Here is a gathering of tools that every serious tube jockey should have in his arsenal. Some may seem down-right strange, and others may not seem all that important. Believe me; each tool represented in the ‘Lesson’ has helped me more than once. Keep in mind that acquiring hand tools may seem like a never-ending adventure, and it is. Also, owning all of the tools in the world will not make you a better technician. What these tools will do, however, is make you and the repair look more professional. In no particular order, let’s look at a few essential tools of the trade! I want to begin with what the essential repair bench should have!
If this isn’t already on your bench, get one ASAP!
Starting off the list, is the venerable VTVM. If you haven’t already done so, please check out the whole ‘Lesson’ devoted entirely to this old favorite of many an old time tube jockey. You can see why this is a valuable ‘tool’, simply by CLICKING HERE. To recap; the analog meter movement and high input impedance make it a priceless piece of test equipment. You can repair Tremolo circuits, check coupling capacitors for leakage, and test variable potentiometers, all with a VTVM. None of this is possible with even the most expensive Fluke digital multimeter.
Now that you know about the VTVM, let’s talk about signal generators. You should have one, unless you graduated from the same ‘school-of-hard-knocks’ as the modern guru. These people tend to be self-taught, which really makes for a boring school reunion. Anyway, having a signal to feed into your amplifier is essential, so let’s discuss a few ways to accomplish this.
Having at least one good signal generator is a must.
Signal generators come in all shapes and sizes. We would be best served with a dedicated ‘audio’ unit, also advertised as a sine/square-wave generator. RF generators used in radio repair work have a dedicated 400Hz sine-wave output, which could be used in a pinch, but is not advised. This is because the sine-wave is usually not very ‘pure’, which is really what we would like to use. Older vacuum-tube audio generators are fine, as long as we restore them to tip-top shape. Modern solid-state units can be much better, with accuracy and purity to the waveform that tube units cannot duplicate. There are the ubiquitous Heathkit and Eico units out there, and there are the ‘Cadillac’ units from Hewlett Packard. Use what you can afford for now, and keep your eyes open for ‘better’ generators as you learn more about them. Having a collection of test equipment is not as silly as it may sound, especially if your one-and-only unit should ‘go down’ at an inconvenient time. And there is really no convenient time for gear to break down, is there? I hope to have a whole ‘Lesson’ dedicated to the signal generator, but for now, we will need a way to hook up the generator to our amplifier. The banana jacks used on all old equipment are always spaced 3/4″ apart. Adaptors are made to convert the banana jacks to other useful connectors. Look into a Pomona catalog, and discover these adaptors for yourself for now.
You do not want to hear your Twin Reverb blasting a 1KHz signal into the actual speakers, so using a dummy load is advisable. Dummy loads also come in all shapes and sizes. Below are just a couple of examples.
A complete test/repair will have a variety of dummy load resistors.
You should have a variety of dummy loads; at least one for each nominal speaker impedance. Remember that any speaker impedance rating is ‘nominal’ or average. Therefore, it is perfectly acceptable to use a 7.5-ohm dummy load, or even a 10-ohm dummy load to test your Deluxe Reverb. There are websites explaining ‘advanced’ dummy loads, that will have a frequency response similar to a typical speaker. You are free to build these, but a simple ‘non-inductive’ resistor will more than suffice. The ‘advanced’ speaker emulators are used for testing bandwidth, harmonic distortion, and such. We are only interested in simple troubleshooting and output tube biasing for now. Use a wattage rating of at least 150-watts; this will help them run at less than egg-frying temperatures. Mount them to a sturdy platform, and give them adequate ventilation. Having a fan nearby is not a bad idea.
Now that you have a signal generator feeding your amplifier, and the amplifier feeding a dummy load, what’s next? You could measure the AC voltage with your VTVM, and survive for about a week. A better tool to use is an oscilloscope. I do have a whole ‘Lesson’ dedicated to the oscilloscope, but I will offer a few pointers here on what to keep in mind.
- Older tube oscilloscopes may seem cool, but are full of surprises. Get one if it is very inexpensive, but be prepared to use it only as a beginner’s oscilloscope.
