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    What does a scratch built VTA ST-70 sound like when it clips?

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    Downhome Upstate

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    What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Wed Aug 27, 2014 7:53 am

    My scratch built ST-70 has all new parts (including the VTA noval driver board) except for the original cloth lead output iron. When I drive it hard, the signal begins to break up, producing static, cracks and pops. That doesn't seem like garden-variety clipping behavior. Any idea what this might be?

    Thanks in advance for any suggestions and ideas.

    Best regards,

    Mike

    kaner

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by kaner on Wed Aug 27, 2014 8:47 am

    I had a similar issue with a guitar amp. In my case it was a cold solder joint. The vibration from the speaker was revealing the issue. Are your speakers close to the amp? Also check your tube sockets to make sure that you have tight pin connections. The vibrations can cause the tube to jostle in its socket.

    corndog71

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by corndog71 on Wed Aug 27, 2014 10:12 am

    Are you sure it's the amp and not the preamp you're overdriving?

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Wed Aug 27, 2014 10:47 am

    kaner wrote:I had a similar issue with a guitar amp.  In my case it was a cold solder joint.  The vibration from the speaker was revealing the issue.  Are your speakers close to the amp?  Also check your tube sockets to make sure that you have tight pin connections.  The vibrations can cause the tube to jostle in its socket.

    Before starting this thread, I reflowed any questionable solder joints that I could see. Who knows. Maybe I missed one. The thing is, we're good at low to moderately high volume.

    All the tube socket pins are good and tight.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Wed Aug 27, 2014 10:49 am

    corndog71 wrote:Are you sure it's the amp and not the preamp you're overdriving?

    Hi Rob,

    No pre in the circuit. It's an Oppo 93; I'm using the digital volume adjustment.  

    I'll insert the Extended Foreplay III in the setup and then hear what happens.

    Best,

    Mike

    corndog71

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by corndog71 on Wed Aug 27, 2014 11:29 am

    Downhome Upstate wrote:
    corndog71 wrote:Are you sure it's the amp and not the preamp you're overdriving?

    Hi Rob,

    No pre in the circuit. It's an Oppo 93; I'm using the digital volume adjustment.  

    I'll insert the Extended Foreplay III in the setup and then hear what happens.

    Best,

    Mike

    According to the manual, they recommend using the volume control like a gain control so that it matches other sources.

    zx

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by zx on Wed Aug 27, 2014 11:57 am

    You may just need more power......I gave up on th e st 70 years ago....an the Eico hf35...not because thay did not sound good but....low power tube amps can sound vary sweet........i have found that tubes can sound so great i had to have at lest 60 watts.....or i did not run tube amps....even in my horns speaker stage with 100db at one watt.....after a beer are 10....hehe i was going to turn it up....i just could not take the clipping no matter what made it happen...for me 60 watts is the starting point......i have here 6ea Dynaco M3s....thay well even run my Apogee Stages....An my ML esls....with some modes......have fun with tube....get more power...Maybe...it just get better



    Thanks for the site Bob.......

    sKiZo

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by sKiZo on Wed Aug 27, 2014 2:21 pm

    I clip my ST120 regularly ... sounds like a bad haircut, short on the sides and prickly in the middle ... much like a Brian Bosworth mullet ... it'll lose separation and get fuzzy up top mostly. None of the great crunching noise of doom my McIntosh MC2125 would be making at the same SPLs.

    Thing is, I don't much mind it as the amp doesn't slam into a clip like solid state ... any distortion builds gradually, and it's on peak transients only, so more times than not, it's not all that annoying, and certainly not enough to start popping cones. I'd go so far as to say most clips go completely unnoticed.

    If yours is still using the "classic" power transformer, it could also be the iron is tired. The original PT is borderline at best and doesn't get better with age. Browning out the peaks can really change what you hear.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Wed Aug 27, 2014 4:28 pm

    sKiZo wrote:I clip my ST120 regularly ... sounds like a bad haircut, short on the sides and prickly in the middle ... much like a Brian Bosworth mullet ... it'll lose separation and get fuzzy up top mostly. None of the great crunching noise of doom my McIntosh MC2125 would be making at the same SPLs.

    Thing is, I don't much mind it as the amp doesn't slam into a clip like solid state ... any distortion builds gradually, and it's on peak transients only, so more times than not, it's not all that annoying, and certainly not enough to start popping cones. I'd go so far as to say most clips go completely unnoticed.

