Correct power on / off sequence for valve amp with standby?

richh

I should know this... but I don't.

With a valve amp that has separate main on / off power switch and also the standby switch, can anybody please tell me in plain English what the correct sequence is to power on and off?

Any recommendations on how long to leave in between would be useful as well please!

Thanks.

ICBM

Power on with standby engaged (ie to 'standby' or 'silent'). Wait any amount of time longer than about thirty seconds.

Standby on and off as necessary in use.

Power off - doesn't matter. Both off together if you like.

That of course assumes that the amp is correctly designed and the standby switch is in the right place when the amp also has a valve rectifier! Many modern ones aren't - Orange and Vox in particular. If your amp has a valve rectifier and you aren't sure if it's one like that, it's probably better to never use the standby at all.

richh

Thanks for that @ICBM - I feel stupid not knowing this already!

The amp is a Fender Blues Deville reissue model.

Voxman

ICBM said: Jim

Power on with standby engaged (ie to 'standby' or 'silent'). Wait any amount of time longer than about thirty seconds.

Standby on and off as necessary in use.

Power off - doesn't matter. Both off together if you like.

That of course assumes that the amp is correctly designed and the standby switch is in the right place when the amp also has a valve rectifier! Many modern ones aren't - Orange and Vox in particular. If your amp has a valve rectifier and you aren't sure if it's one like that, it's probably better to never use the standby at all.

Yup, thats how I switch on my Marshall dsl401 and Laney VC30.  I also switch to standby before turning off just out of habit even though there are mixed views as to whether this makes any difference.  Some techs and manufacturers say turning off an amp like this is better, others say it makes no difference.

Re amps with a valve rectifier but with standby in wrong place eg AC30 I believe, why do manufacturers add a standby when this can actually be more damaging to tubes?

ICBM

Voxman said:

Yup, thats how I switch on my Marshall dsl401 and Laney VC30.  I also switch to standby before turning off just out of habit even though there are mixed views as to whether this makes any difference.  Some techs and manufacturers say turning off an amp like this is better, others say it makes no difference.

It's possibly arguable that in amps without resistors on the filter caps, putting it on standby first allows the caps to drain while the valves are still hot - if you turn off the power first or both together, it's a race between the caps and the valve filaments and if the filaments go cold first the caps can have a fair bit of charge remaining. Whether that's a bad thing is open to question.

Voxman said:

Re amps with a valve rectifier but with standby in wrong place eg AC30 I believe, why do manufacturers add a standby when this can actually be more damaging to tubes?

Good question - ignorance? It's probably because the designers grew up with solid-state rectifier amps (where it's fine to put the standby upstream of the first filter cap) and failed to realise that this will then exceed the rectifier valve current rating when the standby is switched with the valve hot. Basic lack of thorough knowledge of valve circuits, really - surprising, but it's the only explanation I can think of.

Although if you think that's bad, some of the design mistakes in modern Marshalls are quite shocking for anyone with repair experience. Lack of proper separation between high-voltage-difference traces, unnecessary switching in the speaker circuit, fail-unsafe bias adjustment (unlike vintage Marshalls), it just goes on and on...

ecc83

 "why do manufacturers add a standby when this can actually be more damaging to tubes?" ....Promise I won't go on!

No idea where the standby switch came from on guitar amps, virtually no other audio device has or had one. Some say it was a mute function in the dance band era but who knows?

Dave.

ICBM

ecc83 said:

 "why do manufacturers add a standby when this can actually be more damaging to tubes?" ....Promise I won't go on!

No idea where the standby switch came from on guitar amps, virtually no other audio device has or had one. Some say it was a mute function in the dance band era but who knows?

The standby is only damaging to valves when it's in the wrong place! But I won't go on either .

I'm not sure if Fender were the first to use them, although they must have been among the first. Given that Leo Fender was a notorious penny-pincher except when something had a reliability benefit, I'm fairly sure there was a good reason.

One thing it's quite useful for is allowing the amp to warm up and dry out any condensation before applying the HT to things where it might arc, like valve bases… Leo probably worked this out from experience with cold amps being taken into humid dancehalls in the winter.

It's unnecessary as a mute when you can always turn the volume down to zero or just pull the cable halfway out of the jack, so it's unlikely to be that.

Collings

So what is the result of locating the standby in the wrong place is it shortened valve life or potentially more serious?

jpfamps

ICBM said:

ecc83 said:

 "why do manufacturers add a standby when this can actually be more damaging to tubes?" ....Promise I won't go on!

No idea where the standby switch came from on guitar amps, virtually no other audio device has or had one. Some say it was a mute function in the dance band era but who knows?

The standby is only damaging to valves when it's in the wrong place! But I won't go on either .

