Measuring output power and loud crackling noise

[dscottguitars]

Hi,  I'm getting strange results when measuring the output of an amp I built, basically a bassman 6L6 PP, with my own preamp.  I connected an 8 ohm large resistor to a plug and plugged it into the speaker jack.  Hooked up my voltmeter and read a consistent 25+ volts up to 26.1 at the highest I saw while hammering on my guitar.  That works out to at least 78 watts!!  But that's impossible, right??

I have 480v on the plates and 41mA through the tubes.

Is there something I'm doing wrong?

Thanks,

Daniel

[PRR]

What kind of volt-meter?

An averaging meter will read high on a square-wave. Which is what you get when you "hammer".

40 Watts roundy Sine wave can be 80 Watts flat-top square wave.

You really want a controllable signal and some way to monitor output "quality". Organ or signal-generator, not the percussive guitar. With a 'scope, bring it up until it looks bent. With a tapped-off speaker (say 50 ohms in series so it isn't too loud and isn't loading the dummyload), until the unintended overtones are significant.

[dscottguitars]

It's a Velleman multimeter.  I didn't know that about using a signal generator, but I don't have one anyway.  I am getting a very bad sound out of it at loud volumes now.  I took it to a friends where I could open it up and after about 10-15 minutes it crackles really bad, especially on the low notes.  It's both channels.  It sounds like an overload problem, but I don't know enough to determine that.  I tried my line out jack through his amp and it squealed really loud which makes me think it's something there.  I have to open it up tomorrow and disconnect that to see if that's the issue but I'm really stumped now, because it sounds horrible.  The cracking comes with each note pick and sounds like something breaking.  It's not a buzz or hiss, I've never heard this sound before.

[stratele52]

Hi,  I'm getting strange results when measuring the output of an amp I built, basically a bassman 6L6 PP, 

I have 480v on the plates and 41mA through the tubes.

Is there something I'm doing wrong?

Thanks,

Daniel

6L6  or 6L6GC , 6L6GW  ??

41 ma at 480 volts is to hot for 6L6

[SILVERGUN]

I am getting a very bad sound out of it at loud volumes now.  I took it to a friends where I could open it up and after about 10-15 minutes it crackles really bad, especially on the low notes.  It's both channels.  It sounds like an overload problem, but I don't know enough to determine that.  I tried my line out jack through his amp and it squealed really loud which makes me think it's something there.  I have to open it up tomorrow and disconnect that to see if that's the issue but I'm really stumped now, because it sounds horrible.  The cracking comes with each note pick and sounds like something breaking.  It's not a buzz or hiss, I've never heard this sound before.

You could have easily damaged something while performing the output test into the dummy load....running the amp wide open for any extended period of time could expose a weakness in the build (or a component), and the fact that you had the resistor hooked up means that you couldn't hear when something went wrong.....

Where does your direct out come from?.....is it a tap off of the secondary side of the OT?
You could have damaged your OT (I'm just not sure about the fact that it takes 10-15 minutes to show up).......what model is it?...power handling?

Start with posting a full schematic, and more qualified personnel will come along 

I didn't know that about using a signal generator, but I don't have one anyway. .

might be time to get one.....if you're an amp tech, it's on the list 

[stratele52]

I didn't know that about using a signal generator, but I don't have one anyway. .

might be time to get one.....if you're an amp tech, it's on the list 

Sure is not . A amp tech know how to use signal generator it is one of the basic tool when you study electronic at any electronic school.

[dscottguitars]

The tubes are NOS GE 6L6GC's.  The current and plate voltage should be in line with those.

Output transformer is a Weber, rated for 40 watts.  Could it really be I damaged to OT?  Can I test it somehow?
http://taweber.powweb.com/store/018343sch.jpg  

What do you mean "where does your direct out come from"?  You mean the speaker jack?  It comes from the secondary taps, set up with a switch for 4 and 8 ohm.  I was using both to test them with different speakers.

I'm not an amp tech, just learning how to build them on my own, but I will look into the signal generator.

[SILVERGUN]

What do you mean "where does your direct out come from"?  You mean the speaker jack?  

Sorry, I should have said "line out", from what you mentioned in this reply

I tried my line out jack through his amp and it squealed really loud which makes me think it's something there.

Some amps have a "direct out" that can be used in place of a speaker, and it comes off of the secondary of the OT.....(in your case, it's your "line out").
I asked about the OT because I wanted to make sure you had proper primary and secondary impedances (since you were running an 8ohm resistor as a dummy load), and that it was sized properly for your output wattage ......
It is possible to damage your OT by overheating (mismatching impedance or overloading), and potentially creating an internal, possibly intermittent short in the windings,,,,BUT, it's not a common failure...........it takes effort to wreck a perfectly good OT

Here's an article on testing your OT:
http://www.geofex.com/ampdbug/outtrans.htm

My curiosity was based on the idea that the amp was fine until you did the load test,,,but is now showing signs of "shorting out"......there are plenty of other reasons that your amp could be malfunctioning, it's just easy to base a first guess on the facts of the case....
The fact that it happens after it warms up points to a heat related failure, that could be as simple as a bad resistor/capacitor,,,,but very difficult to find......