    If yours is still using the "classic" power transformer, it could also be the iron is tired. The original PT is borderline at best and doesn't get better with age. Browning out the peaks can really change what you hear.

    I like that bad haircut analogy, allthough the great crunching noise of doom bom is more like what I'm hearing, if I understand you.

    My ST-70 uses the new Dynakit power tranny.

    kaner

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by kaner on Thu Aug 28, 2014 9:39 am

    Can you temporarily use long speaker wires to move the speakers or amp into a different room? That way you can take the vibration issue off the table.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Thu Aug 28, 2014 12:20 pm

    kaner wrote:Can you temporarily use long speaker wires to move the speakers or amp into a different room?  That way you can take the vibration issue off the table.

    Sure. It's worth a try. I'll do that, and then report back. Thanks.

    tubes4hifi
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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by tubes4hifi on Fri Aug 29, 2014 3:38 pm

    I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Fri Aug 29, 2014 4:04 pm

    tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    Thanks!  Will do when I get home . . .

    deepee99

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by deepee99 on Sat Aug 30, 2014 1:27 pm

    sKiZo wrote:I clip my ST120 regularly ... sounds like a bad haircut, short on the sides and prickly in the middle ... much like a Brian Bosworth mullet ... it'll lose separation and get fuzzy up top mostly. None of the great crunching noise of doom my McIntosh MC2125 would be making at the same SPLs.

    Thing is, I don't much mind it as the amp doesn't slam into a clip like solid state ... any distortion builds gradually, and it's on peak transients only, so more times than not, it's not all that annoying, and certainly not enough to start popping cones. I'd go so far as to say most clips go completely unnoticed.

    If yours is still using the "classic" power transformer, it could also be the iron is tired. The original PT is borderline at best and doesn't get better with age. Browning out the peaks can really change what you hear.

    Brian Bosworth? Good Lord. Haircuts weren't his only problem and you're showing your age. Skiz, tried to PM you but apparently you've turned that feature off, maybe tired of getting spam from me?
    Only time I've clipped the M-125s was with the cannons on 1812. Was trying to see what they sounded like in my neighbour's living room across the street.
    The street-lights went dim, too, so maybe not a personal problem . . .

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Sun Aug 31, 2014 9:52 am

    tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

    deepee99

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by deepee99 on Sun Aug 31, 2014 2:21 pm

    Downhome Upstate wrote:
    tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

    This will probably get me flamed, but try considering your interconnects, or at least their contact points. Start with a little Deoxit on the cable ends and RCA connectors. I'm not recommending $10k 1-meter cables which are BS, but maybe you've got some resistance that with enough juice finally gives up, starts passing the demanded current, and then everything goes pear-shaped.
    Just a thought.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Sun Aug 31, 2014 2:27 pm

    deepee99 wrote:
    Downhome Upstate wrote:
    tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

    This will probably get me flamed, but try considering your interconnects, or at least their contact points. Start with a little Deoxit on the cable ends and RCA connectors. I'm not recommending $10k 1-meter cables which are BS, but maybe you've got some resistance that with enough juice finally gives up, starts passing the demanded current, and then everything goes pear-shaped.
    Just a thought.

    Thanks for the idea, but I don't think that's it. They are simple but effective stuff: 1 meter Belden 89259 coax with Canare RCAs terminated by Blue Jeans Cable. They have no oxidation at all, and should transfer the signal just fine. Even if capacitance was an issue, they only add about 19 pf/ft.

    deepee99

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by deepee99 on Sun Aug 31, 2014 2:34 pm

    Downhome Upstate wrote:
    deepee99 wrote:
    Downhome Upstate wrote:
    tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
    Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
    Turn it up to 11 . . . . Very Happy

    On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

    This will probably get me flamed, but try considering your interconnects, or at least their contact points. Start with a little Deoxit on the cable ends and RCA connectors. I'm not recommending $10k 1-meter cables which are BS, but maybe you've got some resistance that with enough juice finally gives up, starts passing the demanded current, and then everything goes pear-shaped.
    Just a thought.

    Thanks for the idea, but I don't think that's it. They are simple but effective stuff: 1 meter Belden 89259 coax with Canare RCAs terminated by Blue Jeans Cable. They have no oxidation at all, and should transfer the signal just fine. Even if capacitance was an issue, they only add about 19 pf/ft.

