Tag: Schematic

It’s a long-standing debate about what the standby switches on Fender amps are used for. That’s why Sweetwater‘s own tube amp expert, Greg Bowers, decided to clear things up and end the debate once and for all. This is his story más o menos a few Fomedits:

The Standby Switch

The myth about the lowly standby switch on guitar amplifiers has gone on since they first came on the scene in the 1950s, so no wonder it is still misunderstood. You would think that by now with the internet around everyone would be up to speed, but the myth is too enduring! I have even read articles from educated people that I respect who have not quite gotten the whole story correct because of reasonable sounding, but incorrectly applied details about vacuum tubes. Then the myth gets distorted even more, because everyone thinks these people should know what they are talking about.

These switches are notorious for causing weird problems and numerous questions from my customers like “Why does their amp pop when using it?” (they pop because they are switching anywhere from 300 to 800 volts. WOW!)I have merrily gone on repairing amplifiers over 20 years and decided to break down the mythology of standby switches based on what I know as a technician and amp builder to separate what is folklore and what is fact. At the very least, I would like to explain what standby switches are NOT used for. Here is what I learned repairing amps, doing research and reading history from much smarter people than myself.

Tube Amp History

Back in the 1940s -50’s there were no books or schools for making guitar amplifiers. Amplifying a guitar was a relatively new idea. Most great guitar amp companies were not founded by textbook electronic engineers or scientists, but smart service technicians who experimented with the recommended RCA vacuum tube circuits already published to get a better sounding or louder amplifier. This is true even to this day.

Designers often push the limits of what a tube can handle to see if it will work past its conservatively rated parameters used for AM radios and Public Address amplifiers. This is kind of like what hot rodder’s do to cars. Special effects using very odd looking devices or circuits also find their way into designs. And yes, there are actual technical mistakes made by these self-trained designers that become accepted norm for a given model. So I learned to expect any reason could be possible for just why standby switches exist!

It’s not a mute switch for breaks

Historically, I have yet to see an amp made with standby switches until Leo Fender was around. He is accredited for first inventing the idea and I have no reason to doubt this. Leo Fender adopted the standby switch design from reading vacuum tube service manuals. He was self-trained in electronics and developed his own designs. Basically, his switch disconnects the high voltage from the circuit, but the big question is why?

Leo Fender did not intend them for use during beer breaks as a mute switch (the biggest myth of all), even though this is what everyone thought he meant by the “standby” switch label and used them this way! A “mute” switch is a common switch often used on audio amplifiers but never designed the way Leo Fender’s “standby” switch is wired to the high voltage. A mute switch simply connects the audio signal to ground, stopping it from passing through the amplifier, just like turning the volume control all the way down.

One should note the term “standby” has been used occasionally in place of the word “mute” on other switches that actually are audio “mute” switches for taking breaks, further adding to the public confusion. All guitar amplifier companies are infamous for incorrectly labeling or coming up with cute names for a switch’s function. Leo Fender also is known for mislabeling what technically is a tremolo circuit control as a “vibrato”. This is probably because he did not know how to play guitar? Maybe he could have come up with a better name than “standby” that is less confusing? Too late now…

It’s not for protecting tubes

Leo Fender did not use the standby switch to protect the tubes, because it actually is not good to have the tubes on a very long time in standby, which is a fact from the RCA tube manuals. There are so many people who get this part wrong. Beware advice given by some internet guru who was just regurgitating someone else’s myth that sounds technical, but is just wrong!

This myth started with a misunderstanding of the old RCA tube manual recommendation for using standby switches when running very, very high voltage radio station transmitter tubes. RCA was NOT talking about the tubes used in a guitar amplifier. The tubes used in guitar amps are the same type tubes used in Grandma and Grandpa’s old tube radio receivers, TV’s and record players, etc., which you never see with standby switches, do you? Therefore, why would a guitar amplifier be different than these other devices? Because they are not! Fender’s first “Tweed” amplifiers also did not have a standby switch!

For Leo Fender, tubes were cheap back then and actually made much stronger than tubes we have today, so why would he have this supposed concern for tube life? In order to get the tone he wanted, many of his designs are actually very hard on tubes pushing the limits of their power capabilities, therefore it stands to reason that tube life was not his concern.

The standby switch on a Fender amp was put there by Leo to solve a problem he had later when building the much demanded larger power amplifiers using higher voltages to operate.

The actual reason for standby switches

It’s all about the capacitors!

As the public asked for louder amplifiers, Leo Fender began to build amplifiers with higher power supply voltages. When first turning on the amplifier and before the tubes are warm, tubes do not conduct high voltage, so there is no “load” on the power supply. This phenomenon would allow voltage to rise above the maximum voltage rating for the large capacitors used in the circuit, putting them at risk of shorting out from the stress. This was especially true when Fender started to use solid state rectifier diodes that provided power supply voltage instantly when the mains power was turned on.

While the tubes are warming up, the standby switch removed the high voltage from the circuit until the tubes filaments were warmed up to operating temperature and the power supply voltage would be loaded down by the tubes to the nominal safe operating voltage for the capacitors.

Sure, Leo could have installed much higher voltage rated capacitors that could safely handle the voltage rise, but these were very expensive back in his day. His company’s goal was to produce high quality, but lower cost amplifiers (and guitars), so keeping the price down was important to him. Therefore, the standby switch was a cost-saving design feature much cheaper than the alternative very expensive capacitors.

Takeaways

In my experience, if you want your tubes and the other parts of the amplifier to last longer, put a small fan on the amplifier to get the heat out of it. Excess heat is the greatest problem, so only have the amp on when you need it. Let’s review the takeaways.

There are occasionally a few modern amplifier designs that are taking the problems with conventional plate voltage standby switches into consideration and have put in safer systems for tube warm up purposes. To be fair, these systems do not cause the same potential problems as the old fashioned standby switches. If you have one of these amps, the use of the standby switch may not be causing any harm. You will simply have to inquire about your amps features to know what is used.

However, I still refer to other much smarter engineers than I, including the RCA tube manual which do not list any standby switches in the recommended design of receiving tube power supplies. Don’t expect your tubes to last longer using them.

Don’t use it as a “beer break” switch. For short breaks, simply turn down the volume control (or mute switch if you have one) and don’t use the standby switch, so there is not that nasty pop in the house sound system that could damage speaker drivers. If the time between sound check and performing is longer than 20 minutes, turn the amplifier completely off. You only need 5 minutes at the most to completely warm up a tube amplifier.

It’s as simple as that. Why else would you use something that often pops loudly in the audio when used (remember I mentioned it cuts off the high voltage)? By the way, other brands did not use standby switches until Marshall copied Fender’s Bassman amplifier design and after the two biggest makers used these standby switches, everyone assumed you always had one on a guitar amp. Often, designers put these on amplifiers only because the public asks for them, not that they are needed. This is due to the power of the myth! These days we have other devices available to protect the capacitors and in general capacitors are much cheaper now and can be made to run at higher voltages without great cost.

Don’t put one on your amp because you were told it makes the tubes last longer! Is there a way to help my tubes last longer you say? The correct understanding of vacuum tube operational specifications prove there is no evidence that a standby switch can make your tubes last longer and actually could only hurt them if you overuse the standby mode.