Searched here and the web and find so many different opinions on tube amps and standby switches.
I gather it is good to place the amp in standby mode when switching the power on or off. What about between sets or if the amp is not going to be used a few hours later?
I was taught that the amp should be placed in standby whenever it was at rest and not being used. I was also taught that it was less stress on the tubes to leave the amp on and in standby even if you were taking a break for a few hours (as opposed to turning it off and then back on).
Looking for some science on this one from techs and engineers. Amp in questions is an SVT-CL.
IMHO the real answer probably varies from amp to amp. I use my standby when I turn the amp on and off. I also use it like a mute when I unplug an instrument or the speaker cable.
If I am going to take a 20 minute break I put the amp in standby and leave the power on. If I am going to be take a much longer break I turn the amp off.
Some people think the amp needs to be on for a significant time for it to sound it's best. If you believe this, you will probably leave the amp in operate mode during breaks, regardless of how long they are. The cost will most likely be shortened tube life.
Whether using the standy or leaving the amp running all the time is hardest on the amp would depend on exactly what standby does and how the amp transitions from standby to operate. Some amps develop high bias in standby, so the output section is biased completely off when the amp is set to operate. This prevents the transient that some amps produce. It also means the amp needs a few seconds to for tube current to ramp up before the amp is ready to make good clean power.
Other amps make a pretty significant thump when you set them to operate. This thump is voltage and current being applied quickly. My believe is this quick transition may represent more of a shock to the circuit and tubes. These amp are pretty much ready to play right away, but it will take about 20 minutes for the tubes to reach full operating temperature.
Running the amp in standby only runs the heaters, so the cathodes are ready to emit electrons. The tubes will warm up, but not reach full operating temperature. Once the tube is passing electrons from the cathode to the plate, a lot more heat is generated in the tube.
I think there is some concern with leaving a cathode biased amp running with no signal applied. If you plan to take a break either put the amp in standby or turn it off. The reason is because cathode biased amps actually run cooler when you are playing than when they are idling.
Cathode biased amps are often called Class A, but few if any are actually class A. The Vox AC30 is the prime example. Max bias for one of these amps is up around 90% dissipation but some techs like to bias them over 100%. In other words, the tubes are constantly pulling more current at idle than they are designed to handle. I have read some posts that suggest certain techs like to bias AC30 designs at about 117%. Obviously these amps are very hard on the tubes, but the players who use them can pretty much afford to through in a new set every gig or so.
I have read a couple of threads where it is said these amps cool down when they are cranked. Why? When you play music through the amp, the current level in the tubes goes up and down instead of staying at the constant 117% dissipation level. If you play the amp hard enough, the tubes have a chance to cool during the half cycle when they are at minimal current. Keep in mind the output section is push pull so one set off tubes will have increased current while the other set of tubes will have decreasing current. If run into clipping, the pull circuit may be completely cut off.
Another factor that relates to this is when the amp is asked to make max power, the power supply voltages will sag. Less voltage=less current. Less current=less heat.
There are some fairly scientific articles and possibly even a few studies on this if you hunt hard enough. From what I understand, the types of tubes used in guitar amps do not suffer significantly from cathode poisoning or cathode stripping, but I believe there is some minor effect. Essentially you don't want to leave the amp in standby for days at a time.
As far as whether you should use the standby at all. Some amps do not have a standby. The two amps in my collection that do not have a standby are a JMI era Vox AC30 and a Matamp GT120. The Vox AC30 is cathode biased, so I don't think a standby is really much of a factor. The Matamp has fixed bias. The current ramps up smoothly when the tube cathodes get hot enough so I think it's fine.
On a related note, we had a recent thread on an a vintage V4. If the amp was allowed to warm up in standby for a reasonable amount of time, it would work fine. If the standby was not used and the amp was simply set from off to "on, operate," the output tubes would red plate. Apparently it was taking longer for the bias supply to charge up than it was taking the B+ to charge up. So when the cathodes got hot enough to pass current, the tubes red plated.
We suggested using the standby on this amp. The OP sent the tubes back and got a different brand. IMHO, although the current tube are not red plating, I think there is a good chance they are still being exposed to high current while the bias supply is ramping up, and that ultimately this will reduce tube life IMHO it would be best to let this amp run in standby for a short period of time so the bias supply can charge up fully. Here is the thread if you want to read it.
NAD: Ampeg V4
