Standby switch vs. leaving amp in run position vs. turning off

[agedhorse]

Sweetwater's take:

The Great Standby Switch Myth | Sweetwater

"
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.

View attachment 4141388"

In the battery powered farm radio days standby turned off the high voltage vibrator. The vibrators hummed. It probably also helped when high powered tube amps were not able to use huge filter caps - less hum from the amps on stage between sets.

And when/if you change caps, push the guts out of the cardboard tubes and replace them with a new cap in the tube. They'll look more authentic. It's something many amp restorers do these days. Even for SS amps.

My experience with most Fender amps dismiss the claim of capacitors as a complete myth. On many Fender amps, the capacitors are located BEFORE the standby switch, which makes that argument false.

Additionally, there are many examples of filter caps sized to handle the +10% high line voltage condition.

It’s cheaper to install a higher voltage cap.

Myth busted.

 

[Root 5]

Yes, and as proof just take a look at all the 50-70 year old radios and TVs! Oh wait...

Tube TVs were very large and heavy and new ones of comparable picture size are not. We don't see 50-70 year old TVs because the part supply chain is gone and the picture is inferior. And who would want that? Technology was developed that is superior to tube TVs. As to radios, modern ones are lighter, cheaper, with less maintenance. Parts need to be replaced in tube amps too. We could have 50-70 year old amplifiers out of action today if the supply chain disappeared.

 

[Root 5]

My experience with most Fender amps dismiss the claim of capacitors as a complete myth. On many Fender amps, the capacitors are located BEFORE the standby switch, which makes that argument false.

But wouldn't you think the Sweetwater guy knows this? If he doesn't I would think he has no business working on amplifiers.

 

[agedhorse]

But wouldn't you think the Sweetwater guy knows this? If he doesn't I would think he has no business working on amplifiers.

I suspect that this was influenced by marketing and simple perpetuation of urban myth.

It’s simply not true in most (not all) cases. Certainly not based on the evidence that I have.

 

[beans-on-toast]

There’s a lot of interesting design information in this tube datasheet. I wish that they were all as informative. The 6146B was used in the early SVT.

[Invalid or Expired Link Removed]

They talk about standby operation top right. For maximum reliability RCA recommended that the heater voltage be reduced by 80% of normal when the amp was going to be idle for 15 minutes to 2 hours. I assume that the comment was directed towards transmitter applications.

 

[bigtone23]

these days i mostly labor under the assumption that standbys are somewhat useful for high power amps, not so much for low power amps, and that letting the power tubes warm up a little so the heaters are fully running before kicking in the plate voltage might be a good idea.

i further assume that there isn't much point in any "cooldown" procedures, just turn the stupid thing off already.

hitting the standby just to unplug and switch instruments is silly; just pull the signal cable out of the amp a little, the input jack will typically short itself to ground and give you silence while you switch

This is exactly how I view the standby switch. I sometimes bypass the standby switch on my tube rectified amps, but do use them on the diode rectified ones.

I just remembered a product I designed that uses an automatic standby circuit.

On the Streamliner, there is a standby circuit that disables the power amp while the tube preamp stabilized during warmup.

This is not necessary because on a tube power amp, the delay of the power tubes is longer than the preamp tubes, but on the class D power amp used in the Streamliner, a time delay standby circuit is used to allow the preamp tubes come to quiescent operating conditions before enabling the power amp. It’s all done automatically.

This is a different application of a standby circuit, similar yet different.

I prefer automatic, start up standby circuits and appreciate them.
I read an article from Hartley Peavey where he uses an inrush current limiter to do an automatic standby procedure. My old Bravo didn't have a standby, but certainly had this circuit in it.
It's discussed on page 5 here... https://peavey.com/PDFs/HPW-Chapter-6.pdf
He also discusses 'sleeping sickness'/cathode poisoning. This also a good read.

 

[agedhorse]

This is exactly how I view the standby switch. I sometimes bypass the standby switch on my tube rectified amps, but do use them on the diode rectified ones.


I prefer automatic, start up standby circuits and appreciate them.
I read an article from Hartley Peavey where he uses an inrush current limiter to do an automatic standby procedure. My old Bravo didn't have a standby, but certainly had this circuit in it.
It's discussed on page 5 here... https://peavey.com/PDFs/HPW-Chapter-6.pdf
He also discusses 'sleeping sickness'/cathode poisoning. This also a good read.

