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    Microphonics? Odd tube sounds... ST-120

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    jimmeq

    Posts : 3
    Join date : 2017-02-24

    Microphonics? Odd tube sounds... ST-120

    Post by jimmeq on Wed Oct 18, 2017 2:54 pm

    I've seen a video from Upscale Audio https://www.upscaleaudio.com/pages/Microphonics.html that describes microphony makes a sound and my tubes do not make any noise; they just seem to be acting similar to a microphone so I'm not sure if calling my issue a microphonics issue is correct.

    That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.

    I exchanged four KT-120 tubes in place of the VTA ST-120's  6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker  I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.

    Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.

    I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?

    Thanks,

    Jimme
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    peterh

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    Re: Microphonics? Odd tube sounds... ST-120

    Post by peterh on Wed Oct 18, 2017 3:24 pm

    jimmeq wrote:I've seen a video from Upscale Audio https://www.upscaleaudio.com/pages/Microphonics.html that describes microphony makes a sound and my tubes do not make any noise; they just seem to be acting similar to a microphone so I'm not sure if calling my issue a microphonics issue is correct.

    That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.

    I exchanged four KT-120 tubes in place of the VTA ST-120's  6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker  I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.

    Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.

    I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?

    Thanks,

    Jimme
    I would advice to replace those tubes.
    Tubes that cannot hold bias is a sure sign of problems ahead. Are they in addition
    strongly microphonics suggests to me that things are loose inside.
    I hope you bought them from a reputable dealer that will honor warranty.

    Jim McShane

    Posts : 191
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Microphonics? Odd tube sounds... ST-120

    Post by Jim McShane on Wed Oct 18, 2017 5:40 pm

    jimmeq wrote:I've seen a video from Upscale Audio https://www.upscaleaudio.com/pages/Microphonics.html that describes microphony makes a sound and my tubes do not make any noise; they just seem to be acting similar to a microphone so I'm not sure if calling my issue a microphonics issue is correct.

    That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.

    I exchanged four KT-120 tubes in place of the VTA ST-120's  6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker  I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.

    Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.

    I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?

    Thanks,

    Jimme

    Okay, we'll take this one at a time...

    1) ALL tubes are microphonic to some degree! Unless the tube makes noise while you are playing music then don't worry about it. Almost ALL tubes will make some noise when tapped - if the noise dies out right away then you don't have excessive microphonics. The sudden impulse of a tap will virtually always excite something in the tube and make a noise. The key is that the noise dies out quickly after the tap. Generally for microphonics to be excessive they need to be self-sustaining and/or audible during playing of music/voice program material.

    Be aware that tapping hot tubes CAN damage them - so exercise care!

    There is SO much bad advice and misconception surrounding microphonics...

    2) If the bias on the tubes increases steadily it is not an indication of unstable bias. It usually simply means that before setting the bias you need to let the amp warm up longer. And not just the amp needs to warm - all the circuit parts in the amp change value some as they heat so they need to warm thoroughly as well. So try readjusting the bias after a couple hours and see if it remains stable. Those very large power tubes take a good while to thoroughly warm. And the heat they give off also warms the area around them.

    Another possibility is simply that the tubes you have require more negative voltage at the grid to get them to bias even lower. At a given set of operating points the bias current (which you are actually measuring even though you are reading it out in volts) can vary over 100%. For instance when I burn in and test KT-120s I operate them with 465 volts on the plate and screen grid and negative 50 volts on the control grid. I've measured current in healthy new tubes as low as about 70 ma. or as high about 145 ma. under the same conditions. If the tubes in your amp are in the high current part of that range it may be that you simply need a more negative voltage at the control grid to bring the current down. It's quite possible that cooler (meaning tubes with less current flowing under those conditions) biasing tubes could be adjusted to spec without difficulty if the amp had even more bias adjustment range. If that's the case you either need to get a set of cooler biasing tubes - or ask Bob or someone here how to change the circuit to make the bias more negative so you can use the tubes you have.

    What you are experiencing is what I try to prevent by making sure I don't send very "hot" (meaning high bias current) tubes to VTA amp owners.

    So while there MAY be some issue(s) with the tubes you have (given the footsteps you heard) NO ONE CAN SAY FOR SURE without at least considering and investigating what I just posted. I'd hate to see you throw away healthy tubes only because of what you described in your post. But that's up to you. At least I wanted to make you and other people reading this that there are other perfectly normal conditions that could be the cause and the tubes are the victims of them - not the causes!

    jimmeq

    Posts : 3
    Join date : 2017-02-24

    Re: Microphonics? Odd tube sounds... ST-120

    Post by jimmeq on Thu Oct 19, 2017 11:48 am

    Thanks for the replies and information; here's more about my tube issue.

