Loose socket(s)?

I've been rolling my power tubes a fair amount lately. Last iteration KT120, 2 per amp. Sounded good but Mr "never leave things well enough alone" decides maybe I should revert back to 4 tubes as I'm preferring triode more and more. I would prefer the 120's but only have 4 for now. So in the mean time in with an octet of Sovtek 6550's. But after installing them I'm getting hum on one of the amps? This amp is normally quiet and I've not had issues with these tubes before. Which is more likely? Reduced pin contact due to all the tube changes, or me getting the quads mixed up? I more suspect the former but was hoping to have the experts weigh in.

LeGrace wrote:I've been rolling my power tubes a fair amount lately. Last iteration KT120, 2 per amp. Sounded good but Mr "never leave things well enough alone" decides maybe I should revert back to 4 tubes as I'm preferring triode more and more. I would prefer the 120's but only have 4 for now. So in the mean time in with an octet of Sovtek 6550's. But after installing them I'm getting hum on one of the amps? This amp is normally quiet and I've not had issues with these tubes before. Which is more likely? Reduced pin contact due to all the tube changes, or me getting the quads mixed up? I more suspect the former but was hoping to have the experts weigh in.

Is the bias equal for all 4 tubes in that amp? Unbalance here could
create some hum.

I adjusted the bias to recommended setting of 1 v for each tube pair? Should I try swapping out tube pairs between amps? They came as bundled quads but stupid me didn't maintain them as sets when I put them aside. If I cant figure it out I can always switch back to the KT120's, but I prefer to understand these things.

https://www.pjtool.com/media/catalog/product/cache/7/image/1000x/040ec09b1e35df139433887a97daa66f/3/5/350-4c.jpg

https://images-na.ssl-images-amazon.com/images/I/51oV+D4dbIL._AC_US218_.jpg

First, I suspect that you may have moved something and you are getting some sort of induction hum. Check your cables and either bundle or separate - depending - and see if this helps.

Or, install shorting plugs on all unused *inputs*.

I once diagnosed a situation where a tuner put out so much garbage that if it was within 2 feet of the amp, the entire system hummed (if the tuner were on). Did you move any of the components?

Look for a broken/compromised solder joint under the chassis. Moving tubes will also flex the connections.

Now, the links: the top link is for an inexpensive set of dental picks. I keep several that I got from my dentist (so they are US made) for nothing (after being his patient for 35 years). But the cost of a professional is very high. In any case, they are excellent for tightening socket clamps. The bottom link is for very fine spiral brushes that will help remove skunge without leaving shavings or dirt in the socket.

PLEASE!! tell us what it was when you find it - which I am sure you will.

OPINION: Unless tubes are wildly unmatched, that, alone should not cause a hum. If replacing them does solve the problem, try to find out what is going on with the tubes. If you have access to a tester that has matching capacity (about 4% of what is out there outside of dealers.) I would be fascinated to find out what those results would be.

Peter W. wrote:https://www.pjtool.com/media/catalog/product/cache/7/image/1000x/040ec09b1e35df139433887a97daa66f/3/5/350-4c.jpg  

https://images-na.ssl-images-amazon.com/images/I/51oV+D4dbIL._AC_US218_.jpg  

First, I suspect that you may have moved something and you are getting some sort of induction hum. Check your cables and either bundle or separate - depending - and see if this helps.

Or, install shorting plugs on all unused *inputs*.

I once diagnosed a situation where a tuner put out so much garbage that if it was within 2 feet of the amp, the entire system hummed (if the tuner were on). Did you move any of the components?

Look for a broken/compromised solder joint under the chassis. Moving tubes will also flex the connections.

Now, the links: the top link is for an inexpensive set of dental picks. I keep several that I got from my dentist (so they are US made) for nothing (after being his patient for 35 years). But the cost of a professional is very high. In any case, they are excellent for tightening socket clamps. The bottom link is for very fine spiral brushes that will help remove skunge without leaving shavings or dirt in the socket.

PLEASE!! tell us what it was when you find it - which I am sure you will.

