Tube testing

[bsco]

Looking for some opinions here...good or bad.....Tube testing and not with a tube tester but with the amp, a dummy load and a scope.....Here is what I did.....I took a Fender amp,(actually two different Fender amps).... hooked up an 8 ohm dummy load and then connected up a scope....the scope is set for AC coupling, time base set for 10msec and volts per div set for .1V........when the tube warms up, and you very gently tap the tube and I mean very,very gently, the noise that is generated every time you tap the tube, will cover the entire screen from top to bottom and that is with the scope trace centered.....the amp controls were set with volume at min, tone controls set at mid level, reverb and trem controls set to min as well....the two amps were a Hot Rod deluxe and a 68 custom Reverb....and the tubes we are referring to are the first two pre amp tubes in each amp...the 68 Custom is a brand new amp with GT brand tubes and the Hot Rod Deluxe is a few years old...also with GT brand tubes....Is my testing procedure right or should have I done something different...I had some used tubes kicking around so I tried those....some were better and some were worse than the tubes that were already in the amp.....but these used tubes would not be suitable for use....as they were also too noisy.....I plan to grab a handful of new tubes from the store tomorrow and do the same test....if I am going about this the wrong way then somebody let me know...
Cheers......

[TimmyP1955]

.1V/div is quite sensitive. Connect the amp to a speaker and see what your ears tell you. A little bit of noise is not uncommon.

[bsco]

.1V/div is quite sensitive. Connect the amp to a speaker and see what your ears tell you. A little bit of noise is not uncommon.

I also did that....when you tap the tubes, you will hear noise in the speaker.......sometimes it will be very loud....other times not so loud.... and sometimes it depends on where you tap the tube.....so when the amp is being used at a decent volume, the vibrations from the speakers will cause the tubes to rattle and you will get cracking sounds in the speaker.....and of course the problem is intermittment.....

[olddawg]

I also did that....when you tap the tubes, you will hear noise in the speaker.......sometimes it will be very loud....other times not so loud.... and sometimes it depends on where you tap the tube.....so when the amp is being used at a decent volume, the vibrations from the speakers will cause the tubes to rattle and you will get cracking sounds in the speaker.....and of course the problem is intermittment.....

How do you know it isn't the tube socket or tube pins to some extent? All it takes is a little oxidation. Tapping makes a slightly better or worse connection.

[bsco]

How do you know it isn't the tube socket or tube pins to some extent? All it takes is a little oxidation. Tapping makes a slightly better or worse connection.

Yes sir..I cleaned those suckers...and one of the amps are brand new......and the tube pins are nice and shiny as well.....I am going to pick up some new ones from the store today and just check them out to see what the differences are......Thanks for the reply....
Cheers

[mikeydee77]

Ahhh - signal to chop stick ratio. Why don't manufacturers put this in the specs any more.

I have been recently experimenting with a LED VU meter for measuring/comparing noise levels (hiss/ hum etc) more objectively and it is useful for assessing tube noise too. It is permanently connected to my dummy load and has two scales, one for power and one for noise. I use my ears too of course, but I don't think they are all that consistent (I am talking abouty my ears specifically before I get flamed).

Perhaps I misunderstood what you are trying to achieve, but with the volume set to min it made me think that you will miss the pre-amp valve's noise contribution and just be left with the power valves.

[Leo_Gnardo]

Sounds good to me.

Keep in mind Groove Tube is, and has been for nearly a decade, owned & operated by Fender. All the 12AX7's I see from them lately are Sovtek (and may be marketed under different brand names owned by Sovtek parent company New Sensor.) Amps come in with noise complaints, hisses, clanks & rattles, often enough it's these tubes whether factory installed or bought over-the-counter at music stores or internet. New or with various months/years of use too. Some work for years just fine, others get noisy, still others noisy right out of the box despite GT's claims of superiority, pre-sale testing, and guarantees. Nothing special, same holds for just about all other tube brands too.

[bsco]

Ahhh - signal to chop stick ratio. Why don't manufacturers put this in the specs any more.

I have been recently experimenting with a LED VU meter for measuring/comparing noise levels (hiss/ hum etc) more objectively and it is useful for assessing tube noise too. It is permanently connected to my dummy load and has two scales, one for power and one for noise. I use my ears too of course, but I don't think they are all that consistent (I am talking abouty my ears specifically before I get flamed).

Perhaps I misunderstood what you are trying to achieve, but with the volume set to min it made me think that you will miss the pre-amp valve's noise contribution and just be left with the power valves.

If you hook up a scope to the output and tap the tubes you will see flashes of noise on the scope screen every time you tap the tube...sometimes those flashes or bursts of noise are small but other times they are huge.....I have heard of people testing them with a 20mV per div but that is way too sensative so that is way I picked .1v....

[bsco]

Sounds good to me.

Keep in mind Groove Tube is, and has been for nearly a decade, owned & operated by Fender. All the 12AX7's I see from them lately are Sovtek (and may be marketed under different brand names owned by Sovtek parent company New Sensor.) Amps come in with noise complaints, hisses, clanks & rattles, often enough it's these tubes whether factory installed or bought over-the-counter at music stores or internet. New or with various months/years of use too. Some work for years just fine, others get noisy, still others noisy right out of the box despite GT's claims of superiority, pre-sale testing, and guarantees. Nothing special, same holds for just about all other tube brands too.