Here's some other troubleshooting tips that might help:  
http://www.trinityamps.com/ForumGallery/trinity/resources/geofex_tubeampdebug.pdf

[dscottguitars]

I made a line out from the speaker jack to a voltage divider circuit feeding another jack.  I haven't opened it up yet, too busy.  I do know the OT did not get hot, if that means anything.  I did switch the impedance though while checking it with the dummy 8 ohm load, not thinking it was the wrong impedance.  If that's the problem it's an easy fix, just not cheap, but then I don't have to rack my brain about the rest of the amp...

Thanks...

[SILVERGUN]

I am only suggesting that the OT is one component to consider....the fact that it didn't get hot IS a good sign.
Next, I would check your screen resistors...etc., etc.
I'm just presenting you with an approach for troubleshooting, working from the source of the perceived problem (the speaker, line out), and working backwards, through the power tubes, power supply, etc......

It wouldn't hurt to try a new set of 6L6's (tubes fail, sometimes intermittently).....switching impedances during testing is switching the load on those tubes,,,,, comparable to trying to shift from 5th gear to 2nd gear at 80 MPH  

For now, I still treat an amp like it's a welding machine.....if there's an output related problem I focus on the output side, and vice-versa...
Work your way from the OT inward, and don't take anything for granted....

[Willabe]

switching impedances during testing is switching the load on those tubes

When you change the impedance switch on an amp you only do that with the amp off.


                 Brad     

[sluckey]

When you change the impedance switch on an amp you only do that with the amp off.

That may be a little over the top with caution from a technical viewpoint, but most unknowing people should follow that caution exactly.

But, if you remove the signal going into the amp or just quit framming away, you can safely switch impedance. This statement is only for technicians, not musicians.

[Willabe]

Yes that was the short answer, I was thinking if I should write more but I think when he switched the Z he did have a test signal going in?

Like your saying it's when you have a signal going into the amp that is the problem with no speaker load/flipping the Z switch.

Something to do with a high fly back voltage spike in/from the OT that only happens when an ac signal is across the OT? No ac signal applied no fly back voltage spike so OT is safe.

I think I got this right?


                  Brad      

[SILVERGUN]

Sorry guys I should have been more clear.....my comment came from his comment below

I did switch the impedance though while checking it with the dummy 8 ohm load, not thinking it was the wrong impedance. 

dscott, does this mean that you switched it while testing?...under load?

[dscottguitars]

Hi, yes I switched it while testing.  I had the guitar plugged in and switched it from 8 ohm to 4 ohm and back again a few times.  I wondered why I couldn't see much difference in the voltage coming out, dah!:w2:

I have a lot more to learn and judging by this mistake I'm sure it's going to be the hard.  Thanks for all your suggestions.  I will try changing the tubes and checking all the voltages.  If the screen voltage is good, the resistors should be too, yes?  

It's been a long day and I'm too tired to try anything tonight.  Hopefully more time tomorrow...

Thanks again...

[Willabe]

If the screen voltage is good, the resistors should be too, yes?

Yes, if they burned open no screen voltage.


            Brad   

[dscottguitars]

Hi, I think I may have found the problem  

I checked the transformer with the instructions from that link above and everything seemed fine.  I tested the output tubes on my tester and they measure strong so I put them back in.  I checked the screen resistors-no drift and proper screen voltage.  I did notice a slight popping noise somewhat to a microphonic preamp tube sound and tapped on all of them.  The phase inverter tube was quite loud with a popping sound, not a ringing sound like I've heard before. It tests just fine on my tester-an EMC  model 213.  I replaced that and tried the amp.  It seems fine but I'm going to leave it on for awhile to see if anything changes.  I also tried other 6L6's but didn't notice any difference so I put the originals back in.

Any thoughts on that phase inverter tube testing okay but may still have a problem?  It's a black plate 6201.

Could my testing the output power and switching impedance wrong and such damaged the phase inverter?

[Toxophilite]

By the way
I discovered a while back that you can download a signal generater off the internet that you can play on your computer (basically a wave file or MP3 if I remeber correctly)
Here's one just from a quick search but there's many
http://www.softpedia.com/get/Multimedia/Audio/Other-AUDIO-Tools/Digital-Signal-Generator.shtml
Not sure if this is useful in this application but you should be able to just run it from your computer into your amp