Yes, but cathode poisoning is not a 10-15 minute event. It's a MUCH longer event horizon than that.

Like everything, "it depends".

Also, technology evolves, cathode and heater design evolves, so do some designs evolve, the chemistry evolves. What was 100% true 75 years ago may be only 25% true 30 years ago and 10% true now.

 

[Wasnex]

Tube TVs were very large and heavy and new ones of comparable picture size are not. We don't see 50-70 year old TVs because the part supply chain is gone and the picture is inferior. And who would want that? Technology was developed that is superior to tube TVs. As to radios, modern ones are lighter, cheaper, with less maintenance. Parts need to be replaced in tube amps too. We could have 50-70 year old amplifiers out of action today if the supply chain disappeared.

I have a perfectly functional 26" tube TV in my garage. Of course all of the components are solid state except the CRT. The problem is US broadcast standards changed a few years back from analog to digital, so it will no longer work without some sort of an AD converter box. I used it for a few years after the transition to digital broadcast with my Xbox .

I also have a Tivo with a life long subscription that only works with analog .

I think there is a very real possibility the musical instrument tube manufacturing industry will collapse sometime in the near future. Making tubes is very dirty business and I don't get the impression there is a huge profit margin. There are pretty much three major companies making tubes, New Sensor, Shuguang, and JJ. I believe some of the manufacturing lines are still using old equipment from the glory days of tubes. What happens when this equipment wears out. Ask Winged C (Svetlana)...they stopped production in 2012.

 

[808State]

Some older tube TVs had something called "fast start". Or numerous other names to describe the same function.

Usually it took awhile for the picture tube heater to warm up and would take awhile for the picture to come up.

Fast start" basically kept the heater on all the time, and turning the TV "on" would apply high voltage.
The picture would come up almost instantly. Its basically the same as leaving a amp on all the time on standbye.



All these weird assumptions about the order of switching, is usually plain ridiculous.
If the amp is already on, you just use stand bye to mute things for connecting or reconnecting.

Most Fenders with solid state rectifiers and higher voltages. Uses 2 capacitors in series in the power supply
so the capacitors are capable of handling the startup voltage peak.

Standby is after the capacitors, so yes you wont have a voltage surge on the caps like turning the amp on and off. Depending on the bleeder resistor size it still takes 15 to 20 minutes for the caps to drain.
So its really not that important. not really bleeders, series caps need balancing resistors.

Standbye is to switch channels without making noise. Or instrument change without making noise.
Just call it "mute" instead so nobody freaks out over nothing. Or just leave your heaters cooking for hours and hours for no reason. If you not using the amp, then turn it off....what a concept.

If you want to use the amp...turn it on. What a concept.

Old amps didnt have channel switching, you had to unplug/ replug to switch channels. So a mute switch or standbye was a nice feature, So it did not make obnoxious noise.

 

[808State]

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.

If the amp isnt going to be used for a few hours
There is no need to just leave the heaters cooking away for hours.

Standbye should be considered a mute switch.
Usually its good practice to have a cabinet and the instrument plugged in before the amp comes on.

If for some reason you need to plug in your bass or switch channels when the amp is on.
Stand bye just mutes the signal so it doesnt make horrible sounds plugging in the amp.

People have mentioned older tube TVs. A new " feature" for some TVs was a " fast start"
instead of waiting 5 or 10 minutes for the sound and picture to come on.

The TV would leave the heaters running all day all night all week etc etc.
So when you did turn on the TV. The sound and picture came up instantly.

Find a old TV repair guy. They love fast start TVs because the tubes were replaced often.
Since they sat there cooking all there life when the TV was "off".
Huge waste of energy running heaters constantly. Older TVs would never pass modern
low energy requirements. Specially fast start TVs.
Receiver tubes and sound tubes can be replaced, fast start TVs destroyed the picture tube faster as well.

If you dont plan on using the amp within 10 to 20 minutes. Then turn it off.
It doesnt matter where the standbye switch is at on power up or off.
Just a mute.

