Date: Thu, 22 Mar 90 18:01:01 PST
From: jgro@apldbio.com (Jeremy Grodberg)
Subject: Re: Tube Warmup

In article <19900322212956.5.SSE@UR.bellcore.com> sse@flash.bellcore.com (Samuel S. Epstein) writes:
>I have an MFA Magus A-3 preamp, which contains 4 tubes.  [...]
>Does it make sense to leave the preamp on all
>the time if I listen every day? What if I listen on weekends and once
>during the week?

I used to work for a Radio station that had a lot of tube equipment, and I can
tell you what we decided.  Tubes definitely wear out from simply being left
on, but the inrush current when you turn them on, and the thermal cycling
(heating up and cooling down) they undergo from being turned on and off
cause a lot of wear not only to the tubes, but also to all the electrical and 
even mechanical components (like tube sockets, pc boards, wires).  Also, the
tube equipment is much more able to handle voltage spikes and transients than
the transistor equipment, so we never worried about running them during 
lightning storms, etc.

At the radio station we only really had 2 classes of equipment: stuff used
more than 10 hours a day, and stuff used less than 2 hours a day.  The first
class was left on all the time, the second class we turned off when we were
done.  We found we never needed to replace tubes in either group more than
once a year (except for the 5000w transmitter's final, which, coincidentally,
is one of the few execeptions we made to the above rule: it was used more
than 10 hours a day, but was shut off at night, per FCC regs).  We did not
keep track of how long tubes actually lasted, or do any controlled studies
(again, except for the transmitter final).  As one of the people who maintained
the equipment, I can tell you anecdotally that the number one problem,
before these rules went into effect, was damage due to thermal cycling, and 
the number two problem was bad tubes.  In the two years following the start
of using these rules, we had only one equipment failure: a 30 year old
tube amp suffered some form of component failure, and we threw it out.  No
tubes died, and no other equipment died.

My rule(s) of thumb for tube equipment usage:

1) Don't turn it off if you are going to turn it back on later in the day.
2) Don't leave it on if you are not going to use it tomorrow.
3) If you are done for the day, but will use it tomorrow, leave it on.

For a formula: 
if Time-between-use is <=  2 * (Time-to-warm-up + Time-to-cool-down)
then leave it on, otherwise turn it off.  Time-to-cool-down means time
for the unit to return to room temperature.

Sometimes the formula conflicts with rule 3; If you use something every day
leave it on.  If you use it only on the weekends and the formual says
turn it off, then turn it off.