Standby switches, and why we can (and should) do without

Looking at tube amps from the past 60 years or so, either for guitar or bass, you’ll see a standby switch most of the time. Think of an Ampeg SVT or Fender Bassman for example. Where’s the standby switch on a deBont tube amps? Well, there isn’t one, because they’re not really necessary. At least not anymore. There are a LOT of persistent myths surrounding the standby switch. Why do amps even have them, how and why do people (still) use them and why can we do without just fine?

Fender was the first to provide a standby switch on their tube amps. Note, Fender Tweed including standby switchthe first Fender Tweed didn’t have one either. I believe it was the Tweed Deluxe to be the first and of course, every Fender tube amp after that. The main reason given for the switch at the time was a muting function of sorts by cutting off the high voltage supply to the tubes. No voltage, no sound. Handy, if you’re going for a break while keeping the tubes nice and hot. But there was another reason(*1). I’ll do my best not to get to technical. When tubes are cold at the time the amp is switched on, they don’t conduct any current. Because of this low current demand, the voltages inside the amp are at their highest. When tubes warm up and start to conduct, the high voltage (B+ from now on) will sag a given amount. Electrical components have a maximum voltage rating and exceeding it can have disastrous consequences. Especially with capacitors, who also have a physical limit to what this rating can be. Mind you, high voltage capacitors were very expensive at the time. One way of reducing the costs of an amp was to use capacitors with a voltage rating safely above the amps working voltage (warm tubes), but very close to, or even below the ‘cold switch-on’ voltage. By allowing the tubes to warm-up before applying the B+, their conduction will be (near) instantaneous with the resulting voltage sag down to the working voltage. In other words, the capacitors will never see the higher switch-on voltage and they will live a happier and longer life. But, you were stuck with under-specified components nonetheless.

It’s a know fact that Marshall blatantly copied the early Fender amps. Standby switch and all. Now we have two of the biggest selling names in the industry using standby switches and they became forever linked to tube amps. Brands like VOX and Gibson however didn’t use them. Perhaps most notably the revered VOX AC30. What’s interesting though, when the new re-issues of these ‘vintage’ amps were released, like the VOX AC30 Custom Classic series, they added a standby switch!? ‘Vintage’ image marketing? I don’t know.

Today, high voltage capacitors are readily available at much more reasonably costs. Besides that, I would NEVER try to save a few bucks by using under-specified components. Everything is designed to withstand even the worst case scenarios. Minus one for the standby switch.

Cathode stripping
The main reason coming from standby switch supporters is to protect the cold tubes against the sudden onset of the high voltage. This would cause an effect called cathode stripping. In short, If a tube is cold (not conducting) and B+ is applied, the tube’s cathode is supposedly bombarded with ions and the coating is gradually stripped or even ruptured! Cathode stripping is in fact a real phenomenon… but not in guitar/bass amps.

First and foremost, the voltages seen by the tubes in any given guitar/bass amp are far too low to cause the effect to the extend where it could cause any serious issues. Guitar tube amps tend to use voltages somewhere between 300V and 800V. You need in excess of a 1000V before you even need to start thinking about cathode stripping.

Second, the type of tubes used in guitar/bass amps are called indirectly heated tubes. Remember, a tube needs to be hot to conduct an electrical current. More specific, the cathode has to be hot to be able to emit electrons. There are tube types called directly heated tubes, where the cathode itself is being heated by passing a current through it, just like the filament of a typical light bulb. Indirectly heated tubes use a separate cathode and heating element and are far less susceptible to cathode stripping than their directly heated counterparts.

Third, many tube datasheets state a maximum voltage that may be applied to cold tubes. This number is actually much higher than the max voltage for warm tubes. Why would a manufacturer state such a number at all, let alone an increased value, if applying any voltage to a cold tube would damage it? They wouldn’t.

One extra thing to think about is that guitar/bass amps are not the only equipment using vacuum tubes. In past times, the majority was used in old TV’s (sweep tubes anyone?) and radio equipment, yet, I’ve never come across a TV with delayed B+ and/or stand-by switch… go figure.

Minus two for the standby switch.

Cathode poisoning
Let’s look at it from another perspective, allowing the tubes to warm up without B+ applied and remaining like that for extended periods of time. Or enabling standby mid-rehearsal for that extra long break. I’ve seen many players do this. Now we’re faced with a very real and destructive effect; ‘cathode poisoning’ or ‘sleeping sickness’. If a hot tube is allowed to sit without current passing through it (no B+), slowly but surely, there will be a buildup of barium orthosilicate (*2) between the cathode’s oxide layer and the cathode itself. This ‘cathode interface layer’ is permanent and will dramatically degrade the performance of the tube. With modern and properly designed amps, the use of a standby switch will do no harm (but no good either) to the tubes at start-up, but using it mid-play can reduce the lifespan significantly! Minus three. Doesn’t look good for the standby switch.

Power supply stress
Now that we’ve treated some facts and myths concerning reduced tube life without the use of a standby switch, lets look at the effects on an amp’s power supply. As explained before, a tube needs to be hot in order to conduct current. So it’s easy to see how an amp will draw less current in a cold state compared to a fully heated unit. Allowing the tubes to warm up before applying the B+ will cause an instantaneous inrush and rather high current spike causing stress on multiple components, including the entire power supply. Something you really want to avoid. This effect is not limited to equipment using tubes and lot’s of modern high power equipment use some kind of inrush limiter. Needless to say, this goes for all deBont bass amps as well.

To be thorough, I should make a distinction between the type of rectifier used in the power supply. The rectifier is the first component (or components) used to create the necessary direct current (DC) from the alternating current (AC) coming from the power transformer. The elements responsible for this rectification can be either a tube diode or its modern equivalent, the silicon diode. Silicon diodes are small, cheap, drop (lose) only around 0.6 volts and are most commonly used. The tube diode is big, expensive, drop tens of volts… but is ‘period correct’ and can contribute to the ‘sound’ of an amplifier. Being a tube, it has warm up too before it will pass current to the rest of the amp. So you might say tube amps with tube rectifiers already have a build-in B+ delay and inrush is only an issue once silicon diodes are used. In the earlier mentioned Vox AC30 Custom Classic re-issue, the original tube rectifier was replaced with a silicon variant. The first series suffered from various problems due to the high instantaneous currents. The later production run included some series resistance to limit the inrush.

You’re out!
Minus four for the standby switch! That’s why you’ll never find one on a deBont bass amp. It’s simply not necessary and can even do more harm than good. Sure, the ability to mute your amp is great, but that functionality can be achieved in a multiple of better ways and positions within the audio path. This doesn’t mean you should ignore the one already present on your vintage ’74 SVT or the likes. It might be responsible for keeping other parts besides the tubes in good nick as explained above.

I don’t expect to convince everybody right away. It’s a tenacious myth for good reason. There are lots of interesting reads and discussion on the subject providing tons of (mis)information and I urge you to dive deeper into the subject if there’s still doubt. But be aware. Even leading companies like Fender don’t inform their customers correctly and truthfully about the inconspicuous standby switch.

http://www.diyaudio.com/forums/tubes-valves/168981-lets-settle-b-cold-tubes-issue.html
http://www.diyaudio.com/forums/tubes-valves/219304-cathode-preheating.html
*1 http://www.valvewizard.co.uk/standby.html
*2 http://peavey.com/monitor/pvpapers/Chapter6.pdf
http://www.londonpower.com/standby-switch

2017-11-12T20:06:36+00:00 March 22nd, 2015|0 Comments

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