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 Post subject: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 4:01 am 
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On guitar amps, many seem to have a standby switch, which turns off the B+ to the tubes, the theory being that the heaters can warm up without a signal. And from what I've seen, people think that if you just turn it on and start playing without the tubes warmed up first, the tubes will practically melt down inside or some other tube doomsday event.

My question is what is the deal here? Radio's don't have such a switch and I'd say there's certainly something being fed through at all times from the tuner, at least to the first tube. In fact even tube stereo amps don't have them that I can find. And I certainly can't see why it would really make a difference to just turn the amp on, and if nothing else just don't play for a few seconds. But why put this switch on there?

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 3:06 pm 
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MAybe because a guitar amp might be subjected to very high abusive volume settings right off the bat as compared to those other devices. DO they give a plausible explanation in the user manual?


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 3:25 pm 
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I always thought it was for when the set was over, and you wanted to lean the guitar against the amp and take a break, without changing your settings, otherwise the guitar would start that string-buzz feedback thing. We didn't use to have guitar stands, for some reason. Also for plugging/unplugging the instrument cable, come to think about it.


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 3:45 pm 
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For tube amps with SS rectifiers, the stand by switch allows the tubes to 'warm up' first before hitting them with the high B+ voltages that would be immediately available with a SS rectifier, to help extend the life of the tubes. As a young guitar player back in the day, we used to use it while we were on break (for which was what we thought it was intended for), so you didn't have to warm up the amp. So, these are a few reasons for the SB switch. You rarely had them on 'low end' or low power amps.


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 8:04 pm 
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Tbirdkid wrote:
For tube amps with SS rectifiers, the stand by switch allows the tubes to 'warm up' first before hitting them with the high B+ voltages


Maybe I'm overthinking this, but what would be the issue with letting them have the B+? If the tube is not warmed up, then there should be no current flow, so the B+ is there, but no more so than it's on the wire. As the tube warms up, current can start to flow. I would think it would be better to let the B+ stay on, and it would then start to flow gradually as the tube warmed up, instead of suddenly it's on.

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sat 14, 2017 9:39 pm 
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The standby switch is a fail safe way to mute the guitar amp , with out pops , hum or other speaker blowing repercussions .

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sun 15, 2017 3:39 am 
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Agree - Guitar amps need a way to mute them and be ready to play when the guitarist comes back.

The issue with B+ being applied before tubes warm up is "cathode stripping". When a tube is warming up, parts of the cathode heat first and may supply too much current - possibly damaging the cathode in these areas. Will not happen if fixed bias is applied - only with cathode bias, which needs the full cathode current to provide proper grid bias. Guitar amps mostly use fixed bias, so cathode stripping doesn't occur. And I'm not sure it even happens at the lower voltages used with smaller tubes that use cathode bias.


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Sun 15, 2017 3:49 am 
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battradio@ wrote:
The standby switch is a fail safe way to mute the guitar amp , with out pops , hum or other speaker blowing repercussions .

Yep. And annoying the audience in the process. Keeps the settings and the tubes hot for a break.

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Jan Mon 16, 2017 3:35 pm 
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"Cathode Stripping" is a term or condition which has been seized upon by the Audiophool world and in our audio world, is a non-issue. About the only place that it matters is with high power RF tubes. And yes, the definition is correct, just not applicable to the normal audio world.

If cathode stripping were an issue, HH Scott and Fisher would not have used silicon rectifiers in their equipment (Fisher 500/800 series, Scott 340B/345/380).

I am sure that the stand by switch is very basic in application. Get the filaments "up to speed" and then when you are ready, simply apply B+ and have instant sound. Same would apply as mentioned above about going on a break. Kinda like the "Pause" button on our DVD players. Really, nothing very complicated...

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