under the LTPI wires and above the preamp wires.

I think I need to change that wire out and get it under the preamp wires and further away.

IF that doesn't work, I'll try the bypass stuff & report back. I can certainly change out the filter cap on node E.
Even though my schematic shows that to be a 10uf, it is actually a 16uf filter cap.

If that doesn't work, I am probably going to gut the amp and start over at which point I'll use the Ceriatone type buss wire.

FWIW, I have used Hoffman's buss wire on the back of the pots on every amp I have built with no issues including both of the Tweed Overdrive Special amps that I also have which have even higher gain. However, if I am not opposed to changing that buss wire to a Ceriatone type. It wouldn't be that much effort.

Currently, the reverb is completely disconnected, but I can change the B+ to reverb to node C no problem.

Guys, THANKS to all of you for your help and support and encouragement! You guys are superb and I am truly appreciateive!

With respect,  Tubenit

[tubenit]

Did anybody take my nickel bet? ............... I owe you 5 cents.  

Changing the B+ wire didn't work.  Back to square one.

Between now and Sunday, I have a very busy schedule so I may not get a chance to work on this for a few days?

Picture this,  volume and master volume on "5"  OR  volume and OD controls on
"5", there is not much noise.

However, with those same pots turned up to 7. It makes the same loud obnoxious noise level as one would get if their guitar cable became unplugged from the guitar. It is a HUGE jump in volume/hum. And the chassis will physically vibrate when in the cabinet. It is not the same type of noise as a cable unplugged but it has that type of obnoxious loudness.

For my own curiousity (if not mental health), I am next going to try my bypass plan and see what happens?  Then I'll go thru the other suggestions.

Anybody have another thought of how something in another part of the amp appears to be problematic between V1a and V1b besides the node D to node E dropping resistor?  (which I will attend to)

Is it possible that I have a bad PT but the problem appears to be located between V1a & V1b?

The chassis is out of the amp and the problem still exists. Because it is an open ended chassis, I can easily see under the tagboard and there appears to be  nothing problematic there.

With respect, Tubenit

[Fresh_Start]

Jeff - I know you've used your 1-tube reverb successfully in many builds.  You haven't done a high-gain build before though, have you?  I know the guys who build high-gain amps obsess over the power supply.  They also obsess over grounding. 

If you have TUT 3, figuring out how to get closer to O'Connor's "galactic ground" topology may help.  I can't help wondering where the cap for node "E" is grounded in relation to V1 and to the reverb recovery circuit - including the reverb pot itself.  Can you try just disconnecting node "E" from the reverb recovery stage?

This isn't from personal experience, so please don't take it as gospel.  I just spend some time lurking over on AX84.com and they seem to have a lot more problems with oscillations with those high-gain amps with multiple gain stages than we do on our typical builds.

Good luck!

Chip

[plexi50]

I made a mistake in one of my high gain build a year ago. I had mistakenly wired the ground buss of my preamp section and all pots to a ground lug. That ground lug was on the PT. The amp was noisy and the hum increased along with an increase in volume. I finally saw what i had done and isolated the preamp and pots to the other end of the board at the input jacks. Hum was gone and volume worked as intended. I havent forgotten that one. That was proof for me that grounding is a very sensative thing. That a PT signal or osscillation could actually get injected into my preamp through my bad grounding error

Speaking of Mental Health where is my VOLUME? 

[Dave]

You know, sometimes, reverb pan orientation can have a dramatic effect on humm issues.
It probably a long shot, but if we're stumped, let's look at all possibilities.

Dave

[tubenit]

FreshStart,

Yes, I have done high gain amps with one tube reverb and NO problems. They were quiet.

For example, I have two Tweed Overdrive amps with me & no issues with them. Yes, they are heads instead of combos. However, this Tweed OD Lite still does this removed and isolated from a cab (as far as 4 ft away with the speaker hooked up).  One of the Tweed Overdrive amps did have the one tube reverb & it worked fine with no issues.  It is higher gain than the Tweed OD Lite.  I did convert that though to an active effects loop per request of a buyer who wants it.

Please remember (as I have mentioned)  the reverb is completely disconnected in the amp for the last several days & the problem continues. If the amp essentially doesn't have reverb, then I would anticipate that eliminating the reverb as a problem.
The wires are not hooked up, there is no reverb tube and the pan is disconnected. Node E has been disconnected from the reverb for several days.

