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What you should know about Tube Testers

Tube tester 4 (002)

I would like to talk a little bit about tube testers and explain why we do not use one here at the shop due to their unreliability and limited usefulness in trouble shooting tube equipment problems.

The absolute best test for a tube is testing it in the actual piece of equipment the tube will be used.

The field service tube tester was developed for the telephone, radio-TV, communications and industrial electronics industries to provide basic tube testing capabilities to help technicians and engineers locate defective tubes. The early testers only tested cathode emission. They worked fine in the early days of the industry before the circuits and tubes became more sophisticated.

Over the years there were many approaches to tube tester design. The features, accuracy, and the tests available differ widely by make and model. Some manufacturers wanted to focus on simple low cost units to find weak or just bad tubes. In all cases tube testers were at best a set of balances and trade-offs in the evaluation of the tubes vs. the cost of the tester.

Even the best service testers made trade-offs in design to allow for simplicity of use, the ability to test many different types of tubes, features, and accuracy, balanced by the price of the tester. The service tester was a piece of test equipment to be used by professional engineers and electronic technicians to aid in the process of repairing tube electronic equipment.

Testers were designed with the understanding that those using them were knowledgeable in tube operation, the equipment the tube was used in, and how the tester worked in evaluating tubes. This is more often not the case today when someone acquires a tube tester and attempts to use it.

There are many types of tube testers and most of them date from the early 1950s to late 1960s. The usefulness of each make and model will vary with the type of equipment you are servicing. Considering the fact that these testers are vintage, the age, actual usage, and condition of the tester will have a big impact of how useful it is in testing tubes today. Over the years, moisture, dust, and dirt can be absorbed by tube sockets, switch wafers, and wiring causing many leakage paths that would contribute to false readings. Calibration of the tester is another factor that will affect accuracy and usefulness of the tester. Many simple testers do not have any provisions to make internal calibration adjustments.

Tube testers fall into two main categories- emission testers and mutual conductance testers. Other types of testers would include laboratory and special purpose testers.

Emission testers are the most common testers. Popular brand names include Heathkit, B&K, and Sencore. There are a few problems using any emission tester. The emission test basically connects all of the elements of the tube together, except the cathode, and tests it as a diode for cathode emission. The meter scale is most commonly labeled “Bad-?-Good”. Most tubes depend on the control of the electron flow rather than the amount of electron flow. This important shortcoming means that emission testers will miss the output tube with a cathode ‘hot spot’ which is hidden when the grid is tied to the plate. When a cathode has a ‘hot spot’ most of the emission current stems from this restricted area on the cathode. A control grid does not have the current control action under this condition and when the tube is placed in an amplifier, it draws excessive current and goes into thermal runaway.

Cheaper emission testers used a low current power supply. A tube tested on a tester with a low current power supply may have enough emission to yield a “Good” reading on the meter, but when the tube is placed in an amplifier that is drawing 150ma to 200ma, the tube may not be able to provide sufficient current for operating at peak efficiency.

Emission testers frequently operate at low voltages, with some testers as low as 30 volts. These testers only provide a static test of the tube that does not represent the conditions the tube will be subject to in an actual circuit in which it would be used.

Some emission testers apply too much current to small signal tubes and when the emission test button is held down on the tester for an extended period of time, the cathode is stripped rendering the tube useless. Another shortcoming of many emission testers is the leakage test. In testers where all of the elements are connected together all of the leakage paths are in measured in parallel, which could cause perfectly functioning tube to fail the leakage test. Some testers were advertised as having the most sensitive leakage test in the industry. This many times was more stringent than circuit application requirements and again causing many functional tubes to fail testing.

Most mutual conductance testers work by applying an AC voltage to the control grid of the tube, while maintaining DC voltages on the plate and screen grid. Most of these testers use the 60Hz line frequency transformer coupled as the input test signal. The cathode can be biased with a small positive DC voltage, or the control grid can have a small negative DC voltage. This setup actually dynamically measures the AC gain of the tube, rather than the actual transconductance.

A variation of this type of tester is the “grid shift” approach. This uses a DC voltage on the control grid that is shifted and the change in plate current is measured. Tube theory says that transconductance is the ratio of change in grid potential to the change in plate current.

