Power Tube Matching

Can someone translate the two #’s on the tube boxes?

What does the first two digit # signify? Plate current I think. And I assume millivolts, milliamps? Or what?

And the 2nd four digit # means transconductance I gather. What is transconductance?

How far apart can these #’s be before they are considered unmatched? And if you explain with percentage, I will need to know how high the numbers go, so maybe stick to actual number increases. My point being, on the internet people just say "5% difference." This means nothing if the plate current numbers can go up to 35 million, or only 100, or maybe 50, or 120.

So, if I Bob has 3 tubes that all have the first number of 14.3.....what numbers on a forth tube would be considered matched? 17.1, 18.4, 19.0, 25.8, or even 12.3?

What happens really if Bob used an unmatched pair of tubes in an amp. And it would be nice to hear about direct experience.

Thank you for your time,





IP is plate current , Gm is transconductance .

In your Mid Monos you need matched tubes unless they are the latest version with a trim pot for each tube’s bias.

Bob only has three matched tubes.... if you put one that isn’t close then one amp will have a tube that is not biased correctly because the two power tubes share a bias pot . One will draw more current than the other , not what you want in a Push / Pull amp.

IP plate current is what you are adjusting. It’s basically a math formula that relates to the plate voltage and how much current the tube will / should draw

Your amp has a plate voltage of something like 440 volts. Mike at Quicksilver recommends setting the bias at 53 mA.

You can’t achieve that if both tubes are not electrically " the same" with respect to their current draw. . Quicksilver strongly recommends matched power tubes for best and safe operation.

When you buy a QS product new the tubes are hand matched on their equipment and are very tightly matched. Looking at a pair of EL84 and the current draw is hand written on each box. 24.1 and 24.0. That’s how tightly matched they should be. Those were from their Headphone amp that has no adjustment, it’s Auto Bias so technically they don’t need to be matched . But they do it on all of their amps

Transconductance or Gm is how strong the tube’s emissions are .


Roger Modjeski designed his own proprietary computer tube testers to ensure appropriate matching of power and signal tubes. So here is a tutorial from a very knowledgeable amplifier designer and tube expert: https://www.ramtubes.com/article-5 that should address any questions you may have.


@oddiofyl Thank you.

But, I and still curious.

"Looking at a pair of EL84 and the current draw is hand written on each box. 24.1 and 24.0. That’s how tightly matched they should be."

In this example, how far off could they be and still be considered matched?

24.1 __ 23.7

24.1 __ 24.8

What is the cutoff for what is considered, not perfect, but matched?


It just seems mysterious how the % is gained.

Thanks again


@clio09 Thanks,

From that article, it looks like he is saying that the top, 2 digit, number on a tube box will  be between 32 to 42. Well, I know Ive seen tubes as low as 14 and as high as 34, so.....maybe even 44.

It is quite mysterious.

Were tubes always matched, say in 1950's guitar amps?

If no, when did matching become a thing? And did this correspond to a change in amp design?  And if yes, why the change?





So, when i order tubes, say KT77's for my midmonos, I should seek tubes where the top number is a 53?


From that article, it looks like he is saying that the top, 2 digit, number on a tube box will be between 32 to 42.

@tonydennison No that is not what he is saying. If you read the very last paragraph Roger wrote Bias is matched within 4% and Gm (transconductance) within 8%.

So, if I Bob has 3 tubes that all have the first number of 14.3.....what numbers on a forth tube would be considered matched? 17.1, 18.4, 19.0, 25.8, or even 12.3?

The answer is 13.7 or 14.9 using our method of testing. We actually offer even tighter testing upon request. I personally would not accept anything below 14 or above 14.6. So none of the other values in your example offer a good match to the tube labeled 14.3.

Thank you @clio09 for clearifying, Im no engineer.

What tube shop do you run?

Also, not that Im going to try it, but what would happen using mismatched power tubes? Would there be SQ issues? amp damage? And what type of damage?



No don't worry so much about the Ip value , as long as it is the same on all four.   Some places will put a sticker on the base of the tube.  Apex tube matching puts a label on the base of the tube with the value as measured on their equipment.   

I have had pretty good luck with the JJ tubes I have had that were Apex matched.   There are other places that fully test and grade their tubes in a similar way

On your amps if they have an LED for bias will be around 50mA when it starts to glow brightly.    I use this when I retube a 6L6 or EL34 based amp. 

I use this 


It is a great tool for adjusting bias and safe because you are not using meter probes on test pads 

Quicksilver has all of the bias values except the Mid Mono so I asked Mike and he said 53 mA for best performance.   That VHT tool is more accurate than the LED , although it gets you close if set to where it just starts to get bright. 

Gm is expressed in Micromhos and generally the higher the better because it suggests the tube is strong.   No guarantee,  some tubes can drift.  Draw more or less current 

That's why amps with individual bias for each tube or auto bias are more able to cope with mismatch.   


Hey the milliamp tester looks like a good thing to have.

I see one on reverb for $100 new. I'd like to find a used one for $50 or $60. I will look. Thanks.

I'm go to send you an IM.

@tonydennison RAM Tubes is the company founded by Roger Modjeski and run by me since his passing. You can find it here: https://www.ramtubes.com/


Ok, so Im looking at the Ram EL34's. Tell me about these. Were they made by RAM, or just selected and tested by RAM? if the latter.....where did they come from?

I am intrigued. Are these NOS (from when) or current production (from where)?


Thank you,



@clio09 knows

RAM = Roger Modjeski….sadly RIP

probably the father of modern computerized tube testing and very importantly had ears and greatly understood the various tube applications and SOE….. he knew how to get 10 k hours out of output tubes…


i miss him… greatly 

The RAM EL34s are either from China or Russia. Roger used to purchase large volumes of raw stock from the factories or other sources. Then he would test them and only keep the tubes that tested to his desired specs. The NOS stock EL-34s we carry are Siemens from the old East German factory. During testing each tube is given a serial number and when testing is complete the machine prints a label with the test data for that tube which is then attached to the box.

Power tube matching is overvalued and if you have an amp that can adjust bias individually (every amp should) then almost totally unnecessary.


@tonydennison Not a great design, but common.  So you need 2 matched pairs.  However, they don't have to be perfect, especially if you bias conseratively.


The VHT tools a good way to measure the bias of each tube of your mid mono 

When you adjust the bias with this in place it will confirm whether the tubes are close.   It will also help troubleshoot ....  if you try to bias and one socket is too high or too low with a few different tubes it would suggest a bad bias resistor or something else going on with that socket .


It 's name is some what of a misnomer , not a "tube tester "  more of a  ias adjustment tool.   Not something you'll use a lot but it is a great tool