taking measurements inside a tube amp with old scope

[mikepukmel]

I have an old throwaway TEK 7613 scope. So far it works. The crt is a little gassy but you can see the trace.
I have tried digging around on the internet to find what the maximum input DC and AC voltages are for this scope. The best I could find was a general comment that said something like 50v, but I can't believe that.

I would like to take measurements at the output tubes, and see what the ac trace looks like. The grid is ok obviously a few volts P-P but the plate has the large DC offset.

Is it possible to use the standard 1x/10x 10 meg 14 pf probe, or will these voltages blow up the voltage divider? If so, is it possible to build an external voltage divider and measure that way?

thanks.

[loudthud]

First you need to know the model of the Vertical plug-in(s). It should be on the little plastic pull tab at the bottom of the left or center plug-in, Look for a name like 7A18, 7A26 or similar. Most inputs are protected up to 400V but that's also a general limit for a 10X probe. If you want to look at the plates of the power tubes in a guitar amp, you'll need a 100X probe. A 7613 has a unique screen with Bi-stable storage, but the storage rarely works without calibration.

Were the probes you have made by TEK ? Do you know the procedure for adjusting probe compensation ?

[mikepukmel]

First you need to know the model of the Vertical plug-in(s). It should be on the little plastic pull tab at the bottom of the left or center plug-in, Look for a name like 7A18, 7A26 or similar. Most inputs are protected up to 400V but that's also a general limit for a 10X probe. If you want to look at the plates of the power tubes in a guitar amp, you'll need a 100X probe. A 7613 has a unique screen with Bi-stable storage, but the storage rarely works without calibration.

Were the probes you have made by TEK ? Do you know the procedure for adjusting probe compensation ?

Hi Loud, thanks, yes the probes are TEK, P6120, I don't know the procedure for adjusting probe compensation, (Will google after I get off work).
Also, will look up the model number of the modules in this unit after work as well.

There used to be some great outlets around here (Eastern MA) for old/antique electronics stuff, but one by one they all disappeared. Will poke around for a set of old TEK 100x probes. I saw a set way back when for 50 bucks thought it was too pricey, but should have grabbed them.


Re
"A 7613 has a unique screen with Bi-stable storage, but the storage rarely works without calibration"
Do you mean the screen/trace capture facility? This whole unit has to be very far out of cal. There are old stickers on it from before when my friend got it, probably early 90's.

So, even if I can't take detailed measurements, is it worth it to look at the traces even if its, e.g. 20% off? What Im curious about is artifacts that might be related to oscillation. I started reading up on parasitic oscillation, (yeah, a little knowledge is dangerous!!)

[g1]

So, even if I can't take detailed measurements, is it worth it to look at the traces even if its, e.g. 20% off?

Yes.
My personal use beater scope I've been dragging around for the past 35 yrs is quite out of cal., and it was 15 yrs old when I picked it up. I just verify with meter when I need good numbers.
It's a single trace 20Mhz but does probably 85% of what I ever need it for. I like to say that any working scope is a thousand times better than no scope at all.
For what you would pay someone to spend half an hour showing you how to work a scope, you can buy one and spend years learning how to make good use of it.

[The Dude]

You know, the funny thing about MOST audio circuit repair, as it pertains to scope use, is that MOST of the time you're looking for something that is or isn't there. Where do I lose my audio signal. Is there substantial ACV on a DC rail, Is there gain after this amp stage, etc. I rarely actually look at VPP figures or care about what the specific voltage is at a point in the circuit (not never, but rarely). I'm not advocating for "anti-scope calibration", but I'll say it isn't a big deal. I couldn't tell you when my scope was last calibrated either, but as long as it's close, I don't care. I'm not doing any lab measurements.

[loudthud]

Most TEK scopes will stay pretty close to calibration for decades. Even after the power supply filters start failing, replace the failed part, DON"T ADJUST THE POWER SUPPLY and the scope will still be very close. Generally, there is only one pot to adjust in the power supply, probably the -50V in a 7000 series, and one for the CRT high Voltage. Don't adjust them unless you are doing a complete calibration. In the storage mode there is a small PCB behind the knobs and buttons to the right of the screen. One or two voltages to adjust and then a couple of pots to adjust how the flood guns hit the screen. You'll need to see the service manual to get the particulars.

