Demystifying Tube Amps---The Standby Switch

[1bassleft]

Anecdotal, rather than big science, but my gigging amp was a CMI100W Lead/Bass that Marshall sold ~1976-1979 when the brand was under a restrictive Rose-Morris agreement. It had no standby switch (SS rectifier). I had a habit of switching on with V at 0 a fair while before playing, returning to V=0 during set-break (not to save the bar's leccy bill - heat is usually half of the energy wasted) and again before switch off and tear down. The valves lasted for yonks.

 

[bassdude51]

I used to have a Mesa 400+ and the manual (page 5) recommended having the stand by flipped on before hitting the power switch.

 

[okcrum]

Ask The Amp Tech: Will I damage my tube amp by not using the standby switch? - Guitar.com | All Things Guitar

Folks who get tube amps for the first time usually have a whole lot of questions. Next to "How often should you change tubes," the most common question I see about tube amps is "How to use the standby switch." This article popped up on my news feed today, and I think it says it all.

It really doesn't matter. It's not brain surgery. Most audio devices with tubes have a relay installed that prevents you from being able to make sounds until the tubes are sufficiently warmed up, as playing through tubes that aren't warmed up sufficiently can damage them. However, occasionally you will run into one that doesn't. For example, I have an old Danelectro-made Airline guitar amp from 1962 that doesn't have a standby or a relay. So I have to remember to give it 30 seconds before it warms up before I play through it, and in most cases, that's enough time. They may not be warmed up fully, and you might notice an improvement in tone after 3 or 4 minutes, but it's enough to prevent damage.

After that, game on. I almost never use the standby, and if I do use it, it's for short durations when I'm plugging in or unplugging something in my signal chain. I also use it sometimes if I'm doing AB tests with another amp into the same cab. This prevents signal going through the output section of your amp while it's turned on without a cab, which we all know is a big no-no in the vast majority of tube amps as it can take out your output section in short order.

I've done shows where someone on the crew sees my amp with standby off when I'm not up there and they switch it on. Then I go onstage for the show and panic for 3 seconds till I realize someone turned the standby on. If you're on a crew, please just leave the amps alone so you don't give the musicians heart or panic attacks!

Anyway, read the article, feel free to comment or ask questions, but just know that there is no great mystery to the standby switch. You won't save tube life using it past the first 30 seconds of warmup, and while I think the risk of what they call "cathode poisoning" is greatly overstated if you leave standby on for hours at a time, it has been known to happen on occasion, so IMHO, you're just as well off to ignore it for the most part.

Finally, a clueful article about the standby switch on amps.

FWIW, degradation of emission in receiving tubes (which normally takes thousands of hours of operation except in cases of tube overload) is often mistaken for cathode poisoning, which is actually caused by leaky vacuum seals or manufacturing impurities.

Failure modes of hot cathodes

 

[707GK]

I actually read somewhere that keeping the amp in warm-up or standby mode for too long can potentially lead to cathode stripping. I cannot support nor deny that statement.

I follow your same principle, give it a minute or so to get the tubes up to temp and then it’s fair game. Seems to be working great for me!

 

[JimmyM]

What relay? I've seen them in plenty of SS amps but never in a tube amp.

What musical instrument tube amp(s) are you referring to that have such an arrangement? Not familiar with any.

OK, so whatever the thing that causes amps with tubes to not come on unless the tubes are heated up is what I'm talking about. I thought it was called a relay because of old SVT's that have a relay tube that delays startup for 30 seconds, and are usually replaced with a shorting plug in the socket.

Funnily enough, they still delay coming on for about 30 seconds on startup even without the relay tube.

 

[JimmyM]

I actually read somewhere that keeping the amp in warm-up or standby mode for too long can potentially lead to cathode stripping. I cannot support nor deny that statement.

I follow your same principle, give it a minute or so to get the tubes up to temp and then it’s fair game. Seems to be working great for me!

Read okcrum's post directly above yours.

 

[pbassjbass]

Interesting, I always thought the standby switch was basically a mute switch so we didn't have to go through warm up again. Got rid of the residual hum, allow things to be plugged and unplugged, without restarting the amp or changing settings, and good for those of us who routinely cut it tight getting back on stage after the break.

 

[Seashore]

I use the standby on my Traynor as a mute during shows and rehearsal for the simple reason that it's on the front of the amp with all the other controls, while the power toggle is hidden on the back next to the ground lift switch.

 

[BassmanPaul]

Yes HT is HV in French (´Haute Tension' in French, here used in a loose fashion. The official 'HT' starts higher Haute tension — Wikipédia). The term 'HV' is used in GB and Ireland.

Erm! Not so. HT is High Tension in the UK and, as previously mentioned, is analogous to B+ in the Americas.

 

[okcrum]

I actually read somewhere that keeping the amp in warm-up or standby mode for too long can potentially lead to cathode stripping. I cannot support nor deny that statement.

I follow your same principle, give it a minute or so to get the tubes up to temp and then it’s fair game. Seems to be working great for me!