 

[Root 5]

I suspect that this was influenced by marketing and simple perpetuation of urban myth.

It’s simply not true in most (not all) cases. Certainly not based on the evidence that I have.

I respect your opinions. I’m certainly no amp expert and have never claimed to be. Just repeating what I have heard from respected people.

 

[agedhorse]

I respect your opinions. I’m certainly no amp expert and have never claimed to be. Just repeating what I have heard from respected people.

Agreed, this is the root of much of the mythology, usually based on some truth but not necessarily in context.

 

[808State]

There’s a lot of interesting design information in this tube datasheet. I wish that they were all as informative. The 6146B was used in the early SVT.

[Invalid or Expired Link Removed]

They talk about standby operation top right. For maximum reliability RCA recommended that the heater voltage be reduced by 80% of normal when the amp was going to be idle for 15 minutes to 2 hours. I assume that the comment was directed towards transmitter applications.

View attachment 4142231

Its a interesting tube indeed. One of the longest manufactured tubes by American Producers.
Still manufactured way up till the mid 80's
Basically when most tube manufacturing was long long gone. Was still being made till about 84/85

Its amazing Ampeg had problems with them. Since its considered one of the most reliable tubes ever made.

You would think when the " boutique" thing went wild it the late 90's/2000's
Guys would be buying these up like crazy. Since New Old Stock was available for incredible low prices.
Thousands of the things just sitting on shelfs

 

[beans-on-toast]

Its a interesting tube indeed. One of the longest manufactured tubes by American Producers.
Still manufactured way up till the mid 80's
Basically when most tube manufacturing was long long gone. Was still being made till about 84/85

Its amazing Ampeg had problems with them. Since its considered one of the most reliable tubes ever made.

You would think when the " boutique" thing went wild it the late 90's/2000's
Guys would be buying these up like crazy. Since New Old Stock was available for incredible low prices.
Thousands of the things just sitting on shelfs

The military used the tube so that’s the best way to ensure a good supply.

If you want an amazing sounding tube, look to the 300B. It was introduced in 1938 by Western Electric. Kind of expensive for a musical instrument amplifier although I have seen a couple.

 

[Wasnex]

Its a interesting tube indeed. One of the longest manufactured tubes by American Producers.
Still manufactured way up till the mid 80's
Basically when most tube manufacturing was long long gone. Was still being made till about 84/85

Its amazing Ampeg had problems with them. Since its considered one of the most reliable tubes ever made.

You would think when the " boutique" thing went wild it the late 90's/2000's
Guys would be buying these up like crazy. Since New Old Stock was available for incredible low prices.
Thousands of the things just sitting on shelfs

The SVT originally had a fault in the design that caused blocking distortion in second triode of the driver tubes. Since the drivers control bias in the output section, this was a huge problem. Essentially if the drivers saw a huge transient, bias to the output section was essentially cut off and the output tubes would conduct extremely hard.

The solution was to build in node H of the power supply. Part of this update was to relocate the plate supply of the first driver section from Node C at 320V to Node H at 220V. Reducing the plate voltage on the first triode is supposed to keep the second triode from being pushed into blocking distortion.

Shortly after the factory mod to install Node H was released the SVT was converted over to 6550s; perhaps this is why. Another problem is the 6146 screens are less robust the 6550s. The fragility of the tube's screens is discussed widely in radio maintenance forums. 6550 screens are rated for max dissipation of 6W, while 6146 screens are only rated for 3W. When an amp is asked to produce more than it's rated power, screen current increases quickly. So if you plan to really crank your SVT, maybe a 6146 equipped amp is not the best choice.

On plus side, I have seen several testimonials that say SVTs running 6146s make about 450W clean, while SVTs running 6550s tend to only make about 270W clean.

