[Nocturne79]

HEllo everyone,

So I have a tube amp (using 12at7) and I notice some scratchy noise as I turn the volume knob and so i switched to different tubes and the scratchy noise went away.  I am wondering if my tubes that caused the scratchy noise have gone bad?  I am really new to tubes so any input would be great, thank you

[FullRangeMan]

Two cents:
If the noise appears only when the volume is turn up/down, the potentiometer may be dirty or worn etc, the tubes seems OK.

[medium jim]

Doubt it was the tube(s), more likely a scratchy potentiometer that had some debris that worked its way out.

Jim

[earwaxxer]

I would say the tubes are bad, if you replace them and the sound goes away! Just logical. I have noticed the same thing with my tube preamp. It is also true that a carbon pot will sound like that when it wears out.

[cheap-Jack]

Hi.

HEllo everyone,

 I am wondering if my tubes that caused the scratchy noise have gone bad?

Why not put back the noisy tube & turn the volume pot up & down to see if the same noise would come back again. If it did, then the old tube is noisy & should be replaced.

c-J

[medium jim]

The noise that the OP is describing is most likely from a dirty pot and not from a tube.  Briskly turning the volume knob back & forth usually will dislodge the offending dirt from the wiper in the pot or from its path.  If this doesn't work then a douching of Caig Deoxit will.

Jim

[FullRangeMan]

Reading the post one again where he said puting new tubes the prob gone, can be seen is need some troubleshooting with the Old Tubes x New Tubes x Volume Pot.

[Nocturne79]

wow, thank you so much guys for the input!  I really appreciate it.  I will try to keep the different tubes in for a week as I only had the tubes in for about 5 hours instead of full time like the other tubes, that the scratchy noise came from when i would turn the volume knob.  Again i thank you all for the comments and really am thankful as I am new to tubes heh.  I just know which tubes to get for my amp and how to put them in lol

have a great evening.

[medium jim]

wow, thank you so much guys for the input!  I really appreciate it.  I will try to keep the different tubes in for a week as I only had the tubes in for about 5 hours instead of full time like the other tubes, that the scratchy noise came from when i would turn the volume knob.  Again i thank you all for the comments and really am thankful as I am new to tubes heh.  I just know which tubes to get for my amp and how to put them in lol

have a great evening.

The fact that the noise only happens when turning the volume knob to me is conclusive that it isn't the tubes, rather the potentiometer.

Jim

[raindance]

You don't say what amp you have, but a number of amps using an SRPP gain stage have the first tube directly coupled to the volume pot (no capacitor). The volume pot then becomes the bias circuit for the tube, keeping the grid negative. I have noticed that these circuits sometimes sound noisy and my theory is that there is a small amount of DC on the grid (leakage?) that causes the pot to make a slight scratchy noise. Most Jolida amps do this. If you add a coupling cap between the wiper on the pot and the grid on the tube (and add a high value bias resistor to the grid which a lot of amp makers leave out), the noise vanishes completely.

With some tubes it can be quite noticeable and with others not much at all. However it does not mean that the tube is bad.

Also note that a number of commercial amps with SRPP input stages do not elevate the heaters as they should, and this leakage can be caused be an internal breakdown within the tube as a result (which may only happen in this circuit and not others that are better designed).

As always with tube gear, YMMV.

[Steve]

Hi 79,

If changing the tubes relieves the scratchy problem, then most likely the tubes in question has negative grid current problems, probably a gas problem, and/or some cathode material has deposited on the grid itself.

Cheers.

[Ericus Rex]

Hi 79,

If changing the tubes relieves the scratchy problem, then most likely the tubes in question has negative grid current problems, probably a gas problem, and/or some cathode material has deposited on the grid itself.

Cheers.

79, the new book by Bruce Rosenblat, "Tubes and Circuits," explains what all these things mean pretty thoroughly.  I'm on my second reading and it is a wealth of information for tube equipment owners/DIYers.

[Nocturne79]

Thank you I will looking into that book, heh time I start studying a little on tubes and I really dig my tube stuff.  I atually have been using a 12au7 tube for the past few days and I can hear a scratchy noise, though very little...its more with the 12at7 tubes.  I have another 12at7 tube coming tomorrow so i am going to try that one too. 

Again thank you everyone for the wonderful input I really appreciate it

Nocturne79

[medium jim]

The reason you are hearing less scratchiness with the 12AU7 than with the 12AT7 is the difference in gain between the tubes. The 12AT7 has a gain factor of 60 and the 12AU7 with a gain factor if 40.

Jim

[Ericus Rex]

Here's a link to the book I mentioned:


http://www.amazon.com/Tubes-Circuits-Bruce-Rozenblit/dp/1477532862/ref=sr_1_1?s=books&ie=UTF8&qid=1351298652&sr=1-1&keywords=bruce+rozenblit

[FullRangeMan]

Some small tubes gain and  versions:
12AX7   gain=100,  7025  gain=100
12AX7 - 12AD7,  12BZ7, ECC83, ECC803S, ECC803, E83CC, 5751, 6681, 7025, 7058, B339,

12AT7   gain=60/70, ECC81 gain=60, TFK ECC801S gain=60,  6201, 6679,

12AU7   gain=20/40,  ECC82, ECC802S, 5963, 5814, 6189, 5814, 5963, 6189, 7316,

12AY7 - 6072

12AV7 - 5965

12SN7   gain=20

6SN7   gain=20

6H6P   gain=20

6922   6DJ8,  6N1P,  7308,  E188CC, E88CC, E288CC,  CCa,

[Steve]

The reason you are hearing less scratchiness with the 12AU7 than with the 12AT7 is the difference in gain between the tubes. The 12AT7 has a gain factor of 60 and the 12AU7 with a gain factor if 40.

Jim

If you have a voltmeter handy, you might check the voltage on the no. 1 grid of the tube, at the volume control center pin, with black probe to the volume control ground pin (or signal ground). Set the voltmeter to 2000mv (2v) scale. As you increase the controls resistance (clockwise), see if you measure any DC voltage. If you are reading a little voltage, the tube is definitely the problem. Anytime a DC voltage appears on the volume control, one will hear a scratchy sound with rotating the control.

Cheers.

[medium jim]

Your welcome Jim. If you have a voltmeter handy, you might check the voltage on the no. 1 grid of the tube, at the volume control center pin, with black probe to the volume control ground pin (or signal ground). Set the voltmeter to 2000mv (2v) scale. As you increase the controls resistance (clockwise), see if you measure any DC voltage. If you are reading a little voltage, the tube is definitely the problem. Anytime a DC voltage appears on the volume control, one will hear a scratchy sound with rotating the control.

Cheers.

Steve:

That is all fine and dandy except that the OP is experiencing the scratchiness with different tubes.  The usual culprit is a dirty pot, especially if the tone is otherwise not effected adversely.

Jim

[Steve]

I thought he said in an above post that the scratchiness went away when different tubes were used.

So I have a tube amp (using 12at7) and I notice some scratchy noise as I turn the volume knob and so i switched to different tubes and the scratchy noise went away.

I see a little with different tubes mentioned later, but they could be bad as well.  I apologize, though, if that is not the case.

I still think it is a good idea to measure the grid voltage, just to be sure.

Cheers.

[medium jim]

I thought he said in an above post that the scratchiness went away when different tubes were used.

I see a little with different tubes mentioned later, but they could be bad as well.  I apologize, though, if that is not the case.

I still think it is a good idea to measure the grid voltage, just to be sure.

Cheers.

True, but it is easier to douche the pot in most instances and see if the problem goes away.  Then trouble shoot the tubes.

Jim