Do Not Use Standby on an AC30

[Lyle Caldwell]

I don't know how many times the collective gurus here have said it in various threads, so I'm making a thread just about this subject.

Do not use the Standby switch on an AC30TB/TBX or CC. It is not needed, and can damage the rectifier tube due to inrush current and a voltage spike that over time will cause the tube to fail.

Do not use it. Ever. Just don't. It's dumb.

I'll paste the technical reasons in below after I've had more caffeine.

[tracynorton]

I can see why...but it doesn't look to be a stretch to mod it to come AFTER the rect and BEFORE the choke/OTCT....unless it's all on circuitboard, and involves trace cutting and haywiring.....but we're good at that

[Lyle Caldwell]

I can see why...but it doesn't look to be a stretch to mod it to come AFTER the rect and BEFORE the choke/OTCT....unless it's all on circuitboard, and involves trace cutting and haywiring.....but we're good at that

Doesn't matter where it is - it's not needed.

[tracynorton]

without question, it's not needed, but I get a lot of requests for standby switches on amps that have none.....

on my own toob rectified amps, I don't bother putting them in at all

so in your post caffeine explanation, you might want come up w/ a mod to move the standby for those that still want it (just a request)

[oddjobpeters]

And when using a Weber Copper Cap?

[Lyle Caldwell]

without question, it's not needed, but I get a lot of requests for standby switches on amps that have none.....

on my own toob rectified amps, I don't bother putting them in at all

so in your post caffeine explanation, you might want come up w/ a mod to move the standby for those that still want it (just a request)

It's doable, but still pointless IMO.

[Lyle Caldwell]

And when using a Weber Copper Cap?

The WZ34 has a thermistor inside which replicates the GZ34's slow start, so no standby needed.

[R.G.]

I'm guessing that Lyle is correct and standby is not needed.

But I can see you having trouble with clients who demand it, and who not only will not believe you when you tell them why, they will think less of your technical abilities for having such an heretical idea. I get that a lot with as many heresies as I propose.

There are some alternate ways to go about this. Since Lyle says the inrush current is an issue, you can make the inrush not be severe enough to hurt, or you can make the rectifier immune.

The copper top is the immunity way. The solid state rectifiers should not care about any inrush this amp can generate.

In the way of making the inrush more limited, I can think of a few ways. One is a thermistor on the B+. It's no help for the standby flippers who cycle it back and forth, but if it has time to cool off it should be usable for slowing the inrush after a a minute or so.

Another way is to do a small circuit that will arbitrarily limit the inrush. One way is with a MOSFET current clamp. It is possible to make a small circuit with a high voltage MOSFET which will not higher than X current pass, and X is a value you pick. A little more effort will make it ramp current up slowly. The static voltage drop at full current would be maybe 4V.

Another way to do it is to cold-switch the B+; that is, the standby switch tells a small circuit to allow or disallow B+ to pass. The pass circuit can have built in current clamping to protect the rectifier.

[Lyle Caldwell]

The current inrush is one issue, and could be fixed just by adding series diodes to the rectifier from the PT secondaries.

But there is also a voltage spike (over 700v) which can damage the rectifier tube, the output transformer, the filter caps, and the standby switch itself, though the rectifier tube is the most likely one to fail.

This can also be addressed with a varistor across the switch.

But how many mods must one make to safeguard a non-needed switch?

I posted this in a less technically-inclined forum, but I think it explains the issue well:

Better for the amp if you either just leave it on with the master down or turn it off until 5 minutes before you start playing. Standby offers no benefit but every time you engage the standby switch there is a large current inrush and voltage spike. This gradually damages the GZ34 and eventually the filter caps and output tubes. Nominal B+ on these amps is about 325vdc, and the filter caps are rated 400-500vdc. When you flip the standby switch, voltage spikes of over 700vdc with brief but heavy current surges are common. These put undue stress on the parts mentioned above, as well as eventually causing the switch itself to fail.

Sometimes its a lot of fun when the spike/surge causes the rectifier tube to actually arc weld itself. You get a light show and a funky smell. The light show is over fast, but that smell lingers. And sometimes it takes out other parts of the power circuit! Yay!

And the reason people tell you TO use standby is so there is no instantaneous plate voltage on the tubes until the heater filaments in the tubes are warmed up. Thing is, no B+ (plate supply) flows in these amps until the filaments in the GZ34 have warmed up themselves, by which time every other tube in the amp also has its filaments heated.

So there is no valid reason to use standby, but lots of empirical, explainable, and predictable reasons not to.

Note, for the sake of this argument, I am only discussing amps that use a GZ34 rectifier tube or the Weber replacement. So JTM45, AC30, Bassman, etc. Amps with other tube rectifier types or solid state rectifiers are not part of this discussion.

Though really, unless you're talking about an extremely high voltage amp like an SVT (700+vdc B+) cathode stripping is really not a danger, so there still is no compelling reason to use Standby on a Twin or Marshall either.

