Demystifying Tube Amps---The Standby Switch

[agedhorse]

I think another potential problem, especially with high powered amps like the SVT is that the plate voltage for the power amp is 600V+ and the HT supplies for the preamp tubes are dropped to the 300V range with a resistor. If you slam the HT on when the tubes are not able to conduct, the plates of the preamp tubes see the full plate supply until the tubes warm up a bit because without current there's no voltage drop across the HT dropping resistor or the plate resistors. Can't get around V=IR. And this is made even worse as with no load the HT voltage will be higher than it is under operation because of the series resistance of the transformer. Not as big a deal if the preamp tubes have their own winding on the power transformer or if there are no power tubes. Am I thinking about this correctly?

This can be a problem, but it's kind of self-regulating in a crude way because under the start-up condition, the direct coupled cathode followers can saturate and pull down the supply. This isn't the greatest thing for that tube section, but the duration is fairly short in the overall scheme of thermal time constants.

 

[JimmyM]

Wow. That article completely contradicts everything I’ve been taught and everything I’ve read. I’ve been carefully using standby mode for years to warm up my amp. Are we sure this information is correct? If it is, what a colossal waste of time for all of us to be concerned about nothing. I’d really like to read more about this point of view.

As long as you don't put signal through your tubes for about 30 seconds, that's all you really need. Use standby or don't...it's fine.

 

[JimmyM]

Wow! With six pages to this thread already, it appears not much has been "de-mystified."

Threads like this always get into high level tech discussions. By page 2 it was already over my head But the info in that article about standby mode that I posted pretty much says it all IMHO and is pretty simple to understand. Just give your amps with tubes 30 seconds or so to warm up and it really doesn't matter whether you use standby or not.

 

[JMV]

Threads like this always get into high level tech discussions. By page 2 it was already over my head But the info in that article about standby mode that I posted pretty much says it all IMHO and is pretty simple to understand. Just give your amps with tubes 30 seconds or so to warm up and it really doesn't matter whether you use standby or not.

Me too, quickly over my head!! So, my simple take as for good reason to use standby - as AGCurry points out, use of the switch can help avoid spontaneous feedback when on breaks. I remember the squeals coming from stage back in the day when I had a nice small Fender tube amp and really cheap hollow body guitar

 

[okcrum]

And hopping we talk the same language (as I.m not a native language speaker btw.) The screen voltage is not what is measured in the top of its supply decoupling cap (this is the screen supply point), but the voltage present after the limiter resistor right on the tube screen. To say the screen voltage is xxx V it have no absolute sense in actual context because in fact the screen voltage is anywhere between its iddle voltage and the voltage resulting after substract the voltage dropped on screen limiter resistor. Eg: if the screen at iddle is 450v and at full throtle 350v (meant 100v will be dropped over the limiter resistor) meant the screen voltage is anywhere between those values during tube operation, and is a very dynamical characteristic. You can mention the screen voltage just for a point of its operation because the screen voltage is not regulated. Add the dropped voltage over the B+ supply rail during the operation. This do the tube operation very non linear but good enough for geetar aplication. On the other hand hi-fi amps have voltage regulators in this position which keep the screens voltage stable. In this case we can talk about a screen voltage cause those voltage will be consistent over the whole operation of the tube.
Keeping the screen supply voltage to a low value if will help to keep the tube more linear cause the voltage will not swing so much over the limiter resistor. An other way is to use not individual screen resistors but a half value common one. As time it works in a push-pull circuit one tube will draw more the other less keeping balanced more consistent the dropped voltage over the limiter and in consequence over the screens. This was used as Mullard patent in Hiwatt 100 circuit.
So it have to do a difference between supply screen voltage and screen voltage. What is determinant for unregulated screen voltage it is screen voltage as it is base of calculation for the power dissipated by the screen in respect with current draw. Of course voltage will be not affected at small amplification level the serious current will be drawn at high level of amplification
Nothing related with stb sw.

Yep, I'm talking volts at the tube electrodes. I'd run less aggressive in a hifi amp. Or ultralinear.

It's in the Fender/Traynor/Marshall tradition.

 

[okcrum]

I might know a thing or two since that's my job.

Amps are (supposed to be) designed to handle the rated nominal AC voltage plus/minus roughly 10%.

Because many products (especially in that era) were designed by the seat of the pants without full recognition of the safety issues associated with such decisions, regulations and safety agencies came into being to test and verify that unsafe conditions are not designed into products like amps.

The good old days were full of poor designs (from a safety and reliability perspective), things are indeed different today.

That said, tolerances on parts like caps and tubes may have been more liberal to accommodate such design sloppiness, so some of this way average out.

Not to mention that to many people, there are only two line voltages, 110 and 220.

Time for the inevitable meme, any moment now...

 

[iiipopes]

Go back and read.

Nah, I quit dealing with tube amps decades ago. I did all this discussion before way back when.

 

[iiipopes]

Threads like this always get into high level tech discussions. By page 2 it was already over my head But the info in that article about standby mode that I posted pretty much says it all IMHO and is pretty simple to understand. Just give your amps with tubes 30 seconds or so to warm up and it really doesn't matter whether you use standby or not.

