Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Yes, good thinking.

Posted by cheap-Jack (M) on February 21, 2007 at 09:22:50

In Reply to: Simple! posted by NoWay on February 20, 2007 at 18:11:45:

Hi.

That's what I would do.

(1) measure the DCR of the tube heater, OUT of its socket.

(2) calculate the theoretical inrush current of the heater.
Since I do not have a 6C33C-B with me right here, so I just measure
EL-34 (6.3V 1.5A) which I have a few sleeping in my parts bins, which is 0.7R. So the theoretical cold start heater current inrush factor will be 6.3V/1.5A/0.7R=6 ie. 6 times the rated heater current.

So taking this aproximation guideline factor, the theoretical inrush
current of a cold start 6C33C-B heater (12.6V 3.3A) would take 12.6V/3.3AX6=22.9A !!!!

(3) get the right SMPS to handle such huge inrush.

It is hard to find a convention analogue PS to deliver such cold start inrush 6 times of rated current. In fact I blew a few conventional anaologue PS before I switched to SMPS now, to charge up my SLA cells. I learnt it the hard hard way.

But surprisingly a quality designed SMPS can handle very tough inrush load punishment.

Let me take a look at the specs of a 210W (70KHz with 72% efficiency)
external switching adaptor (rated to IEC801-4 transients specs).

It is rated 5A 115V I/P norminal max load & O/P voltage 30V 7A peak. It is also rated 50A max cold start I/P current at 115V, which will give cold start I/P power surge of 5,750W!

To test how 'robust' a SMPS can be, I tried with my 9.86V2.7A SMPS (scraped from a used camercorder battery charger), it delivered up to 4.5A on initial charging my 6V5AH WIHTOUT adding any current limiter !!!! It got very hot but worked fine.

So get the right SMPS to deliver 3.3A which is a piece of cake, & get a limiter resistor of the right value, to prevent damaging the PS on cold star. For seried 6C33C heaters, I would recommend adding an inrush current limiter, e.g. Ametherm MS355R025 rated max SSI 25A, R(hot) 0.03R upstream of the heaters.

c-J

This post is made possible by the generous support of people like you and our sponsors:

Herbie's Audio Lab

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---