Update :73 Silverface Twin

[gtrplayr1976]

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.

Yes they do seem to assume the user is already familiar with the instrument. I had printed some of the user pages for initial settings.

[km6xz]

Set the trigger mode to No D'ly
The A&B Int to Vert(this will trigger on either Channel 1 or Channel 2, which ever has focus at the time if Chop or Alternative sweeps
Set the Trigger level to the center point
Set the A Source to INT (upper position)
Set A Ext to anything since you are not using external trigger....yet.

In the Vertical Mode section Set the mode to Chop
Set the Channel display to Ch1 if using Channel 1 for input, if using both set it Both

Locate the tiny gold plate pin socket between the screen and Cha 1 input attenuator.
Remove the probe hook assembly, it just pulls off with light tension to reveal the pin center conductor of the probe, which just fits that Probe Adjust socket. The socket has a 500mv peak to peak square wave at close to 1khz. You can use that to calibrate the scope and the probe. The square wave should accurate enough to verify sweep speed, rise time, frequency, vertical amplifiers and frequency compensation of the probe. Use that reference signal to verify that you have everything adjusted to how you intended it to be.

Your signal generator is a classic, and ought be calibrated pretty accurately for an analog meter for actual output level such that you do not have to double check with your DMM. It is calibrated in volts and dbm. We do not use dbm as much as when that meter was designed, which was a power ratio. Now we do not use terminated matching impedance for measuring audio signals in terms of power because power calibration required a reference impedance, usually 600 ohms, or some systems used a standard reference of 500 ohms. Nowadays we interface many devices in bridging mode, where there the reference is voltage at a higher impedance, 1000 ohms, that is not matched for best power transfer from the source. The source is a low impedance so the voltage level does not change when loaded by 1 or more higher impedance loads. I am explaining this because meters calibrated dbm read lower than meters calibrated in dbv.
1 volt on the volt meter will be 0dbv and on the scale calibrated in dbm(like yours) will read the same 1 volt RMS as .775 dbm. Check your signal generator meter, set the generator output to 1volt RMS read off your DMM and see if the voltage scale reads 1 volt and the dbm scale reads .775dbm. There is a switch to insert a 600 ohm load so the meter will have the correct load for dbm power calibration. For your work, turn the 600 ohm termination off. If you verify that 1 volt is displayed on the voltage scale, great, you do not have to use an external meter to verify the output level, you just dial what level you want by adjusting the level displayed on the analog meter. If it is far off, it needs calibration which should be easy since you have an accurate DMM as a calibration standard.
It is important to verify your test instrument calibration so you can trust each piece to tell you reliable information.
Your assembly manual for the generator has detailed, but simple calibration procedures for the meter.
If you have a guitar tuner, it is very accurate, and can be used to calibrate the signal generators output frequency. Unless someone replaced some parts it should be quite accurate.
Now you have two essential elements of making a diagnosis, a known level, at a precise frequency, going in, and an accurate measurement of the frequency, amplitude and timing, plus a visual indication of distortion content. Before jumping in and taking measurements randomly, check your test instruments instead so any later measurement will be more informative of what is really going on in the circuit.
If the calibration is good enough, and the process of checking calibration calibration teaches you how to read the different meters and understand what their reading represents, you are ready to measure parameters of an amplifier's performance.
Have fun.

[gtrplayr1976]

Set the trigger mode to No D'ly
The A&B Int to Vert(this will trigger on either Channel 1 or Channel 2, which ever has focus at the time if Chop or Alternative sweeps
Set the Trigger level to the center point
Set the A Source to INT (upper position)
Set A Ext to anything since you are not using external trigger....yet.

In the Vertical Mode section Set the mode to Chop
Set the Channel display to Ch1 if using Channel 1 for input, if using both set it Both

Locate the tiny gold plate pin socket between the screen and Cha 1 input attenuator.
Remove the probe hook assembly, it just pulls off with light tension to reveal the pin center conductor of the probe, which just fits that Probe Adjust socket. The socket has a 500mv peak to peak square wave at close to 1khz. You can use that to calibrate the scope and the probe. The square wave should accurate enough to verify sweep speed, rise time, frequency, vertical amplifiers and frequency compensation of the probe. Use that reference signal to verify that you have everything adjusted to how you intended it to be.

