I don't know. Should I hook the DMM with AC setting between ground and tip ?
Well, I'm not sure myself but I constantly see people mention 100mV and when I scope my amps I do just what you mentioned. I just measure tip-to-sleeve and look for 100mV on my DMM. When I do a similar thing with a cable plugged into my guitar (i.e. measuring the output AC voltage of my guitar signal) I get in the 100mV ballpark when strumming a chord lightly with low-ish output humbuckers. FWIW, this gets me in the 5v ballpark on the plate of V1a. I think the important thing is to input a signal that's somewhat comparable to a guitar signal. I use 880Hz and I've seen some people use 440Hz.
Also, when I'm putting that 100mV signal into the amp I measure the output of the Send jack. First, I set the preamp and Masters to where I usually set them (I start with my Masters at about 1 o'clock and then adjust/balance from there) and then adjust the Send Level pot such that I'm getting a Send signal level comparable to a guitar level signal (i.e. about 100mV...because that's what my FX pedals expect to see and I don't want them to clip).
If you're looking for other ideas on diagnosing the problem...check out this 'amp stethoscope' thread. I decided that I wasn't going to touch anything until I built this amp stethoscope and went through the amp. It would be a shame to mod my amp to hell and back only to find out I have a bad resistor somewhere or something else very simple.
I don't know. Should I hook the DMM with AC setting between ground and tip ?
Well, I'm not sure myself but I constantly see people mention 100mV and when I scope my amps I do just what you mentioned. I just measure tip-to-sleeve and look for 100mV on my DMM. When I do a similar thing with a cable plugged into my guitar (i.e. measuring the output AC voltage of my guitar signal) I get in the 100mV ballpark when strumming a chord lightly with low-ish output humbuckers. FWIW, this gets me in the 5v ballpark on the plate of V1a. I think the important thing is to input a signal that's somewhat comparable to a guitar signal. I use 880Hz and I've seen some people use 440Hz.
Also, when I'm putting that 100mV signal into the amp I measure the output of the Send jack. First, I set the preamp and Masters to where I usually set them (I start with my Masters at about 1 o'clock and then adjust/balance from there) and then adjust the Send Level pot such that I'm getting a Send signal level comparable to a guitar level signal (i.e. about 100mV...because that's what my FX pedals expect to see and I don't want them to clip).
If you're looking for other ideas on diagnosing the problem...check out this 'amp stethoscope' thread. I decided that I wasn't going to touch anything until I built this amp stethoscope and went through the amp. It would be a shame to mod my amp to hell and back only to find out I have a bad resistor somewhere or something else very simple.
Thanks Fisherman for the explanation. I'll do just that.
Btw. Ive built the DIYstompboxforums 'audioprobe' (with 630V cap). I'll put in a 1M pot there also and listen to the different sections of my build. I've troubleshooted several pedal builds with it.
All in all, if nothing else, it is quite cool to look at the scopes screen while adjusting the knobs
Mat, I think all is OK, don't sweat the bias I think it is fine.
To measure the voltage of the input signal - use your scope! Just connect it up to the input jack, adjust the scope to get a good reading and based upon your amplitude scale you can read it off the scope.
For oscillation, just connect your scope to the output jack and look at the waveform. An oscillation will look like a little compressed blip on part of the waveform. What I saw earlier showed no oscillation.
Mat, I think all is OK, don't sweat the bias I think it is fine.
Ok, it was scaring the hell out of me to see the DMM going over 200mV's Good to know it is normal phenomena.
To measure the voltage of the input signal - use your scope! Just connect it up to the input jack, adjust the scope to get a good reading and based upon your amplitude scale you can read it off the scope.
Thanks for the tip, actually I already measured it with DMM. I'm much more comfortable with the DMM than the scope.
For oscillation, just connect your scope to the output jack and look at the waveform. An oscillation will look like a little compressed blip on part of the waveform. What I saw earlier showed no oscillation.
