Update :73 Silverface Twin

[gtrplayr1976]

Have you cleaned the pots & switch contacts yet as discussed earlier? Test equipment needs maintenance service too.

No sir I have not ,but I will do that this evening.

[52 Bill]

For the frequency setting, set the upper multiplier to X100, the middle knob to 10 and the right knob to 0 (zero).

The two right side knobs set the voltage output (signal strength/volume). The lower switch sets the maximum voltage output and the upper control is like a volume pot that lowers the output from the maximum setting on the switch.

So to set 100mV set the switch to .1 volt (-20) and turn the upper control fully clockwise. The meter should read 1 on the lower scale (0-10). Or you can set the switch to a higher setting like 1 volt and then use the upper control to lower it down to the desired level.

[gtrplayr1976]

For the frequency setting, set the upper multiplier to X100, the middle knob to 10 and the right knob to 0 (zero).

The two right side knobs set the voltage output (signal strength/volume). The lower switch sets the maximum voltage output and the upper control is like a volume pot that lowers the output from the maximum setting on the switch.

So to set 100mV set the switch to .1 volt (-20) and turn the upper control fully clockwise. The meter should read 1 on the lower scale (0-10). Or you can set the switch to a higher setting like 1 volt and then use the upper control to lower it down to the desired level.

Thanks Bill ! Gives me a better idea of this. You ALL have been a big help.

[gtrplayr1976]

Good photos. They really help support the discussion.


It appears that the Sig Gen frequency is set to 100 Hz.



Yes. But to be precise you are reading 80 mVpp. Which is 80 mV peak to peak. To convert to V rms, which is the scale used by your DMM you would divide by 2.828 which gives you ~28 mV. You can connect your DMM to the sig gen output at the same time the scope is connected and verify these numbers.

Experiment with the settings on the sig gen and scope to learn the effect of each control.

I think I have a decent feel for the instruments now. So if i connect to the input of the Twin at 1KHz and 100mv ? do i just follow the signal ? if so how do I do that ?

[Tom Phillips]

...do i just follow the signal ? if so how do I do that ?

Yes. You just follow the signal.

Really though, you have asked a loaded question. Kind of like “Now I understand how to hook up my guitar. How do I play a song?”

You have been watching tutorials about how to use your test equipment, select the proper scope settings etc. You need to trace the signal by following the signal flow through the circuit. That means you should be able to read the schematic and find the signal path test points in the physical circuit. Are you up to that point?

The good first experiment is to connect your signal to the input then trace it through the first stage pre-amp. Such as

• observe the input signal at the input jack
• verify that it reaches the grid of the first triode stage
• probe the plate of the first triode and note how the signal has been amplified. (Note: you will be using AC coupling on the scope)

A big part of this process is knowing what the waveform should look like. There are signals within the amp that are “correct” but are not perfect looking sine wave shapes.

You will, of course be working on powered high voltage circuits. In addition to your own safety you need to keep the amp safe by insuring you don’t slip and short out anything (such as plate pin 3 which is adjacent to heater pin 4) while you work.

If the above still seems confusing then just review some additional tutorial information. I could go on and on if time allowed but it’s would turn into a very long thing. Part of the process is being able to really understand the circuits. The book and information at the web site How to design valve guitar amplifiers shows actual scope waveforms. Do you know about that resource? And others?

[gtrplayr1976]

Yes. You just follow the signal.

Really though, you have asked a loaded question. Kind of like “Now I understand how to hook up my guitar. How do I play a song?”

You have been watching tutorials about how to use your test equipment, select the proper scope settings etc. You need to trace the signal by following the signal flow through the circuit. That means you should be able to read the schematic and find the signal path test points in the physical circuit. Are you up to that point?

The good first experiment is to connect your signal to the input then trace it through the first stage pre-amp. Such as

• observe the input signal at the input jack
• verify that it reaches the grid of the first triode stage
• probe the plate of the first triode and note how the signal has been amplified. (Note: you will be using AC coupling on the scope)

A big part of this process is knowing what the waveform should look like. There are signals within the amp that are “correct” but are not perfect looking sine wave shapes.

You will, of course be working on powered high voltage circuits. In addition to your own safety you need to keep the amp safe by insuring you don’t slip and short out anything (such as plate pin 3 which is adjacent to heater pin 4) while you work.

If the above still seems confusing then just review some additional tutorial information. I could go on and on if time allowed but it’s would turn into a very long thing. Part of the process is being able to really understand the circuits. The book and information at the web site How to design valve guitar amplifiers shows actual scope waveforms. Do you know about that resource? And others?

I guess that was a bit of a loaded question. I guess I need to learn what waveform I'm looking for ,and how to know when it isn't right. Can I place a signal through both inputs to compare waveforms ,or would that even help?

[gtrplayr1976]

Have a few more pics of the test equipment ,and playing with the scope. I also have a short video that shows something I'm not familiar with. If I can get it to load. Here is the link.
Sine wave - YouTube

[g1]

What is your concern with the video? Is it the "ghost" double image (inverted) of the sine wave?
If so, it is just not triggering perfectly. Try adjusting the 2 pots in the trigger section, you will probably be able to correct it.

[gtrplayr1976]

What is your concern with the video? Is it the "ghost" double image (inverted) of the sine wave?
If so, it is just not triggering perfectly. Try adjusting the 2 pots in the trigger section, you will probably be able to correct it.

Yeah that's what I was wondering about. It only did that at certain settings ,but if it replicates I will try adjusting the trigger. Thanks g-one.

[Tom Phillips]

I also suggest that you set the scope's Horizontal Mode Switch to "NO DLY" (No Delay). You don't need that function at this stage and the NO DLY setting will keep things simple and prevent possible confusing display effects.

Again, your clear high rez photos are really helpful.

Cheers,
Tom

[Tom Phillips]

Here are a couple of links to some YouTube Oscilloscope Triggering control use Tutorials
Tektronix Oscilloscope Triggering controls and their usage - YouTube
Oscilloscope AUTO Triggering explained - YouTube

[gtrplayr1976]

I also suggest that you set the scope's Horizontal Mode Switch to "NO DLY" (No Delay). You don't need that function at this stage and the NO DLY setting will keep things simple and prevent possible confusing display effects.

Again, your clear high rez photos are really helpful.

Cheers,
Tom

Thanks Tom. If I recall ,I tried to use the "NO DLY" and lost my beam ? Went blank. I can try again to be sure I'm not mistaken.

[Tom Phillips]

Thanks Tom. If I recall ,I tried to use the "NO DLY" and lost my beam ? Went blank. I can try again to be sure I'm not mistaken.

Hmm...That might be caused by an associated control setting. Try again and report. At least you have a setting that works fine for now. Do you have a copy of the operator's manual for the scope?

[gtrplayr1976]

Hmm...That might be caused by an associated control setting. Try again and report. At least you have a setting that works fine for now. Do you have a copy of the operator's manual for the scope?

I have a service manual in a pdf file. I think it has some operators section, but I will have to look.

[Tom Phillips]

I have a service manual in a pdf file. I think it has some operators section, but I will have to look.

That should be fine. I have found that, even when a separate operator's manual was published by Tektronix, the operator's information was also included in the service manual. Tek manuals, however, do seem to assume that the user is already knowledgeable about scope use so the various YouTube tutorials remain a valuable resource.