- Higher bandwidth oscilloscopes generally have a higher accelerating potential on the CRT. This means a brighter, clearer, and more accurate trace.
- Make sure to have the appropriate manual. It will come in handy at some time or another.
- Use the proper probes. Your displayed signal will be cleaner and more accurate.
There is no need to rush out and purchase a multi-thousand dollar 500MHz oscilloscope. But do try and get one with at least a 20MHz bandwidth. The dedicated ‘Lesson’ will explain this in a lot more detail. But for now; these newer oscilloscopes will have a brighter, more accurate trace, which will make your work go a lot smoother. Trust me. You can often find these oscilloscopes at your local Ham Radio Swap Meet, and at a fraction of eBay prices. Visit the oscilloscope ‘Lesson’ by here.
In a round-about way, you can test coupling capacitors for leakage with an oscilloscope! However, a much ‘better’ method uses a dedicated piece of test equipment. Every ‘guru’, without exception, advises you to change your filter capacitors ‘just because they are forty years old’. This isn’t always necessary, but the sad part is not one guru has written about testing the coupling capacitors for leakage! These capacitors will often leak, and you should have a method for checking this. You can use your VTVM (as outlined in the VTVM ‘Lesson’), or splurge and purchase one of the Sprague capacitor ‘analyzers’. My Sprague TO-5 is seen below.
Sprague capacitor ‘analyzer’ has many uses for the guitar amplifier technician.
The 6E5/1629 ‘cat’s eye’ tube seen in the upper left corner ‘opens up’ when the balanced bridge indicates the value of the capacitor being measured. This in and of itself is no big deal, because many fancier digital meters have a dedicated capacitance range. However, the Sprague units will have a ‘Power Factor’ dial (seen just under the 1629 indicator tube and to the right of the binding posts) that takes into account the ESR of electrolytic capacitors. Also, your DVM uses a piddly 9-volt battery, while the Sprague unit utilizes a high-voltage power supply. You can also measure actual leakage current of electrolytics, and the ‘pièce de résistance’; the leakage test (or insulation resistance test) of electrostatic capacitors (paper, mica, ceramic, etc.) at operating voltages (up to 600VDC, anyway) is an invaluable servicing aid. The TO-5 (as seen above) will also measure transformer turns ratios, making it even more handy than the latest Sprague capacitor analyzer (the TO-6) that somehow omits this ‘feature’. You can easily ‘get away’ with using the TO-4, but my advice is to use either the TO-5 or the TO-6. These are wonderful units to add to your bench. Many of the ‘classic’ tube oscilloscopes I am repairing for the biasing ‘Lesson’ had leaky coupling capacitors inside. These capacitors measured ‘right on’ for capacitance value, but my TO-5 detected leakage, and pointed out the capacitors I needed to change. The oscilloscopes ended up working fairly decently, thanks to my TO-5. A similar unit is available from Heathkit, but is simply not built to the same quality standards as the Sprague unit. There is a Heathkit IT-11 and an IT-28. I have the IT-11, and it can be seen below.
The Heathkit unit also ‘features’ a high voltage power supply, but I just seem to prefer my Sprague TO-5. The first leaky coupling capacitor you find in your tweed Fender amplifier will convince you this is a worthwhile investment. After you remove the offending capacitor, check it for capacitance value in any fancy Fluke digital meter that has the capability to measure capacitance. Then sell that paper capacitor on eBay, and recoup some of the money you spent on your capacitor analyzer. Just don’t breathe a word to anyone about how leaky the capacitor is.
Many gurus babble incessantly about using a VariacTM to help Marshall users achieve ‘the tone’. The VariacTM was never designed to use as a tone-shaping tool, but as a service aid. There is a piece of test equipment to help you do one better than a ‘plain’ VariacTM, and I will introduce you to it. The Sencore Powerite PR57 consists of a variable AC power supply, similar to a VariacTM, plus many extra features. It really is a very useful tool for the designer/builder or service technician.
Sencore Powerite PR57 is a very useful tool.
