Thermister - what value?

[johnm]

Hi folks!

Not sure if I'm posting this in the correct place, so apologies if so! Been a long while since I posted here - good to see it's still going strong!

I have nearly completed building a Dynaco ST-70 using all new parts, except the two original output trannies. The Mains transformer is the following:

http://store.triodestore.com/pastpotr24.html

I'd like to add a thermister to prevent inrush current (and protect my NOS GZ34!), but am unsure of the specs of the 'ideal' unit for this particular mains transformer.

Any help would be gratefully received

Mucho thanks!

- JohnM

[John Caswell]

Hi JohnM,
First of all measure the cold DC resistance of the primary, then if possible measure the current drawn when on.
Then select a thermistor/Negative Temperature Coefficient (NTC) from Farnell/RS that is suitable
ie High cold resistance, equal if possible to the DC resistance of the transformer
Low hot resistance
Current rating of say 3-4 amps.
I have used/recommended the following all from Farnell
606-753 10R hot - 0R2 cold - 3Amp
606-765 7R hot - 0R15 cold - 4Amp
606-777 12R hot - 0R22 cold - 4Amp.

Mount in free air away from any component if possible and wrap the wires around the tags before soldering.

John - Dr John

[johnm]

Thanks for that Dr John! I'll do that when I've completed the amp - hopefully today or tomorrow.

Was also told that as I'm using an indirect rectifier (GZ34) I probably wouldn't need an inrush limiter anyway as it takes 10-15 seconds to apply high voltage?

- John

[systemx]

I would use one, most of the load is from the valve heaters, and it is the first few cycles applied to the transformer primary that leads to the surge

[John Caswell]

Hi all,
What SystemX says is also true and to add insult to injury ideally the GZ34 should have some resistance in the anode feeds to make up Mullard's minimum recommendation of 150R per anode.
So measure anode to centre tap, deduct that value from 150R and that will give you the limiting resistor value and if it comes to a silly number eg 139R then the next preferred value upwards will fine eg 150R. Needs to be wirewound as well 7Watts will be adequate.

John - Dr John

[johnm]

Thanks for the advice everyone.

I've just measured the cold DC resistance across the primary, and this is 03.3 on the 200-ohms setting.

Still building the amp (another couple hours should do it) so haven't been able to take the warm dc resistance yet.

- J