Should 12AU7 tubes in Foreplay III be hot?

[Mr. Lin]

Hello, I'm new to the Bottlehead forum.  Two weeks ago I completed my Foreplay III kit, and now I'm working on Paramours.  Due to a number of circumstances, I purchased the kits nearly five years ago and they've been sitting in boxes until now.  I'm thrilled with the Foreplay btw.

My question is this:  I've noticed that after use - playing music for a while - the 12AU7 tubes get very hot.  Hot enough that they're uncomfortable to touch for about 5-10 seconds after the Foreplay is turned off.  I've never known small signal tubes to get that hot, and I'm concerned because a lot of the 12AU7 tubes I own are very expensive NOS ones, and I wonder if something is wrong that could shorten the tubes life or outright damage it. 

My voltage and resistance checks are ok.  I did notice nearly all my voltages are a little higher than what's in the manual, but well within the 10-15% indicated as being acceptable there. 

Thanks!

[Grainger49]

David,

Tubes get hot.  That is all there is to it.  So what you are experiencing is normal.  It looks like you are fine.

If you need to remove a tube that has been working just use a soft cloth to pull it out.  That is what I do and is handy for comparing the sonic signatures of different tubes. 

There are a large number of tubes with different numbers that roll into the 12AU7 spots.

Here is a thread to expand your horizons:

http://www.bottlehead.com/smf/index.php/topic,1027.0.html

[Mr. Lin]

Thank you very much Grainger.  I figured it was probably ok, it's just that I have a lot of other audio equipment that use 12AU7 or 12AX7 tubes, and I've never noticed them getting as hot as in the Foreplay.  

I do have some of those 12AU7 variants, but contrary to other peoples' experiences, I've found I prefer a really good 12AU7 tube - although I've yet to try them in the Foreplay.

Thanks again!

[Grainger49]

There is a tube rolling thread in the FP III folder.  Some like one tube some like another.  Makes life interesting.

[Paul Joppa]

It is odd, since the 12AU7 in Foreplay is operating at a low plate dissipation, so most of the heat comes from the heater power. That should be about as low a temperature as is possible with that tube.

Your high line voltage will slightly increase the voltage to the heaters and thus the temperature. IF you will measure the voltage at the heater pine (between pin 9 and pins 4-5 on either tube socket) I will calculate a revised resistance value for the heater power supply (at the back of the power transformer). I do not expect that to make much difference though.

[Doc B.]

  Hot enough that they're uncomfortable to touch for about 5-10 seconds after the Foreplay is turned off. 
Thanks!

If they are cool enough to touch 5-10 seconds after you turn them off I'd say they are running about right.

[Mr. Lin]

  Hot enough that they're uncomfortable to touch for about 5-10 seconds after the Foreplay is turned off. 
Thanks!

If they are cool enough to touch 5-10 seconds after you turn them off I'd say they are running about right.

Gotcha Doc.  Come to think of it, I suppose the 12AU/AX7 tubes in my Wright phono preamp can get pretty hot as well.

And Paul I'll double-check those voltages later on and get back to you.