What is so special about the Sencore Powerite? It consists of an isolated variable AC power supply, up to 140VAC, and metered! You can meter the current consumption. This is handy when determining what size of fuse is correct for a repair or an amplifier you are building from scratch. Also, abnormally high current consumption indicates a problem that may not be immediately recognized, but could lead to serious repercussions soon enough. Is your Marshall blowing fuses intermittently? Monitor the current consumption as you play! You can measure leakage current to the chassis. This is an invaluable test when servicing that old amplifier that still utilizes a line bypass capacitor.
The Sencore Powerite was an expensive piece of equipment it its day, and still commands a fair price on eBay today. You can do a lot of the same measurements with a good DVM, but to have everything in one unit is a definite plus. The Sencore Powerite takes up a fair bit of real estate on your bench, but I still really enjoy using it.
You need to use the proper pliers to remove and replace the AC cord strain relief clip.
Next up is the one tool no one else seems to have heard of. If your amplifier uses an AC cord, you really should have this tool in your toolbox. Although manufactured by a few people, the most popular trade name is Heyco. These pliers are officially called strain relief pliers, because they are used to insert or remove the strain relief clip. We should know that the chassis is punched out with what is referred to as a ‘D’ knockout. This is easy to understand why; the shape is like a capital letter ‘D’. This is necessary so the cord cannot twist around in the chassis. The other style you may see is the ‘Double D’. These ideas are crudely represented below.
The shape of the ‘knockout’ prevents the AC cord from spinning around.
I have no idea why they would have two different shapes to the knockout, although perhaps the ‘Double D’ is even less prone to ‘spinning’. Regardless, the Heyco clip itself is a ‘Double D’ shape, and fits either knockout hole. Without the proper pliers, ‘hacks’ resort to using Channel-Lock pliers (shudder). Below is a representative picture of the ‘proper’ pliers in action.
The ‘proper’ Heyco pliers make for a much more professional job.
There is a ‘repair’ video available that demonstrates the complete ‘unprofessional’ way to replace the AC cord on your vintage amplifier. Technical problems aside, the declaration is that replacing a thinner gauge 2-conductor plug with a heavier gauge 3-conductor plug forces the unprofessional approach to the repair. Bullshit. What you should learn, if you are not too busy producing such garbage, is as follows.
- There are a plethora of 3-conductor AC cords available. You can just as easily find an 18-3 AC cord as you can find a 14-3 AC cord. Seek and ye shall find.
- There is also a variety of Heyco clips available. If your new AC cord does not fit in the old Heyco clip, try a bigger Heyco clip! They are cheap enough that it pays to have a variety on hand. You will need to enlarge the ‘D’ knockout, and this can be done carefully be hand, or…..
- If all else fails, using a Greenlee knockout punch (see below) to enlarge the hole is not half as bad as doing the repair half-assed. They are available in different sizes, and should be owned by the serious tube amplifier technician.
Next up, we’ll look at the aforementioned chassis punches. They were marketed by such names as Olson, Pioneer, Walsco, and the very common Greenlee. In Great Britain you can buy Q-Max. All are virtually the same. Below is a sample of a Greenlee chassis punch.
You should have at least a few popular Greenlee punches in your toolbox.
The concept is simple. You drill a 3/8″ hole (5/16″ with a Q-Max), and with a ratchet or crescent wrench, draw the cutter into the die. The hole is cut cleanly, and accurately. Far superior than using drill bits or a hand file, chassis punches come in a wide variety of sizes. Your toolbox should have at least the following punch sizes.
- ¾” for 9-pin tube sockets (12AX7).
- ½” for common on/off switches, indicator lights, and fuse holders.
- 1″ for common octal sockets (6SN7, 6L6).
- 1 ⅛” for larger octal sockets, and for Mallory FP filter capacitor ‘cans’.
The punches come in a variety of shapes, as well, and below left is a 1/2″ ‘D’ punch from Pioneer. Later on, Pioneer and Walsco somehow became a ‘couple’, and the result was a Walsco-Pioneer chassis punch, as seen below right. I have a feeling Walsco was more of a distributor than a manufacturer, but I can’t find any information to either substantiate or discredit that theory.
1/2″ ‘D’ punch is essential for smaller AC cord Heyco clips.
