Microphonics? Odd tube sounds... ST-120

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

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.

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!

deepee99 wrote:

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

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" .

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