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How to use a Digital Oscilloscope...to test your tube amp! AT THE BENCH!
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- Опубликовано: 7 авг 2024
- The previous video detailed how I use the multimeter to test voltages in a tube amp, this clip deals with the Oscilloscope. I cover Scope basis, generating a test signal, tracking the signal through the amp and finally how to measure RMS output power.
Our test amp is a Mojotone 45 kit that was built by a local player and brought in to me for completion.
0:00 Intro
0:53 Scope basics
6:43 Signal generator
9:22 Connecting to the amps input
11:35 1st gain stage
14:00 Volume wiper
15:42 Mix resistors
17:03 V2a gain stage - AC Coupling
20:03 Cathode follower - tone stack in
22:50 Treble wiper
23:27 Phase inverter in
24:05 Speaker output
26:24 How to measure output power
28:32 Outro - possible mods?
headfirstamplification.com
If you have more than one input channel on your scope, a good idea is to colour-code all the channels, use Ch-1 to monitor your input signals, and Ch-2 to monitor the signal at various points in the circuit, this will give you more of an idea of how the circuit you're testing is behaving, also useful for tracking down faults too. you can also do phase-measurements with a scope that has more than one input channel.
would love to see the dirty Shirley schematic and your docs :) Thank you for all that you do! :)
thanks, you can never have too much detail in vids like this, more detail the better.
Awesome Jase thanks for explaining that, yeah it would be great to see the mods you do on that JTM.
Thank you so much for these videos, really appreciate how to the point they are with no clickbait or ad’s or anything else.
Would love to see the DS mods!
Nice video and walk through of signal tracing. Very well explained! It was good you discussed AC Coupling and knowing the max input voltage of your scope as well. It's worth mentioning the need to know the voltage rating of your probes as well as the X10 setting :)
Very helpful guide for those who might be interested though nervous about about taking to an amp with a scope
Stumbled onto your videos recently. I have to say your mods are pretty amazing!
Thanks!
Great job on the Oscilloscope Jason. Very comprehensive & well presented toward what we do. Thank you for taking the time. David Hopkins
Thank you kindly!
Dude...this is brilliant. Thank you.
Thank you so much for this lecture! - Just bought oscilloscope (4x channel, 2x signal gen) just the other day - even though I have no know how on how to use it (..sound stubid, but that is how it is). So this video really helped me - thanks again. Best regards, from Iceland
Glad it helped!
**Thank you** for these long format style videos you do- in depth, step-by-step, and with comparative graphics of schematic and board layout! (As someone who used to edit videos, it's a lot of time and energy you've given here!). I've always enjoyed fixing my own amps- cap jobs, re-tubes, but anything beyond replacing known bad parts I'm a bit lost and it often takes way longer than it probably should lol! .... I'm one of those people who find it more difficult learning through reading vs visual learning... so I've learned more about working on Marshall tube amps, and overall understanding of how they function in the month I've discovered your channel than in 20 years of tinkering on my amps, scanning internet forums, etc. Watching your videos has really reawakened a desire to get an old '76 fawn JMP I've had for years sounding better, and the confidence to finally dig in! Have you given any thought of an even longer format version you'd produce- say a complete walk through of a stock healthy 2204, walk through of an entire scematic while comparing the board layout, voltages at each stage, as a PAID patreon type deal? I would sign up in a heartbeat.
Hey Michael, thanks for your feedback and support. Really appreciated!! Great idea for the full 2204 walkthrough - happy to do that. Patreon? Well one day I think. I need more subs make that worthwhile I think...
Very helpfull to understand the signal flow! Thank you. Great channel!!!
Glad it was helpful!
thank you, I bought the scope, made the dummy load and now waiting for the signal generator. I hope to learn much more about tube amps repair
Mate your timing is impeccable! Haha thanks heaps for these videos Jase!
No worries!
Thank you again Jason. Very informative and extremely useful! I’d be interested in seeing the mods to the JTM45.
You got it!
I have the same scope, and it seems to work well for this kind of work. I'm glad it has a signal generator. If it didn't I'm thinking you could use the headphone jack from a cell phone, tablet or lap top, then download a signal generator app.
Very helpful, thank you, much appreciated.
