Help using scope to track hum

[hesamadman]

I have not used my scope yet. Not in any practical sense. I can turn it on and put it in spots of my amp and see the sine wave on the screen but thats as far as it goes.


I have an AC30 based off of Hoffman layout. I have some crazy hum going on. I built this thing a while ago. It was extremely quiet. I had a post PI master. There were two instances where an EL84 would arc like crazy. The tube just went bad. Twice. I don't know for sure if one went bad and the its replacement went bad OR if one went bad and another went bad. Now this could have been an issue from factory. I got Apex Matched JJ's. I had this problem with a new set of 6L6's once. Well I didnt take a chance. I thought maybe something was acting screwy with the PPIMV. I changed it to pre PI master just after my treble wiper of tone stack. The hum was there with the post PI mv and its there now. So what I am getting at is I do not have a matched set of EL84's installed right now. I do know from experienced that these amps will be noisy if the output tubes are not matched well. I am curious if there is any way to test if it is the output tubes or not. I pulled two of the tubes and ran it to se but the hum was still there. Maybe its not as loud but its still there and loud.


So I hooked up my scope and probed around, having no idea what I was looking for. I have an old scope. I dont quite understand how its suppose to work. When I think of a sign wave, I think of a center line (zero), numbers on the left to show the amount of voltage, and a positive and negative swing. Mine doesnt have numbers on the left. It just shows the sign wave. So I dont understand how your suppose to know what the voltage swing actually is. It also has a vertical adjustment so the sign wave can physically be moved up or down which makes the actual measurement even more confusing to me. I put my probe on the speaker out and there was a sign wave. Guitar was plugged in. volume was on maybe 3. Gains was on 3. Obviously sign wave increased as MV was turned up. I actually had TWO sign waves. They were close to being the same but one had a bit more negative swing than the other.


Any help would be appreciated AND do you suggest a better scope for future projects?

[pompeiisneaks]

Not knowing the exact details of the scope makes it hard to say for sure, but most old scopes are more than enough.  First off, the tubes arcing and dying aren't related to the hum likely, but could be an indication that the amp is biased a bit hot.  I built an AC30 from hoffman's layout and I had to adjust the bias resistor up a bit, as I recall.  it calls for a 50 ohm but that's based on Vox's original designs and they ran them a bit hot.  I think I ended up using like a 125 ohm resistor, but I can't recall 100%.  Effectively it would be a good idea to go buy multiple values around that range at 10 watts, say 75 ohm, 100 ohm, 125 ohm and 150 ohm.  Then take measurements of the current going through the resistor.  The process is pretty straight forward.  Measure the voltage drop across the resistor, and the resistor value exactly (It may say 50, but a DMM may show 48 or 53 or whatever, get the exact measurement) and then do the Ohms law to calculate the current.  You can also just use a simple calculator site like weber's: http://www.tedweber.com/webervst/tubes1/calcbias.htm

then get the current you're seeing and that's for 4 tubes and the above is for 1 tube.  Make sure you're within that tolerance or if you really like the tone now, and don't mind burning up tubes regularly, you can leave it as is.

Ultimately for the hum, it depends on the type.  If you can turn the volume up, with no jack inserted and hear the hum, then you can put the scope on the signal pathway and look for where it 'arrives' that would then indicate the trouble location.  Is it at the input, or is it at the PI, etc. make sure to measure the signal after the coupling cap or you'll get no reading because the DC will throw a kink into the measurement (I'm still new at scopes, so maybe I've been doing that part wrong, not sure)

~Phil

[SILVERGUN]

A good place to start is to identify the frequency of the hum.
 
Also:
http://www.tubebooks.org/Books/smith_oscilloscope.pdf
 
 
http://www.tubebooks.org/Books/Middleton_troubleshooting.pdf
 
 
EDIT: fixed link and added one

[hesamadman]

Good info guys. I really think it is 60hz. Could I try the lantern battery trick? Does that have enough current for all the el84's?

[jjasilli]

Search the Forum.  Sluckey & others have given great pointers on how to use a scope.  There's also plenty of info on the web.  There's a learning curve. It's probably bets to first learn how to use a scope and achieve a comfort level, before jumping right in and using it as diagnostic tool on a broken amp.


My suggestion:  When you have time & patience 1st learn to use the scope as a glorified volt meter.  Using known voltages learn how to measure various DC voltages and scale them on the screen.  Then measure AC voltages sourced from a signal generator.  Don't forget the 1.414 factor.  Again learn to scale & locate the sine waves on the screen.  Learn how to see how the waveform on the screen matches the frequency set on the signal generator.


