Can't get a consensus what a cutof reading on a B&K 465 means

[Chris K]

I have 2 10BP4 CRTs, On a Sencore Super Mack on the cutoff setting, one of them has no needle deflection, the other has deflection about 1/3 up the scale. According to the instructions, the higher needle deflecting CRT has better contrast resulting in a better picture.

I tested the same CRTs on a B&K 465. The instructions for measuring cutoff are below:



The cutoff voltage range for the 10BP4 G1 bias should be between 18 and 53 volts.

The low or no cutoff tube on the SuperMack gives a reading of 18V on the 465




The good cutoff tube on the Sencore gives a reading of 40V on the 465 according to the instructions




However, the instructions for the 465 say the LOWER the G1 voltage is needed to move the needle to the cutoff position is a better thing for indicating better tube cutoff.

I am confused. Why does my bad tube on the Sencore have a low cutoff G1 bias cutoff on the 465 indicating, by the instructions, better cutoff?

Dazed and bewhildered. Is this a typo?

[old_tv_nut]

What is the procedure on the Sencore?
I'm looking at a Super Mack manual and don't see a cutoff setting, just four fixed grid bias settings.
http://nestreble.ca/Sencore/CR31%20Manual.pdf

[Chris K]

Sorry for not responding sooner. Tomorrow I'll show you the dame procedure using the Sencore. It's not complicated. Choose the right socket, set filament volts to 6.3v, turn the multi test select dial to check for HK shorts, then select the che k for G1 shorts, then select "Gun Balance" (this is Sencore's term for cutoff) and watch the needle deflect. If it doesn't move, little or no cutoff. NOS 10BP4 tubes deflect the needle to the midpoint of the scale "12 O'clock) To check emissions you turn the gun balance pot so the needle goes back to essentially zero then you measure emissions. It's a commercial diagnostic product. A well deflected needle in Gun Balance mode means good cutoff. A needle that doesn't move means little or none. The bad one doesn't move the nee, the good one deflects it 1/3 up the scale.

[Chris K]

BTW the correct bias setting on the Sencore for testing the 10BP4 is "C"

[old_tv_nut]

I re-read the Gun Balance section fo the Super Mack manual, and I think the criterion for "good" is poorly written. It says:
"A tube that fails to adjust TO the GUN BAL line should be consdered questionsable..."

It doesn't say from which side it should adjust TO the line. I'm thinking that the tube has too little gainl if it draws too much current and reads to the right of the line, but if it reads lower (or zero) the gun has higher gain than minimum acceptable and is OK *at least for a B&W tube) - just out of spec in a good direction. Of course, if you are looking at a color tube, it's more important that all three guns are similar, and you need SOME current on all three to see if they are equal.

Just for yuks, I would take the tube that tests good on the B&K, put it on the Mack, and try the next (less negative) bias setting if there is one, to see if the GUN BAL control can show a usable meter reading with less negative bias.

[Chris K]

On their YouTube channels, both Shango66 and Bob Andersen have the Super Mack and celebrate a tube as having good cutoff if the needle deflects more than a quarter of way up the scale when switched to gun balance. The B&W/Red pot should be turned all the way up when switching to gun balance and then adjusted down to the gun balance line on the meter. That's done before switching to emissions to test that. Using the sencore and the 465 as I've seen them used, the tube I'd considered good was good on both and the bad tube was bad on both. I have a pristine NOS 10bp4 that shoots the needle 3/4 of the way up the meter when switched to gun balance. Maybe I'll put that on the 465 and check how it reads. It produces a fantastic picture.

[old_tv_nut]

Originally Posted by Chris K

... Using the sencore and the 465 as I've seen them used, the tube I'd considered good was good on both and the bad tube was bad on both...

I misunderstood. I thought you were saying you had opposite results on the two testers.

Glad it's cleared up.