Is this describing a blown tube, or two blow tubes?

From what you describe I would say it's a typical tube failure if you did not have any problems with the old tubes. Unfortunately, tube infant mortality is quite common as tube factories do not have the greatest quality control, which is why the better tube sellers burn in tubes when they're delivered. But sometimes one just slips through the cracks.

Plug in the old tubes and if all is normal, then get another from Tube Depot. Those guys are one of the best and they'll stand by what they sell. 

Also, with push-pull amplifiers a tube tester doesn't offer much practical use. If a power tube can't hold its bias then that is all you need to know to replace the tube.

tony,

yes, I always post things that other's as well as OP might find interesting, and finding out about stuff is fun.

I have hundreds of tubes, two of the GE Orange and Gray Porrtable Cases repair men brought to the home, and a few drawers full, a mix of used and new.

I test all my active tubes bi-annually as a matter of course, test any from my drawers of tubes when needed for me or my friends, test my friends tubes, test tubes I order.

Like an inexpensive sound pressure meter, they do not need to be 'calibrated' to give you 'relative' answers. If two tubes show the same strength on the meter, they are in fact equally strong, a bit more or less irrelevant unless close to bad.

My Hickock was calibrated by the tech who sold it to me. My little Accurate 157 always closely matched the Hickock. When I got the Accurate 257, both 157 and 257 matched, I preferred the smaller one because my first wife got it for my birthday way back when. 257 is quite portable as well.

I also have a set of quicksilver mid mono and have lost a tube before. You described my experience to a t. For what it's worth I was running mine with kt120 for a long time until I decided to try the el34 which as I understand the amps were voiced for. My humble opinion is that JJ stands for junk junk. JJ tubes are the only brand that I have bought that were bad out of the box and I have an audio buddy who's experience has been the same. I run my mid mono with Tung Sol and could not be more pleased. I am not sure what they are getting for those tubes now after the embargo but I seem to remember not paying a lot of money for them. I hope this was helpful.

@marantz33 

120's ? wow, I didn't think that was possible with MidMonos, I assume you have a more modern version than mine. I have a simpler bias mechanism than I assume you have.

I generally like to stick to Gold Lion.

T

Tube testers are not very good for matching tubes, you may be able to weed out a shorted tube though.

I’m not sure vintage tube testers were meant to match tubes. I doubt TV repair techs needed to match tubes as a rule.

I have a Hickok 6000A Mutual Conductance tube tester that I use to test tubes.

User Manual

http://www.grandpas-shack.com/parts/datasheets/6000A(Hickok)_manual.pdf

It doesn’t match tubes per say, example plate current, but it is useful in others ways.

Tests:

Shorts. (A must) See page 6, manual.

(Quality). Good, marginal, bad, test. (Very useful.)

Gas (Grid Current) Test.

Mutual Conductance Test. (A must for me matching Mutual Conductance of the dual triode sections of a 6922/7308 signal tube.)

Life Test. When a tube tests low good on the meter, or marginal, the life test tells the user how much life is left in the tube or if it should be replaced.

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