VTA ST-120 Crackle Mystery

Hi All - I'm diagnosing an issue with my VTA ST-120. It has been a phenomenal amplifier in the two years (I think?) that I've had it.

I've been running it with a pair of Wharfedale Diamond 10.1's (specs below) with great results. Everything is crisp, no ear strain, really lovely.


Recently, I came into a pair of PSB Image 2B's (specs:)


On connecting the PSBs, I noticed there's a LOT of static and crackle in the output. Confused, dismayed, I tried re-biasing the amp. No change! I noticed the static is lessened if the amp's stepped attenuator is left all the way open... but then things are quite loud, and they still don't sound very good. I know they were working a couple weeks ago, because I demo'd them. I've got no idea what's going on.

Some more details, FWIW: I had similar results when I was testing the amp out with some Large Advents about a year ago, and just chalked up the poor performance to a problem with 50 year-old speakers. (I hadn't ever tested them on another system.) Now I'm wondering if it was the amplifier there too. I'm using Mr. Latino's stock tubes (Sovteks for power, GE for the little guys), with a vintage Sylvania 12bh7a in the center. I'm running the whole thing off a variac, because the wall voltage in my house is about 110vac and Bob recommends 117, but I would be surprised if that's the culprit. It certainly doesn't affect how the amplifier drives the Wharfedale speakers.

Does anyone have ideas of what might be going on here? The PSBs and the Wharfedales have almost exactly the same specs, if anything the PSBs are allegedly more efficient... The whole thing is bizarre!

Albert wrote:Hi All - I'm diagnosing an issue with my VTA ST-120. It has been a phenomenal amplifier in the two years (I think?) that I've had it.

I've been running it with a pair of Wharfedale Diamond 10.1's (specs below) with great results. Everything is crisp, no ear strain, really lovely.


Recently, I came into a pair of PSB Image 2B's (specs:)


On connecting the PSBs, I noticed there's a LOT of static and crackle in the output. Confused, dismayed, I tried re-biasing the amp. No change! I noticed the static is lessened if the amp's stepped attenuator is left all the way open... but then things are quite loud, and they still don't sound very good. I know they were working a couple weeks ago, because I demo'd them. I've got no idea what's going on.

Some more details, FWIW: I had similar results when I was testing the amp out with some Large Advents about a year ago, and just chalked up the poor performance to a problem with 50 year-old speakers. (I hadn't ever tested them on another system.) Now I'm wondering if it was the amplifier there too. I'm using Mr. Latino's stock tubes (Sovteks for power, GE for the little guys), with a vintage Sylvania 12bh7a in the center. I'm running the whole thing off a variac, because the wall voltage in my house is about 110vac and Bob recommends 117, but I would be surprised if that's the culprit. It certainly doesn't affect how the amplifier drives the Wharfedale speakers.

Does anyone have ideas of what might be going on here? The PSBs and the Wharfedales have almost exactly the same specs, if anything the PSBs are allegedly more efficient... The whole thing is bizarre!

Albert do you have other means of testing the speakers with another amp ? If so try that out first and see if the problem still exist with the speakers with a different amp. This way you can narrow down if it's the speakers or the VTA ST-120 amp. Then you can work from there.

Albert do you have other means of testing the speakers with another amp ? If so try that out first and see if the problem still exist with the speakers with a different amp. This way you can narrow down if it's the speakers or the VTA ST-120 amp. Then you can work from there.

I did demo them a couple of weeks ago, and they sounded great. It would be odd if they both failed in the same way during that time. Still I suppose I can’t rule anything out until I’ve tested the possibility...

Albert wrote:

Albert do you have other means of testing the speakers with another amp ? If so try that out first and see if the problem still exist with the speakers with a different amp. This way you can narrow down if it's the speakers or the VTA ST-120 amp. Then you can work from there.

I did demo them a couple of weeks ago, and they sounded great. It would be odd if they both failed in the same way during that time. Still I suppose I can’t rule anything out until I’ve tested the possibility...

Yes Albert.... It's the process of elimination... In this case the speakers are the weakest link or the easiest to check. Once you test the speakers on a different amp if they sound good then it's not the speakers it's the 120.

If it's the 120 the first thing I would do if you have some extra tubes is switch them out or have the tubes tested and make sure the tubes are seated in the sockets.  If The tubes check out ok then it's time to go after the electrolytic capacitors first then the coupling capacitors and see if one has failed and/or is shorted or leaky.
When electrolytic capacitors or coupling capacitor start going out shorting or leaking they will arc inside before they completely go bad and they will make static arcing noise on your speakers when they're going out.

Does the crackle happen on both channels?
Bob has a forum sticky for trouble shooting. It's helped me in the past.

Bob Latino wrote:Usually something like you are experiencing are problems at the "front end" of the amp

Things to try ..

1. Resolder all 6 connections on the stepped attenuator and the input jack connections
2. Remove all the driver tubes, clean the pins and reinsert the driver tubes
3. Replace each driver tube one at a time. One could be defective. If the static/noise is on BOTH channels then the center driver tube may be a suspect.

Bob

Thanks Bob.... That's very helpful.

This Is common with all tube amplifiers regardless of brand, make or model.

Tube amps produce a lot of heat, high-voltage, and are notorious for being dust magnets and need maintenance and cleaning from time to time. This is common nature for tube amps and our hobby.

The advantages that you get with tube amplifiers with regular maintenance and cleaning is they produce the best analog sound of any other type of amplifier and that is just my own opinion.

I have worked on solid-state and tube amplifiers for many years and there is nothing that sounds better than an old tube amplifier when it's running at its best.

Well, I'll be a monkey's uncle... it was the speakers after all! I'd heard them a couple weeks ago and they sounded really great. I'm not sure what's happened in the meantime, but all I can say is I'm glad I didn't pay for them.

Thank you all for your help! It's so encouraging to know that there's a community here. I hope one day I'll know enough to be able to contribute to it myself.

Of course, while working on this I power cycled the amp really fast (just out of clumsiness) & now I think I'm in the market for a new rectifier tube. I hope that's all it is anyway!

Albert wrote:Well, I'll be a monkey's uncle... it was the speakers after all! I'd heard them a couple weeks ago and they sounded really great. I'm not sure what's happened in the meantime, but all I can say is I'm glad I didn't pay for them.

Thank you all for your help! It's so encouraging to know that there's a community here. I hope one day I'll know enough to be able to contribute to it myself.

Of course, while working on this I power cycled the amp really fast (just out of clumsiness) & now I think I'm in the market for a new rectifier tube. I hope that's all it is anyway!

The VTA ST 120 Wins Again !!! :-)

Glad to have been some help Albert.......:-)