testing 6DJ8s

[Geek]

Hi Mark,

Originally posted by hailteflon
Also, Geek said he cleaned the grids with 15mA. Was this d.c.? Seems easy enough to put a load resistor from the plate to B+.

I used a CCS load with the grid + cathode grounded.

You can also use a load resistor. In early experiments, I used an 8.2K, 5 watt resistor with 240VDC from a bench supply, then grounded the grid and adjusted the cathode resistor until the Va was 120V.

I switched to the CCS and lower voltage since the heatsinked transistor didn't melt my breadboard and felt safer with the tube at 90V instead of 120

Cheers!

[hailteflon]

Thanks for the reply, Geek.

I did a little more holding of the button on the 707 and one half of this 6DJ8 is now staying in the green and the other half is staying in the yellow. This tester uses a.c., d.c is probably better.

On the scale they have gone from 26 to about 50 and 62. They tend to want to go down again, but they aren't taking the dive of death. They start out in the 70s on the scale and will probably stay there in the end. The original 707 meter has been replaced with one that goes from 0-100 instead of 0-120 on the scale.

I will put your advice in the tube box and wait until I can configure a CCS.

Great advice, Mark

[t_blizzard]

Have you tested your tubes in a real circuit?
Do they still oscillate?
Trenton

[hailteflon]

I never found any evidence that they were oscillating.

This is a batch of used Amperex 6DJ8s from Tek scopes that were scrapped years ago. Most of them check either strong or weak, but OK for transconductance.

Unfortunately, I don't have any true high fidelity tube equipment, yet.

I have some monophonic phonographs, but that's about it. Nothing that uses a 6DJ8, except one Tek scope that I still have.

[Dave Cigna]

If it were me I would want to rule out the test equipment as the source the problem. Do you see the meter dipping with only some tubes? All of them? All, but some only a little?

[hailteflon]

The one scope that I didn't scrap works fine. It works whenever I use it.

I am going to take the Amperex 6DJ8s out of it though.

It is only a few tubes that do this. The rest of them test properly in that they hold whatever reading they give.

The scopes that these 6DJ8s were in have ceased to exist.

I was reading about the 6DJ8 on the Brent Jesse site. He says that they have an especially fine wind on the grid. This may make them susceptible to this problem.