And all my builds except this one have been successful using the same type layout format with paralleled terminals.  Including a cathode biased D_mble HRM and both Tweed Overdrives, Plexi 25W with revergb & an 5881 HoSo56. The EL84 56T and 56T with 6BM8's were quiet also.

And I've always used Hoffman's grounding scheme with success in over 20 different builds. So why wouldn't it work this time?  I am not aware of what would make it different. The preamp filter caps are connected to the buss wire like Hoffman shows. The power filter caps are connected to a PT bolt.

Again, the problem "appears" to be related between V1a & V1b. Which is confounding me because there are so few parts there.  Using Sluckey's suggestion, grounding V1-2 does NOT eliminate the noise but grounding V1-7 makes the amp super quiet.

And while this seems to indicate that between V1a and V1b is the problem, I have literally replaced everything from the input jack up to the V1-7.

With respect, Tubenit

[VMS]

This probably won't help either, but it looks like the heater wiring is very close to the grid resistor of V1b. I would bend those away from the other wires and that resistor.

[phsyconoodler]

Ok,what about NOT concentrating on V1 and looking to the output section.The output jacks in particular.
1)does the amp have negative feedback and do you have a ground reference for the output jacks?
2) did you use isolated jacks or switchcraft style?
3) have you tried loosening off the bolts on the output transformer and moving it around a bit?
4) then back to grounds.I had a nasty hum on a 5E3 until I completely separated preamp filter cap grounds from PS cap grounds.
  Just thoughts added to an already frustrating scenario for you.

If you take a look at the ground scheme on a D'Lite layout,it shows a great separation of those grounds,even using really long ground wires.
  I even had a nasty hum on a V-Verb build that turned out to be the unused bias tap wire wrapped around the filament wires!
Go over the amp and don't concentrate so much on the preamp.It may not be there,but it gets amplified by the preamp!
   I do not star ground anything and use Doug's ground method sometimes,but not always.Do not assume it works with every amp.
 

[mresistor]

I wonder if it could be a bad V1 tube socket?

[tubesornothing]

I wonder if it could be a bad V1 tube socket?

He already tried that.  Multiple tubes too.

[tubenit]

Phsyconoodler,

Your input has given me another couple of ideas to try.

The amp has a passive effects loop. I'll try using that and see if I can hook up another amp preamp ........... or feed into another amp poweramp to more conclusively pinpoint the area.

I may have time to do that tonight.  And if so, I'll post the results.

And I can disconnect the effects loop and  remove it to eliminate that as an option also.

Since this is a 4th or 5th incarnation in the same chassis, I should point out that the PT, OT, grounding scheme, layout, etc............ has worked in all the previous incarnations of the amp.

I pretty much changed the preamp and left the majority of it alone on this last conversion. 

As TON mentioned, it has a brand new socket. No change.

With respect, Tubenit

[tubeswell]

Maybe a dodgy switch contact on one of the FX loop jacks?  Sometimes when you tighten up the jacks, the switch and tip get squeezed. I suggest this because on a recent BFPR build I did, the same thing was causing similar symptoms to what you describe is going on with your vol control. Just a wild hunch (but given the other hunches around here to-date - I figured what the heck?) Or it could be the contacts in your O/D switch perhaps.

[tubenit]

I thought about the clean/OD switch also? It's a mini-dpdt. And I don't remember if it was one I rewired or new? I will work to eliminate that in the list of things to try. I have bypassed it and the noise is still there. But perhaps bypassing wouldn't eliminate it?

I can easily remove the effects loop.

I am still wondering about the on/off switch? It seems to be making a louder pop turning it off and on. OR it may be that I am hypersensitve listening for noise and it was already that way? 

I already tried taking a chopstick and moving the heater wires around. No difference at all.

Guys, I sure appreciate the ideas and support alot!  Thank you!

With respect, Tubenit

[tubenit]

UPDATE:

Using the passive effects loop, I ran the Tweed OD Lite preamp into the Tweed Overdrive and it still makes the hum.

I ran the Tweed Overdrive into the Tweed Lite OD power amp and that sounds fine.

It is definitely in the Tweed OD Lite preamp section PRIOR to the LTPI.

Then I removed the effects loop just in case that was a factor and it still makes the noise, so the effects loop isn't the culprit.

I think Sluckey has it figured out with the V1a to V1b area. 

I am going to try the bypass idea next and further pinpoint where it is.  I may not be able to work on it again til Sunday?
However, after I try it, I'll give another update.

