need help. unable to bias 250tl

[tasmaniandevil]

Hello, I have tested my 250tl like the below. i can get the right current pretty much inline with the eimac manual. However the current gone crazy on my fixed biased amp.

here"s how i tested them. got appro 45mA. neg grid 17vdc

1. 900 vdc on the plate with a simple clc filter.

2 ac filament with a 400 ohm resistor to ground.

3. grid connected to ground.

is this method correct ? i dont own a tester can test these tubes properly. Any possibility that the tube is defect ? The fixed biased amp works perfecty with all my other 250tl except this one.

Help is appreciated

 

[Bandersnatch]

How is 'grid connected to ground'? It could be you're on the edge of grid circuit resistance and the new tube is just more intolerant of this. As in try it with half the grid circuit resistance( unless you are already using an inductor/TX secondary ).
cheers,
Douglas

 

[DavesNotHere]

Its hard to tell what's going on. post a simplified schematic of the stage in question.

 

[hpeter]

https://frank.pocnet.net/sheets/088/2/250TL.pdf
according to DS , it should be -30V ?
And you will need A2 capable driver. (i presume this is SE)

 

[DavesNotHere]

Hello, I have tested my 250tl like the below. i can get the right current pretty much inline with the eimac manual. However the current gone crazy on my fixed biased amp.

here"s how i tested them. got appro 45mA. neg grid 17vdc

1. 900 vdc on the plate with a simple clc filter.

2 ac filament with a 400 ohm resistor to ground.

3. grid connected to ground.

is this method correct ? i don't own a tester can test these tubes properly. Any possibility that the tube is defect ? The fixed biased amp works perfectly with all my other 250tl except this one.

Help is appreciated

The Idle current is within reason, depending on the class of amp.
Your tube is defective if it tests bad with excessive current when others are OK. They commonly develop a short from grid to cathode btw.

 

[PRR]

> according to DS , it should be -30V ?

He says "fixed bias" but then:
"got appro 45mA. neg grid 17vdc
"900 vdc on the plate
"filament with a 400 ohm resistor to ground."

45mA in 400r is another 18V of bias; total about 35V. Not the 30V you figured but close-enough for tube work.

> you will need A2 capable driver

Agree it looks like silly-small available power in negative-grid zone. 10mA-15mA of peak grid current is wanted. However the question so far is about bias, not yet getting sound out.

 

[hpeter]

> according to DS , it should be -30V ?

He says "fixed bias" but then:
"got appro 45mA. neg grid 17vdc
"900 vdc on the plate
"filament with a 400 ohm resistor to ground."

45mA in 400r is another 18V of bias; total about 35V. Not the 30V you figured but close-enough for tube work.

> you will need A2 capable driver

Agree it looks like silly-small available power in negative-grid zone. 10mA-15mA of peak grid current is wanted. However the question so far is about bias, not yet getting sound out.

You forgot to quote this :

3. grid connected to ground.