Lots of techs "know" things based on what they have been told. That really is not knowledge. That is just a good memory.

Let me put this in less technical terms.

It's rare for you to be sitting in a room reading a book and the light in the lamp just goes off.

But it's common to switch on a lamp and have the bulb die at that point.

In the first scenario, the light has a steady voltage and steady current through it. Unless physically vibrated, that filament inside that bulb will keep on going a long long time.

But that same bulb when subjected to the voltage spike and current increase due to the electricity quickly being switched on will fail. The filament that could withstand hours and hours and hours with a steady voltage and current will fail instantaneously when the voltage and current briefly exceed its threshold of failure.

Now, that same bulb on a dimmer would not fail at power up if the dimmer was all the way down when the switch was turned on, and then you slowly brought the voltage up with the dimmer.

This last scenario is what happens when you let the filaments of the GZ34 control the voltage sent to the rest of the amp, rather than using the Standby switch.

Standby = instant on.

GZ34 = slow dimmer increase.

Hope this helps.

[Lyle Caldwell]

And that last point - the slow voltage rise rather than the instant on voltage, even if you eliminate the current surge and the voltage spike, is preferable IMO.

With a power scaled amp you can also do a slow voltage drop before powering off the amp, eliminating any speaker pops.

[Lyle Caldwell]

Tracy- there's your solution, btw. If a client wants "standby" then install power scaling instead. You will be able to offer them a valid equivalent with huge tonal benefits and gain, not lose, "tech cred".

With a PS amp, turn the PS level down to 0 before turning on, then turn back down to 0 before powering off.

[Digam11]

Not to hi-jack this thread and hopefully this isn`t a stupid question but .... the same goes for an AC15CC ?

Thanks, Mike.

[R.G.]

The current inrush is one issue, and could be fixed just by adding series diodes to the rectifier from the PT secondaries.

Well, that keeps the PT and OT from dying, but the rectifier tube is still toast.

But there is also a voltage spike (over 700v) which can damage the rectifier tube, the output transformer, the filter caps, and the standby switch itself, though the rectifier tube is the most likely one to fail.

I'm wondering - where does that voltage spike come from?

But how many mods must one make to safeguard a non-needed switch?

That's probably the most salient issue. Why use it if it's not needed.

The problem is, if it's there, people will use it.

Here's a thought - un-wire the standby switch and use it to mute the audio instead of interrupting power. The same amp behavior results - warm amp, but guaranteed quiet while the standby is on - but the bad effects of the standby are sidestepped. Now a semi-literate guitar player can flip it all he wants and not harm the amp.

Though really, unless you're talking about an extremely high voltage amp like an SVT (700+vdc B+) cathode stripping is really not a danger, so there still is no compelling reason to use Standby on a Twin or Marshall either.

Yeah. Switching the cathodes with a MOSFET also works.

Again - the reason I keep putting in alternate suggestions is that it's only rare guitar players who will understand and follow the reasoning that says don't use the standby switch if it's there. If you modify the standby switch to make it do something that they think is right, you remove the social engineering problem at the same time as the technical problem.

[Lyle Caldwell]

The current inrush is one issue, and could be fixed just by adding series diodes to the rectifier from the PT secondaries.

Well, that keeps the PT and OT from dying, but the rectifier tube is still toast.

How so? The diodes would be between the rectifier tube and the secondary of the PT (which is switched on/off by the standby switch in the stock schematic).

But there is also a voltage spike (over 700v) which can damage the rectifier tube, the output transformer, the filter caps, and the standby switch itself, though the rectifier tube is the most likely one to fail.

I'm wondering - where does that voltage spike come from?

From the standby switch itself.

But how many mods must one make to safeguard a non-needed switch?

That's probably the most salient issue. Why use it if it's not needed.

The problem is, if it's there, people will use it.

Here's a thought - un-wire the standby switch and use it to mute the audio instead of interrupting power. The same amp behavior results - warm amp, but guaranteed quiet while the standby is on - but the bad effects of the standby are sidestepped. Now a semi-literate guitar player can flip it all he wants and not harm the amp.

Again - the reason I keep putting in alternate suggestions is that it's only rare guitar players who will understand and follow the reasoning that says don't use the standby switch if it's there. If you modify the standby switch to make it do something that they think is right, you remove the social engineering problem at the same time as the technical problem.

The trouble there is most players think standby extends tube life, uses less energy, etc. Using the switch as output mute would please the player until he learned that the amp is being muted but the tubes are still running full tilt.

Maybe the answer, short of full power scaling, would be to do a MOSFET circuit than slowly ramps the B+ up and down, emulating the behavior of a standby but with none of the downsides. It would be easy to do - a very miniaturized version of power scaling, but with just "on/off" options and it would affect all the B+ supply, not just the output. Or maybe MOSFETs would be overkill here. Just thermistors could suffice if the circuit is clever enough.

Anyway, I'm heading to a kid-free afternoon at the pool. I'll ponder this in the deep end.

[AC50]

Same for the AC30HW?