    The tubes have been sent back to the depot for replacement, and I hope to have the new ones next week. The 6550's are back in with all biasing to .500.   I'm new to tubes, decided to jump into tube rolling and thought a good first roll would be with tubes much different than the stock 6550's I selected when I built my ST-120. Hence, my choice for the Tung-Sol KT-120's. Sometime next year I'd like to try Gold Lion KT-88's and would prefer working with you, Jim as it appears from some threads that you supply VTA with their tube stock. BC me any information you have on your stock.

    1. There sure is a lot of (mis)information on microphonics out there! From what I have gathered, it is not a classic microphonics issue; I certanily don't know what it is; it seems that I should not hear tapping on the table though the speakers. I was tapping VERY lightly with a tiny old screwdriver that was included with a Shure phono cartridge I purchased about 45 years ago. It was surprising how light the tap was, yet heard very clearly from the speaker. I now wonder if the "ping" I heard was actually "ringing" which would imply microphonics. The 6550 tubes don't make any noise. One experiment I did was having two 6550's for the left channel and the 'OK' two KT-120's in the right channel. The 6550's were quiet and there was a slight sound from the KT-120's. I didn't play the amp in this configuration. So far I see 74 views with two replies so I must be pretty much solo with this tube issue.

    2. Bob has recommended a bias setting of .500 for the ST-120 and I was trying to get to that with the tubes. In addition to a fifteen minute pre-bias warm-up, I'm pretty sure I've tried to bias after listening for 45 minutes to an hour or so yet could not get the one tube below .515. The bias adjuster was "pegged" full counter clockwise. I'm not a perfectionist audiophile, so if it's not detrimental to the sound or amp if one tube is at .515 and the rest are at .500; that is acceptable. I am wondering if the fact that it would not bias below .515 was a problem.

    Jim McShane

    Posts : 191
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Microphonics? Odd tube sounds... ST-120

    Post by Jim McShane on Thu Oct 19, 2017 12:36 pm

    jimmeq wrote:Thanks for the replies and information; here's more about my tube issue.

    The tubes have been sent back to the depot for replacement, and I hope to have the new ones next week. The 6550's are back in with all biasing to .500.   I'm new to tubes, decided to jump into tube rolling and thought a good first roll would be with tubes much different than the stock 6550's I selected when I built my ST-120. Hence, my choice for the Tung-Sol KT-120's. Sometime next year I'd like to try Gold Lion KT-88's and would prefer working with you, Jim as it appears from some threads that you supply VTA with their tube stock. BC me any information you have on your stock.

    1. There sure is a lot of (mis)information on microphonics out there! From what I have gathered, it is not a classic microphonics issue; I certanily don't know what it is; it seems that I should not hear tapping on the table though the speakers. I was tapping VERY lightly with a tiny old screwdriver that was included with a Shure phono cartridge I purchased about 45 years ago. It was surprising how light the tap was, yet heard very clearly from the speaker. I now wonder if the "ping" I heard was actually "ringing" which would imply microphonics. The 6550 tubes don't make any noise. One experiment I did was having two 6550's for the left channel and the 'OK' two KT-120's in the right channel. The 6550's were quiet and there was a slight sound from the KT-120's. I didn't play the amp in this configuration. So far I see 74 views with two replies so I must be pretty much solo with this tube issue.

    2. Bob has recommended a bias setting of .500 for the ST-120 and I was trying to get to that with the tubes. In addition to a fifteen minute pre-bias warm-up, I'm pretty sure I've tried to bias after listening for 45 minutes to an hour or so yet could not get the one tube below .515. The bias adjuster was "pegged" full counter clockwise. I'm not a perfectionist audiophile, so if it's not detrimental to the sound or amp if one tube is at .515 and the rest are at .500; that is acceptable. I am wondering if the fact that it would not bias below .515 was a problem.

    Just a bit more...

    1. I'm not sure what you mean by "classic" microphonics. As I said - ALL tubes are microphonic to some degree. The fact that a tap causes noise is not an indicator of anything. If you hear a noise only when you tap that's not excessive microphonics; that's normal behavior. The fact that some tubes make less noise when you tap does not mean that all tubes should make less noise.

    2. Your bias voltage readings are the voltage across a 10 Ohm resistor. Using Ohm's Law which says that the current is equal to the voltage divided by the resistance you can compute the current through the tube. .515 volts divided by 10 Ohms equals 51.5 ma. That's about a 1% current difference as compared to the .500 volt/ 50 ma spec. It's nothing to be concerned about at all.