    I use BJC's better stuff as well, never had any grief. And you have not pre-amp issues. So I'm stuffed here. Anyone else got a better idea? (No, sKIzo, don't blow your amp up again Evil or Very Mad)

    audiobill

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by audiobill on Mon Sep 01, 2014 7:17 am

    Could your CD player be at fault?

    Try one of your video disc players and see if a difference.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Mon Sep 01, 2014 8:44 am

    audiobill wrote:Could your CD player be at fault?

    Try one of your video disc players and see if a difference.

    Thanks for the suggestion, but the Oppo does not cause any issues in its usual spot as part of my main hi-fi rig. Its not the culprit here.

    Downhome Upstate

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    Maybe its time to try different EL-34's before asking VTA whether I can ship it in for diagnosis and repair

    Post by Downhome Upstate on Mon Sep 01, 2014 8:57 am

    The first thing I did trying to fix this problem was to get a new triplet of driver tubes. That didn't help. But I never tried to substitute the power tubes. I'm going to look around for 4 used but decent EL-34s and see if that changes things.

    tubes4hifi
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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by tubes4hifi on Tue Sep 02, 2014 12:31 am

    I can't think of any possible way that 200mv p-p signal would cause the amp to clip. It should take nearly 10X that level to cause any clipping.
    Do you have a test CD or sig gen that you could set to that output level and then check the output side of each capacitor?

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Tue Sep 02, 2014 5:47 am

    tubes4hifi wrote:I can't think of any possible way that 200mv p-p signal would cause the amp to clip.   It should take nearly 10X that level to cause any clipping.
    Do you have a test CD or sig gen that you could set to that output level and then check the output side of each capacitor?

    I'll get one and give that a try.

    Downhome Upstate

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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Downhome Upstate on Tue Sep 02, 2014 7:47 am

    tubes4hifi wrote:I can't think of any possible way that 200mv p-p signal would cause the amp to clip.   It should take nearly 10X that level to cause any clipping.
    Do you have a test CD or sig gen that you could set to that output level and then check the output side of each capacitor?

    I'm using an iPhone signal generator app called Sig Gen, by Audio Artillery. The maximum I can get out of this thing is 140 mv p-p; the sine wave is flattened on top.

    With a 1 KHZ 0.14 p-p (distorted as above) sine wave input, I get the following results:

    RIGHT CHANNEL -

    Measured at the output of the front-side right channel 0.22 uf PIO coupling cap: 1/2 half of a 2.4 v p-p 1 khz sine wave, followed by a 17v p-p 125 KHZ burst for 0.5 msec (instead of the other half of what should be a clean 1 khz sine wave signal), repeating.

    Measured at output of the the back-side right channel 0.22 uf PIO coupling cap: a clean 3.5 v p-p 1 khz sine wave, shaped like the input waveform but otherwise undistorted.

    LEFT CHANNEL -

    Measured at the output of the front-side left channel 0.22 uf PIO coupling cap: 1/2 half of a 2.4 v p-p 1 khz sine wave, followed by a 17v p-p 125 KHZ burst for 0.5 msec (instead of the other half of what should be a clean 1 khz sine wave signal), repeating.

    Measured at output of the the back-side left channel 0.22 uf PIO coupling cap: a clean 3.5 v p-p 1 khz sine wave, shaped like the input waveform but otherwise undistorted.

    So, what component or components is/are common to both sides of the front-half of the amp (something on the driver board, right?) that could behave like this? It would be great if we could troubleshoot this to the component level, and then I could get this baby up and running right here without shipping it out.


    Last edited by Downhome Upstate on Tue Sep 02, 2014 8:01 am; edited 1 time in total (Reason for editing : Clarification)

    Bob Latino
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    Re: What does a scratch built VTA ST-70 sound like when it clips?

    Post by Bob Latino on Tue Sep 02, 2014 8:15 am

    Downhome Upstate wrote:My scratch built ST-70 has all new parts (including the VTA noval driver board) except for the original cloth lead output iron. When I drive it hard, the signal begins to break up, producing static, cracks and pops. That doesn't seem like garden-variety clipping behavior. Any idea what this might be?

    Thanks in advance for any suggestions and ideas.

    Best regards,

    Mike

    Mike,

    You said here in your first post (above) that this was a scratch built ST-70 that used a VTA noval driver board. Could you post a photo of the inside wiring and let us look at how the amp is wired ? One of us may be able to pick up on some issue in the wiring of your scratch built ST-70 that could cause the problems you are having with the amp.

    Bob

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