The Steel Guitar Forum

http://www.bunkerofdoom.com/tubes/rc25/

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<font size=1>“Back then, everything was different, and you only saw it once; now everything’s the same and you see it over and over again"
-Peter Case
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Regarding tube testers, it's important to understand that there two basic tube tester designs. These are the "emission" tester and the "dynamic conductance" tester.

Emission testers simply heat the tube up and measure the flow of electrons from the hot cathode. They are only slightly better than checking the heater resistance with an ohmmeter for an open circuit. Most of the old "drugstore" tube testers were emission testers. In fact, they had a hidden pot so that they could be set to whatever the owner wished the meter to show.

Dynamic conductance testers actually place regulated voltages on the various components within the tube and supply a small signal to the tube's grid. The gain of the tube is then measured in a manner very similar to what the tube would see in an actual amplifier circuit. They require a setup chart because there are so many combinations of pin connections and internal components to test for.

There were many dynamic conductance tube testers made in the vacuum tube era (40's to 70's) which are quite accurate and reliable. If you decide to try to find one for purchase, the most sought-after units are made by B&K and Hickok. The Heathkit TT-1 is also a very good tester using the Weston 981 design.

I have a B&K 707 and a Heathkit TT-1A, and they are both very reliable and accurate.

My comment about the ubiquitous 12AU7 merely comes from the fact that, for the past 50 years, we've had a drawer full of a hundred of 'em in the garage. We always need 12AX7's, and hardly ever need a 12AU7. It's a great tube, just never the one I need and always the one I have plenty of...

Mike, unless your B&K and Heathkit slap the tubes with 400+ plate volts, they are not all that much use for guitar amp power tubes other than detecting shorts.

I have a couple nice old Hickoks, and they are great testers - but all of the garden-variety tube testers max out at about 180 volts on the plates. you can't match tubes...or even test for durability...with those testers. they are nice to have if you own a lot of tubes and need to weed out duds, but the only way to really do final tests and match them is in a specialty tester built for the purpose, or in an amp with a bias tester (BiasRite, BiasKing etc.). The transconductance testers ARE far better than the emmision types - but still extremely limited, and most people are better off NOThaving one so they don't get overconfident about old tubes....and blow an output transformer in a vintage amp.

One other major issue is calibration - ALL transconductance testers must be calibrated every 5 years or so or they are useless. 90% of the tube testers you find on eBay or at garage sales have not ben calibrated - and that little job runs about $100-150 plus shipping. Forget price shopping, there are only a few shops that calibrate them.

So while the unit ARE accurate and reliable, that is true only when calibrated, and only within a rather limited scope.

Again - for most players, a tube tester is a huge waste of money. You'll SAVE money buying tubes from a qualified source.

Speaking of tube testers... I have a B&K Dyna-Jet Model 707, but the tube chart is missing some pages. Without knowing the settings for a particular tube the tester is useless. The rest of the book is in pretty bad shape too. Does anyone know where I might find a replacement book?

Mitch - the RCA tube manual or any of the others will give you the right pinouts for a particular tube, and the normal voltages. You can also find the tube data on many websites. It at least gets you in the ballpark for basic "not-junk/junk" testing.

I found this on the 'net and thought it might be of interest to some reading this topic...

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Tube Characteristics and substitutions
Some quick and dirty subs and some tube data such as recommended bias current and appx voltages. These subs are all taken from the Tube Substitution Handbook sold by Antique Electronics Supply. or provided from the net.

A (short) catalog of tubes you are likely to see in a guitar amp:

12AX7[A, WA] and substitutes - preamp and driver tubes
12AT7, 12AU7 and subs, preamp and driver tubes
12AY7 - driver tubes
6EU7 - dual triode used in some older amps for preamp tube
6L6 types - power output tubes, up to 50 watts/pair, a mainstay of Fender
EL34 - Euro power pentodes, up to 50 watts/pair, many Marshalls
6V6 - smaller, lower power cousin of the 6L6, 10-14 watts per pair; used in smaller Fenders
EL84 - fits a 9 pin socket like a 12AX7 but twice as tall; miniature power pentode, good for 12-18 watts per pair; used in smaller Vox amps, and a quad of these drives the Vox AC-30 for 30 watts.
Substitutions:
* means appropriate for parallel filament circuits
# means may not work in all circuits
Preamp and driver tube substitutions:

12AX7 (high gain dual triodes with pinout 9A)
12AD7* 12DT7 7729
12AU7# 5751* B339
12AU7A# 5751WA* B759
12AX7 6057 CV4004
12AX7A 6681 E83CC
12AX7WA 6L13 ECC803
12BZ7* 7025 ECC83
12DF7 7025A M8137
12DM7* 7494
12AU7 (moderately high gain dual triodes with pinout 9A)
12AU7[A,AW,] 6189 7730
12AX7* and subs 6670 ECC186
5814[A,AW]* 6680 ECC802
5963 7316 ECC82
6067 7489 M8136
12AT7 (medium gain dual triodes with pinout 9A)
12AT7[many suffixes] 7492 E81CC
6201 7728 ECC801
6679 A2900 M8162
ECC81 B152 QA2406
12AZ7[A]* B309 QB309
6060 B739
6671 CV4024
12AY7 (low gain dual triodes with pinout 9A)
12AY7(and suffixes) 6072
2082
Power tube substitutions:

6BQ5/EL84 (miniature pentode with pinout 9CV)
6267 7189 EF86
6BQ5 7189A EL84
6BQ5WA 7320 N709
6P15 E84L Z729
6L6 (beam power tube with pinout 7AC)
6L6(many suffixes) 7581(A)
5881 WT6
5932 EL37
EL34/6CA7 (power pentode with pinout 8ET)
EL34 12E13
6CA7 KT77
7D11 KT88
6550 (power pentode with pinout 7S)
6550[A] 7027A#
7D11 KT88
12E13