B+ ramping up before CCS filament - danger?

[phrarod]

This is my first time with CCS filament supply. The problem is the good part of CCS - slow ramp up - is so slow that even my slow start 5AR4 rectifiers hit the tube with voltage that begins to climb to almost 100VDC over the final stabilized voltage once the heaters come up to full voltage.

Is this a potential problem that could cause cathode stripping?

The only way to further slow the B+ would be to use a thermistor but I don't know if that would have sonic implications.

 

[Rundmaus]

... delay the filament voltage for the 5AR4 until the other filaments are up.

Greetings,
Andreas

 

[phrarod]

With a thermistor? I would need about a 15 second delay

 

[Rundmaus]

Depends on the question if you find a thermistor capable of 1,9A filament current for the 5AR4 and reasonably low hot resistance...

On my amp project, I will do the delay "manually" via a rotary switch: Pos1 - tube heaters current limited, Pos2 - tube heaters on, Pos3 - tube heaters, rectifier heaters and HV on...

Greetings,
Andreas

EDIT: Not the most practical solution, but I decided to do completely without semiconductors. Using them, you can built beautiful delay circuits instead...

 

[phrarod]

Thanks Andreas. I think I would like a built-in auto solution that is 'wife friendly'.

I promised she could actually use the new preamp and not be afraid of it. lol

 

[Arnulf]

Time relay ?

 

[phrarod]

Yes, as mentioned I need around a 15 second time delay.Tell me how Arnulf?

 

[bequerel]

What about this!

http://www.sophiaelectric.com/pages/parts/delay_action_board.htm

 

[phrarod]

That is AWESOME. Unfortunately I don't have a 6.3v tap for it and no room for another transformer. I'll call them though and see if there's any alternatives.

 

[phrarod]

I just spoke with Sophia and an issue would be getting power to the module. I would have to tap off my 12.6V ac tap that is going to my filament supply. He said to put it on the filament of the 5AR4. With this in mind wouldn't a thermistor be an easier solution?

 

[Knarf]

I'd just hit the preamp tubes with 6V (or 6.3V) DC from a regulator. You don't really need inrush current protection with these small tubes. Finding old tubes with a broken filament wire doesn't happen very often, not even in tubes from consumer equipment like radios and TVs. Don't think I have one in my 'trash' box of dead tubes.

With simple DC heating, the tube cathodes are hot before the 5AR4 heats up. Not sure though whether you risk cathode stripping with your current setup. It is usually a problem seen with much larger tubes (amps of filament current).

 

[phrarod]

Thanks Frank. I would like to make the current set up work if possible mainly because I made the investment with the CCS modules. I know the tubes I use FLASH when hit with full fil DC and I do like the ramp up. Just not so slow.

In the future I won't do this set up again in a preamp.