ST-120 noise issue

Recently my VTA ST-120 with 800 hours of trouble free listening has developed a hum/crackling noise issue that is best described in the sticky note "quick trouble shooting guide" as a combination of problems #5 & #6

5. Problem - Amp has a hum that doesn't change in level as you advance the volume control of your preamp
Areas to check - bad quad cap, bad ground inside the amp, bad tube filament that is leaking AC into the audio

6. Problem - Hiss, noise, crackling sounds on idle
Areas to check - Noisy driver tubes, noisy resistor on driver board (especially older carbon composition resistors), bad solder joint or bad solder trace on the driver board.

I have isolated the hum/noise to the amp by shorting the input leads from the preamp to the amp, my amp has a stepped attenuator so I'm able to use the amp without my DAC and preamp and have noted that hum is consistent and doesn't seem to vary however the hiss/crackling noise seems to come and go and is not consistent.
Both channels have the hum, crackle noise so i started by changing out the center driver tube with no improvement then I tried swapping side to side then different set's of driver tubes with no improvement.
I have pushed firmly on the PT while the amp is in operation and did not detect any notable difference in the hum I also tried tightening loosening the mounting screws with no positive results.
just for the heck of it i also changed from my KT-120s to Kt-88s and from GZ34 to 5AR4 to a WZ68 copper cap, no improvement or change.
I have noted that when I shut the amp down the hum stops but the crackling noise is still present for a few seconds (I'm guessing caps bleeding off?)
I have pulled the cover and visually inspected for an obvious bad solder joints using a magnifying glass at this point I have not re flowed any solder joints
Would the next logical step be to check quad cap and resistors on the driver board?
Keeping in mind that I'm a novice what would be the next trouble shooting steps should i take any tips or advise would be greatly appreciated
Thank You:confused:

Your noises would typically be two different issues, so it's a bit surprising that they both appear at the same time. I'm not a big believer in conicidenses when it comes to tube amps. I would focus on the crackle first, as that's a bit easier to diagnose. Use a wooden chopstick or plastic probe (not a pencil or pen!) and with the amp open and running see if you can poke around to create the crackle. BE VERY CAREFUL AND KEEP ONE HAND IN YOUR BACK POCKET. THERE"S SEVERAL HUNDRED VOLTS RUNNING THROUGH YOUR AMP. Make sure that you are NOT touching the chassis or anything other than the opposite end of the chopstick. Start around the middle drive tube socket and work from there.

It's probably a solder joint that's opened up or a bad resistor. Find and fix. I guess I'd be surprised if it fixed your hum and am curious to hear what happens. Hum sounds like AC getting into the signal. Maybe a bad solder joint on your middle driver filament leads??

Good luck!

Kaner

Bob Latino wrote: You might get lucky and one tap might cause the hum/noise to go away?

Or you might get UNLUCKY and the mere vibration of picking up and moving your amplifier might
"fix" the bad connection temporarily.

Don't ask how I know.

No bench should be without one of these for getting up close and personal with your gear ...



Don't forget to look for long tails on capacitors and such, especially around the driver board. Those can cause all sorts of grief. Trim everything tight to the solder. Even if they're not the original source of the problem, poking around inside could move one to where it bites ya.

Also check for good clearance around the filter caps if you're using the Russian PIOs. Wouldn't be unheard of to have some intermittent arcing from the exposed metal cases, even if only static.

Which brings to mind humidity levels. Winter heating, lots of static in the air, metal chassis ... good potential for seasonal noise issues. First step is to use a humidifier - I keep the house at around 30% here. Might also help to add a common ground between the amp and the rest of your gear (the phono ground on most receivers is a good point as long as that's also connected to earth). I've also got a flat washer glued to the rack and connected to earth so I can discharge any static buildup due to sock shuffling and such - forced air from winter heating and heavy woolies, and we're talking veritable lightning bolts we are ...

*** Hummmm ... Don't forget the easiest potential fix ... two wire plug, reverse it in the socket and see if that helps. If the blades are polarized, I don't imagine you'll have black helicopters hovering overhead if you snip the ends of the wide one ... ;-}

Update
Kaner & Bob

I bought myself a handy little insulated screw driver rated to 1000V for probing around under the hood I made a cover for the exposed tip using two pieces of folded over shrink tube (Rated to 600V) and it slips off and on when needed.
I poked and prodded all wiring and parts with no discernible changes after further diagnoses it does in fact seem that the hum is an amp only issue, So for the heck of it I decided to check voltages as recommended in the kit instructions everything seem to check out fine except for the Rectifier PIN 4 & 6 checks were 16.7 & 36.6 V AC and all V2, V3, V6 & V7 PIN 1 to ground and 8 to ground all measured the same at 0.6 V DC
I have the sticky note diode upgrade, using a Weber WZ 68 copper cap and KT120s
All checks with speakers input cables from preamp hooked up, the amp seems to play fine.
I'm not sure what to do next, Help  

Bob,

I have the sticky note diode upgrade and just figured out that if I take my measurements on Pins 5 & 7 then I come up with 418.9 V AC on each I'm guessing this would be correct?
I still only get 0.6 V DC on Pin 1 & 8 to ground on V2, V3, V6 & V7

Maintarget wrote:Bob,

I have the sticky note diode upgrade and just figured out that if I take my measurements on Pins 5 & 7 then I come up with 418.9 V AC on each I'm guessing this would be correct?
I still only get 0.6 V DC on Pin 1 & 8 to ground on V2, V3, V6 & V7

pins 1 & 8 are shorted and your BIAS resistors are connected to those, what you are reading is your BIAS voltage of 600mV, so depending on your power tubes, yes, thats correct. KT120's are good at 600mVDC, KT88's should be about 500-550mVDC.
As has been already advised, go through EVERY solder joint and triple check it, anything that looks even vaguely suspect, touch it up!

MW
I'm obviously new at this and learning as you said what I'm reading is the bias voltage, I'm using KT120s biased at .600mv
Bob is right in his earlier post stating
"The person rationalizes to themselves that >"it played fine for weeks, months etc. so why NOW do I have a problem and why are you saying it's a bad solder connection?"
I will be re-soldering all connection in the power supply ASAP.
Thanks for your patients you guys are the best!

Roscoe

Update:

Re flowed ALL solder connections in three stages over several days until all were redone long story short no changes until i re-soldered connections on driver board and at the same time loosened the power transformer mounting hardware, sweet success I'm hum-less again!
Dam this thing sounds awesome!
Still have a Whooshing crackling sound in between tracks that is UN detectable when audio is present
Thanks Bob, Kaner & MW