Tektronix 2205

[riz]

There is one available locally for $125. Is this a good deal, and, more importantly, a decent scope for a guy whose interests lead him to lurk on this forum? I want to go to the next level and start using a scope for basic tube amp analysis.

[g1]

From what I can tell, that is a 20Mhz dual-trace scope, should be more than enough for analog audio applications. Price seems good if it is in good shape. See what kind of probes are included, you will want two 10:1 probes (or switchable between 10:1 and 1:1). If no probes are included, price them out first.

[km6xz]

If it is in good condition and has 10:1 probes with it, grab it. For high voltages 100:1 probes come in handy but overall it is a reliable 20Mhz dual trace scope that will last for years. It does not have calibrated delay sweep like the 2236 but you probably will never need that. good probes are expensive so without probes the value is less certain. Cheap Chinese probes might only be $20 each on eBay but they are not going to hold up or handle the hight voltage as well as the Tektronix probes.

Edit: By the way, it cost about $1500 new so it is inexpensive as Tektronix goes but a quality piece of gear by any measure.

[riz]

Thanks for the replies! It comes with one 10x probe, so I think I will pursue it. Ad says it is very clean.

[km6xz]

If the trace is bright go for it.

Now, look for an old VTVM, $20 max. Vacuum Tube Volt Meters are well suited to amp work, analog reading is better for seeing trends and null points, and do not require constant calculating differences between current and the reading .5 a second before in varying values. The most important reason, other than price is that high voltage will not kill them as it will DMMs. Draw an arc and that do-all digital box is toast.

A signal generator is a must. Sine wave only is enough but ringing and loss at high or low freq is best seen with Square waves without sweeping.
An excellent service tool is the Heath IG-5218. It does all you could ask for in a service setting: accurate metered output, accurate step attenuator, accurate decade fixed frequency selection, low distortion, separate square wave output terminals and attenuator. eBay or a ham swap meet should have them. It is so accurate that no independent monitoring is needed for output level or frequency. Very few much more expensive generators have that.

A must have for any new builds or repair....a Variac. It will pay for itself in the first amp that you DIDN'T blow up not having one. Metered lab type are pretty expensive, with accurate AC current and voltage meters, cheaper ones can had as general purpose autotransformers which often have voltmeters built in. You need both voltage out and current out. So get one as cheap as possible and build a case for it and install your own panel meters and save the difference between $100 for a bare autotransformer in a case and a $500 lab type with both current and voltage meters. After using one with current meter built in(analog, not digital because you need instant recognition of jumps or trends). create a log book and record the idle current of every amp you work on. Later if you want to see if an amp is setup close, just check it at idle and compare. If the current from the mains has not changed, the operating point of the output tubes are pretty close to what they were, so you don't have to pull it out of the cabinet.

The last item that makes a competent basic bench is a set of power resistors for use as dummy loads. Non-reactive are best but any resistance wire high power resistors will work since even the most inductive will still have lower inductive reactance of a real speaker. Don't put speakers on any suspect amp until you know for sure it is stable and has no possible defect that could destroy a speaker.

Other little gadets to look for: an old electric tooth brush for tracking down intermittent connections. A heat gun. A big soldering gun for soldering to chassis when needed. A CRL bridge, preferably an old one hat uses higher voltage and power factor. Don't bother with a tube tester, your scope, VTVM and DVM will tell you a lot more about a tube's functionality. A very well insulated handle pair of needle nose pliers. When working on crackle noise problems in a vintage amp, a light squeeze on carbon composition plate resistors will tell you instantly if that is the one. I have a pair of ceramic tongs used in chemical labs for reaching into strong chemical solutions. Works great for putting compression pressure on comp resistors while listening to the noise. Saves hours of cut and try. A metal sewing thimble is a great tool for applying light tapping to potentially microphonic tubes. Most use a wooden handle screw driver but a thimble on your finger is much more subtle and you can control very finely the force of the impact.
That is enough, I am going to bed....
Good luck