You can't strip a cathode with or without B+ applied in a receiving tube. That can happen in transmitting tubes, which are most definitely not in commercial guitar amps. Whoever wrote what you read was confusing the two basic tube types.

Receiving tubes are designed with controlled warmup characteristics, expressly for the purpose of being able to be lit up when B+ is already present.

 

[okcrum]

Erm! Not so. HT is High Tension in the UK and, as previously mentioned, is analogous to B+ in the Americas.

To complete the explanation of terms, the reason we call it B+ here in the US is this:

Back in the early days of tube radio, and before power supplies with big enough filter caps, the units were powered by batteries. Portable tube radios always used batteries, of course. These batteries were classified as A batteries for heaters, B batteries for plates, and C batteries for grid bias. B+ survives as the only artifact of that bygone era.

 

[beans-on-toast]

I use my standby switches. I paid for them, I’m going to get my money’s worth. When turning on the amp, if there’s a short on the input it’ll make a buzz.

When shutting down, going into standby for a while can leave the fan running. It depends on the amp. This can be good.

 

[JimmyM]

Interesting, I always thought the standby switch was basically a mute switch so we didn't have to go through warm up again. Got rid of the residual hum, allow things to be plugged and unplugged, without restarting the amp or changing settings, and good for those of us who routinely cut it tight getting back on stage after the break.

Basically, yeah, although I do the cutting it tight thing by just turning down the volume knob till I'm ready to go again or turning off my wireless when I use one.

 

[okcrum]

As Merlin points out (quoted in the article: The Valve Wizard) there is still the DC coupled cathode follower who may not like being slammed by the HT before being warm. (The Valve Wizard - See A Useful Mod' for DC...).
The diode trick solves the problem but maybe some people do not like because it 'may' affect the sound of the overdriven DCCF (I have never tried it - except on a reverb, where it is not overdriven).
But for most amps, I suppose one can do without indeed.

I've designed DC coupled cathode followers, and there's no difference in sound from the diode, because the cathode is swinging up and down along with the grid from the input signal. As you say, the diode is there to prevent the tube's grid from momentarily going positive on power-up, and degrading tube life.

 

[JimmyM]

I use my standby switches. I paid for them, I’m going to get my money’s worth. When turning on the amp, if there’s a short on the input it’ll make a buzz.

When shutting down, going into standby for a while can leave the fan running. It depends on the amp. This can be good.

You are a pro tube amp user and tech. I think this article was geared mainly to those going from a SS amp to a tube amp for the first time. You know how scared of the glass they can be sometimes

 

[Ric Vice]

You are a pro tube amp user and tech. I think this article was geared mainly to those going from a SS amp to a tube amp for the first time. You know how scared of the glass they can be sometimes

Yep, they're afraid it will keep their spirit image imprisoned, when they see their own likeness, reflected from the surface, of a glowing glass mirrored tube.

 

[Wasnex]

I used to have a Mesa 400+ and the manual (page 5) recommended having the stand by flipped on before hitting the power switch.

Poor choice of language. In most instances, putting an amp in standby involves putting the switch to the off/open position, and that is the case with the 400+. One caveat, you have to turn an amp on for it to be in standby mode. Otherwise the amp is simply off. Standby means the amp is ready to operate, but it is not operating. After the tubes have warmed up, move the standby switch to the operate position.

Setting the amp to operate mode is the same as taking the amp out of standby.

I have the schematic for the 400+ power supply. It looks like the tube heaters and negative supplies are energized when the amp is in standby. The standby switch breaks continuity after the bridge rectifier and main filter caps, so none of the positive voltages are supplied to the circuits. When you close the standby switch (or take the amp out of standby), the voltage is provided to a voltage dropping ladder formed by a choke and a 10K resistor to supply 480V and 340V to the circuity.

 

[beans-on-toast]

I think this article was geared mainly to those going from a SS amp to a tube amp for the first time. You know how scared of the glass they can be sometimes

You’re certainly right about the basic questions the new tubies ask. Many think that tubes are fragile things which they aren’t.

One thing that irks me is the inconsistent standby labelling on amps. Some manufacturers label standby “on” as the amp is in playing mode. Others label “on” as the amp is in standby. It’s confusing.

 

[ToneMonkey]

Tube amps with tube rectifiers generally don't need Standby switches because the rectifier must heat up in order to provide high voltage to the power tubes.

What OP states is generally correct. But if you dig deeper, you'll find many folks who have much more nuanced opinions about Standby switches.

One tip if you do have a Standby switch, don't engage it when you power the amp down. Just turn off the main power. It's a minor safety thing. The HV will still be connected to the speakers and will drain through the speakers after powering down. This is especially helpful if you plan to work on the insides.

 

[vvvmmm]

IME, another good reason for the amp to be in standby is for when you switch the amp power to "on" and then knock yer axe over. Now, I've only ever done that with a 6-string (so we don't really care, right? ) but it would mebbe sound even worse with a bass thru a 500 w. head.