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Amplitrex Tube Tester: Help!

dagordon

Platinum Supporting Member
Messages
110
Hey,

Does anyone have experience with an Amplitrex AT1000 tube tester? (Specifically, using one to test tubes for use in guitar amps?)

I just got one, and I'm now massively confused in terms of how to use it and what the results mean. In the stock Fixed Bias mode, a ton of tubes I've tested which are either new or (I'm told from reputable dealers) NOS are testing low on emissions in particular...

Googling has only confused me more... some people saying it should be in Fixed Bias mode, others that it should be in Auto Bias mode, others that this tester gives false low readings for tubes, one guy saying you have to use the computer mode and do all sorts of fancy software mods in order to get accurate results...

Can anyone tell me: a) should it be in fixed bias more or auto bias mode and b) how to interpret the results (what are the thresholds for emission, gm that correspond to "good", "on it's way out," "weak")?

I'm testing tubes for use in basically Fender-style amps that have fixed bias, FWIW...

Thanks!
 

RussB

low rent hobbyist
Messages
11,158
Did you buy it new? If not, does it require calibration?

I'd LUV to have one of those, but they're WAY out of my affordability range
 

dagordon

Platinum Supporting Member
Messages
110
Yes I bought it new, and calibrated.

Yes, there is a manual. I'm not technically knowledgeable enough (read: I'm a dumb sh** when it comes to this) to know, based on reading the manual, whether I want Fixed or Auto bias mode. The manual suggests that 80% is a reasonable cutoff for what counts as a "good" tube but at least in Fixed Bias mode I have many testing below this (particularly on emission) that are either new or were NOS from some of the most respected tube dealers around (including I believe yourself) so I don't know what to think.

Any help from someone who has actually successfully used this unit would be much appreciated!
 

RiftAmps

Member
Messages
207
If you want to know.....

1) 'What condition is this tube in and how well does it perform under set conditions, like those found in a guitar amplifier?' - use Fixed Bias mode. This is how most valve testers work. The Amplitrex actually does two tests here:-

a) A high voltage, low current test
b) A low voltage, high current test

You will get different results here for each test (of course you will, you're testing the tube under different conditions!) and you can use the results from each test to determine the overall performance of the tube. This mode usually gives lower than expected emissions but that's because it's testing a 'worst case scenario'.

2) 'What grid voltage and other settings do I need to achieve my expected plate current?' - use Auto Bias mode (for clever clogs who like this stuff)

This test gives you a higher 'score' than the Fixed bias test does, but that's because the tester is optimizing the operating conditions of the tube to achieve a good score (useful for designing amplifiers etc) - it's more of a 'please find the maximum performance of this tube and then tell me the settings!'


The tube test data is stored on board the Amplitrex on a flash drive, however you need to connect a computer to it to access it.

When you test a tube using the computer software, you get a results sheet with one column showing the expected results and the other showing actual.


I've tried to explain this in simple terms and of course there's far more to it, but this should at least help explain what's going on.

Good luck!
 

J M Fahey

Member
Messages
2,665
Hey,

Does anyone have experience with an Amplitrex AT1000 tube tester? (Specifically, using one to test tubes for use in guitar amps?)

I just got one, and I'm now massively confused in terms of how to use it and what the results mean.......
Googling has only confused me more... some people saying ......., others that ....., others that ........ one guy saying ......
Can anyone tell me: ...... how to interpret the results .......

I'm testing tubes for use in basically Fender-style amps that have fixed bias, FWIW...
With due respect, such a fine piece of equipment is useful for, say, a Tech which goes through a ton of tubes daily and wants to sort them quick, or a reseller who wants to match or rate them, etc.

Not much use, definitely overkill for an average user and, as you see, the ton of data available may confuse rather than help.
 

dagordon

Platinum Supporting Member
Messages
110
If you want to know.....

1) 'What condition is this tube in and how well does it perform under set conditions, like those found in a guitar amplifier?' - use Fixed Bias mode. This is how most valve testers work. The Amplitrex actually does two tests here:-

a) A high voltage, low current test
b) A low voltage, high current test

You will get different results here for each test (of course you will, you're testing the tube under different conditions!) and you can use the results from each test to determine the overall performance of the tube. This mode usually gives lower than expected emissions but that's because it's testing a 'worst case scenario'.

2) 'What grid voltage and other settings do I need to achieve my expected plate current?' - use Auto Bias mode (for clever clogs who like this stuff)

This test gives you a higher 'score' than the Fixed bias test does, but that's because the tester is optimizing the operating conditions of the tube to achieve a good score (useful for designing amplifiers etc) - it's more of a 'please find the maximum performance of this tube and then tell me the settings!'


