Unplugging cabinets while on standby?

gmm52

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I know that tube amps want to see a load and that they shouldn't be turned on without a speaker connected, but what about standby? Can one safely unplug and change cabinets while an amp is on standby?
 

dsutton24

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I can't see what it would hurt. The standby switch removes the B+ supply from the power amp, so there shouldn't be any way to generate damaging energy through the output transformer.
 

rdjones

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On some amps, notably the '60s Silvertones, the "standby" was simply a mute function that did not remove the B+.
Some of those Silvertones were noted to have high incidences of output transformer failures.
Coincidence ? I think not.

I'd verify for certain your particular standby circuit before doing this.
 

SPUDCASTER

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Are you just switching cabinets? Do you need to unplug the cabinet for some reason with the power on?

Would an A/B switching work if you're only switching cabinets?
 

2L man

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Play and listen when switching Stby Off. If you hear volume quieting while usually also distorting it means Stby remove high voltage and tubes use the energy from filter capacitors and it is very safe to change loudspeaker/cabinet
 

gmm52

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Play and listen when switching Stby Off. If you hear volume quieting while usually also distorting it means Stby remove high voltage and tubes use the energy from filter capacitors and it is very safe to change loudspeaker/cabinet
Yes, this is the characteristic when the standby's hit. The amp is a 50w Marshall clone, a hybrid, JCM800 preamp section with a JTM45 power amp section.
 

gmm52

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There is zero advantage of using standby instead of mains power to switch cabinets, and as @rdjones noted it, not all standby circuits work the same way.

(Dons preacher robes, waves arms): Can you handle the truth? There is zero advantage of standby at any time:

http://www.valvewizard.co.uk/standby.html
Here's the thing. I want to do extensive cabinet testing, and my assumption is powering down completely, repeatedly, when changing cabs would only take mere seconds seems more taxing on the amp than using just the standby.
 

King Fan

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Yeah, I get that. :) You could certainly analyze what kind of standby you’ve got and decide. There’s an active debate whether it’s *actually* harder on the amp to switch on and off repeatedly, but we all feel like it might be. OTOH on a really busy testing day I might switch on and off 20 times. Not a big deal — I’ve never lost a tube doing that; most I’m 'just playing along' and pfffft…
 

2L man

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Yes, this is the characteristic when the standby's hit. The amp is a 50w Marshall clone, a hybrid, JCM800 preamp section with a JTM45 power amp section.
I do lots of speaker testing and have built all my amp and know that second switch removes the High Voltage. Then continuing playing few seconds deplete capacitors fast and there is no need to switch Mains Off for loudspeaker change.

I have worked with electronics all my life and can confirm that most electrics seem to prefer break when they are turned On. But I think they would have failed some day anyway! However after taking systems Off it is good practice to put them running early enough that there is time to replace something.

I recall that cold tube filament surge current shorten filament life an hour but I think it is not a problem for instrument amp tubes. Some (military) tube datasheet list tube estimate life when used "nominally" and how many starts certain persentage of them last.
 

printer2

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Heck, I don't even turn the amp off when I am changing speakers, and none of my amps have standby switches. If you have no signal going to the speakers the amp is not really going to notice. The trouble comes if you are playing through the amp and it looses the load.
 
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andrewRneumann

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I swap between resistive load and a speaker all the time without powering down or engaging standby. I just make sure there is nothing being amplified by turning down the gain and master volume to 0. DC current flowing through the OT primary is oblivious to whether a speaker is connected.
 

Nicko_Lps

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Yes, this is the characteristic when the standby's hit. The amp is a 50w Marshall clone, a hybrid, JCM800 preamp section with a JTM45 power amp section.
Here's the thing. I want to do extensive cabinet testing, and my assumption is powering down completely, repeatedly, when changing cabs would only take mere seconds seems more taxing on the amp than using just the standby.
You can play it safe in the long run and... Protect yourself against the mistakes you might do on the future by adding a switching jack that switches to a load resistor when no cab is plugged in, its like a 15 minute job once you have the jack + power resistor:
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Tip: This 8ohm 50watt aluminum housed resistor comes pretty cheap in car modification sites because they use them for swapping to LED headlights, i bought this one for... 3.5euro while electronic sites have exactly the same brand/type for 12 or so.
 

gusfinley

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As I understand it, certain amps do benefit from a Standby - especially those with a bias tap on the power transformer feeding a low-filtered Half-wave rectified bias power supply - Such as the Tweed Bassman and the Early Marshall amps. This is reported to take up to a few minutes to stabilize, so it is recommended to leave the amp on standby for a few minutes.

If you want to be lazy, you can put a TRS jack at your output and wire the cathode of your power tube circuit into the Ring and the Sleeve to ground. This way, if there is no TS jack inserted there is no connection and it is essentially in stand-by.
 




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