soft startup for preamp tubes

[Alf]

Because I have quite a few of my nos preamptubes , ecc83's that flash when they are started up cold, I am looking for a way to limit the inrush current.
I think it would benefit their lifespan if they could just start glowing up gently.

Has anyone ever designed a circuit to do this ?

I'm talking about 3 to 4 preamptubes so about 600 mA .

Could this be done with powerresistors in the filament wires ?

Any help is highly appreciated.

Regards Alf

[stokes]

The flash you are seeing is the filaments,I have a couple of old Bugle Boy AX7's that give the bright flash when first turned on and have been in my Ampeg since the late 1960's and are still working fine.Some preamp tubes will flash bright and some dont,but it obviously doesnt have any adverse affect on the tube.I wouldnt suggest putting resistors in the filament wires,just leave it be.

[Alf]

That's reassuring Stokes !

I heard that American tubes like 12ax7a don't flash , is this always true ?

Alf

[Sweetfinger]

IIRC The reason some tubes 'flash' and some don't has to do with the formulation of the filament. Remember that those tubes were used in EVERYTHING back in the day, and who wanted to wait around while their TV, radio, or missile tracking system warmed up? The tubes that flash have a quick start filament for faster turn-on. Tubes were probably available with/without the quick start option, but I'm guessing that consumers probably got whatever was handy for the manufacturer to stuff into boxes to meet demand.

[Amp Kat]

There are soft start circuits which could involve a RC time constant to ramp up the voltage. Many tubes will flash but most won't. I've seen the Telefunken,EI Yugo and a few others but most don't do it

[Alf]

Ampkat do you know where I could trace such a circuit ?

I've been looking around but so far not what I wanted.

Alf

[stokes]

I have only seen the "flash" in European tubes,never in any American tubes.I am talking of NOS preamp tubes here.I have never heard of there being an option of a quick start or not.Since you have to wait for the entire tube to warm up anyway,I dont see the point in the filament taking an extra second to warm up,but who knows,I suppose it is possible that the option was there at some time.Anyway Alf,I wouldnt be concerned with modding the filament to slow down the voltage.

[Enzo]

The options were for the designers. If you wanted faster turn on, specify a brand that did so. If that was not an issue, then specify any brand availble. That is why you see "COntrolled warmup characteristic" in the tube manuals.

[Alf]

Enzo ,

Can these " controlled warmup characteristics" be found in any tubeamp book ?
I don't have one but suppose they can also be found online ?

Alf

[Enzo]

I can't bring specific tubes to mind. Usually it is one of the later alphabet additions. Like 6**7-A, -B, -C. ANd the -C would have the heater spec.

I don't see WHAT the chontrolled chracteristic is in manuals, just that it has it. Full engineering data on the tube would include it. My old Sylvania manual calls for 11 second warm up on a 12AX7. The book is too old for listings of 12AX7WB and such.

[Amp Kat]

Alf the one I saw was in KOC Priciple Of Power book although the version 5 may have one also. I'll see what I can find.

[Alf]

Amp Kat ,

I have that book too , so I looked it up on page 3-18 .
There are some reservations though. If one of the filaments opens up Kevin says that the others will suffer serious damage.
And it will take as long as ten times longer start up.

So I think I'll pass for now , I might try it some day just to see how long it really takes.
Kevin also states that it will make your tubes last a lot longer.

Thanks, Alf

ps Very nice shop you have there in Baton Rouge !!

[Frankli]

This project is perfect


I have mount this power supply regulated and stabilized. (never hum problems)
The startup is soft and delay time. You can use at 6.3v or 12.6v. This project is to use with preamp tubes (all amps (hi-fi or other)).
The minimum voltage of secondary of transformer is 15v.
The only problem is the transistor (BUZ11). Is very hot. The great heatsink is need. lol
My build:

Sorry by my english

Frankli

[Alf]

Thanks Frankli for you schematic and picture .
How much current can it handle and the buzz 11 what part is that ?
Looks like something from the Elektor magazine is that right ?


Alf

[Frankli]

The project is configured to support a maximum chain of until 5A.
I do not know the origin of the diagram. A friend sent me it.