Oscope ?

[gtrplayr1976]

What are you most likely to try fixing?

Mostly amplifiers, and hopefully mostly tube amps. Right now it's just a hobby ,and only for friends.

[Leo_Gnardo]

Lots of good info here, but now I'm thinking I may be better off without one.

Whaaaaat? Crikeys, how could you NOT want to see what's going on? There's no extra points for flailing around with circuits, not knowing what's going on in them, all revealed by the mighty o-scope. And yes, a sig gen would be an excellent adjunct to your scope. The ability to see ultrasonic oscillation (parasitics, "snivets") and flyback spikes on the output = priceless!

I like Salvarsan's suggestion of the old Tek mini 122. Don't know how much one would cost or where besides ebay to find one, but it's a good little scope. One fine day in 1975 I had the (highly educational) experience of working with sound reinforcement legend Don Pearson, and he had one. He told me it was only $1200 (in 1975 dollars!) so if you can pick one up now for cheap, that would be a hit. It's a cute li'l thing and won't eat up acres of space on your workbench. In fact I'll be looking for one myself. Thanks for the reminder Salvarsan!

[Tom Phillips]

...Do I really need one ? Someone could sell me a used one ,and I would have little idea if it actually worked correctly. I could learn to use one I would just need to know where to hook it up , what to set it too, and what the wave looks like...

That’s insightful of you. If your goal is to set up an electronics work bench and keep using it then an oscilloscope is a basic and essential piece of equipment. If you just want to complete the repair of the Fender Silverface Twin then maybe you don’t want to own a scope of your own. As you said, learning to use it is a doable but not insignificant task.

...Signal generator sounds like it may be more useful?

That depends on what you intend to do with it. For the service bench they go together like a fork and a knife at a steak dinner. You could be by with only one or the other (or without either) but it wouldn’t be a pretty sight.

[gtrplayr1976]

Whaaaaat? Crikeys, how could you NOT want to see what's going on? There's no extra points for flailing around with circuits, not knowing what's going on in them, all revealed by the mighty o-scope. And yes, a sig gen would be an excellent adjunct to your scope. The ability to see ultrasonic oscillation (parasitics, "snivets") and flyback spikes on the output = priceless!

I like Salvarsan's suggestion of the old Tek mini 122. Don't know how much one would cost or where besides ebay to find one, but it's a good little scope. One fine day in 1975 I had the (highly educational) experience of working with sound reinforcement legend Don Pearson, and he had one. He told me it was only $1200 (in 1975 dollars!) so if you can pick one up now for cheap, that would be a hit. It's a cute li'l thing and won't eat up acres of space on your workbench. In fact I'll be looking for one myself. Thanks for the reminder Salvarsan!

I'll probably get one if I find one I know works ,and I can afford it . I like toys. I still have an old Eico tube tester I haven't fooled with yet.

[gtrplayr1976]

That’s insightful of you. If your goal is to set up an electronics work bench and keep using it then an oscilloscope is a basic and essential piece of equipment. If you just want to complete the repair of the Fender Silverface Twin then maybe you don’t want to own a scope of your own. As you said, learning to use it is a doable but not insignificant task.

That depends on what you intend to do with it. For the service bench they go together like a fork and a knife at a steak dinner. You could be by with only one or the other (or without either) but it wouldn’t be a pretty sight.

It wouldn't be for just the SF Twin. I have a few myself ,and I'm now building some. I will continue to help my friends if I'm capable ,so it looks like I'll be finding some test equipment.

[km6xz]

Leo is right, how could you prefer not knowing what is going in the circuit under test? Tinkering blind with your own gear is one thing but accepting a repair from friends is pushing the risk factor if proper test instruments are not available.
At a bare minimum you will need dummy loads, a signal generator, variac and scope. You already have a DMM. All that can be had for $200 or less. Getting lower end gear will not save money since quality lab type gear used is cheap, but will make it harder to tell what a circuit is doing.

There are lots of books and videos on use of all this gear so don't be intimidated by not remembering.