OPINION: Unless tubes are wildly unmatched, that, alone should not cause a hum. If replacing them does solve the problem, try to find out what is going on with the tubes. If you have access to a tester that has matching capacity (about 4% of what is out there outside of dealers.) I would be fascinated to find out what those results would be.

You can buy a hook and pick set at Harbor Freight.

http://t.harborfreight.com/4-piece-pick-and-hook-set-66836.html

HF also has sets that are more like the ones Peter W posted but the ones for $1.99 work just as well.

Great suggestions. Was waiting to hear back before starting the diagnosis process. Going to go mess around now. Thanks!

LeGrace wrote:I adjusted the bias to recommended setting of 1 v for each tube pair? Should I try swapping out tube pairs between amps? They came as bundled quads but stupid me didn't maintain them as sets when I put them aside. If I cant figure it out I can always switch back to the KT120's, but I prefer to understand these things.

I have no access to st120 schematics, i assumed that there is a cathode resistor for each cathode.
If resistor is shared between 2 tubes you cannot say with confidence if the tubes
draw the same current unless they are properly matched.

Is the am quiet with the kt120 then you have some kind of issue with the 88's

But i remember you saying that it was an octet: are they matched as an octet? Then it should not matter if you swap them around.

At the end of the day, remember that tubes are expendables and not supposed
to live forever. Tuberolling certainly does not make life easier for the tubes.

Anyway turns out it was the tubes. I cleaned all the sockets and pins, no change. Carefully tightened the pin cups in the sockets, no change. Then I replaced the 6550's with 4 x KT88's. Bingo, amp is whisper quiet again. The 6550's don't have a lot of hours on them so I'm a bit surprised.

As far as I know Sovtek does not make a KT-120 so the pin diameters of your KT-120 and 6550 tubes are likely different. They probably have different profiles too.

Before you pull your tubes I suggest you serialize them with a Sharpie so you can better monitor your situation.

As you know, gripping the tubes by the base when you handle them you will reduce the likelihood of damage to the tubes themselves. In the absence of ZIF (zero insertion force) sockets you run the risk of damaging your sockets and compromising solder joints each time you swap tubes. This is the darker side of tube rolling.

You can do a "seat of the pants" matching exercise by sequentially inserting and measuring the bias of each of your output pentodes--one at a time.

Power down between tube changes. If you have built-in or socketed solid state B+ rectification this is a good time to use it. If you have a tube rectifier avoid wearing gloves when changing pentodes.

-Set your preferred bias current with just pentode #1 inserted, i.e. three empty sockets in the case of M-125.
-Measure the voltage drop across the cathode resistor for the occupied socket. Write this voltage on tube #1 with a Sharpie.
-Leave the bias setting unchanged. Remove pentode #1 and insert #2 in the same socket.
-Measure the drop across your cathode resistor. Write this voltage on tube #2 with a Sharpie
-Repeat this process for the remaining six tubes, one at a time, using the same socket without moving the bias pot. Write the voltages on the tube with a Sharpie.

Ideally all eight would measure exactly the same. If they do you are probably doing something wrong.

Do you have a socket saver? This would be a good time to use it.

Never heard of a socket saver before.  Looked it up, cool.

My first thought was the KT120's had streched the sockets a bit.  I've read where the pin size on these can be slightly larger.  All I know for sure is the KT88's are making good contact as replacing the 6550s the hum goes away.  I could try the problem 6550s in amp 1 that is currently quiet with the other 6550 quad, but I'm getting leery of changing tubes so many times. Think I'm starting to see the point of a tube tester.

It's been noted before that all tube pins aren't created equal, and this seems to be especially true with the KT-120's. They're just that much larger diameter that going back to most any other tube is problematic unless you re-tension the sockets.

Not a bad idea any time you roll anyway, as there's no additional stress or wear on the sockets if you do it right.

When the amps were brand new I struggled with hum issues. Now I think know why, these were the original tube set. Not to say the 6550 is a crappy tube to avoid. Right now I have the 4 good 6550's in one M125, and 4 x KT88 in the other. Both amps are silent, and sound overall is remarkably good. Better then with just 2 tubes per amp.

Different tubes in the left amp versus right? Bad juju!