 

[walterw]

I read an article from Hartley Peavey where he uses an inrush current limiter to do an automatic standby procedure. My old Bravo didn't have a standby, but certainly had this circuit in it.
It's discussed on page 5 here... https://peavey.com/PDFs/HPW-Chapter-6.pdf
He also discusses 'sleeping sickness'/cathode poisoning. This also a good read.

huh! this article is almost bizarrely on-target to my situation

i have that very amp, a peavey bravo, and it lives basically built into my repair bench (chassis up under a shelf in easy reach, speaker in a cab down by my feet) as the test amp, where it's left on for like 8 hours a day every day for many years now.

i added a standby switch forever ago with the goal of leaving the amp on all day without cooking the tubes too much; it's been so long ago now i don't remember how i wired it, i'm sure it was just interrupting the B+ voltage.

as it is i get a couple years out of a set of EL84 power tubes and longer with the three 12AX7s; after literally two decades of daily use i just last year had to replace the filter caps.

now i'm curious about this idea of the standby cutting the screen grids rather than the plate

 

[808State]

The SVT originally had a fault in the design that caused blocking distortion in second triode of the driver tubes. Since the drivers control bias in the output section, this was a huge problem. Essentially if the drivers saw a huge transient, bias to the output section was essentially cut off and the output tubes would conduct extremely hard.

The solution was to build in node H of the power supply. Part of this update was to relocate the plate supply of the first driver section from Node C at 320V to Node H at 220V. Reducing the plate voltage on the first triode is supposed to keep the second triode from being pushed into blocking distortion.

Shortly after the factory mod to install Node H was released the SVT was converted over to 6550s; perhaps this is why. Another problem is the 6146 screens are less robust the 6550s. The fragility of the tube's screens is discussed widely in radio maintenance forums. 6550 screens are rated for max dissipation of 6W, while 6146 screens are only rated for 3W. When an amp is asked to produce more than it's rated power, screen current increases quickly. So if you plan to really crank your SVT, maybe a 6146 equipped amp is not the best choice.

On plus side, I have seen several testimonials that say SVTs running 6146s make about 450W clean, while SVTs running 6550s tend to only make about 270W clean.

6146B and 6550 can both do 35 watt plate. But if you wanted to abuse or hot rod a tube like many musical instrument amps have done. The overall size of the 6146B and the top plate connection. Just screams please abuse me Im ready lol.

RCA highly recommended " Pentode" operation for Audio use. But you had to read in further cause often
Pentode would mean Ultra Linear. Since apparently the details of that patent had work arounds by name changing.

But if you look at the Schematics and transformer requirements. You realize Ultra Linear was recommended for audio use. And required a dual transformer with extra taps. The plate could go to 750 volts but the screen only 200 volts. So the ultra linear tap also needed separate power tap for only 200 volts.

Was a rather expensive transformer to do it right. Normal pentode mode if the screen had a large AC ripple. It would cause problems. The DC had to be very clean, usually a regulator or large inductors.
6146 did not tolerate AC ripple on the screen. If there was AC it had to follow the input signal, or modulated screen or Ultra Linear. Ampeg probably just used Pentode mode. If the screen was over 150 volts and was dirty with AC ripple. 6146 wont last very long. Needs to be Ultra Linear, or the DC supply has to be very clean 150 volts to the screen.
When they said clean, it was very clean. Inductor might work, but all the RCA applications use a Regulator tube for the screen supply.

 

[Wasnex]

6146B and 6550 can both do 35 watt plate. But if you wanted to abuse or hot rod a tube like many musical instrument amps have done. The overall size of the 6146B and the top plate connection. Just screams please abuse me Im ready lol.

RCA highly recommended " Pentode" operation for Audio use. But you had to read in further cause often
Pentode would mean Ultra Linear. Since apparently the details of that patent had work arounds by name changing.

But if you look at the Schematics and transformer requirements. You realize Ultra Linear was recommended for audio use. And required a dual transformer with extra taps. The plate could go to 750 volts but the screen only 200 volts. So the ultra linear tap also needed separate power tap for only 200 volts.

Was a rather expensive transformer to do it right. Normal pentode mode if the screen had a large AC ripple. It would cause problems. The DC had to be very clean, usually a regulator or large inductors.
6146 did not tolerate AC ripple on the screen. If there was AC it had to follow the input signal, or modulated screen or Ultra Linear. Ampeg probably just used Pentode mode. If the screen was over 150 volts and was dirty with AC ripple. 6146 wont last very long. Needs to be Ultra Linear, or the DC supply has to be very clean 150 volts to the screen.
When they said clean, it was very clean. Inductor might work, but all the RCA applications use a Regulator tube for the screen supply.

