Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel

Jeepnmon

Jeepnmon

AK Subscriber
Subscriber
Hi all,

I recently purchased a VTA ST120 from a member that looks amazing and it has been working great. I knew there was a possible issue with it as disclosed by the seller in our discussions and thought this could be a great way for me to learn about tube amplifiers by jumping right in. Not sure that's always a wise course of action but I decided to do it.

Here's my issue. The amplifier was working great until a few days ago when I began hearing a scratchy sound coming from the right channel that increases and decreases in volume somewhat randomly. It subsides sometimes but then comes back. I've tried swapping power tubes and interconnects which didn't make a difference. My understanding is that this was VTA ST-120 kit that the seller built.

Any thoughts or help from the members here would be greatly appreciated. Thanks!
 
Register to hide this ad
Dirty socket pins, and noisy/failing plate resistors are the most common culprits. Try cleaning and/or tightening the socket pins first. Did you also swap the preamp section tubes? Those can get noisy.
 
Thank you! I forgot to add that I swapped the preamp tubes also with no change.

My understanding is that this kit was purchased this past spring and built shortly afterwards. Could the socket pins be dirty on this new of a unit? Could a plate resistor/s fail on a relatively new unit like this?
 
Jeepnmon said:
Thank you! I forgot to add that I swapped the preamp tubes also with no change.

My understanding is that this kit was purchased this past spring and built shortly afterwards. Could the socket pins be dirty on this new of a unit? Could a plate resistor/s fail on a relatively new unit like this?
Click to expand...
Yes to both questions. Tighten the pins and clean them first is the fist easy step that cost nothing. Did you swap the power tubes as well? This is a pretty robust amp. Also there is a forum dedicated to VTA equipment you can check it out here https://dynacotubeaudio.forumotion.com
 
Yes, I swapped the power tubes also with no change.

Interestingly, when I swapped all the tubes I noticed that there was some back and forth play in the socket pins/holders. More than what I would've expected. How do I go about cleaning them? Can the plate resistors be tested and how specifically would I do that? Remember I'm a layman

Thank you for the link!
 
Just swapped the driver tubes and power tubes again along with the speaker cables with no change. Crackling scratchy sound is still coming from the right channel. I also just tested the bias on all four power tube sockets and three read .500 and the one on the right rear fluctuates between 14.00 and 17.00 and I can't get it biased correctly. When I turn the bias POT all the way clockwise I get a readout of "1" on mu multimeter. I then tried swapping tubes and get the same reading from that socket, which tells me that it's not a tube issue and probably has to do with the right rear socket, right?

I've also noticed that the output volume on the right channel is very low compared to the left channel, which I'm thinking is related to this issue.

Another thing I've noticed is that when I place a tube in any of the sockets there is a gap of about an 1/8" between the base of the tube and the socket, where the tube doesn't seem to be seating all the way in allowing them to rock back and forth in the socket. Shouldn't the tubes fit tighter into the socket?

Here's a picture for reference.

SmwXYdD.jpg
 
Last edited:
With the amp off, ohm check from ground to the bias test points. Fair bet you'll get 3 that read about 10 ohms and the one that shows funny results will be a much higher value. If thats the case, you've got a bad resistor on the bottom of the output tube socket.

The question is whether that burned out because a tube failed or what. I'd probably want to at least test those tubes just to make sure its not going to burn out another resistor.


tubes ought to plug all the way in, but they might be hitting the socket screws. Lower profile heads may be the fix here.
 
can you solder? If you can I would reflow all the joints to that tube socket that cant be BIASED. Also on that site I mentioned there is a ton of info on your problem. I would check it out. The protruding screws don't allow the power tubes to sit flush so they wobble a bit. Bad design yes. But it does not hinder the reliability or the function of the amp. Most issues that I have read up on that cause issues on this amp are related to cold solder joints.
 
With the amp off, ohm check from ground to the bias test points. Fair bet you'll get 3 that read about 10 ohms and the one that shows funny results will be a much higher value. If thats the case, you've got a bad resistor on the bottom of the output tube socket.

I just tested as you suggested and the readings on the 3 are about 10 ohms and the "bad" sockets gives a reading of "1".

The question is whether that burned out because a tube failed or what. I'd probably want to at least test those tubes just to make sure its not going to burn out another resistor.

How do I test the tubes?

tubes ought to plug all the way in, but they might be hitting the socket screws. Lower profile heads may be the fix here.

Looks like that's what's happening.
 
Eminence1963 said:
can you solder? If you can I would reflow all the joints to that tube socket that cant be BIASED. Also on that site I mentioned there is a ton of info on your problem. I would check it out. The protruding screws don't allow the power tubes to sit flush so they wobble a bit. Bad design yes. But it does not hinder the reliability or the function of the amp. Most issues that I have read up on that cause issues on this amp are related to cold solder joints.
Click to expand...

I have soldered a few times over the years and have done some online research on it. I also posted this question on that site as you suggested. Good to know that the tube wobble is commonplace and doesn't affect performance. Going to do some more research into soldering so I can get this fixed.
 
"1" as the left-most digit with nothing else on the screen usually means out of range, so its either completely open or just beyond whatever resistance range you had the meter set on. Either way its not reading 10 ohms, so its bad.

You test the tubes with a tube tester. Maybe someone in your area has one, or someplace that works on guitar amps can do that. Mostly I'd just want to know if they are shorted or otherwise bad before powering the amp back up if at all possible. I worked on a Luxman amp not terribly long ago that broke in a similar way. One of the tubes had some internal failure that caused it to draw massive amounts of current. It burned open the resistor. New resistor and tube and it was happy again.

if you don't have one and can't come up with someone who can check them, you can just put a new resistor in and monitor the voltage across the bias test point. If it climbs too high and can't be brought down with the control, there is either a problem with the tube or a problem with the circuit at that socket. Moving the tube will tell you. The problem will either move with the tube, or it will stay with the socket.
 
Here is a picture of the bottom of the amplifier with the base removed and one focused on the bad socket area. Where is the "LM334" that someone recommended I test or replace?

20211213_135112.jpg 7QZC7YC - Imgur.jpg
 
Jeepnmon said:
Here is a picture of the bottom of the amplifier with the base removed and one focused on the bad socket area. Where is the "LM334" that someone recommended I test or replace?
Click to expand...

I don't see one. It might have been a voltage regulator mod of some sort.
 
You must log in or register to reply here.
Back
Top Bottom