Fender claims to test those tubes but I have seen many of them that are very noisy...even in brand new amps and I have one here that if you tap the tube, the noise and popping generated is way too much...i do realize that there would be some noise but not that much.....I was into the store this morning and pick up six GT ones and four Mesa branded ones as well....I am going to compare them with the scope just to see what the differences are ......I could be wasting my time but I would like to know.....and maybe higher plate voltages will bring out the inherent noises more.....I am going to look around for a simple circuit to build which has a lower plate voltage and then I can preform the same test when I run into the same issue and compare the results of the two different plate voltages.....I guess this is how you learn about stuff that is never taught in class......

[Enzo]

Gain of the stage circuit matters. A noisy tube in V1 might be perfectly fine in high signal level stages like a phase inverter.


If your input stage gets 10mv noise and 10mv guitar, then the noise is as loud as the guitar. Put that same 10mv of tube noise in the phase inverter where you have say 30v of signal, and that 10mv disappears.

[g1]

You will need to record some benchmark data and use some constant test methods. Like always one position in one particular amp (first or second gain stage usually most sensitive). Go through some by ear and sort into good or noisy. Now do the scope check and record your benchmarks. Should give you a go/nogo idea you can now use without having to listen. Of course this is for microphonics only. The advantage of the listening test is you will also notice excess hiss or hum.

[bsco]

You will need to record some benchmark data and use some constant test methods. Like always one position in one particular amp (first or second gain stage usually most sensitive). Go through some by ear and sort into good or noisy. Now do the scope check and record your benchmarks. Should give you a go/nogo idea you can now use without having to listen. Of course this is for microphonics only. The advantage of the listening test is you will also notice excess hiss or hum.

That is what I was doing downstairs for the last few hours.....I have noticed that the Mesa Boogie branded hi quality replacement for the regular 12AX7, was much quieter than the GT brand.....I also noticed that when a noisy pre-amp tube was installed, and you tapped the output tubes they showed some extra noise and ringing....I have to do some more testing and then I should be able to have a guide of what to look for on the scope and I have also listened to each tube with a speaker connected as well....I do find that you have to be careful when tapping the tubes so as not to tap them too hard........you just have to do it lightly and you also have to use the same tapping instrument whenever you do this.....there is no point in using a pencil and then use a chop stick and the next time use something else......everything has to be the same....I also noticed that when moving tubes around while they are operating will produce some static noises which can be fixed quite easily with some good contact cleaner like deoxit....and you can't rely solely on the scope...you still have to listen to the sound as well...
Cheers....
P.S. The snow is finally going away......I should have the deck cleaned off by the weekend and then the BBQ season will start....

[Leo_Gnardo]

bsco, let's look "beyond the paint" and find out who really makes the tubes you have branded GT and Mesa. IDK what Mesa's using these days, how about a photo?

On tapping, our man Stan made a tube tapper for Ruby many years ago. A calibrated tube tapper, that is. At least set up so it imparted the same force to the tube under test each time it was used. He described it a bit in one of his posts a few years ago. No photo or diagram though. I got the idea, it was sort of like a piano hammer, rigged on a dowel or axle. The tester lifts the hammer to a pre determined position then either lets it drop, or presses a trigger that lets it drop and hit the tube. I can imagine Ruby's tube tester team sitting there in the lab with a row of these gadgets... . It wouldn't take too much effort to make a home-made version, for tubes in a test rig. I don't think it's would be too easy to make one that would test them while in any amp.

[bsco]

bsco, let's look "beyond the paint" and find out who really makes the tubes you have branded GT and Mesa. IDK what Mesa's using these days, how about a photo?

On tapping, our man Stan made a tube tapper for Ruby many years ago. A calibrated tube tapper, that is. At least set up so it imparted the same force to the tube under test each time it was used. He described it a bit in one of his posts a few years ago. No photo or diagram though. I got the idea, it was sort of like a piano hammer, rigged on a dowel or axle. The tester lifts the hammer to a pre determined position then either lets it drop, or presses a trigger that lets it drop and hit the tube. I can imagine Ruby's tube tester team sitting there in the lab with a row of these gadgets... . It wouldn't take too much effort to make a home-made version, for tubes in a test rig. I don't think it's would be too easy to make one that would test them while in any amp.

I returned the tubes this evening with a couple of amps for them as well...those Mesa tubes come with some kind of black coating....looks like some sort of heat shrink maybe with the Mesa name printed on it...I can get you a pic Monday but I can't cut off the black coating as these are brand new....I'll check downstairs through the junk pile I have collected and see if I have one kicking around...if not I will have to buy one and remove it....I'll get back to you with a couple of pics so you can take a look.....Fair enough?? I would be curious to know where all these different brands of tubes are made....sounds like you guys would have more info on that than me.......you can probably tell by just looking at the construction...

[Leo_Gnardo]

I returned the tubes this evening with a couple of amps for them as well...those Mesa tubes come with some kind of black coating....looks like some sort of heat shrink maybe with the Mesa name printed on it.

Mesa sold what they called SPAX7's, that's the description. 25-30 years ago they were supposed to be pick of the crop, that is low noise, low hum & low microphonics selects, then they put on the heat shrink which further reduces microphonics. I have some from late 80's - early 90's that are Shuguang. For a while they were selling EI. I wouldn't be surprised if they're using JJ these days.