Screen current during warm-up

[kodabmx]

I have an amp I've built that uses glow regulators for the screen grids in a 6550 PP amp.

I don't delay B+ and as the tubes warm up to conduct there is a peak in screen current seemingly before the plate takes over.

B+ is 380V, screens are 300V, The glowtubes get brighter and brighter until they return to an idle condition. Is this a normal thing? Usually I don't have a visual indicator of current.

I have the plate of the VR75 connected to regulated B+, and the screen to the cathode of the VR75 which is connected to ground through a 100K resistance.


It's similar to the set up in this schematic, but B+ is regulated.

 

[kodabmx]

Here's a video:
YouTube
Sorry for the quality.

EDIT: When warmed up and pushed I assume this is also normal.
YouTube

 

[gofar99]

Hi, Looking at the circuit ...during the time from turn on to fully operational the B+ on both the anodes and screens will be well over the operational value. This is normal as the 5R4 will most likely deliver full voltage before the others can utilize it. So my belief in what you are seeing is that the regulator is handling more voltage and because of this more current during that period. Thus the greater glow. As the tubes warm up the B+ will drop into the proper range and the regulator will stabilize in its final state. Hopefully the design was well done and the combination of voltage and current applied to the regulator during this period does not exceed its ratings.

 

[kodabmx]

But B+ is at 380V the whole time. And the 5R4 isn't there, it's been replaced by an SMPS. 12V to 380V.

 

[sser2]

It is quite normal for VR tubes to draw excessive current during warm-up. Current stabilization may take as long as 10-15 min.

 

[kodabmx]

I thought it was based on drawn current though. A VR tube with no load won't vary. And since when did a gas regulator tube "warm up"? Isn't the point of a cold cathode that is doesn't require warm up time?

EDIT: it doesn't regulate current, it's a voltage source.

 

[Eli Duttman]

A gas discharge tube's cathode is "cold", in the sense that a filament/heater is absent. After the device "strikes", the cathode heats up. I²R heating is present, any time electricity flows.

Look at this data sheet. What's allowed in the "steady state" is different than that during start up surge.

 

[kodabmx]

I see what you're talking about. Cheers for the link.