Depending on what I find, I might  rebuild V1b and change B+ from node D to node E.

with respect, Tubenit

[Danskman]

If it was me, I would let it alone and forget it for three weeks.
Then, as you'll be fresh-minded, it will be the right time to debug it again.
Maybe you'll be tempted to dismount it entirely and re-build it from the start... I did this on a 6G13A and it helped a lot 
Good luck and BR,
Danskman

[tubesornothing]

Your close Jeff, don't give up now!

[tubenit]

What I lack in knowledge, I try to make up for with persistence!

 

I'll get it fixed!  Will post again Sun or Mon with an update.

Thanks guys!

With respect, Tubenit

[tubeswell]

I still wonder about vibration and a dodgy switch contact(s). If you brush your finger-tip against the tip contact(s) of the input jack(s), does it/they rattle? I had this problem once with an amp, and it annoyed the feck outta me until I found it. I found that the tip and switch contacts were ever-so-slightly on the verge of being unconnected, and this induced a buzz/rattle in the chassis at certain frequencies or certain volumes. Worth a look at maybe.

[tubenit]

Tubeswell,

From a previous post on the thread

And while this seems to indicate that between V1a and V1b is the problem, I have literally replaced everything from the input jack up to the V1-7.

Input jack was replaced with a brand new switchcraft. It's not the issue. It's making good contact and no rattle from it.

I'll get back to trouble shooting it probably tomorrow & post what I find out.

with respect, Tubenit

[tubenit]

I bypassed V1-b into LTPI & noisey.  No OD

I bypassed V1-a into LTPI & noisey.  No OD

I concluded it was either the mini-dpdt or the dropping resistor between node D and node E.

I removed the DPDT from the wiring altogether & left OD wired in and changed the dropping resistor from 1k to 4.7k.

Amp is VERY quiet at idle now with only typical hiss and NO hum! 

That resolved it!

THanks guys!  with respect, Tubenit

[jjasilli]

 

[Fresh_Start]

Congratulations!  You went through the trials of Job on this one.

So which was it?  The switch or the under-sized power rail resistor?


BTW you really need a "listening amp".
http://www.el34world.com/Hoffman/tools.htm

I built a little box similar to what Doug shows at the bottom there and plug it into a TradeMark 10 amp I don't like much anymore. http://www.el34world.com/Forum/index.php?topic=3950.0  It's fantastic for figuring out where noise is coming from... at least sometimes.

Cheers,

Chip

[tubesornothing]

Yippee!  Congratulations Jeff!

[tubeswell]

Yay! another victory in the struggle for tube amp perfection

[jojokeo]

Do you think the DPDT switch was overheated when soldered to causing it to go bad?

[tubenit]

Guys,

Unfortunately, I don't know which it actually was?  Here is what happened ............

Neither bypassing V1a (and using V1b as 1st stage) or bypassing V1b eliminated noise. So I thought at that point, that I'd just try a couple of last things and then gut it and start over.

I was not anticipating the last few things to resolve the issue. I was presuming the amp would be gutted.

So, I removed the master volume, the DPDT & replaced the dropping resistor between node D and E ........ all at once. Something in that removed the hum and it is very quiet like my amps are typically.

Wished I had done those one at a time, but I can say that I am pleased with having a really quiet amp again.
 

THANKS for all the guidance, support and encouragement!!!!!!  You guys are fantastic!

Best regards, Tubenit

[Dave]

Boy I wouldn't have suspected either of those. Congrats.

Dave

[sluckey]

So, I removed the master volume, the DPDT & replaced the dropping resistor between node D and E ........ all at once.

It may not have actually been any of those. You may have fixed a problem solder joint in the process. Of those 3 components, I think the dropping resistor would be the most likely suspect. Dropping resistors for the preamp are usually fairly large to drop the voltage and to provide a higher degree of decoupling from the previous B+ node.

Glad you got it. Now to start stitching all the bell and whistles back in. 

[plexi50]

AMEN! I have a similar issue myself now but a little different. When i turn up the volume in my Bassman normal channel with no input i have a hum that increases as the volume is turned CW. The vibrato channel is dead quiet and does not do this. Is this Hum month?

[sluckey]

You have a Bassman with a Vibrato channel?   

[plexi50]

You have a Bassman with a Vibrato channel?   

Sorry i ment a Bandmaster Reverb.  That is a big difference isnt it?

[sluckey]

Actually, I knew what you meant just from your recent Bandmaster thread. They both start with "B". I mix up words like that all the time. Ask my wife! Don't know why she doesn't understand what I'm talking about.