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Leaving tube amp on standby issues

garmenteros

garmenteros

Bass Enthusiast
Aug 24, 2008
3,007
697
41
Dominican Republic
metalurgia.bandcamp.com
#1
I accidently left my rig on standby for 4 hours after playing for about 2 hours. I turned it off as soon as I noticed. My rig consists of an all tube mesa400+ head into a powerhouse 1000 cab. I didn't unhook any of the cables or anything because I know having a tube head without a load is issues but Im worried about my amp being on that long on standby. Should I be worried?
 
#5
Do not keep tube amps in standby for long time (hours), power tube damage will occur. Search for "cathode poisoning".

On the other hand, leaving a cathode hot without any anode current flowing does lead to the very real effect of cathode poisoning, which reduces the gain (transconductance) of valves. Fortunately this phenomenon really only becomes significant if the valves are left on standby for hours on end.
Click to expand...

from http://www.freewebs.com/valvewizard1/standby.html
 
#6
^^^What he said.

Based on what I've read an engineer here on TB say, (paraphrasing) leaving a tube amp on standby for extended periods can lesson the life of the tubes, due to prolonged pent up heat.
 
#9
Giaime said:
Do not keep tube amps in standby for long time (hours), power tube damage will occur. Search for "cathode poisoning".

from http://www.freewebs.com/valvewizard1/standby.html
Click to expand...

Hmm. Sounds like folklore to me. The lifespan of the cathode in VT's is limited by the normal deterioration of the material due to heat itself and the slow, normal breakdown of the coating on the cathode which aids transconductance.
I don't see that this lifespan would be adversely affected by simply turning off the plate PS.....

I also disagree that a standby arrangement serves no useful purpose. Each time you heat-cycle the filament, you use up a little more of its life. With the standby circuit you can shut off the plate current and thus silence the amp and then turn it back on again without heat-cycling the filament. This can actually increase the tubes' lifetimes.

Also, don't forget that the filaments aren't the only things that generate heat in a tube. The current flow when the tube is on also generates heat and a lot of it when it's near saturation. Keeping the filaments lit at all times keeps the whole tube warm and reduces thermal shock when they start conducting after having idled for a while. Thermal shocking the tubes is hard on all the little metal parts in there, especially the grids - eventually the grids can deform and short against each other or against the cathode (common failure modes in most tubes with 2 or more grids). So you want to keep this down to a minimum.

Standby circuits are SOP and pretty much vital for tube life in tube-style RF transmitter amplifiers, for example, for these very reasons. These also accumulate weeks (and probably more) of time on standby over the life of the tubes in normal use in applications where the amps are frequently in shut down state. I've never heard of any affect on tube lifetime of this kind of usage (tho there's a very definite negative affect on lifetime with increased heat-cycling of the filaments or heat-shocking the tube by making it conduct heavily while still "cold").

Go ahead and use that standby switch - it's there for a reason and good for your amp...
LS
 
#10
heath_r_91 said:
Correct me if I'm wrong (I probably am) but can't you disconnect the cabinet and basically do whatever you want with a tube amp when it is in standby mode?

Something about it not turning on the actual power supply or something like that?
Click to expand...

Absolutely, and another reason a standby circuit is essential in a tube amp.

In standby mode, the power supply to the output tubes is shut off, so there's no danger of the output circuit conducting when there's no load on the output transformer.

So you can disconnect your speakers with no worries of damage in standby mode without having to shut it completely down (and thus heat-cycle the filaments as I mentioned before).

LS
 
#12
garmenteros said:
Fired her up and everything was ok. What a relief. Tubing this thing up with decent tubes is atleast 200 250 bucks.
Click to expand...

There's no problem with leaving the amp on in standby mode. I left my old bassman on around the clock for a while. It had a couple bad sockets in the preamp which would "static" horribly when you first turned it on as it warmed up. Once the whole thing was warm that would stop.
I didn't feel like replacing the sockets (pain in the a$$) so I just left it on 24x7 and turned it on/off with the standby switch. I kept it in standby overnight for weeks. Tubes were fine and the next owner to my knowledge used it with that same set of tubes until I stopped hearing from him.

I'd just turn it on at the beginning of rehearsal/the gig and then turn it completely off when you're going to move it, it'll be fine (use standby for breaks too).

