A new hum...

[BigSteve22]

Just turned on my amp, VHT Special 6 Ultra, and I'm getting a hum at just about 120Hz. (Strobe tuner app on my phone shows it to be just slightly flat of a 123.5Hz, B2.) Wasn't there last night, but definitely there now. Other than the hum, amp appears to be working as always, all controls work as before. Guitar tone appears unaffected. Interestingly enough, the volume control has no effect on the volume of the hum, but the "Watt" control seem to attenuate it at lower settings. Tried swapping out tubes. no effect, same for trying a different outlet. That's about as far as I'm prepared to go on a tube amp without some sort of guidance.

If any of you engineer types have a few moments to spare, I've attached a copy of the schematic for your perusal. Any help would be greatly appreciated. Thanks.

 

[Verne Bunsen]

Are there any new environmental factors? New A/C unit, refrigerator, anything with an electric motor running that wasn’t there before?

 

[Cagey]

It's possible one of your power supply filter capacitors have gone belly-up. One of the batch you see down in the lower right of the schematic. Open the amp up and see if any of them are bulging/leaking. They'll be the larger, round electrolytic units. If they're not right, your power will look like pulsating (at about 120hz) DC instead of a nice, flat DC. Kinda need a 'scope to see it, unless the cap is obviously failed. Happens a lot on tube amps due to the high voltages and heat involved, which electrolytics struggle with at higher levels.

Note that electrolytics can hold a fairly high charge even when the amp is off, so be careful what you touch or it'll be a very memorable experience.

 

[BigSteve22]

Are there any new environmental factors?

No, nothing new, I even unplugged some of the old stuff just to make sure.

It's possible one of your power supply filter capacitors have gone belly-up...

That's what I was thinking too. I remember reading somewhere that they have a habit of burning out. As I recall, your suggested method, "look at them", is how I found the problem with a solid state stereo receiver I had many years ago. Opened it up, and one of the caps was discolored. Replaced that one, and everything was good. I should be so lucky this time!

Found a picture of the turret board:

I'll take a look, and see what there is to see. If nothing is evident, and I check the voltage across the caps, would I be right in assuming that a blown cap would have no voltage present, if the others are all charged?

 

[Cagey]

Chances are good there will be voltage present, but it won't be nice smooth DC, which is why I said it's easier to see on a 'scope. But, it'll probably read low, since the dips between half-waves will cause an average lower level.

I should also mention that if they are obviosly failed/failing, turn the power off right away. The little rascals have been known to explode due to overheating, and it can be like a little aluminum hand grenade  :icon_biggrin:

 

[BigSteve22]

Chances are good there will be voltage present, but it won't be nice smooth DC, which is why I said it's easier to see on a 'scope.

What I meant was, after powering down the amp, the power caps remain charged. Would I be correct in assuming that a blown cap would not retain a charge like the functioning caps do?

...The little rascals have been known to explode...like a little aluminum hand grenade  :icon_biggrin:

I've actually seen the results of just such an aggressive failure! Friend who owned the equipment, an old 60's era stereo amplifier, said it sounded like a little firecracker went off, just before it stopped working! When I took it apart, it looked it too..., little bits of oily paper stuck to everything! At least it was obvious what had failed. (And thankfully, there was a schematic glued to the inside of the cover, otherwise I would have had no idea what the value was!)

 

[Cagey]

Chances are there's no charge left on a blown cap, but if it was me, I'd find a piece of wire and connect it to the chassis and touch the positive side of the caps. That'll discharge anything left in there. Note: be sure the amp is unplugged.

I've seen my share of catastrophic failures of those things, too. Not a pretty sight.

 

[Rick]

I use a voodoo labs pedal power 2 plus.  It eliminated excess noice.  Sure if you listen carefully you can hear the 60 cycle hum, but nothing excessive.

 

[stratamania]

I use a voodoo labs pedal power 2 plus.  It eliminated excess noice.  Sure if you listen carefully you can hear the 60 cycle hum, but nothing excessive.

I am not sure how that helps with a problem of hum in an amp being discussed in the thread?

 

[Rick]

Remove environmental factors before doing surgery.

 

[stratamania]

Remove environmental factors before doing surgery.

I am sure that he would not be checking it with pedals plugged in.

 

[TBurst Std]

I believe he is referring to using the receptacle out on the PP+ For the amps main

 

[BigSteve22]

I am sure that he would not be checking it with pedals plugged in.