Then learn to read a waveform.  Seemingly tiny & insignificant visual differences in the shape of the waveform make a big difference in tone. 

[SILVERGUN]

So I hooked up my scope and probed around, having no idea what I was looking for. I have an old scope. I dont quite understand how its suppose to work. When I think of a sign wave, I think of a center line (zero), numbers on the left to show the amount of voltage, and a positive and negative swing. Mine doesnt have numbers on the left. It just shows the sign wave. So I dont understand how your suppose to know what the voltage swing actually is.

It is nowhere near as difficult as most 1st timers imagine/fear.
 
-The horizontal lines are there to give you a visual interpretation of voltage/amplitude based off of where you set the Volts/Div. switch (so that control is 'voltage per each horizontal division line')
-So, the reason they don't put numbers on there is because:
- YOU set the range based on where you set the Volt/Div. switch
 
-The vertical lines are there to give you a visual representation of time (= frequency) and you set that by adjusting the Sec./Div. switch (so that control is 'seconds per verticle division line')
- YOU set that range by where you set the Sec./Div. switch   (might be labeled Time/Div - same thing)

[shooter]

a better scope

any scope that works should be good enough.

the lantern battery trick? Does that have enough current for all the el84's?

You can || multiple ones.  You just want it on long enough to know it's gone, or not.

Don't over-look the normal things like pull all but power tubes.  Voltage checks, looking for high ripple where it shouldn't be, broke wire, etc.

[hesamadman]

Ive read everyones responses and took them all into consideration when tracking the hum. I played around with the scope a while. It was hard to really determine much with the scope. I put my 120 incoming on the scope, got my 60hz sine wave centered up on the graph and then put my scope on the MV wiper since the hum is increased by that. Then I turned it up, hit the X5 button on my scope and increased it even more but it was hard to really tell much about the noise because it was a very jagged sign wave. Well all the scope play was fun. I intend to keep going with that a little more in depth later.


I started to take AC measurements on the grids. My assumption is that if I am trying to find a 60hz signal on my grid, I would have to reference to chassis or Neutral? I did end up clipping to my neutral and measured on the first grid of the preamp and got 40v. Went to next grid of next stage and it was 70v. I honestly didnt do much else. What I would like to know from you guys is where should I have referenced??


Ultimately I said screw this and cut the leads of the heaters going to all 12ax7 including the phase inverter. Put two lantern batteries in series and connected it to the heaters of just the preamp. MAGIC. Hum was WAAAAYYYY down. Let me back up a bit. I built this amp originally with an extra gain stage. Very jcm800 like preamp. I did not put a high/low input jack ONLY a high gain jack. I realized this didnt give me any clean at all so my recent project was to make a switchable high low gain switch. Bypasses the first stage just like an 800. With this in mind, this hum may have always been there from the beginning. When in high gain mode, its not as noticeable. But when in low gain you have to turn the volume up a lot more since its lacking the one gain stage. So its MUCH more noticeable here. Plus its a clean signal.


Anyway, found the problem. Now I had a hard time determining the hum. Couldnt tell if 120 or 60. I used a tone generator to try and see if I could tell. Is it possible for both 60 and 120 to be present? Its almost as if the 120 was present when volume was turned up but when volume was all the way down there was a slight 60hz sounding hum. Slight not too bad. Even after connecting lantern battery there was still a very small hum. It was something that I need to address but just barely there. Im assuming its either the mismatched EL84's or the fact that the EL84's were still using the AC source for heaters.


So my assumption at this point is I need to elevate the heaters. Sooooo thats next.

[SILVERGUN]

...... Now I had a hard time determining the hum. Couldnt tell if 120 or 60. I used a tone generator to try and see if I could tell. Is it possible for both 60 and 120 to be present?

Yes...because 120hz is the second harmonic of 60hz

[hesamadman]

Elevated heaters did the trick. Center tap moved to cathode of one of el84's.

[shooter]

Elevated heaters did the trick

Nice!, now go play with your scope

[trobbins]

The hum may just have been a poor 12AX7 in the input stages - sometimes just swapping that part a few times can alleviate a hum problem without recourse to circuit changes.  That could certainly have been your situation, given that elevation alleviated the hum (rather than some capacitive coupling of heater wire signal over to input signal circuitry).