A lot of the old radio techs insist the screen voltage on the 6146 must be kept below 200V for the tube to be reliable. Although I am sure this does extend tube life, I think it limits power quite a bit.

The 69 SVT schematic says 220V on the screen supply. Based on the readings I have gathered, the real world operating voltage on the output of the screen supply should be around 230V. Some say 235V is ideal.

All six tubes pull a total of about 2ma on idle, so the voltage on the screens is only slightly lower than the screen supply.

I do know of one member who had a 6550 PT in his blueline wh ran an inductor to filter the supply and drop the voltage down so it was safe for the 6146s. The spec for the inductor he used are as follows, 15H, 600ohms, 50mA. He said the inductor dropped the voltage down to about 260V, and the amp was reliable for backline services where it was used for 2-4 at a time, but not pushed into distortion.

 

[808State]

A lot of the old radio techs insist the screen voltage on the 6146 must be kept below 200V for the tube to be reliable. Although I am sure this does extend tube life, I think it limits power quite a bit.

The 69 SVT schematic says 220V on the screen supply. Based on the readings I have gathered, the real world operating voltage on the output of the screen supply should be around 230V. Some say 235V is ideal.

All six tubes pull a total of about 2ma on idle, so the voltage on the screens is only slightly lower than the screen supply.

I do know of one member who had a 6550 PT in his blueline wh ran an inductor to filter the supply and drop the voltage down so it was safe for the 6146s. The spec for the inductor he used are as follows, 15H, 600ohms, 50mA. He said the inductor dropped the voltage down to about 260V, and the amp was reliable for backline services where it was used for 2-4 at a time, but not pushed into distortion.

He was on the Right track to use the inductor.
RCA recommended 200 volts with 500 to 600 plate.
If you ran 750 volts plate no More than 150 volts on the screen.

Its a amazing tube really, 135 watts with a pair no problem and 750 volt plate.
screen only needed 150 volts for 135 watt pair.
RCA had a lot of side notes for this tube, and recommended interstage transformer for the grids.
And specified a regulated separate source for screen.

Far as radio guys saying under 200 volts. They are very correct.
Very Clean 150 volts is all that is needed. Its just expensive to make a 400 watt amplifier work.

Later magazines that published audio designs for the 6146.
Went all out, Transformer driven grids, Ultra Linear Operation, 150 volt regulated supply for screens.
The output transformer had 2 power taps. One for plate, One for UL screens. Inductor for plate supply as well.
Power Transformer had 5 volt and 6.3 volt screen supply

So its a ridiculous amount of iron to make a 135 watt pair lol
Be beautiful amp though. 50 and 60's most home Hifi was still 15 to 30 watts.
So a magazine that published a 135 watt amplifier was insane at the time.
Wish I could find the article, they called it the Law breaker or Cop Caller or something lol

 

[Wasnex]

He was on the Right track to use the inductor.
RCA recommended 200 volts with 500 to 600 plate.
If you ran 750 volts plate no More than 150 volts on the screen.

Its a amazing tube really, 135 watts with a pair no problem and 750 volt plate.
screen only needed 150 volts for 135 watt pair.
RCA had a lot of side notes for this tube, and recommended interstage transformer for the grids.
And specified a regulated separate source for screen.

Far as radio guys saying under 200 volts. They are very correct.
Very Clean 150 volts is all that is needed. Its just expensive to make a 400 watt amplifier work.

Later magazines that published audio designs for the 6146.
Went all out, Transformer driven grids, Ultra Linear Operation, 150 volt regulated supply for screens.
The output transformer had 2 power taps. One for plate, One for UL screens. Inductor for plate supply as well.
Power Transformer had 5 volt and 6.3 volt screen supply

So its a ridiculous amount of iron to make a 135 watt pair lol
Be beautiful amp though. 50 and 60's most home Hifi was still 15 to 30 watts.
So a magazine that published a 135 watt amplifier was insane at the time.
Wish I could find the article, they called it the Law breaker or Cop Caller or something lol

The is may be what you are talking about.

Source: http://www.bunkerofdoom.com/xfm/ACROSOUND_1955/acrosound_1955.pdf (page 11)