Ok, I took few pics of the scope with following settings (o'clock):
vol 1
bright off
mid off
rock on
treble 3
middle 12
bass 9
drive 2
od master 9
clean master 9
hrm:
treble 12
bass 9
middle 11
dlator send pot adjusted at 100mV at send jack (PAB off, clean channel)
A bit of scope advice - adjust the focus, and keep the brightness as low as possible.
Also, you should learn how to measure your max clean power output. Adjust input and controls to obtain the largest possible clean sine wave on the output, then determine its peak-to-peak voltage. (count the lines on the scope, multiply by the volts/division setting).
Knowing the P-P voltage and the load resistance, the formula for power is:
Vrms = Vp-p / 2.818
Power (watts) = (Vrms ^ 2) / R (ohms)
Once you know the max peak-to-peak voltage you can cleanly get out of the power amp, you'll know when you're pushing it too hard. A sine wave of max clean power is not too hard on the amp, but a fully squared off wave of the same amplitude is pushing everything much harder. That's when you'll likely hear your transformer buzzing.
tele_player wrote:I'm not sure what you're expecting people to see.
Well, I read earlier people said they can see a lot of things from the shape of the wave. I hoped to see myself an ugly line on the scope that would tell there is something wrong with the build..
A bit of scope advice - adjust the focus, and keep the brightness as low as possible.
I did. Actually it might be my crappy digital camera that has moved while taking the pictures.
Also, you should learn how to measure your max clean power output. Adjust input and controls to obtain the largest possible clean sine wave on the output, then determine its peak-to-peak voltage. (count the lines on the scope, multiply by the volts/division setting).
Knowing the P-P voltage and the load resistance, the formula for power is:
Vrms = Vp-p / 2.818
Power (watts) = (Vrms ^ 2) / R (ohms)
Once you know the max peak-to-peak voltage you can cleanly get out of the power amp, you'll know when you're pushing it too hard. A sine wave of max clean power is not too hard on the amp, but a fully squared off wave of the same amplitude is pushing everything much harder. That's when you'll likely hear your transformer buzzing.
Thanks for the info. The scope is not mine so I have it only a limited time. I don't see myself using it alot. I just try to debug this build and when done I try to keep away from building/tweaking/troubleshooting
no oscillation there! The square waves are very heavy distortion. Here is a pic of what an oscillation might look like. The red trace, right at the beginning causes a short oscillation to happen which sounds like marbles in a tin can.
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One more scope-noob-question: would the oscillation (if there is any) be seen always when connecting the scope to the speaker output ? Or could there be oscillation on some section of the circuit 'hiding' and not be seen on the output but on other sections
Would there be any other places worth to scope now when the chassis is open ?
The oscillation can occur anywhere in the circuit. Basically any place one signal can be fedback into another area of the circuit. I see them most often when a strong signal lead is parallel or close to a weak signal lead. I have also had them with power tube grid wires too close to each other.
I had an oscillation in one build where the feedback wire (from the output xfmr) was run beneath the power supply board and was coupling into the preamp filters. Sounded absolutely HORRID. Rerouted the wire and all was well.
Wife: How many amps do you need?
Me: Just one more...
mlp-mx6 wrote:I had an oscillation in one build where the feedback wire (from the output xfmr) was run beneath the power supply board and was coupling into the preamp filters. Sounded absolutely HORRID. Rerouted the wire and all was well.
You mean the wire from presence pot via resistor to 4ohm speaker jack ? It goes between the preamp board and powerboard to the speaker jack in parallel with the wires from the bias adjustment pots on my build.
d2camero, the amp sounds very cornery and barky. It has tons of feedback and harmonics. Softly picking the strings it sounds nice but with little more pressure it sounds ugly. Funny enough it has very little overdrive. It is like a singing (and barking) clean amp I've struggled with the build about half an year but I'm not giving up .. yet
I'll start to use 'bias is XXmA'.
Good to know it is normal phenomena.
1218]
I've struggled with the build about half an year but I'm not giving up .. yet 