Many believe the AC (dynamic) method is superior because it reflects true RMS values, regardless of waveform distortion. If the line voltage is not a true sine wave, which is common in heavy industrialized areas, the dynamic tester will still indicate correct values. Hickok held the patents for and made most of the mutual conductance testers.

Many low priced “conductance” testers used AC voltage on all of the elements can actually damage high transconductance tubes. The heavy rectification current caused by driving the control grid positive can overheat the grid wires, resulting in critical spacing to be upset. The tube then actually loses transconductance.

There are special purpose tube testers that are usually special test fixtures that are used to test a specific tube for a specific purpose. These special test fixtures are usually not available commercially and are usually built by the manufacturer or individual who intends to use them. These are usually found on assembly lines for grading tubes to be used in a certain application.

As was stated at the beginning of this article, the best test for a tube is to test it in the equipment in which it will be used. If you intend to acquire a tube tester, be aware of its limitations and realize that some good tubes will test bad and some bad tubes will test good under certain conditions.

When comparing labor rates are you comparing apples to apples, or apples to Kumquats?


We charge $77.50 per hour here, which for the level of experience, the efficient manner in which we are able complete the repair, and the quality of the repair work we are able to provide in my opinion the best deal in town. 

What is the effective labor rate you are paying, that lower price that other shop quoted you sure sounded good .. But is it really? 

A lower level of experience may actually mean you pay more for your repair.

An experienced well equipped shop may complete a repair in an hours’ time that may take a less experience poorer equipped shop and hour and a half to accomplish. So if the less experienced shop is charging 60.00 an hour you are in effect paying them 90.00 per hour.

If a shop is not factory authorized for most brands they will not have access to all the factory service materials.

This will again cause them to spend more time than needed to repair the unit and again increase the effective labor rate you are paying.

Sadly, some shops quote a lower labor rate and a quick turnaround time to get you in the door, but then actually charge a higher rate, and then take longer than quoted, sometimes much longer to complete your repair.

I was on good terms with a tech that used to work out of Showcase Music, and he freely admitted that he engaged in this practice. Many shops will also give low ball estimated repair cost on the phone just to get you in the door, but then will find a reason to charge you more, sometimes much more.

I hope you found this information useful and encourage you stop in sometime and check us out.


Randy Morgan


Speaker phasing

This is a simple, but important topic.


All speakers as most of you know have a positive and a negative terminal. If these terminals are hooked up backwards the speaker will still work, and if only one speaker is used you will probably not even notice the speaker is out of phase.

The problem comes into play when using more than one speaker cabinet, or multiple speakers within one cabinet.

If one of the two speakers is hooked up backwards then the two speakers will be out of phase and when played together they will sound very thin as they are essentially cancelling each other out. This is because one cone is moving out while at the same instant the other is moving in.

In cabinets with multiple speakers it is also important if you have to replace a speaker that you match it up as closely as possible to the original, or replace both at the same time. The reason is if the two speakers have different efficiencies and cone materials then the cones will move at different rates which creates another type of phasing issue.

When hooking speakers up you cannot always rely on the markings on the speakers. I have seen many speakers over the years that the positive & negative leads were labeled backwards.

The good news is that it is extremely easy to check for phasing issues and proper speaker polarity, all it takes is a 9 volt battery.

With nothing else hooked up to the speaker touch the positive lead of the battery to the positive lead of the speaker, and touch the negative lead of the battery to the negative lead of speaker. As you are looking straight into the front of the speaker it should move towards you.

The motion when the speaker pops when you hook up the battery is what you are looking for. . Do not leave the battery hooked up for more than a few seconds at a time.

When doing this with a cabinet you can simply use the input terminals of the cabinet to perform this test.

Randy Morgan …..  Owner & head technician at All Service Musical Electronics Repair


Mam … Step away from the super glue.

I just spent and extra 30 minutes time carefully extricating a jack from a circuit board so as not to damage the board because someone attempted to glue the plastic jack back together.

If something breaks on your gear super glue may sound like a good idea at the time, but 99.9% of the time it is not.