I don’t know the reasoning why this next practice was stopped, but here goes. Older punches (especially Pioneer) came in a tin can filled with a light oil. The theory was to always keep the punch lubricated, even when not in use. Perhaps it became too messy, but I like the idea. Another thought is that the oil eventually became ‘gummed up’ with metal filings, unless you cleaned off your punch before returning it to the can. Below is an older Pioneer square chassis punch. These are great for starting the hole for a ‘Z’ mount transformer (the power transformer in most Blackface and Silverface Fender amplifiers). If you can find a 3/8″ square punch, they are the only thing to use if you plan to use ‘cage nuts’ (seen holding a Marshall chassis in the cabinet). To view some Vintage Advertising for chassis punches, CLICK HERE.
Older Pioneer punches were kept in a light oil bath.
Next up we’ll look at the ‘poor man’s Greenlee’, or the Unibit. It has many names, depending on the manufacturer (Unibit is the brand name of the ‘original’, as far as I know). Most I have seen were meant for a 3/8″ chuck, and they come in various ‘sizes’. Below is one example.
‘Stepper’ drill bit is very useful if chassis punches are not available.
The typical ‘small’ bit will allow you to drill ‘in steps’ from 1/8″ to 1/2″, and a ‘large’ unit will drill from 3/8″ to 7/8″ in 10 equal incremental sizes. The only ‘drawback’ is they do not allow for drilling into thick material. If you try to drill through 14 gauge steel (as an example), the ‘front’ of the hole may be the desired 1/2″, but the ‘back’ of the hole will only be 7/16″. The solution? I use a large hand-reamer (another essential tool), and ‘clean up’ the drilled hole (from both ‘sides’) to a uniform size and smoothness. In these cases, I also usually drill my hole 1/16″ smaller than the desired size, and clean it out the rest of the way. These bits work well, but do keep them (as well as any chassis punch) lubricated with something; even WD40 in a pinch.
Another ‘must have’ item on any good technician’s workbench is the substitution box. These are very useful for trying different resistor and capacitor values in prototype work and mod ideas. You can figure out, without tedious trial-and-error, if that extra gain stage sounds best with a 1.5K cathode resistor or a 2.2K cathode resistor. Is your homebrew ‘5E3’ motorboating? Try different coupling capacitors while you watch your oscilloscope (you do have an oscilloscope, don’t you?). This is why I have a dozen or so decade boxes around; you can hook one up to different points and experiment. This is what the audio business is all about. Below is one of many resistor decade boxes I have.
Basic resistor decade box helps when designing or modifying tube circuits.
Some decade boxes, like one I have from Radio Shack, have resistors on one rotary switch, and capacitors on a second rotary switch. This is the uncommon version; usually your decade box will have ‘either-or’, and the SPDT switch on the bottom can possibly select either from the ‘Low’ range (usually 15-ohms to 10K) or the ‘High’ range (usually from 15K to 10M). You can make up your own decade box easy enough. Some scenarios present a problem. High powered resistor decade boxes aren’t easy to find, so I make one up for Class A amplifiers I am building. If you follow this route, use a large enclosure (the resistors will throw off some serious heat), and drilling a few vent holes is advised. Typical values you can use are 47, 56, 68, 100, 125, 150, 180, 200, and 250. Use 10-watt resistors at least, or if your enclosure has the room, use 25-watt resistors. If you have a 12-position switch, think up a few combinations of resistors in series to get odd values between 200 and 250-ohms. If you only have a 6-position switch, take three possible cathode resistor values from say an AC15 (100, 150, and 180-ohms), and three possible values for biasing an AC30 (47, 56, and 68-ohms). That will make a very usable decade box, and as your needs change, you can always build another box. I also have decade boxes built with zener diodes (to try diode clipping for possible mod work). This box also has a second rotary switch to select ‘bounding’ resistors. The possibilities are endless, so it should be clear that you cannot be doing your best work without a decade box.
Well, there you have it. I have said you need most of the ‘tools’ here, and in a way you do. Many good technicians do not have elaborate test equipment, relying on years of experience to tell them what is happening in a given circuit by observing what is going on. There are of course the essential tools; soldering station, certain hand tools, and what not. The tools presented here are of the uncommon variety, or at least to most ‘young’ technicians I know.