LS
 
#13
unclejane said:
There's no problem with leaving the amp on in standby mode. I left my old bassman on around the clock for a while. It had a couple bad sockets in the preamp which would "static" horribly when you first turned it on as it warmed up. Once the whole thing was warm that would stop.
I didn't feel like replacing the sockets (pain in the a$$) so I just left it on 24x7 and turned it on/off with the standby switch. I kept it in standby overnight for weeks. Tubes were fine and the next owner to my knowledge used it with that same set of tubes until I stopped hearing from him.

I'd just turn it on at the beginning of rehearsal/the gig and then turn it completely off when you're going to move it, it'll be fine (use standby for breaks too).



LS
Click to expand...

Thats great to hear, did you have a furman or any other power conditioner infront of the amp just in case. I've got a furman I run my mesa rig, my b15, and my laptop thru. Voltage in the DR is terrible.
 
#14
Cathode poisoning is a real concern, however you need a whole lot more than a few hours to be a problem. We are talking 100's of hours inbetween B+ supply being applied. This was studied by Motorolla when they came out with the "Instant-On" tube TV's. The heaters were always lit when the set was plugged in.
 
#16
garmenteros said:
Thats great to hear, did you have a furman or any other power conditioner infront of the amp just in case. I've got a furman I run my mesa rig, my b15, and my laptop thru. Voltage in the DR is terrible.
Click to expand...

Nope just plugged it into the wall. Not a bad idea to insure you have the right line voltage tho (too high is worse than too low but it'll always sound better with the right 115-120v)....

LS
 
#17
B-string said:
Cathode poisoning is a real concern, however you need a whole lot more than a few hours to be a problem. We are talking 100's of hours inbetween B+ supply being applied. This was studied by Motorolla when they came out with the "Instant-On" tube TV's. The heaters were always lit when the set was plugged in.
Click to expand...

Some transmitting tubes (as well as some early rectifier tubes) don't have a separate cathode element around the filament. The filament itself has a coating put on it and acts as the cathode. They light up to full heat in one or two seconds and the tube can go from cold to full blast in pretty much that amount of time.

Still good to have a standby mode even with this type of tube, tho, since cycling the filaments still reduces their life in any case....

LS
 
#18
unclejane said:
Some transmitting tubes (as well as some early rectifier tubes) don't have a separate cathode element around the filament. The filament itself has a coating put on it and acts as the cathode. They light up to full heat in one or two seconds and the tube can go from cold to full blast in pretty much that amount of time.

Still good to have a standby mode even with this type of tube, tho, since cycling the filaments still reduces their life in any case....

LS
Click to expand...

Thanks I was aware of this, used to design/build AM and FM transmitters. Several old direct heated cathode triodes are popular in class A SE HiFi these days as well.
 
#19
With unclejane on this one. Until someone shouts me down with proper engineering. Indirectly heated cathodes were done with thorium paste from memory, though like old tubes i'm fading as well ( nothing wrong with my getter however...). If by some chance there was damage from cutting the screen 2 voltage, then it would be far less than leaving all the HT applied while having a break... as for power cycling the valves on and off all night...thermal cyclic stress would be a real concern. Both are more damaging than "Standby" in my view. As for directly heated cathodes.....theres another storey. Used to keep one directly heated bottle on my desk to alarm students in my lair....powered up of course it made a great hand warmer...

Regards John
 
#20
Here's a quote from Hartley Peavey:

".....The early rectifiers used “directly heated cathodes” (which were themselves, merely the heater filaments
coated with a barium/strontium oxide). They produced output voltage VERY RAPIDLY when turned on. Since these
directly heated cathodes were often used to power other tubes (valves) whose cathodes were “INDIRECTLY
HEATED,” a problem arose when the full plate voltage (B+) was applied BEFORE the “indirectly heated” cathodes of
the audio amplifier’s tubes reached their operating temperature. When the full high voltage (B+) is applied
significantly BEFORE a tube reaches operating temperature, the cathode coating can be “stripped” of its ability to
emit electrons. This destructive process is called “cathode stripping” and it is mainly for THIS REASON that socalled
“standby” switches were provided on numerous electronic devices using DIRECTLY HEATED RECTIFIERS
(with INDIRECTLY HEATED TUBES for operation)!
 

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