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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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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Wed 08, 2017 11:15 pm 
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I think the basic purpose can be pretty much deduced from the fact that the first designers to put them in guitar amps referred to them as "standby" switches, rather than "HV" switches or "warm up" switches

They don't tend to appear in PA equipment from the same era, even though it used the same type of circuitry. to me, the main purpose was simply so I could put the amp into "standby" and bring it back up instantaneously as the same volume as before .

I have met players who are utterly convinced that you will damage a amp if you don't use the two switches in the correct order, both when turning it on AND when turning it off.

Even Fender themselves push the idea

http://www2.fender.com/experience/tech-talk/the-standby-switch/

An alternative view is here

http://www.valvewizard.co.uk/standby.html

IMO there is an ungodly amount of mystical nonsense talked about tube guitar amplifiers nowadays that ,as a tech who is the son of a tech, I never heard discussed when this equipment was popular the first time around.

If applying HV before the tube is warm causes damage, the failure rate on general tube equipment with SS rectifiers should be substantially higher than that of guitar amplifiers. I see no evidence for this. Further, the first Fender standby switches appeared while they were still using tube rectifiers, which indicate that the designer were not worried about HV being available before the tubes heated up.


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Thu 09, 2017 6:11 pm 
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martin halstead wrote:
IMO there is an ungodly amount of mystical nonsense talked about tube guitar amplifiers nowadays that ,as a tech who is the son of a tech, I never heard discussed when this equipment was popular the first time around.

If applying HV before the tube is warm causes damage, the failure rate on general tube equipment with SS rectifiers should be substantially higher than that of guitar amplifiers. I see no evidence for this.


No offense, but you need to get out more. Tube audio equipment, particularly tube power amps, are very marginal things at startup even with tube rectifiers, and the failure rate of the associated power supply parts goes up astronomically if you merely replace tubes with diodes - and not just because the B+ is higher.

And at least with a tube rectifier, it's usually just rectifier tube and the fuse that blows, rather than the power supply caps, the output tube, the power transformer, etc. Unless some uber-nitwit has decided his power supply fuse "sounded bad" and shorted it or replaced it with a much larger fuse for "better transcendency" or some equally nonsensical gibberish.

The failure rate of the standby switch itself isn't too good, either, because you never really want to be putting step functions into B+ at any time, tubes conducting or not. People screw around with diodes because of the rectifier tube failure rate, but that alone just moves the failures downstream. The real solution is to leave the tube in there and add diodes in series with the plate, the tube acts as the soft-start circuit and a backup for the diode shorting, and the diode does the rectifying.

Brett


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Sun 12, 2017 7:28 pm 
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The problem is not having the B+ on the tube before warmup, rather it is the surge when a switch is closed and charging the filter caps. This sudden inrush is like putting a momentary short across the rectifier. This results in eventual problems with contacts arcing in the switches. (As a tech I have replaced numerous ones that have simply burnt contacts)

As far as B+ on the plates of the output tube, that happens with both tube and SS rectifiers. If you put a meter on the B+ line of a typical tube rectifier (5U4, 5Y3, etc.) you will find that warm up time is about 3 seconds on the rectifier but 10 - 15 on the typical beam power tube. Thus for that interim period the B+ line will reach full, unloaded potential. Once the tubes begin to conduct it will fall to its normal ratings. The output tubes suffer no real damage as a result, they are designed to accept that condition.

That said there is a concern with filter caps. This is why when replacing filters in an upstream circuit that is intended to operate at 200 V or so, one must use filters rated at the full voltage of the B+ source which feeds the output tubes, maybe 400-500V. At power up, and before tube conduction begins the preamp may reach well reach the same voltage as that applied to the output tube plates. (Assuming the amp uses the typical dropping resistor circuits between each section of the filter)


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Mon 13, 2017 9:37 pm 
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Brett_Buck wrote:
. Unless some uber-nitwit has decided his power supply fuse "sounded bad" and shorted it or replaced it with a much larger fuse for "better transcendency" or some equally nonsensical gibberish.



Brett


Ha! reminds me of a conversation I had many years ago

What's the problem?

Its blowing a 10 amp fuse

But this is only supposed to have a 3 amp fuse

Yea, but that kept blowing, so ......


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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Thu 16, 2017 7:25 pm 
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If the switch is in series with center tap of the high voltage winding of
the power transformer, with an amplifier using, for example a 5AR4
rectifier, there are several factors:

The surge to the capacitors is limited by the resistance of the transformer
winding.

The capacitors are being charged by two steps of voltage separated by 8.3
milliseconds. (first one half of the wave, next the second half.

In the Ampeg below the filters are staggered across the voltage divider with
the input value 30 uFd.

Is anything, use of the standby switch eliminates the primary winding
transformer magnetization bump, which on stage during a show can
throw back a dip through power snakes to other gear, especially digital.

Any transformer that (normally) audibly grunts when initially plugged in can be
checked for inrush effect with a Fluke 33 peak recording meter.

Attachment:
Ampeg B-22-X  Darr Jack p 122.jpg
Ampeg B-22-X Darr Jack p 122.jpg [ 166.37 KiB | Viewed 153 times ]

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Fri 17, 2017 3:13 pm 
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martin halstead wrote:
They don't tend to appear in PA equipment from the same era, even though it used the same type of circuitry. to me, the main purpose was simply so I could put the amp into "standby" and bring it back up instantaneously as the same volume as before.

+1 to this. I have only limited knowledge of guitar amps, but I think we may be over-thinking this issue.

Consider the case of televisions: Even into the 1970s, many tubed TVs were still being manufactured. Particularly lower-priced B&W tabletop sets. In order to compete with the "instant-on" feature of SS gear, some of these kept the tubes constantly heated when the set was plugged in. Thus when the set was turned on it came to life almost instantly.

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 Post subject: Re: Guitar amp stand by switch???
PostPosted: Feb Tue 21, 2017 3:36 am 
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Poston Drake wrote:
... but I think we may be over-thinking this issue. ...


My thoughts exactly...

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