Super glue

Most of the time it just makes things 10 times worse than they already were.

It is so useless in electronic repair I don’t use even a small tube of it in two years’ time.

Do yourself a big favor and think at least twice before you try to super glue something.

Randy Morgan …..  Owner & head technician at All Service Musical Electronics Repair

Cleaning controls or “pots”


Almost every piece of equipment be it a keyboard, a mixing board, a guitar amplifier, or an effect unit has some good old fashioned potentiometers (Pots) that at some time in its life will need to be cleaned.

Cleaning controls is a pretty simple process; it is the getting to the controls to clean them that can sometimes be complicated.

Fist a warning about faders or slide controls. Do not clean them unless the absolutely need it, and only use special fader only cleaner, and finish up with some fader lube. If you use anything else the controls will become so stiff you will barely be able to operate them. Even when you use the proper products on them they will still be stiffer than they were before they were cleaned. That is why I advise not cleaning them unless it is necessary.

Fader controls unlike rotary controls can be cleaned without taking the equipment apart. You just spray the fader cleaner in the slot that shaft rides in that the knob it attached to. Then work it back and forth over its entire sweep several times, apply some fader lube in the same way, and you are done.

Cleaning rotary controls requires disassembly of the unit being worked on. Once you have accesses to the controls the process is pretty simple. Most, but not all rotary controls have a slot or other opening on the back, or side that you can spray your preferred brand of control cleaner into.

Clean the controls one at a time, do not spray them all first and then go back and rotate them as it is not nearly as effective as spraying the cleaner into the control and then immediately rotating it vigorously several times over its entire sweep.

Here at the shop we generally clean each control twice, the reason we do this is if you have to spend several ours on cleaning the controls on say a mixing board that you had to pull all the circuit boards on to get to the controls you sure as heck don’t want to have to pull it all back apart because you were not thorough in your first attempt.

A few additional notes on control cleaning.

When cleaning controls on keyboards, and synthesizers be careful not to get any control cleaner in the area of the keyboard display, or the keyboard contacts.

When cleaning controls on guitar amplifiers try not to get any cleaner on the tubes or any of the power resistors.

When cleaning controls on mixing boards there will generally be a lot of cleaner run off after cleaning the controls. Put the board on some old newspapers or something else absorbent and let the cleaner drain off before you put the cover back on it. It generally helps to do this with the mixing board left sitting in a variety of positions to facilitate draining of the excess cleaner out of the mixing board controls.

Randy Morgan …..  Owner & head technician at All Service Musical Electronics Repair

Buyer Beware!

The internet is like the old Wild West, pretty much out of control in many respects.

If you are buying something new on the internet please check to see that who you are buying it from is an actual authorized dealer for the product. If they are not there are two possible problems.

One of them being is the product could actually be counterfeit. Yes it is true counterfeiting is not just limited to Gucci handbags any more.

The other is they are second sourcing the product which makes them the original purchaser and you are considered as far as the manufacturer is concerned to be buying it 2nd hand from the internet seller.

In either of these cases the manufacturer will not honor the warranty on the product. In the case of counterfeit products many times the manufacture will require a serial number to order repair parts and if the serial number is not in their system the will not sell the parts to repair it.

Randy Morgan …..  Owner & head technician at All Service Musical Electronics Repair

Broken USB ports

usb port

Many pieces of gear these days have USB ports of this style.

Keyboards use them, DJ mixers Use them, DJ control surfaces use them, some standard mixing boards use them, some effects use them, Digital recorders use them … On and on the list goes.

The problem is the connectors were really designed to be plugged in and left alone.The music industry really needs to come up with better connector to use. The big problem is about 25% of the time when these break off the circuit board they also damage that circuit board.

Unfortunately when these break you must take them to a well-equipped and experienced shop trying to change these yourself, or having an ill equipped shop try it will only end in your having to replace the entire circuit board. Which in most cases is the Main PCB. This is a most definite do not try this at home situation.

My best advice of course is to be as careful and as gentle as you can with these USB ports. As I said they are a poor design choice for this gear and break easily.

Randy Morgan …..  Owner & head technician at All Service Musical Electronics Repair