[gtrplayr1976]

Leo is right, how could you prefer not knowing what is going in the circuit under test? Tinkering blind with your own gear is one thing but accepting a repair from friends is pushing the risk factor if proper test instruments are not available.
At a bare minimum you will need dummy loads, a signal generator, variac and scope. You already have a DMM. All that can be had for $200 or less. Getting lower end gear will not save money since quality lab type gear used is cheap, but will make it harder to tell what a circuit is doing.

There are lots of books and videos on use of all this gear so don't be intimidated by not remembering.

I agree. So what kind of signal generator, dummy load ect...what it the variac used for ? Is it like the light bulb limiter ?

[km6xz]

Dummy loads can be wire wound power resistors for $5-10 each that will act like a high power speaker so you do not have to risk an expensive speaker when testing a malfunctioning amplifier. A variac is actually a trade name that has become generic to mean a variable power transformer. It allows watching a circuit's behavior while slowing increasing the AC mains voltage from 0 to all the way up to 130V or so. It allows you to fire up an amplifier that has a high current drain due to a defect to help isolate the problem without causing more damage. The beauty of a variac is you can get current measurements and be able to stop bringing the voltage up when you see the circuit behaving badly. For example you plug in a tube amp and bring the voltage up slowly by turning the big variac know and notice that the current draw is higher than expected for a low main voltage, say, 10 volts. Nothing is hurt by having that higher current drawn because little power is being dissipated. You can do preliminary tests to isolate the source of the higher current being drawn. It might be a shorted filter cap or rectifier or whatever.
Reworking old amps with old filter capacitors which have not been used for a long time is best down with a variac to bring up the voltage slowly to let a possibly depleted oxide layer reform inside the caps to allow them to take full voltage later.
A light bulb limiter is a light bulb in series with the load(amp under test) so the maximum current that can flow is the addition of amp load added to that of the light bulb. If the amp is shorted and having a very low resistance, the bulb will still offer its normal current drain as if the amp was not there. It is a safety item that limits total current that the combination can draw to that of a bulb alone. A variac is a variable voltage source that can supply a lot of current if the amp was shorted but watching the current drain while increasing the mains voltage above 0 volts, you have more control and data about what is happening.
Variacs are used in every repair shop, and shops that repair or calibrate any electronics that is AC mains powered. They used to be quite expensive for high power metered variacs but there are low cost Chinese made variacs that are available for $50 or less that can supply enough current for any guitar amp. These cheaper ones come with a volt meter but you have to add a current meter. You need both. I bought a 4 digit panel ac ammeter plus 50 amp shunt for $12 including shipping.
Another use of the variac is simply to control the AC mains voltage input to an amp to the level the spec sheet specifies by the amp manufacturer that was used for the amp when they adjust bias or display typical values on a schematic. If your home is 130VAC on the day you are biasing an amp, you can set it down to 120 that the manufacturer specified as their testing voltage. Power of 130 volts will change the readings.

[gtrplayr1976]

Dummy loads can be wire wound power resistors for $5-10 each that will act like a high power speaker so you do not have to risk an expensive speaker when testing a malfunctioning amplifier. A variac is actually a trade name that has become generic to mean a variable power transformer. It allows watching a circuit's behavior while slowing increasing the AC mains voltage from 0 to all the way up to 130V or so. It allows you to fire up an amplifier that has a high current drain due to a defect to help isolate the problem without causing more damage. The beauty of a variac is you can get current measurements and be able to stop bringing the voltage up when you see the circuit behaving badly. For example you plug in a tube amp and bring the voltage up slowly by turning the big variac know and notice that the current draw is higher than expected for a low main voltage, say, 10 volts. Nothing is hurt by having that higher current drawn because little power is being dissipated. You can do preliminary tests to isolate the source of the higher current being drawn. It might be a shorted filter cap or rectifier or whatever.
Reworking old amps with old filter capacitors which have not been used for a long time is best down with a variac to bring up the voltage slowly to let a possibly depleted oxide layer reform inside the caps to allow them to take full voltage later.
A light bulb limiter is a light bulb in series with the load(amp under test) so the maximum current that can flow is the addition of amp load added to that of the light bulb. If the amp is shorted and having a very low resistance, the bulb will still offer its normal current drain as if the amp was not there. It is a safety item that limits total current that the combination can draw to that of a bulb alone. A variac is a variable voltage source that can supply a lot of current if the amp was shorted but watching the current drain while increasing the mains voltage above 0 volts, you have more control and data about what is happening.
Variacs are used in every repair shop, and shops that repair or calibrate any electronics that is AC mains powered. They used to be quite expensive for high power metered variacs but there are low cost Chinese made variacs that are available for $50 or less that can supply enough current for any guitar amp. These cheaper ones come with a volt meter but you have to add a current meter. You need both. I bought a 4 digit panel ac ammeter plus 50 amp shunt for $12 including shipping.
Another use of the variac is simply to control the AC mains voltage input to an amp to the level the spec sheet specifies by the amp manufacturer that was used for the amp when they adjust bias or display typical values on a schematic. If your home is 130VAC on the day you are biasing an amp, you can set it down to 120 that the manufacturer specified as their testing voltage. Power of 130 volts will change the readings.

