Tube Rolling Questions

[onion_breath]

Hello all,

I've been happily using my Eros for the past 1.5 years, and was just bit by the "tube-rolling bug" and wanted to run some questions by y'all. I ended up finding some nicely priced Amperex Bugle Boy EF86s on the bay - being sold as NOS. Not having tube testing machine (and wanting to really see if they were as advertised), I'd like to see if there was a way to find out how strong each tube is by doing measurements while they're installed on the Eros. If I am able to find their measurements/strengths I can at least pair up the best ones. Additionally - is there any way of knowing what strength / number reading I should be looking for when it comes to NOS tubes?

Thanks in advance,
Greg

 

[Doc B]

It's pretty straightforward. You just plug them into the Eros and make sure they run at the voltages listed in the voltage checks page 76 of the manual.

 

[onion_breath]

Thanks Doc,

I see on page 78 it mentions taking measurements at Kreg, which connects to A3 (EF86 cathode) and OB, which connects to A6 (EF86 plate). Would these two locations give me the measurements needed to determine strengths and which tubes I should pair together?

-Greg

Sorry for these simple questions, I just wanna make sure I'm not gonna mess something up.

 

[Paul Birkeland]

Yes, the A3 and A6 voltage measurements will be informative.

 

[onion_breath]

Thanks PB!

I just ran some measurements on the tubes that came with my Eros kit:

socket A side :
A3 cathode - 1.88 v
A6  plate - 97.8 v


socket B side:
A3 cathode - 1.65 v
A6  plate - 97.3 v


Do these numbers seem okay, and should I be looking for higher numbers from both on cathode and plate on NOS tubes?

-Greg

 

[Paul Joppa]

The plate voltage is controlled by the servo bias. Your A6 measurements indicate the tube is strong enough to be regulated. That's the most important thing.

The cathode measurements indicate what bias the servo produces. A low bias (less than 1.0 volt) usually indicates very low transconductance, though there can be other causes. A tube that meets new-tube specifications will usually run around 1.5 volts bias. Your A3 measurements indicate good transconductance.

Usually there is no need to match tubes as long as they can be biased properly by the servo. But if you want to explore this, the A3 number would be the one to match.

 

[Paul Birkeland]

I would also add that a cathode voltage under 1V may just meant that you need to run your Eros for a few days to get the cathode emission to come back up to factory spec. 

 

[onion_breath]

Thanks again for all the assistance Doc and PB. All tubes seem to check out well with strong measurements. Gonna give the best matching set some time to burn in... hopefully these Bugle Boys live up to the hype.

Cheers,
Greg