I'm not sure if Fender were the first to use them, although they must have been among the first. Given that Leo Fender was a notorious penny-pincher except when something had a reliability benefit, I'm fairly sure there was a good reason.

One thing it's quite useful for is allowing the amp to warm up and dry out any condensation before applying the HT to things where it might arc, like valve bases… Leo probably worked this out from experience with cold amps being taken into humid dancehalls in the winter.

It's unnecessary as a mute when you can always turn the volume down to zero or just pull the cable halfway out of the jack, so it's unlikely to be that.

Fender introduced the standby pretty soon after they started using a DC-couple cathode follower to drive the tone stack.

This can arc over if the HT comes up before the valves are conducting, so it could have been introduced to protect the valve in the cathode follower.

It could also have been introduced to protect the filter caps from the unloaded HT.

ICBM

Collings said:

So what is the result of locating the standby in the wrong place is it shortened valve life or potentially more serious?

Blowing the rectifier valve.

DJH83004

I think Merlin Blencowe's excellent article puts it all into context,  http://www.valvewizard.co.uk/standby.html

Ancient_Mariner

Oops.

ICBM

DJH83004 said:

I think Merlin Blencowe's excellent article puts it all into context,  http://www.valvewizard.co.uk/standby.html

Yes.

Although he doesn't mention one fairly useful advantage to a standby switch, for techs… it's extremely handy for troubleshooting and valve swapping .

The big problem with quickly troubleshooting an amp with no standby switch is that it's hard to tell where the problem is if it just blows fuses when you turn it on.

DJH83004

Totally agree IC,  the more you can isolate circuits the easier to determine the fault, maybe that's what Leo had in mind after all he was very much into designs that were serviceable, unlike today where we are bottom of the pile!    

fretfinder

ICBM said:

Power on with standby engaged (ie to 'standby' or 'silent'). Wait any amount of time longer than about thirty seconds.

Standby on and off as necessary in use.

Power off - doesn't matter. Both off together if you like.

That of course assumes that the amp is correctly designed and the standby switch is in the right place when the amp also has a valve rectifier! Many modern ones aren't - Orange and Vox in particular. If your amp has a valve rectifier and you aren't sure if it's one like that, it's probably better to never use the standby at all.

Do you know if the standby switch is in the 'wrong place' on a Marshall 1974X combo?

ICBM

fretfinder said:

Do you know if the standby switch is in the 'wrong place' on a Marshall 1974X combo?

Yes. It's on the DC side of the rectifier upstream of the first filter cap, which is the worst of all possible places.

It wouldn't be a huge job to fix it though, unlike some of the more complex PCB amps, eg the AC30CC.

ecc83

There are a couple of mods you can do to an AC side S/B switch  circuit which protect* the GZ but I would still rather do without them! As a valve TV tech in a former life with "live" chassis and later all chassis at 1/2 mains input I got used to shit or bust power.

*The point about protecting a DC coupled stage I had not thought of (Merlin tells of it) Leo F did not of course have silicon diodes back in the day!

Dave.

fretmeister

My Mesa Prodigy 4:88 instruction manual demanded a delay of 4 minutes between turning on, and flipping the standby!

It also had a noisy fan that was intolerable at low volumes. I suppose I could have asked a tech to install a far better fan, but it annoyed me, so it went.

Collings

So in general then would it just be better to leave the standby in the "play" position all the time and just use the mains on/off switch to power on and off. I assume that would then get over the potential problems of the standby being in the incorrect position in the cct?

ICBM

Collings said:

So in general then would it just be better to leave the standby in the "play" position all the time and just use the mains on/off switch to power on and off. I assume that would then get over the potential problems of the standby being in the incorrect position in the cct?

Yes.

If you wanted to fix it properly you could do one of three things:

Move the first filter cap to upstream of the switch. (Not totally straightforward given the layout.)
Add a resistor across the switch to 'trickle charge' the cap, as suggested in the link above.
Add an extra smaller filter cap on the upstream side of the switch.

If it was me I'd probably do the last - since that requires the minimum of work and disturbance to the amp but retains the advantage of having no HT on the circuitry if you want to change valves or troubleshoot.

For what it's worth I wonder if the bad standby switch location is responsible for the PT failures I've heard about with these amps… as Merlin Blencowe says it can cause a flyback voltage in the PT. (But which having the cap on the right side of the switch also stops.)

ICBM

For what it's worth I was working on an original 18W Marshall today, and the standby was in the wrong place back then too... so the reissue is historically correct if nothing else!

But since part of the job was replacing the filter caps, it was obviously best to simply rewire it correctly - with the first filter cap upstream of the switch - at the same time.

Today's other job is a 90s Marshall-built Vox AC15TBX - with the standby in the wrong place - and a blown power transformer. I wonder if that's related... the arc is in the HT winding so it's quite likely.