How much clearance around 12AU7s?

[GregH2]

Hi all,

I'm designing a preamp PCB and am wondering how much clearance (from other valves and capacitors) I should have around my 12AU7s. Your advice is appreciated.

Regards,

Greg.

[ashok]

At least one tube diameter ?

[MellowTone]

I think what ashok said is about right, one tube diameter should do it, perhaps a touch more to be safe. I'd be inclined to keep caps at least two diameters away, as I believe they are affected a bit by heat.

[ashok]

Yes as Mellow Tone says , caps should preferably be kept further away. A caps life depends also on operating temperature and as always elevated operating temperature shortens useful life for almost anything on Earth !

Remember that power tubes generate a lot of heat and things might need more clearance. I see some designs very tightly packed. I don't think they will last very long if used on a continuous basis. Ambient temperature and air flow also matters a lot. In cold countries many compact things might work well as it's so cold most of the time(?). Here in the tropics it's 35+ deg C ambient temp all through mid summer . Can't guess how long compact power tube amps will work here. Right now we are almost mid way in December and it's.....cold.....21degC .

[Ivey]

Truth:

There is no set rule for novar base tubes of 3/4" dia. In designing military tube equipment in the 1950's and 60's. Most tubes had a metal shields that dissipated the heat to the chassis. So I/we never worried about heat.

Your caps are also an item not to worry about as well. If your chassis is ventilated. The warm air will rise and natural cooling will occur. Your biggest concern is air moisture. Too little or too much. This dry out your power main caps and mare your plastic caps.

But then again, this will happen over years of time.

So, there is no fear in how you design tube preamp, but a power amp is not the same.

If using a 12AU7 in a "phase inverter/driver" it will draw a heavy current load. Cooling is needed. But in a preamp, there is no need.

Again on caps. Most plastic caps are sealed. Like Wima, Epco, Evox, etc.,
So you are pretty safe there. Electros are a different breed, but use good ones and you are looking at 6 to 10 yrs of service.

Take Care

Ivey