Tube amps? People still use tube amps? I don't even take an amplifier of any kind on stage. It's just a direct box, the Behringer digital wi-fi board, and in-ear or hot-spot monitors. I let the sound man take everything out front. I quit tube amps decades ago when it was going to cost as much to completely re-tube the SVT head I owned then as it was to get a new amplifier head.

 

[JimB52]

With my various tube amps over the years, Ampeg Fender Guild Silvertone Traynor, and currently a Marshall Superbass 100w, I always switch to standby during breaks to avoid feedback and when powering down. That way, standby is active when I power up next time. I strongly dislike unintentional loud noises, and this regimen prevents all the ugly sounds I don't cause myself.

 

[JimmyM]

Tube amps? People still use tube amps? I don't even take an amplifier of any kind on stage. It's just a direct box, the Behringer digital wi-fi board, and in-ear or hot-spot monitors. I let the sound man take everything out front. I quit tube amps decades ago when it was going to cost as much to completely re-tube the SVT head I owned then as it was to get a new amplifier head.

Don't bother me with your personal problems

 

[iiipopes]

Don't bother me with your personal problems

Oh, believe me: divesting of tube amps solved more personal problems than you will ever know.

 

[BaltBass5]

See mute function, instrument feedback on break, save a few joules of energy, etc. I never saw a mute switch on my YBA3.

why not rename it mute on modern tube amps? Nostalgia?

 

[BaltBass5]

https://www.sweetwater.com/sweetcare/articles/great-standby-switch-myth/

 

[Mark Reccord]

This can be a problem, but it's kind of self-regulating in a crude way because under the start-up condition, the direct coupled cathode followers can saturate and pull down the supply. This isn't the greatest thing for that tube section, but the duration is fairly short in the overall scheme of thermal time constants.

Never thought of that. Of course this falls apart if the amp doesn't have any direct coupled CFs in it. If the amp has a standby switch, there's no downside to letting the tubes come up to operating temperature before engaging the HT.

As long as you don't put signal through your tubes for about 30 seconds, that's all you really need. Use standby or don't...it's fine.

It's not a matter of putting signal through them because there's no harm in putting signal on the grid of the first active stage with the tube not warm, it just won't do anything. There are potential damage issues with just slamming on the HT on cold tubes in an amp that was designed with a standby switch. If I was going to design an all-tube amp (and I might one of these days) I'd use an internal delay circuit to power the HT on after about 30s with just the filaments on.

 

[beans-on-toast]

I think another potential problem, especially with high powered amps like the SVT is that the plate voltage for the power amp is 600V+ and the HT supplies for the preamp tubes are dropped to the 300V range with a resistor. If you slam the HT on when the tubes are not able to conduct, the plates of the preamp tubes see the full plate supply until the tubes warm up a bit because without current there's no voltage drop across the HT dropping resistor or the plate resistors. Can't get around V=IR. And this is made even worse as with no load the HT voltage will be higher than it is under operation because of the series resistance of the transformer. Not as big a deal if the preamp tubes have their own winding on the power transformer or if there are no power tubes. Am I thinking about this correctly?

A design feature of the SVT....

From Bill Hughes: The SVT grid bias starts at -150V at power up and settles at a value around -50 hot, if you leave the standby switch always on. The bias is held to cutoff (-150) by the 12BH7 cathode follower driver.

So use the standby switch.

 

[agedhorse]

A design feature of the SVT....

From Bill Hughes: The SVT grid bias starts at -150V at power up and settles at a value around -50 hot, if you leave the standby switch always on. The bias is held to cutoff (-150) by the 12BH7 cathode follower driver.

So use the standby switch.

Bill is a very clever and insightful guy, but wickedly smart too.

 

[JimmyM]

why not rename it mute on modern tube amps? Nostalgia?

A mute switch only mutes while the standby turns off the high voltage to the tubes as well as mutes. Using it as a mute switch is only a by-product of the standby, and as that article states, there are much simpler ways of incorporating a mute switch, which wouldn't turn off the high voltage.

 

[feterpatch]

On a somewhat related note... after a 3 hour show my SVT II pro is pretty dang hot. I always leave it on and in standby mode with the fan running for about 15 minutes before shutting it down and putting its road case covers on. Is a cool down period even necessary?

 

[JimmyM]

It's not a matter of putting signal through them because there's no harm in putting signal on the grid of the first active stage with the tube not warm, it just won't do anything. There are potential damage issues with just slamming on the HT on cold tubes in an amp that was designed with a standby switch. If I was going to design an all-tube amp (and I might one of these days) I'd use an internal delay circuit to power the HT on after about 30s with just the filaments on.

Not arguing that, Mark, but I've always heard that the filaments are quite susceptible to damage in amps that allow signal to pass before they're warmed up, such as my old Dano Airline 15w amp. Is that truth or myth?

 

[JimmyM]

On a somewhat related note... after a 3 hour show my SVT II pro is pretty dang hot. I always leave it on and in standby mode with the fan running for about 15 minutes before shutting it down and putting its road case covers on. Is a cool down period even necessary?

It is best to give it 5 minutes or so before you move it once you're done, as filament damage can occur when it's hot. It's not always possible at some gigs, especially gigs where gear changeovers are very quick and you're being rushed by the stage manager, so you'll want to be very careful not to bump it in those cases. But usually 3-5 minutes of being off, not in standby, covers you. Leaving it in standby still keeps the tubes hot, so the best thing is to turn it completely off.