I'm just getting started on testing my way through a 1967 Fender Showman amp (AB763 circuit). Looking at the amp on JTM 45 kit amp, it looks a lot like my Fender Showman. A lot of similarities. I have a noisy volume pot (maybe the pot, maybe the 7025 preamp tube) which I will need to trace out and track down the offending component. I will acquire an oscilloscope to help me with this task. Great video.
Thanks Man you do a lot of good work and information. God bless you. Cheers.
Thanks!
Really nice video! One minor thought re: calculations on output at the end, I have those same 8 ohm resistors and they are not exactly 8 ohms. So I guess (if it mattered) you could measure it before calculating. Thanks for the excellent presentation!
very helpful! subscribed!!!!
Thanks for the sub!
Would like to see how it deform the sin with higher gain.
Thanks man watching a digitech qc-1936 ebay uk! great help thanks!
Would be cool to see you probe through a master volume amp like a 2203 or 2204. Love your instruction. Thanks!
Thank you!
Thank you very much
Thank you.
I've been seriously thinking of buying myself a new Digital Scope for a long time, Jaycar Electronics stock some good ones, in the meantime I'm making do with an old DSE Q1804 analog Dual Trace 20MHz Scope I bought in the late 90's, fixed quite a few things with it.
Hey, thanks - that was very helpful.
I would be also interested in how to identify dc leakage in the coupling caps and parasitic oszillation. Would you mind showing this also in the next video?
Noted
You can use the 2 probes conveniently to see the input and the output signal, and check if the signals looks alike, and if the output is in or out of phase.
how do you use a scope to track down noise in a high gain preamp circuit?
Thanks man. Really helpful video. Now I'm curious how Dirty Shirley is!
Is his wife named Shirley?
Hi Jason. Great channel and great video on using the oscilloscope.
I was wondering why you moved the scope back to DC coupling when you were still only interested in the AC signal. Could you please explain that.
Also wanted to know if you use an isolation transformer between the amp being tested and mains power ? Cheers
just found your great video on troubleshooting this amp. I want to do this on my non-working transistor radio.can your built in waveform gen be followed with placing 2nd probe on speaker output to hear the sine wave tone in addition to seeing on the scope?
thanks
John
great explaination, just a question, on the speaker output section (24:00), you change the location of the ground probe (from the ground input jack) to where exactly ? i suppose its the ground bus soldered on the back of the pots, i'm right ?
Hey Jason I'm curious to know if you can probe the high treble channel like that as well......that's what I am trying and I'm getting a distorted wave at the mixer resistors.....
Hey Jason, love your stuff....
Is your signal gen 50 ohm source impedance? Do you terminate it in to 50 ohms and feed off that to the high impedance amp input and does that help with output voltage accuracy? For example, you set 100mV and actually get 100mv at the input...
Keep up the great work! Loved your recent stream with Dave & Bruce!
Got enough subs to make a transistor Class a & ab power output from start to finish tutorial or a tutorial on grounding specifics as to not short amplifiers when using oscilloscope grounding leads?
Any need to be concerned about the isolation of the scope ground to protect the scope? John
Excellent video ! I built a few guitar amps and always want to « see » the effect of each gain stage. You made it 👍
One question : why do we have an amplification after the 1st grid stopper resistor (320 mV versus 150 mV of the generator) ?
Thanks! The generator is 150mV from zero to peak, which is ~320mV pk-pk.
That makes sense ! Thanks for your answer.
This is absolutely great! Thank you so much for doing this. I'm almost a complete beginner though and there are already some things i don't understand. At around 10:16 you turn the signal generator on, but you've turned on gen on/gen off. I've got the same scope as you're using here and I'm not getting what I see you are. I don't get a wave unless I've turned on gen/dso. I don't think it makes sense for me to keep going until I get this figured out.
One reason for that is with the wave displaying (gen/dso on) I get a complaint from the scope telling me to push gen/dso to quit dds first.
Very confusing!!!
At that point in the clip, I have the signal generator on (the button on the right is ON) but the scope display set to the probe (button on the left is OFF)
@@HeadfirstAmps Yes, thanks, I see that. Thanks for your reply. I'm wondering if my meter is defective - I don't get a sine wave on the display with the probe hooked up as shown and the signal generator turned on. All I get is a jaggy straight line. I'll contact the manufacturer. Thanks again, though. Your videos and your help are very much appreciated.
