Determine Strength of the Power Tube

[Kris]

Question to PB.

Paul,

A while ago you explained that by measuring voltages on driver tube socket pins we can determine how strong the particular tube is. This info is part of Crack FAQ also and I used that method few times already, since I don't possess tube tester. My question is if there is a similar way to determine strength of the power tube as well?
Sorry if this question was answered already... Just couldn't find it.

Thanks,
Kris

P.S. Paul, congrats on your new design... Quicksand is extremely tempting. I hope it's going to be as popular as your Crack amp.

[soundsgood]

i apologize for cutting into your post, just wanted to share my experience. my crack started to make noise in one channel after 6-8 weeks. i could hear a faint high pitched sound when standing in front of amp but could not hear it off to the side. when i put the 6080 on a tube tester (i own 3) emissions measured 30%. the tubes in the kit are not claimed to be new but i would not have used a tube that weak considering the low cost of a new one. in no way do criticize bottlehead, i applaud their ability to offer such performance to the masses at so little cost. if you suspect you have a weak tube, just replace it.

[Paul Birkeland]

For the power tube, drawing more current across the 3K cathode resistors (to a point) will indicate higher emissions (given the same bias voltage). This is a little counter intuitive, as more current through the driver tube means a lower plate voltage. 

[Kris]

Thank you PB for another great info.

Are there any particullar current values I should be looking for?
... and how about speedballed crack? Will using terminals 3, 7 and 9 for current measurements on crack with speedball, be adequate as well?

Cheers,
Kris

[Paul Birkeland]

Hello Kris,

With the Speedball, subtract the voltage at terminal 7 from the voltage at terminal 2.  This is the plate voltage on your 6080.
Take the voltage at terminal 1 and subtract from terminal 2, this is your grid bias.

Open the datasheet for the 6080, go to the plate curves, you can place a point at 30mA and your plate voltage, then observe how far off your measured grid bias is from the data sheet grid bias.  (There will be a lot of variation!)

[Kris]

That sounds like a fun experiment.
Many thanks PB.

Kris