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Standby switch on tube amp?


RudyH

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I've always had Fender tube amps, and ALWAYS use the standby switch. A minute or so for warm up, then at least 30 seconds or so for cool down before powering off.

 

If you don't use standby during set breaks, or if you have an amp that leans back on kick back legs or in a leaning stand when you play, you really should install a small cooling fan to protect the components from over heating. I learned that the hard way. Had to replace a transformer in my old Twin once.

 

Never again. I bought a 3 inch fan from Radio Shack and installed it in the bottom of the amp, facing up toward the chassis. I put a small on/off switch in the line to the fan, and wired it directly off the current coming into the amp, so it will run even with the amp turned off. It circulates enough air to keep the heat from building up and melting the goo out of the components, and cuts down the time it takes for the amp to cool for transport. A hot tube will succumb to vibration and shock much faster than a cool one.

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I never heard that before.I have a 1972 marshall amp that I've had since about 1983' date='and I have used the standby switch all the time and no issues with anything so far[/quote']

 

 

I imagine so... some people down here have old amps that they use (with their standby switch) and have never run into trouble... I used another tube amp a couple of years and used the std by switch with no issue... but there's something about the ac30's circuit that makes the standby switch irrelevant (and even dangerouos)... I'll see if I can get Lyle C. to give me some more info on that and post it here... the guy really knows about stuff. (He gave the explanation to this a while ago but I cant remember well) anyway, my tubes are working fine... (I keep some spares just in case).

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In the vintage amp forum most guys say you shouldnt use the standby switch' date=' a very knoledgeable guy over there called Lyle Caldwell (that really knows his stuff, hell he knows MY stuff better than me!) told me that (at least on the VOX AC30) the standby switch should be left in the "on" position allways, as turning it up can lead to damage in the transformer or the rectifier of the amp... the pre and power amp tubes can be damaged too... so I just leave my stdby switch on all the time and use the power on-off switch only... never had a problem that way.[/quote']

 

there's something about the ac30's circuit that makes the standby switch irrelevant (and even dangerouos)...

 

I agree about the standby not being needed but I don't know about dangerous. DST makes what is basically a righteous AC30 clone and added a standby switch - "And yes' date=' I know the standby isn't really needed, but everybody asks for it." (source: Bob Dettorre of DST Engineering, ToneQuest Report, June 2007 issue).

 

DST-30

 

Now, while I do get a lot of good information from these forums and some others, I tend to take what most people say with a grain of salt as rumors have a very frustrating way of being perpetuated on the Internet and guitar players are like golfers - they'll believe anything you say if the they think it will improve their game or make them look smarter whether they have any first hand knowledge themselves or not (I include myself with that broadly painted overview but I do try to keep myself grounded and in check).

 

I don't know who Lyle Caldwell is except after doing a quick Google search he's a guy who has at least modded an AC30CC2x. Maybe he knows a thing or two or maybe he doesn't. But, the guys at DST build a version of the AC30 and I don't think they'd add the switch if they thought that people were going to have problems with it.

 

Here is the thread with what Lyle Caldwell had to say about the matter:

 

Do not use the Standby switch on an AC30TB/TBX or CC. It is not needed' date=' 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.[/quote']

 

The current inrush is one issue' date=' 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.

 

[b']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.[/b]

 

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.

 

I'm no tech so, for all I know, what he's saying might be spot on but, again, who is this guy? We've got some people who post on these forums who talk a good game but, IMO, don't know crap. I'm not saying that I am The Lord of Guitar, but if I don't know the answer, I simply won't write anything or, if I'm guessing or speculating, I'll say as much rather than speak like I'm some sort of authority on a subject. We also have some people who recommend gear as if one size fits all. There have been plenty of threads where somebody asks for recommendations for an amp without giving any indication of what sort of sound he's looking for and right away people start recommending this or that without even bother to ask the dude what he's looking for - "Oh yeah, get a 100w Marshall with a master volume - it'll work great in your bedroom! Just crank the preamp!" After a few questions, it turns out the guy is into jazz or something that would suck with a Marshall. My other favorite is "Just replace the caps/pickups with paper in oil/Burstbuckers/Dimarzios/whatever because they're 'better'" (whatever "better" means) as if what they use is the cure for everything - again, not even bothering to find out what sort of change the person asking is looking to achieve.

 

Sorry for the rant.

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It's been common practice to use the standby on tube powered equipment for ages. I can't speak for Vox products, since I have never owned one, or learned anything about their strengths/weaknesses.

 

Most old military tube type radio sets and such are equiped with a standby, since they tend to be powered on for long periods of time, and needed to be ready to use quickly. I think that's where the standby circuit first came into use, as a means of extending power tube life.

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Directly from the my Mesa Boogie Blue Angel Manual:

 

Power Up:

 

Connect your favorite guitar to the instrument input jack. Flip the power switch "ON" while leaving the standby switch set to "STANDBY". (It's always a good idea to practice this startup procedure) as at least 30 seconds of warm-up time lessens the shock on cold power tubes, thus prolonging their life substantially.

 

Directly from my Marshall JTM30 user manual:

 

To prolong the life of the valves it

is always advisable to switch on

the Mains Power Switch (item 16)

about 2 minutes before switching

on the Standby (item 15).

This allows the valves to heat up to

full working temperature before

use. On switching off, the Standby

should always be switched before

the power switch.

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I use the standby on my Boogie to power up. A few minutes, standby on. Quick sound check, then leave it on, turn down guitar volume.

 

Standby is never used again. Cathode poisening. Power off/standby on.

 

I get decades out of power tubes.

 

Best of luck.

 

Murph.

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