Awesome vid. Next one, is it possible to show the sine wave deform when you crank it?
Thanks. Sure can!
Do you isolate the power supply of the amp when probing around; If so, how are you doing that?
So do you connect the speaker output to a dummy load then from the dummy load to the scope to measure
Did you plug your guitar into the scope to measure the peak voltage swing to use? Also do the results vary a lot when using a speaker cab vs. a simple resistive load? (Due to the freq/impedance curves of the speaker)
Yes on both counts!
Hi Jason. This was very enlightening, thank-you. Just a couple of quick queries re your power calculations at 27:11 - why is the 10VAC divided by 2? Is this always a part of the formula or does it depend on the amp? Thanks!
Divide by 2 to go from peak to peak reading to zero to peak
@@HeadfirstAmps Thanks Jason. I also watched Uncle Doug’s output power video and he didn’t do this part of the calculation. Am I missing something? See from 6:36 onwards.
ruclips.net/video/38bj6-MSx58/видео.html
Hi Jason,
Good video... Forgot all the basics I learnt as an apprentice, good to get a handle on it again.
Just one fault U made... Calling measured mV - milliamps in beginning of setup?
Hi - well it's possible I said one thing and meant another! If you have timecode I can check the part you are referring to.
I would just add a note of avoiding ground loop conditions. I assume you powered this from an isolation transformer?
is device under test floating ? is it isolated ? I just watched 20 videos that ask these questions- I dont understand how is that not even an issue here?
Hi ...could you recommend a similar Oscilloscope like yours? Cant find yours over here in Europe.
Thanks
I know a guy here in Australia that found one by Hantek on AliExpress that was very similar if not identical. AliExpress ship worldwide
Would a hantek digital oscilloscope around £100 or so. Be ok ?
It has a sig gen.
Being a hobbyist,I'm not at the pros price point. And I'm just looking for a bit of help on that
Yes, will be fine
Do you use an isolation transformer when performing this testing?
No. But they are advised if you’re starting out. However you need to be very aware of grounding, as if your amp chassis is grounded (they all normally are) then the isolation transformer is not doing much to protect you or your scope.
did I miss the x prob settings?
Nope, I didn't mention it. I have mine set on x10.
I have a question. Is the output power meant to be the same at different frequencies?
Yes, but the relationship between voltage and current will change with frequency,
@@HeadfirstAmps Yeah ok. I'm just trying to understand the numbers from a recent trip to a Yamaha Service Centre. Amp is under warrantee, and after a week, it sounded like the bass wasn't producing as much loudness or power. I took it in to a service centre and they said I was right... pretty unusual problem, but I didn't fully understand what I was seeing. Hopefully I can send a link to the video here and you can see if his diagnosis is correct.
@@HeadfirstAmps ruclips.net/video/tsjXcQc9h6Q/видео.html
@@HeadfirstAmps I thought he was testing the bass frequency vs a higher midrange frequency... but now I'm not so sure..? He starts with 1khz at the amps full power and it was outputting 28v, then he switches to 10khz and it was only putting out 16.5v. 10khz isn't a bass freqency though is it? Anyway, was the 2nd readying meant to be outputting 28v also?
Signal-level is in milli-volts, not milli-amps.
I know
Monotone? Should that read a Mojotone 45?
Auto correct
150 mA...😮
150 mV!
150 miliVOLTS
Great video! Signal generators can be a hassle. I just use a sine wave video on youtube and pipe that out my USB audio interface via a DACCAPO re-amplificaiton box. Cheap and easy. I had the reamplification box already. You can buy a wave generator PCB kit on fleabay for $10 if you're so inclined. Needless to say you don't need a $500 unit! You are paying for the extra channels you won't be using. Get something in the $200 range: I recommend the Hantek DSO5102P. Can get them as cheap as $150 if you go direct along the silk road. A lot of people will say just get an old analogue scope off ebay for $50. Hey, if its working/refurbished, that's very much OK too. But if not, you can expect to find at least $100 of leaky caps in those things from the 80's, so it can work out rather false economy..
Let me see if I"m following this: SG's are a hassle, but you're recommending a Hantek (overkill for most) and yet you yourself use a signal from RUclips. You wouldn't recommend buying an analog SG because you'd find "at least $100 of leaky caps". REALLY? Did I get that right?