Demystifying Tube Amps---The Standby Switch

[Mark Reccord]

Not arguing that, Mark, but I've always heard that the filaments are quite susceptible to damage in amps that allow signal to pass before they're warmed up, such as my old Dano Airline 15w amp. Is that truth or myth?

I'm relatively certain that's a myth but I also stand to be corrected by one of the real experts. The only thing that would even see that signal would be the grid of the very first stage because it wouldn't get through to the plate and to the rest of the amp until the tubes began to conduct. I've been doing it pretty much every day on my preamp prototypes for a year now with no issues. I've also done lots of not great things to the tubes because I wasn't paying attention. 10k plate resistor on a 12ax7 instead of 100k... Oops. Luckily I caught that before destroying the tube.

 

[okcrum]

There are potential damage issues with just slamming on the HT on cold tubes in an amp that was designed with a standby switch.

If there are, the amp is designed wrong.

 

[JimmyM]

I'm relatively certain that's a myth but I also stand to be corrected by one of the real experts. The only thing that would even see that signal would be the grid of the very first stage because it wouldn't get through to the plate and to the rest of the amp until the tubes began to conduct. I've been doing it pretty much every day on my preamp prototypes for a year now with no issues. I've also done lots of not great things to the tubes because I wasn't paying attention. 10k plate resistor on a 12ax7 instead of 100k... Oops. Luckily I caught that before destroying the tube.

Wow, that could have cost you $15 Unless of course it was an old collectable tube...and that would have been a severe wallet ouch.

As for the tube warmup myth or fact, I have yet to damage a tube in that amp by not waiting, but I've only done it a couple times because I get paranoid.

 

[okcrum]

why not rename it mute on modern tube amps? Nostalgia?

I suppose you could label it anything you like in consumer gear. We seem to be gradually remapping all the other words, so why not?

Standby is a literal term for this function. It has a clear meaning in terms of safety. The Standby function's meaning gains weight as the equipment does, so it's descendant from that. Standby actually means something on a broadcast transmitter, X-ray machine, or other large power system.

 

[agedhorse]

Allowing a little bit of time for the amp to cool down can help, because the interior tube structures expand and contract with temperature, and there can be a little slackness in the grids too.

A couple of minutes should be fine.

 

[Wasnex]

I haven't seen any relay. The tubes just have to be warm enough for it to amplify. Randall Smith (Mesa Boogie) knows a thing or two about bass tube amps (Ampeg too but the original founders are long gone) and he recommends it. In fact his latest and probably last tube bass amps, he recommends a longer warm period, instead of the 30 seconds or so, he has extended it to 5 minutes if I recall. I have the Prodigy 4:88. Usually single ended class A do not have it but my THDs have it.

The Mesa hifi tube amp I have (BAron) has some sort of soft start which could probably mean a relay

I have a few amps with some relays but they are not time controlled.

As far as the soft start, I don't think you necessarily need a relay. I have a couple of amps with soft start as well. With one, the schematic shows a fairly high value resistor in the bias supply is shorted when the amp is switched from standby to operate. I am not 100% sure because the schematic is hand drawn and pretty rough, but I think the switching is done with a relay. In standby the bias is high enough to cut off tube current. In operate, bias ramps down with the RC time constant of the circuit. So basically standby turns on the DC voltages and simultaneously gently drops the bias.

I have another amp that also does a soft start, but I am not sure this one is intentional. It has a huge amount of resistance in the grid and bias netork (too much). When you switch the amp to operate, the bias slowly drops until it stabilizes. It's possible this amp has a resistor that is brought into the bias circuit as well, but I haven't found it. I don't have a complete schematic this amp as I have only traced out the signal path.

 

[Wasnex]

I think another potential problem, especially with high powered amps like the SVT is that the plate voltage for the power amp is 600V+ and the HT supplies for the preamp tubes are dropped to the 300V range with a resistor.

I agree this is a valid reason to use standby. The plate voltage is actually closer to 700V when the amp is idling. If you were just to set the amp to operate instead of using standby mode, all of the capacitors in the preamp supply ladder would begin charging towards to unloaded voltage, which is probably around 730V. AFAIK the only components that shunt any current to ground before the tubes start conducting is the series 220K balance bleeder resistors at the first filter node.


I plan to continue using the standby switches in my amps.

 

[Wasnex]

why not rename it mute on modern tube amps? Nostalgia?

Because that not what it really does. As discussed earlier in the thread, a mute function would be implemented in totally fashion than a standby circuit.

 

[BassmanPaul]

Just give your amps with tubes 30 seconds or so to warm up and it really doesn't matter whether you use standby or not.

This business of harm to a non warmed up tube by inputing a signal makes no sense to me. With a cold heater the tube section is passing no current. A signal applied to the grid in such a state is akin to a piece of wire in a vacuum. There are literally thousands of Hi Fi amps out there with no standby switch and SS rectification. How many of them fail?

 

[Wasnex]

Use standby or don't...it's fine.

Maybe this will kick up another hornets nest or not. AFAIK, the whole argument that standby is not useful hinges on the assertion that cathode stripping does not occur and cathode poisoning does. These are two good points in favor of not using standby.