On the front panel there is a test signal, usually 1KHz at 0.4V peak. You use that to adjust probe compensation (make square waves) and tweek the gain of your plug-ins.

[Tom Phillips]

You know, the funny thing about MOST audio circuit repair, as it pertains to scope use, is that MOST of the time you're looking for something that is or isn't there...

So true. Especially with respect to guitar amps.

I just diagnosed a problem in a Super Reverb that had low gain and dull tone in the "Vibrato" channel although the normal channel sounded as it should. It turned out that the reverb circuit was breaking into ultrasonic oscillation. The oscilloscope made it easy to figure out the problem. I didn't need to measure the oscillation amplitude or frequency once I saw the issue so scope calibration didn't matter.

[Helmholtz]

What Im curious about is artifacts that might be related to oscillation. I started reading up on parasitic oscillation, (yeah, a little knowledge is dangerous!!)

When a tube amp fully oscillates plate voltage may exceed 2kV peak, which might be over the limit of even an 100x probe.
As oscillation will show at the output there's no need to scope the plates.

[Helmholtz]

I just diagnosed a problem in a Super Reverb that had low gain and dull tone in the "Vibrato" channel although the normal channel sounded as it should. It turned out that the reverb circuit was breaking into ultrasonic oscillation. The oscilloscope made it easy to figure out the problem.

How did you cure the problem?

[Tom Phillips]

How did you cure the problem?

I suspect it's a bad aged cathode bypass cap in the "Vibrato" channel or further down stream in the reverb recovery circuit. I'm currently awaiting a response from the customer about other issues so have set the amp aside for now while I work on other things. I'll post the actual resolution later. The amp is full of bad workmanship from several lazy, inexperienced or amateur "techs."

[mikepukmel]

First you need to know the model of the Vertical plug-in(s). It should be on the little plastic pull tab at the bottom of the left or center plug-in, Look for a name like 7A18, 7A26 or similar. Most inputs are protected up to 400V but that's also a general limit for a 10X probe. If you want to look at the plates of the power tubes in a guitar amp, you'll need a 100X probe. A 7613 has a unique screen with Bi-stable storage, but the storage rarely works without calibration.

Were the probes you have made by TEK ? Do you know the procedure for adjusting probe compensation ?

Here's my old throw away pride and joy. Yep, exactly what you wrote, a 7A18 and a 7A26.
As Im looking at the zoomed photos, Im very surprised that the cal date says due 10/98. I thought the timeline was a decade earlier.

Found a few of these on ebay:

"Tektronix P6009 100 MHz 100X Probe"
various prices, some as low as 30.00 "as is", some guaranteed tested for 100.

Reading all of the posts above . . . way cool.

[mikepukmel]

When a tube amp fully oscillates plate voltage may exceed 2kV peak, which might be over the limit of even an 100x probe.
As oscillation will show at the output there's no need to scope the plates.

Thanks! So, once you see that there is oscillation problem, if you're probing say at the speaker terminals, how do you narrow down the source of the problem from there?

[Helmholtz]

Thanks! So, once you see that there is oscillation problem, if you're probing say at the speaker terminals, how do you narrow down the source of the problem from there?

Oscillation is a kind of noise (i.e. an unwanted signal) even if inaudible.
So troubleshooting is similar to other noise problems: Effect of controls, isolating preamp and power amp problems, scoping preamp plates and supply voltages, pulling tubes, adding caps, chopsticking wires, checking ground connections, etc.
Might be worth an own thread.

[mikepukmel]

Oscillation is a kind of noise (i.e. an unwanted signal) even if inaudible.
So troubleshooting is similar to other noise problems: Effect of controls, isolating preamp and power amp problems, scoping preamp plates and supply voltages, pulling tubes, adding caps, chopsticking wires, checking ground connections, etc.
Might be worth an own thread.

Re "Might be worth an own thread" that would be fantastic!