# GTA ma tube tester?



## fernieite (Oct 30, 2006)

Hi guys,
I have a blown power tube in my Traynor YBA-1. The other one is still good and strong, (Rogers branded Mullard) but my amp tech's tester doesn't measure ma. Is there anyone in the GTA who has that capability?
I'd like to get another vintage 6CA7 (EL34) that's a close match.
Thanks.


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## Alan Small (Dec 30, 2019)

Tim @ Superfuzz


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## fernieite (Oct 30, 2006)

Thanks, but apparently Tim's tester only tells you if it's strong or not. (Same as my amp tech's)


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## Latole (Aug 11, 2020)

Many tubes tester are not reliable to test power tubes.
Buy a new matched set of tube, don't waste you time.

Before you must know why amp blow a tube


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## jb welder (Sep 14, 2010)

You could do it in your amp. You can run it on one power tube for test purposes. Just compare them and find 2 tubes that give the same idle current.
You would need to either install a cathode resistor (usually 1 ohm is used) or use a bias-probe


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## dtsaudio (Apr 15, 2009)

jb welder said:


> You could do it in your amp. You can run it on one power tube for test purposes. Just compare them and find 2 tubes that give the same idle current.
> You would need to either install a cathode resistor (usually 1 ohm is used) or use a bias-probe


This is the best and most reliable way to do this unless you have a very expensive test set up for matching tubes, similar to what the Tubestore uses.
Your tech should have known this.


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## telle-tom (Nov 8, 2021)

A knowledgeable amp tech can test the idle current using a volt meter on the ma setting. It only takes a few seconds if you know what your doing. You read the current by inserting the meter between the plate and the center tap (edit: of the output transformer) for each tube. Easy to see if the two tubes are close at idle.


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## Latole (Aug 11, 2020)

telle-tom said:


> You read the current by inserting the meter between the plate and the center tap of each tube.



What is the center tap of a tube ? I never hear that before.

Dangerous milliamp reading is from the tube's plate and center tap of the Output transformer,
It is very dangerous. I never do that .
I work on kathod and ground with 1 ohms resistor


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## telle-tom (Nov 8, 2021)

Latole said:


> What is the center tap of a tube ? I never hear that before.
> 
> Dangerous milliamp reading is from the tube's plate and center tap of the Output transformer,
> It is very dangerous. I never do that .
> I work on kathod and ground with 1 ohms resistor


Yes, the center tap of the OT for each tube. Essentially you just shunting the current through your milliammeter. 
Not "dangerous" at all for someone such as a knowledgeable tech. It's fast and accurate.


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## Latole (Aug 11, 2020)

telle-tom said:


> Yes, the center tap of the OT for each tube. Essentially you just shunting the current through your milliammeter.
> Not "dangerous" at all for someone such as a knowledgeable tech. It's fast and accurate.



Good you wrote '"for someone such as a knowledgeable tech."

There is lethal voltage it can kill you.


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## DavidP (Mar 7, 2006)

Latole said:


> Good you wrote '"for someone such as a knowledgeable tech."
> There is lethal voltage it can kill you.


+1001 on this!!! BE CAREFUL, even if you are knowledgeable -- you're playing around with 400-500V here...


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## jb welder (Sep 14, 2010)

telle-tom said:


> Not "dangerous" at all for someone such as a knowledgeable tech. It's fast and accurate.


Maybe so, but it is the most dangerous method of measuring bias. And not the _most_ accurate when you have an OT with low primary resistance (some are less than 50 ohm per half).
I use this method myself but try not to recommend it in public. I've got an ear-full when ever I've done that.


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## Paul Running (Apr 12, 2020)




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## Jerome (Dec 4, 2015)

I think when all is said and done , you will have to buy a matched set of tubes .


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## nonreverb (Sep 19, 2006)

FYI This tests current and it's expensive.


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## fernieite (Oct 30, 2006)

Jerome said:


> I think when all is said and done , you will have to buy a matched set of tubes .


OP here. That's what I did. 👍


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## Paul Running (Apr 12, 2020)

nonreverb said:


> FYI This tests current and it's expensive.
> View attachment 386934


Here's the unit that I picture in your shop, Rich:


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## nonreverb (Sep 19, 2006)

Paul Running said:


> Here's the unit that I picture in your shop, Rich:
> View attachment 386983


Paul!! I would love that! I could keep that thing busy for a looong time.


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