What the argument does not seem to address is the way power supplies are laid out and how voltage ladders work. Typically one set of transformer taps feeds a voltage ladder to develop a range of voltages. You have the highest voltage at the beginning of the ladder and the lowest voltage at the end of the ladder. The different voltages develop due to current passing through the various resistors in the voltage ladder.

With no functioning tubes to pull current through the ladder, all of the filter capacitors will charge towards the highest voltage. Note it takes time for this to occur rather than happening instantaneously. If you turn on an amp in standby mode, the DC voltages are not allowed to develop through all, or part of the voltage ladder, so this problem is eliminated.

To summarize, turning on your amp with the standby switch in the operate position allows higher voltages to develop in the power supply. Long term this should reasonably be expected to causes more wear and tear on the components. In order to design for this, more expensive components are required, and also some extra components are required in certain types of direct coupled tube circuits.

So my conclusion is standby switches are probably included because it reduces the cost of making amplifiers by allowing the use of fewer and lower spec'ed components.

I plan to continue using the standby switches in my amps during turn on and shut down. It's really not that big of a hassle.

 

[okcrum]

Maybe this will kick up another hornets nest or not. AFAIK, the whole argument that standby is not useful hinges on the assertion that cathode stripping does not occur and cathode poisoning does.

Nah, the contention is that it's not necessary. Neither is television. You can choose to avail yourself of either if you want, and make a cogent argument either way.

There is no standby function on the most common tube equipment of all: the microwave oven. So it goes both ways.

 

[Wasnex]

Nah, the contention is that it's not necessary. Neither is television. You can choose to avail yourself of either if you want, and make a cogent argument either way.

There is no standby function on the most common tube equipment of all: the microwave oven. So it goes both ways.

Of course you didn't address the rationale for why I think so many amps use standby switches . My contention is, in a round about way, the standby function is sort of a soft start circuit for guitar amps, but its more to protect the filter caps than the tubes.

I don't think the operation of a microwave really has all that much in common with a guitar amp .

 

[JimmyM]

Of course you didn't address the rationale for why I think so many amps use standby switches . My contention is, in a round about way, the standby function is sort of a soft start circuit for guitar amps, but its more to protect the filter caps than the tubes.

I don't think the operation of a microwave really has all that much in common with a guitar amp .

Now I understand why it took 9 hours to cook a hot dog in my SVT.

 

[agedhorse]

Now I understand why it took 9 hours to cook a burrito in my SVT.

I fixed it for ya Jimmy

Ok, how many of you guys have cooked something on the exhaust manifold on the way to of from a gig? There are sound guy recipes for cookin things on the way to a gig.

I once heated up a can of chili driving to NAMM about 25 years ago. It was a little messy to open but was better than fast food

 

[byacey]

I fixed it for ya Jimmy

Ok, how many of you guys have cooked something on the exhaust manifold on the way to of from a gig? There are sound guy recipes for cookin things on the way to a gig.

I once heated up a can of chili driving to NAMM about 25 years ago. It was a little messy to open but was better than fast food

Alternatively, out on the highway I've cooled down canned beer in a wet fabric bag hanging from the side mirror on the truck. When we arrived at the fishing spot, it was quite cool; you just have to remember to keep the bag wet.

Oh, in the winter I drink rum, which you can also cool on the mirror; especially well when it's -35C!

 

[byacey]

What the argument does not seem to address is the way power supplies are laid out and how voltage ladders work. Typically one set of transformer taps feeds a voltage ladder to develop a range of voltages. You have the highest voltage at the beginning of the ladder and the lowest voltage at the end of the ladder. The different voltages develop due to current passing through the various resistors in the voltage ladder.

Maybe this is a regional dialect thing, but we call them voltage dividers around these parts.

 

[JimmyM]

I fixed it for ya Jimmy

How about we combine the two and make it a burrito dog?

Damn, I sure know how to torture myself when I'm trying to lose weight!

 

[Bertr]

I fixed it for ya Jimmy

Ok, how many of you guys have cooked something on the exhaust manifold on the way to of from a gig? There are sound guy recipes for cookin things on the way to a gig.

I once heated up a can of chili driving to NAMM about 25 years ago. It was a little messy to open but was better than fast food

My Mum used to warm up my milk bottle on the manifold of the car during long trips. Not recommended: I have developped a case of severe Flat-6, V8 and Italian twin-cams addiction...

 

[Wasnex]

Maybe this is a regional dialect thing, but we call them voltage dividers around these parts.

Nah, that's just the term of art that came to mind. I have not worked in electronics since about 1989. Also, I was just a technician, rather than an engineer or instructor, so i hads sum limited book smarts to begin with .

 

[okcrum]

Of course you didn't address the rationale for why I think so many amps use standby switches . My contention is, in a round about way, the standby function is sort of a soft start circuit for guitar amps, but its more to protect the filter caps than the tubes.

That's because I felt no need to argue with your rationale. I consider it valid, even though it differs from my opinion.

I don't think the operation of a microwave really has all that much in common with a guitar amp .

I dunno, it takes about the same amount of expertise to operate either a microwave or a tube guitar amp. The microwave also has a vacuum tube of considerably higher power output than a guitar amp, and no standby/soft start function, in addition to being ubiquitous.