    Also - generally the bias current will tend to increase as the amp (and the tube) warms. So what you want to do is get the amp to be fully warmed (15 minutes is not long enough IMHO) - let it run until the bias is stable - and then set the bias.

    Finally keep in mind that if the AC line/wall voltage changes the bias will change along with it. So small bias changes may simply be the result of higher or lower line voltages than the last time the tubes were biased. And small bias changes are not an issue - they are not a concern at all.
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    deepee99

    Posts : 1759
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Microphonics? Odd tube sounds... ST-120

    Post by deepee99 on Thu Oct 19, 2017 12:39 pm

    Jim McShane wrote:
    jimmeq wrote:I've seen a video from Upscale Audio https://www.upscaleaudio.com/pages/Microphonics.html that describes microphony makes a sound and my tubes do not make any noise; they just seem to be acting similar to a microphone so I'm not sure if calling my issue a microphonics issue is correct.

    That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.

    I exchanged four KT-120 tubes in place of the VTA ST-120's  6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker  I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.

    Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.

    I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?

    Thanks,

    Jimme

    Okay, we'll take this one at a time...

    1) ALL tubes are microphonic to some degree! Unless the tube makes noise while you are playing music then don't worry about it. Almost ALL tubes will make some noise when tapped - if the noise dies out right away then you don't have excessive microphonics. The sudden impulse of a tap will virtually always excite something in the tube and make a noise. The key is that the noise dies out quickly after the tap. Generally for microphonics to be excessive they need to be self-sustaining and/or audible during playing of music/voice program material.

    Be aware that tapping hot tubes CAN damage them - so exercise care!

    There is SO much bad advice and misconception surrounding microphonics...

    2) If the bias on the tubes increases steadily it is not an indication of unstable bias. It usually simply means that before setting the bias you need to let the amp warm up longer. And not just the amp needs to warm - all the circuit parts in the amp change value some as they heat so they need to warm thoroughly as well. So try readjusting the bias after a couple hours and see if it remains stable. Those very large power tubes take a good while to thoroughly warm. And the heat they give off also warms the area around them.

    Another possibility is simply that the tubes you have require more negative voltage at the grid to get them to bias even lower. At a given set of operating points the bias current (which you are actually measuring even though you are reading it out in volts) can vary over 100%. For instance when I burn in and test KT-120s I operate them with 465 volts on the plate and screen grid and negative 50 volts on the control grid. I've measured current in healthy new tubes as low as about 70 ma. or as high about 145 ma. under the same conditions. If the tubes in your amp are in the high current part of that range it may be that you simply need a more negative voltage at the control grid to bring the current down. It's quite possible that cooler (meaning tubes with less current flowing under those conditions) biasing tubes could be adjusted to spec without difficulty if the amp had even more bias adjustment range. If that's the case you either need to get a set of cooler biasing tubes - or ask Bob or someone here how to change the circuit to make the bias more negative so you can use the tubes you have.

    What you are experiencing is what I try to prevent by making sure I don't send very "hot" (meaning high bias current) tubes to VTA amp owners.

    So while there MAY be some issue(s) with the tubes you have (given the footsteps you heard) NO ONE CAN SAY FOR SURE without at least considering and investigating what I just posted. I'd hate to see you throw away healthy tubes only because of what you described in your post. But that's up to you. At least I wanted to make you and other people reading this that there are other perfectly normal conditions that could be the cause and the tubes are the victims of them - not the causes!

    Jim, would your higher-bias current outputs do OK in a Sachs amp with its lower plate voltages?

    Jim McShane

    Posts : 191
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Microphonics? Odd tube sounds... ST-120

    Post by Jim McShane on Thu Oct 19, 2017 1:22 pm

    deepee99 wrote:

    Jim, would your higher-bias current outputs do OK in a Sachs amp with its lower plate voltages?

    I don't know any reason why not.

    Guys - a 1 or 2 ma. difference in bias current is of no consequence. Unless the amp is a poor design (not like Bob's, Don's, or Roy's) and is running the power tubes at the limit of their dissipation rating/capability - that small difference is of no consequence. Even 5 ma. difference isn't a big deal.

    FYI -  a tube's dissipation rating (in watts of heat dissipation) is found using the formula P = IE where P equals power in watts, I = current in amps, and E = the voltage from the plate to the cathode (NOT to ground!).