The tube test data is stored on board the Amplitrex on a flash drive, however you need to connect a computer to it to access it.

When you test a tube using the computer software, you get a results sheet with one column showing the expected results and the other showing actual.


I've tried to explain this in simple terms and of course there's far more to it, but this should at least help explain what's going on.

Good luck!

Thanks so much, that's very helpful! Sounds like I want fixed bias mode.

Any suggestions as to what thresholds make sense in terms of when a tube is "bad"? Is the 70% or below suggested by the manual reasonable?
 

dagordon

Platinum Supporting Member
Messages
110
With due respect, such a fine piece of equipment is useful for, say, a Tech which goes through a ton of tubes daily and wants to sort them quick, or a reseller who wants to match or rate them, etc.

Not much use, definitely overkill for an average user and, as you see, the ton of data available may confuse rather than help.
You may be right -- there are certainly a lot of features I won't use. Though it's clearly designed to be easy to use and relatively foolproof, the only question is what are reasonable guidelines around how to interpret the results (what results should make me reject a tube that was advertised as new or NOS).

It would be surprising to me if it were impossible to give simple guidelines around this, and I would have a lot of questions around how the people who are selling the tubes are able to test and interpret the results if it's really so complicated, but I suppose anything's possible.
 

skytrench

Member
Messages
240
Simple guidelines for a test of a 12ax7 with fixed bias method would be:

(These values are taken from the 12ax7 datasheet)

1.Set plate voltage to 250V
2.Set grid voltage to -2V
3.Read the emission current. 100% would be 1.2mA. Typical reject at 70% would be 0.84mA.
4.Read emission current for the other triode.
 

Blue Strat

Member
Messages
30,192
I bought a "matched pair" of NOS 6V6s that were Amplitrex tested. They weren't matched, and one had a short. ;)
 

J M Fahey

Member
Messages
2,665
Personally I build my own simple test jigs depending on what I'm interested into, no need for a 1000 tricks dog but one which does a couple I'll actually use, the way I need.

Nothing against Amplitrex of course, which looks cool and advanced.
 

dagordon

Platinum Supporting Member
Messages
110
So I have a bunch of Tube Amp Doctor 12ax7s that failed the fixed bias test by quite a margin...

I contacted TAD tech support and they're saying that auto bias is the more meaningful test as "any preamp tube in any application operates on auto bias mode which a) balances any differences and b) regulates the amplification to a generally high level".

Now, I dont know that tese tubes will even pass in auto bias mode and can check, but assuming they do, is this right?
 

Blue Strat

Member
Messages
30,192
So I have a bunch of Tube Amp Doctor 12ax7s that failed the fixed bias test by quite a margin...

I contacted TAD tech support and they're saying that auto bias is the more meaningful test as "any preamp tube in any application operates on auto bias mode which a) balances any differences and b) regulates the amplification to a generally high level".

Now, I dont know that tese tubes will even pass in auto bias mode and can check, but assuming they do, is this right?
It's true that all preamp tubes in guitar amps are cathode biased which could mean "auto bias".

Whichever method gives you the most "pass" results for tubes you know to be good is likely the one you want to use.
 

robrob

Member
Messages
409
It sounds like Amplitrex's "Auto Bias" test mode does not refer to the common meaning of a cathode biased tube circuit and the TAD support guy doesn't know this.
 

skytrench

Member
Messages
240
The 12ax7's you measured don't meet the datasheet spec, simple as that. That doesn't mean they won't sound good.
I would be surprised if you didn't find several of your tubes emitting 1mA or more. New production tubes are often weak. The tubes with lower grid voltage compared to the datasheet, will have less headroom, since they will reach cutoff sooner.
 

RiftAmps

Member
Messages
207
From the manual:-

You may ask, what constitutes a “good” tube? Well, this is difficult to say, and depends more upon your own criteria.
In the days when vacuum tubes were the mainstay, most component values and supply voltages could be off as much as 20%, and the equipment would still work satisfactorily. You can apply this rule to a minimum acceptable Emission and Transconductance of 20% low, or a display of 80%.
Tubes showing emission or GM less than about 70% can be classified as bad. However, you must be the one to determine for yourself if a tube with 70% emission or GM is suitable for use or not. For example, a tube showing 70% may perform very poorly in one piece of equipment, yet be OK in another.


If you wanted a tube tester that gave a simple 'Good/Worn/Fail' then the Orange VT1000 would have more than sufficed for common guitar types. The Amplitrex is a very advanced tester that requires a lot of user knowledge for setting up and interpreting the results. Most of the time, you need to manually adjust the test settings for each tube type in order to tailor it to your needs. For example, if you wanted to test a 6V6 tube for use in a radio then the settings you might use would be different than those in a Deluxe Reverb.
 




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