With everything unplugged, even the guitar, hum starts several seconds after turning on. Tried unplugging everything that has a motor, tried different outlets, and a lifted ground. No love....

Thank you all, I appreciate all the suggestions, and hope you continue to post anything that comes to mind. For the moment, I've switched back to my little Mustang I, just so I don't go into withdrawal. Tuesday or so I'll have a further look under the hood of the VHT.

You guys are the beast!  :yourock:

 

[stratamania]

I believe he is referring to using the receptacle out on the PP+ For the amps main

Perhaps but I do not think that is an isolated out and thus would not help. It is an auxiliary power outlet and is not be used for plugging in an amplifier. For that, something like a Furman product would be needed.

http://www.voodoolab.com/manuals/pedal_power_2plus_manual.pdf

 

[TBurst Std]

I believe he is referring to using the receptacle out on the PP+ For the amps main

Perhaps but I do not think that is an isolated out and thus would not help. It is an auxiliary power outlet and is not be used for plugging in an amplifier. For that, something like a Furman product would be needed.

http://www.voodoolab.com/manuals/pedal_power_2plus_manual.pdf

I agree. It is a convenience outlet.

 

[mayfly]

Hey Steve!  Can you do me a favor and plug something into the effects return jack?  Ideally a plug that has the tip shorted to the sleeve.  Failing that, how about a guitar with the volume all the way off.  Once you've done that, is the hum still there?

I'm suspecting something...

 

[Rick]

My point was to introduce as much isolation and filtering into the the signal chain.  I'd even try plugging it in to a different house.  But it looks like now it's something to do with the amp.  When it reaches that I bring it to a wizard.

 

[Cagey]

What Mayfly is suggesting will reduce the areas noise could come from pretty fast and is very fast/easy/free to do. If the hum goes away, it absolves your power supply and power amp.

 

[BigSteve22]

Hey Steve!  Can you do me a favor and plug something into the effects return jack?  Ideally a plug that has the tip shorted to the sleeve.  Failing that, how about a guitar with the volume all the way off.  Once you've done that, is the hum still there?

I'm suspecting something...

What Mayfly is suggesting will reduce the areas noise could come from pretty fast and is very fast/easy/free to do. If the hum goes away, it absolves your power supply and power amp.

Sorry for the delay, been running around all day. Just tried your suggestion, used a patch cable with a piece of bus wire shorting the contacts of the loose end. Hum is attenuated, but still present.

 

[BigSteve22]

Testing Update...
Took the amp out if the cabinet and checked a few things. Other than some sloppy assembly work, nothing stood out. There were a couple of places where wires were burned with a soldering iron, (definitely burned from the outside), but no bulges or leaking components noted. (I even stripped off the plastic from C47 after testing, just to check, no cracks or leaks.)

So, referring to the turret board photo:

The electrolytic filter caps, from left to right are: C47, C39, C41 & C43, C45 & C3. Took some readings, Here's how the numbers shake out...
 
                              C47        C39        C41        C43          C45        C3

VDC/Pwr On      +6.5        -396      +233      +232        +323      +300

VDC/Pwr Off          0          +3.75    +2.75      +2.5        +1.9        +1.75

R(Ω)/Pwr Off        3Ω          OL          OL          OL            OL          OL

Cont. Test:          Short        OL          OL          OL            OL          OL

Testing Notes:
1) All readings taken with a multimeter connected directly across the component leads.
2)VDV/Pwr On: Readings were taken twice: Once as soon as the hum started, (Appx. 5 sec. after power on), and once at appx. 15 minutes. Readings were within .1V.
3)VDC/Pwr Off: Readings taken as soon as amp was powered off, from left to right. Voltage continued to drop across all caps until reaching 0V. Gorunds for all these caps are bused together, so it appears that they were draining through the shorted C47.
4)R(Ω)/Pwr Off: Readings taken as soon as all caps read 0V. C47 was the only cap with a measurable resistance.
5)Continuity Test: Show "OpenLine" for all caps except C47, which showed a short.

--------------------------------------------------------

So, to me anyway, it looks as if C47 is the culprit. This would seem to make sense, as it's the filter for the filament heater circuit, which would introduce a hum without effecting overall operation of the amp. Installed cap is 10,000µf/10V, according to the schematic, it should be 12,000µf/16V.

Thoughts, corrections, observations?