Would I need different resistor values as far as wattage ,and ohms ?

[salvarsan]

Would I need different resistor values as far as wattage ,and ohms ?

It helps.

Buying 60 watt resistors can be expensive.

I scavenged a heating element from a thrift store space heater.
The amp sees a well-behaved load and the 'heating element' barely gets warm.

1500 watts at 120 VAC implies a 12.5 ohm resistance, so tap (not cut) it at your target resistances of 8, 4, and 2 ohms.

[Leo_Gnardo]

Would I need different resistor values as far as wattage ,and ohms ?

Scout those surplus dealers (Fair f'rinstance) & ebay too. My faves are the big gold Dales, that have a flat side and tabs that can be bolted down. (Some other manuf's make similar resistors so no need to stick to one brand.) Buy bigger wattage than you think you'll need and bolt 'em to rack panels or castoff heat sinks then you'll never wreck a resistor or run them so hot they'll singe or set alight anything nearby. Some jacks (quarter inch or banana) on panels & matching plugs on speaker wires & bob's yer uncle.

I also like Salvarsan's solution (AGAIN!). Salvarsan, you da man! Yes you could clamp wires to a heating element to create a multi choice load.

By my calculation, P=Vsquared/R you could use a 900W heater (intended for 120VAC use) for a 16 ohm load. If you can find one.... 1000W I suppose would be "close enough for rock and roll." Good luck finding a guitar amp that will challenge a 900-1000W heater. Such a rig will likely never break, and will deliver a little warmup to your shop room on a chilly day.

[km6xz]

You can use one large wattage tapped resistor or a couple smaller resistors mounted on a wood base that allows jumpering to get 2, 4, 8 or 16 ohms. I use some 300 watt 10 ohm resistors with taps set at 8 ohms which can be connected in series for 16 ohms or parallel for 4 ohms. Wire wound resistors have some inductance but for that is less than a voice coil so power measurements are going to be accurate without calculating for inductive reactance.
You only need one rated at least as much in dissipation as the amp is.

25 watt ceramic resistors are less the $2 or the last 300 watt ceramic tubular resistors were $15 each. Ceramic box resistors are in a metal heatsink and need to be mounted on a metal panel or heatsink to get the power rating specified.

There are non-reactive resistors are more expensive and really not needed.

Do not use a light bulb or heating element since they change in resistance with heat. Nichrome has a negative temperature coefficient so it is not the value calculated until at operating temperature, which is not going to get to with only 10-30 volts that an amp will put out.

[Tom Phillips]

Tom,
I suggest you search "Oscilloscope Tutorial" on YouTube.com and watch some selections. Not only will you learn about how to use an O'scope but you will then know how to evaluate a used one. Some tutorials even show testing of actual amplifiers thus showing various waveform shapes and explaining how to interpret them.
Cheers,
Tom

[gtrplayr1976]

Would this be enough as a signal generator , or do I need something with more range ?

[JoeM]

If that generator works, it's a nice unit for audio.