Your signal generator is a classic, and ought be calibrated pretty accurately for an analog meter for actual output level such that you do not have to double check with your DMM. It is calibrated in volts and dbm. We do not use dbm as much as when that meter was designed, which was a power ratio. Now we do not use terminated matching impedance for measuring audio signals in terms of power because power calibration required a reference impedance, usually 600 ohms, or some systems used a standard reference of 500 ohms. Nowadays we interface many devices in bridging mode, where there the reference is voltage at a higher impedance, 1000 ohms, that is not matched for best power transfer from the source. The source is a low impedance so the voltage level does not change when loaded by 1 or more higher impedance loads. I am explaining this because meters calibrated dbm read lower than meters calibrated in dbv.
1 volt on the volt meter will be 0dbv and on the scale calibrated in dbm(like yours) will read the same 1 volt RMS as .775 dbm. Check your signal generator meter, set the generator output to 1volt RMS read off your DMM and see if the voltage scale reads 1 volt and the dbm scale reads .775dbm. There is a switch to insert a 600 ohm load so the meter will have the correct load for dbm power calibration. For your work, turn the 600 ohm termination off. If you verify that 1 volt is displayed on the voltage scale, great, you do not have to use an external meter to verify the output level, you just dial what level you want by adjusting the level displayed on the analog meter. If it is far off, it needs calibration which should be easy since you have an accurate DMM as a calibration standard.
It is important to verify your test instrument calibration so you can trust each piece to tell you reliable information.
Your assembly manual for the generator has detailed, but simple calibration procedures for the meter.
If you have a guitar tuner, it is very accurate, and can be used to calibrate the signal generators output frequency. Unless someone replaced some parts it should be quite accurate.
Now you have two essential elements of making a diagnosis, a known level, at a precise frequency, going in, and an accurate measurement of the frequency, amplitude and timing, plus a visual indication of distortion content. Before jumping in and taking measurements randomly, check your test instruments instead so any later measurement will be more informative of what is really going on in the circuit.
If the calibration is good enough, and the process of checking calibration calibration teaches you how to read the different meters and understand what their reading represents, you are ready to measure parameters of an amplifier's performance.
Have fun.

Thank you for all the detail. Very helpful. One question I would like to ask. How do you use a guitar tuner to calibrate the generator ?

[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?

Tom , it seems to work ok now with this setup. I think It may have been when I had the probe in the test port. So I have it set in No DLY ,and seems to be working. Seems like the readings on the scope may not be right with the settings on the generator. I used the probe set at 10X , 200mv from the generator, .2 Volts/Div ,and .1 Sec/Div. The last picture is with probe in adjust port set at .1 Volts/Div.

[km6xz]

The tuner has a frequency reference that is accurate so you only need to set the generator to a frequency of a fundamental and the turner displays whether the external signal is sharp, flat or right on the correct frequency. There are calibration adjustments in signal generators to coordinate the number silk screened values of the switch positions to the actually frequency.
For example the lowest A on a guitar is the second octave, is 110hz, middle C is 261.626hz which is harder to remember so use A notes, such as the 4th octave A is 4 times the 2nd octave A, so it is 440hz, the 5th octave A is 880hz.

Frequency counters and clocks in electronics are the most accurate reference we have access to commonly. A crystal oscillator in your computer, the tuner, you DMM, CD player is 2-4 orders of magnitude more accurate and stable than other reference you will have laying around. I have a timebase, TCXO in my scope that cost me $400 extra, above the normal clock oscillator but the result is very low drift with aging and temperature so frequencies readings are good to 0.00005%(0.5 parts per million) while the standard time base is 0.001%(10 parts per million). I have a crystal oven time base in my RF service monitor that is 15 times better than that. The square wave level accuracy from that probe calibration socket on the front is +/- 5% and the frequency accuracy is +/-20%.
So any value that can be derived from time, is going to be more accurate than one derived from current or resistance.
For many decades the government has transmitted time and interval reference signals by which you could calibrate everything from your watch to your signal generators. The easiest to receive was station WWV broadcasting on 5, 10, 15 Mhz and one that did not bounce off the ionosphere so had a more predictable distance and signal travel time from Colorado to your antenna, WWVB on 60khz. It signal was generated by atomic clocks.

I see from from your meter reading on the generator and DMM they both very close so you can just dial in the level you want watching the generator meter, which shows just under 2volts AC while your DMM says 1.99. Unless you are just curious, the meter on the generator will be more than accurate enough for audio work.

[gtrplayr1976]

The tuner has a frequency reference that is accurate so you only need to set the generator to a frequency of a fundamental and the turner displays whether the external signal is sharp, flat or right on the correct frequency. There are calibration adjustments in signal generators to coordinate the number silk screened values of the switch positions to the actually frequency.
For example the lowest A on a guitar is the second octave, is 110hz, middle C is 261.626hz which is harder to remember so use A notes, such as the 4th octave A is 4 times the 2nd octave A, so it is 440hz, the 5th octave A is 880hz.

Frequency counters and clocks in electronics are the most accurate reference we have access to commonly. A crystal oscillator in your computer, the tuner, you DMM, CD player is 2-4 orders of magnitude more accurate and stable than other reference you will have laying around. I have a timebase, TCXO in my scope that cost me $400 extra, above the normal clock oscillator but the result is very low drift with aging and temperature so frequencies readings are good to 0.00005%(0.5 parts per million) while the standard time base is 0.001%(10 parts per million). I have a crystal oven time base in my RF service monitor that is 15 times better than that. The square wave level accuracy from that probe calibration socket on the front is +/- 5% and the frequency accuracy is +/-20%.
So any value that can be derived from time, is going to be more accurate than one derived from current or resistance.
For many decades the government has transmitted time and interval reference signals by which you could calibrate everything from your watch to your signal generators. The easiest to receive was station WWV broadcasting on 5, 10, 15 Mhz and one that did not bounce off the ionosphere so had a more predictable distance and signal travel time from Colorado to your antenna, WWVB on 60khz. It signal was generated by atomic clocks.