    So if you have a tube running 50 ma. (.05 amps) current and 450 volts from plate to cathode the tube is dissipating .05 x 450, or 22.5 watts. That's watts of heat the tube has to be able to dissipate.

    If (as in the original poster's situation) the bias is 51.5 ma. instead of 50 ma. the tube would be dissipating .0515 x 450, or 23.175 watts. That's less than 1/2 watt difference. So even if the plate to cathode voltage was 500 volts you'd still only be dissipating .0515 x 500, or 25.75 watts. And the KT-120 can dissipate 60 watts (or a bit more) total. So as long as the B+ (high voltage) power supply can handle the extra current you have no worries at all. In Don's amp and the VTA amps you need not worry! Yes, the tone may or may not change a bit but the amp is not in any danger

    Proper bias adjustment is important - but don't get crazy/obsessive about it. If you'd like to read more about it and tube stuff in general check out this post of mine:

    https://www.audioasylum.com/messages/tubes/253369/quot-tube-problems-quot-that-aren-t-tube-problems

    Make sense?
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    peterh

    Posts : 874
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Microphonics? Odd tube sounds... ST-120

    Post by peterh on Thu Oct 19, 2017 5:23 pm

    There is one more thing that might cause bias instability :
    The data sheet for st120
    ( http://www.tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf )
    the maximum "grid 1 resistance " ( page 2 ) is 51kohm whan using fixed bias.
    Looking at my schemativ for vta70 i see that 270k is used . Possiby vta120 has
    51 k , if not this might cause bias runaway due to grid current leakage.
    kt-88 ( in the GEC version) is a little more forgiving here.

    One could reduce R29 and R31 to 51k, the coupling cap ( 0.22 ) is large enough. The loading
    in the V2 increases however, we have to ask bob if this is "the right thing to do" .


    Jim McShane

    Posts : 191
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Microphonics? Odd tube sounds... ST-120

    Post by Jim McShane on Thu Oct 19, 2017 5:36 pm

    peterh wrote:There is one more thing that might cause bias instability :
    The data sheet for st120
    ( http://www.tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf )
    the maximum "grid 1 resistance " ( page 2 ) is 51kohm whan using fixed bias.
    Looking at my schemativ for vta70 i see that 270k is used . Possiby vta120 has
    51 k , if not this might cause bias runaway due to grid current leakage.
    kt-88 ( in the GEC version) is a little more forgiving here.

    One could reduce R29 and R31  to 51k, the coupling cap ( 0.22 ) is large enough. The loading
    in the V2 increases however, we have to ask bob if this is "the right thing to do" .


    You are absolutely correct - IF he is having instability...

    jimmeq

    Posts : 3
    Join date : 2017-02-24

    Re: Microphonics? Odd tube sounds... ST-120

    Post by jimmeq on Tue Oct 31, 2017 1:03 pm

    I received the replacement tubes, and all are biasing and working fine! All is well again with my ST-120.
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    sKiZo

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    Location : Michigan USA

    Re: Microphonics? Odd tube sounds... ST-120

    Post by sKiZo on Tue Oct 31, 2017 7:36 pm

    Not to forget common mechanical vibration, which can affect most any sensitive electronics. In many cases, "microphonics" can be cured with some simple isolation. Vibrapods, isolation legs, mats, that sort of thing.

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    deepee99

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    Location : Wallace, Idaho

    Re: Microphonics? Odd tube sounds... ST-120

    Post by deepee99 on Wed Nov 01, 2017 12:17 pm

    Guess I'm either lucky or I'm next, but I've never had a microphonic output tube. Signal tubes, that's another story.
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    sKiZo

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    Re: Microphonics? Odd tube sounds... ST-120

    Post by sKiZo on Wed Nov 01, 2017 6:08 pm

    Somebody say microphonic tubes?

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    Bob Latino
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    Re: Microphonics? Odd tube sounds... ST-120

    Post by Bob Latino on Wed Nov 01, 2017 7:14 pm

    peterh wrote:
    the maximum "grid 1 resistance " ( page 2 ) is 51kohm whan using fixed bias.
    Looking at my schemativ for vta70 i see that 270k is used . Possiby vta120 has
    51 k

    The VTA ST-70 does have 270K grid resistors BUT > The Tung-Sol KT120 is NOT recommended for use in the VTA ST-70. The VTA ST-120 and VTA M-125's both use 100K grid resistors. The Tung-Sol KT120 tubes work fine in both the ST-120 and M-125 amps (using 100K grid resistors) despite Tung-Sol saying that 51K is the max grid resistance for an amp using fixed bias.

    Bob

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