Did my tubes go south ?

[Nikon1975]

I have a Tubelab SE for 300B. It's a rather new build but I have been using the tubes with another buil for a while.

Today both tubes went in runaway. I have to admit that I was squizing them a bit, 400 V on plate with 90mA, buy it lasted maybe 20 seconds, I switched ti off.

Then I checked everything on the circuit, and it looks like it works correctly, but when I start to pull up the current in the 300B the bias at the beginning raise slowly, then all of a sudden jumps to 100 - 120 mA.

Now the question is: did I damaged the tubes, or I can have hope that is something else ?

Thanks,

Davide

[toprepairman]

Sounds like the tubes have become gassy.
At the point when the current rockets up, do you get a pretty blue glow from within the tubes? Has any of the silver gettering turned white?

[Nikon1975]

What seem suspicious, is that it is affecting both tubes

Thanks,

Davide

Sent from my N-04C using Tapatalk

[SY]

If it's both tubes, I would carefully check anything which relates to the biasing.

[Nikon1975]

I will, my hope is that the mosfet are doing something strange, and for some reason the bias jumps up.

Thanks,

Davide

[Nikon1975]

Actually I did check check the bias circuit without output tubes, and the B- changes smoothly turning the pots.

D.

[toprepairman]

Classic case of a tiny amount of gas ionising and then forcing positve bias on the control grid. It doen't mean the valves are bad but not suitable for the HT supplied.
I've seen new valves do this, but more likely older ones.

[dcgillespie]

I there any chance the unit is going into some sort of parasitic oscillation to cause the bias increase? You might look at the output on a scope to confirm that all is well in this respect.

Dave

[DF96]

Oscillation is possible, but gas is more likely. What is the value of the grid bias resistors (plus any grid stoppers)? Maybe they could be smaller.

[overtheairbroadcast]

Originally Posted by Nikon1975

I have a Tubelab SE for 300B. It's a rather new build but I have been using the tubes with another buil for a while.

Today both tubes went in runaway. I have to admit that I was squizing them a bit, 400 V on plate with 90mA, buy it lasted maybe 20 seconds, I switched ti off.

Then I checked everything on the circuit, and it looks like it works correctly, but when I start to pull up the current in the 300B the bias at the beginning raise slowly, then all of a sudden jumps to 100 - 120 mA.

Now the question is: did I damaged the tubes, or I can have hope that is something else ?

Thanks,

Davide

Hi Davide,

I am new here, but just from my little knowledge, what were the vacuum tubes looking like? Red? Pink/purple between the grid wires? I read something way back about the grids being able to run away because of biasing (covered already above) or because of a mismatch with the grid leakage resistor to the tube (apparently there has to be a resistive load to the grid). Make sure that the lower Rg1 value on the manufacturer's tube data sheet is used and then checking that value against what resistor is really there.

If I can find the link (it was either here or at diyaudioproject.com) then I will leave it here.

Do you have pictures? Do you have access to a tube tester to see if your tubes are damaged? And have you tried swapping out tubes (do not do this if it might cause damage to another set of tubes)?