I see from from your meter reading on the generator and DMM they both very close so you can just dial in the level you want watching the generator meter, which shows just under 2volts AC while your DMM says 1.99. Unless you are just curious, the meter on the generator will be more than accurate enough for audio work.

Thank you . This helps. Actually the meter is reading 200mv . The generator is set on the 0-1 Volt scale.

[km6xz]

Another point> According to your photos, the scope is pretty well calibrated(as I would expect a Tektronix to be) for both vertical attenuator and for time base. One minor issue, see how the squarewave has a leading edge ski slope look, it should be flat. This means the high frequency harmonics of the 1khz square wave are being displayed with more sensitivity. That is a probe adjustment to take care of that and flatten the frequency response. While the probe and scope are set up just the way it is now, take a small non-metallic screw-driver if you got one with the probe or just a jeweler's screw driver and adjust the little trimmer capacitor the small hole in the BNC connector end of the probe cable. Adjust it slightly to flatten that top horizontal line. You can do that easily with the plastic adjustment screwdriver that comes with the probe of if you are using a metal screw driver, just touching the adjustment with the screw driver will change its value so make a little change and remove the screw driver to see if it went in the desired direction. Keep making adjustments until after removing the screw driver, the line is flat. Done. Do that with both probes. Now, sending the same level signal at say 100 hz will display the same line height at 200khz also.

[gtrplayr1976]

Another point> According to your photos, the scope is pretty well calibrated(as I would expect a Tektronix to be) for both vertical attenuator and for time base. One minor issue, see how the squarewave has a leading edge ski slope look, it should be flat. This means the high frequency harmonics of the 1khz square wave are being displayed with more sensitivity. That is a probe adjustment to take care of that and flatten the frequency response. While the probe and scope are set up just the way it is now, take a small non-metallic screw-driver if you got one with the probe or just a jeweler's screw driver and adjust the little trimmer capacitor the small hole in the BNC connector end of the probe cable. Adjust it slightly to flatten that top horizontal line. You can do that easily with the plastic adjustment screwdriver that comes with the probe of if you are using a metal screw driver, just touching the adjustment with the screw driver will change its value so make a little change and remove the screw driver to see if it went in the desired direction. Keep making adjustments until after removing the screw driver, the line is flat. Done. Do that with both probes. Now, sending the same level signal at say 100 hz will display the same line height at 200khz also.

I wasn't aware of an adjustment on the probe. I will take a look at it this evening. Thank you !

[Tom Phillips]

I wasn't aware of an adjustment on the probe...

You can find more information about this by looking up the topic "Probe Compensation" in your scope manual.

[gtrplayr1976]

You can find more information about this by looking up the topic "Probe Compensation" in your scope manual.

Got them adjusted last night. Yes I need to read more of the manual. I will probably start poking around with it on the Twin once I get the load resistors mounted on something.

[gtrplayr1976]

Anyone have a suggestion on how I could mount my load resistors ? I guess I need some threaded rod to go through them ,and maybe use some strap material ? Is there insulation between the resistor ,and the metal mount where the rod would go through ?

Thanks !

[Tom Phillips]

Anyone have a suggestion on how I could mount my load resistors ?...

The easiest solution is to use power resistor mounting brackets as shown on the bottom of the page at OHMITE Power Resistors . I have seen through rods used too but they were kept centered with large shoulder washers.The body of the resistor is insulated from the solder tab.

[gtrplayr1976]

The easiest solution is to use power resistor mounting brackets as shown on the bottom of the page at OHMITE Power Resistors . I have seen through rods used too but they were kept centered with large shoulder washers.The body of the resistor is insulated from the solder tab.

Thanks Tom. I'll get some of those.
Back to the post(s) 35,36 ? I was poking around on the Twin yesterday getting it set up to use the scope. I may have found the reason the Normal channel sounds fuller, or different. I did not have the .1 ,and .047 caps connected on the Normal channel as you can see in the schematic in the red circle. I would think that missing this would make the amp sound bad ,but it seems to sound fuller/louder ?

[g1]

Looks like you were bypassing most of the normal ch. tone controls.
You should have noticed that the tone controls weren't doing as much as they do for the other channel?

[gtrplayr1976]

Looks like you were bypassing most of the normal ch. tone controls.
You should have noticed that the tone controls weren't doing as much as they do for the other channel?

Well it definitely took the .047 cap out of the circuit , but I swear it seems to sound better than the Vibrato channel. That schematic isn't exactly how the tone controls are wired either. It may be the same in the end ,but they are different. Maybe I need to edit that schematic to show how it is wired ?