Tube Amp - Standby Switch Position

[Whoops]

Hi,
I have a Roland Bolt 60 tube amplifier, the circuit at the time had a mistake in the standby switch implementation so not all high voltages were disconnected when in stand by. Roland addressed this issue at the time.

Modifying the placement of the standby switch is highly recommend.

I attached a pic of the respective part of the PSU schematic,
where should the standby switch be connected in position 1 or position 2?
does it make any diference if its before or after the rectification diodes?

Ive seen this mod made both ways so I just want to be sure

thanks

 

[CJ]

put the switch on the cathodes of the power tubes,

they use a 120/240 sw on a 500 volt line,

ac is even worse so sip mod 1,

having hv to the preamp tubes is normal

 

[Iskrem]

Hi,

I'd put the switch in the first position. The switch is probably rated for AC, not DC.

more info here:
http://www.valvewizard.co.uk/standby.html

 

[Whoops]

Thank you so much for your replies, I was able to see the main problems and investigated a bit on it.

thanks for the link Iskrem, thats a great article.

they use a 120/240 sw on a 500 volt line,

ac is even worse so sip mod 1,

I think DC is worse in this case

"When comparing switching devices for AC and DC use you can specify the acceptable maximum voltages for a common current eg

10A at 230 VAc or 32 VDC
or the acceptable maximum current for a common voltage eg

230V at 10A AC or 1A DC."

http://electronics.stackexchange.com/questions/53310/calculating-current-load-for-a-switch

"Just as with fuses, switching a DC supply is much more stressful on a mechanical switch than switching an AC supply, because of arcing. Arcing leads to corrosion of the switch contacts and in extreme cases may even weld it shut. The switch will be subject to less arcing if is placed in the AC part of the circuit, e.g. prior to the rectifier."

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

 

[Whoops]

they use a 120/240 sw on a 500 volt line,

posted a pic of the service manual info on the switches used in the amp, theres a 100/117 and 220/240 designation there, although I dont know if this is really the rating reference of the switch.
But if you're right I will change the standby switch for a proper one in that application.

I will open my amp and see if there's something written on the switch itself.

I never seen anyone with a damaged standby switch on these amplifiers, so the switches used by Roland somehow managed in the last 35 years to stand the 500VDC.

 

[PRR]

AC may stop arcing 120 times a second as the wave goes through zero.

A DC arc may go on forever. DC ratings are usually much lower. The amount to de-rate can not be guessed, depends on many details we don't know.

Although we are outside ratings, I don't think it is wrong to use a hefty wall-power switch inside an amplifier. We can NOT get the insanely high fault currents possible in a lamp or vacuum-cleaner switch, because of PT impedance. "230V" wall-juice switch separations are ample for 600V, because a 230V motor will kick-back far over 600V when switched-off. Finally, make it serviceable and keep a spare switch on hand.

 

[Whoops]

I was able to work on my amp today,
the switch used by Roland for the Standby and Power are rated  at 3A 250V.
It doesn't say if it's 250V AC or DC. I'm assuming is 250V AC
Strangely they used that switch for standby on 500VDC rails.

So if I maintain this switch and modify the Standby placement , It seems safe to use it in Position 1 (AC side)
The tap of the Power transformer is 450V at 250mA so it's under the switch max rating.

Correct me if I'm wrong but if the max rating is 250V AC at 3A , at 500V AC it would be 300mA max
is this right?

thanks

 

[Iskrem]

Well, usually you divide amps by two if you double voltage, so 1,5 amps. But since it's way over "normal" voltage, it's maybe not that strong.
Anyways, I would not hesitate to use it. I would probably bypass it with a big resistor, say 47k/3Watt. Less inrush current, less thump and tubes live longer with a tiny current flowing at all times to avoid cathode poisoning.
Most tubeamps I've worked on have AC rated switches for switching high voltage DC, and it's usually not a problem.

 

[Whoops]

Well, usually you divide amps by two if you double voltage, so 1,5 amps. But since it's way over "normal" voltage, it's maybe not that strong.
Anyways, I would not hesitate to use it. I would probably bypass it with a big resistor, say 47k/3Watt. Less inrush current, less thump and tubes live longer with a tiny current flowing at all times to avoid cathode poisoning.
Most tubeamps I've worked on have AC rated switches for switching high voltage DC, and it's usually not a problem.

yes I got confused with the AC and DC accounting

so 250V AC at 3A

should be approximately:

500VAC at 1.5 amps
and
500VDC at 300mA max

does this seems right?

 

[Iskrem]

For the DC rating,
yes, because it was already worked at those voltages/currents and survived,
but:

AC may stop arcing 120 times a second as the wave goes through zero.

A DC arc may go on forever. DC ratings are usually much lower. The amount to de-rate can not be guessed, depends on many details we don't know.

Although we are outside ratings, I don't think it is wrong to use a hefty wall-power switch inside an amplifier. We can NOT get the insanely high fault currents possible in a lamp or vacuum-cleaner switch, because of PT impedance. "230V" wall-juice switch separations are ample for 600V, because a 230V motor will kick-back far over 600V when switched-off. Finally, make it serviceable and keep a spare switch on hand.

 

[Whoops]

For the DC rating,
yes, because it was already worked at those voltages/currents and survived,
but:

AC may stop arcing 120 times a second as the wave goes through zero.

A DC arc may go on forever. DC ratings are usually much lower. The amount to de-rate can not be guessed, depends on many details we don't know.

Although we are outside ratings, I don't think it is wrong to use a hefty wall-power switch inside an amplifier. We can NOT get the insanely high fault currents possible in a lamp or vacuum-cleaner switch, because of PT impedance. "230V" wall-juice switch separations are ample for 600V, because a 230V motor will kick-back far over 600V when switched-off. Finally, make it serviceable and keep a spare switch on hand.

Thanks,
so can I conclude that it's better to modify the standby switch to Position 1 on the the schematic I posted on the first post?

that way it's on the AC side and there would me less stress on the switch, would you agree?

 

[trobbins]

A ‘standby switch’ can provide a number of features such as: muting the output; instant on capability;  reducing power consumption; extending the life of amp parts (high operating temperature, emission).

Whether a ‘standby switch’ achieves any of those benefits, and similarly whether there are counteracting disadvantages, depends on the design of the switch. 

The schematic doesn't show where the power rails go?  Does the original switch remove anode supply, but not screen supply?

Any switching of an inductive energy supply should really be assisted in minimising over-voltage stressing that could allow an arc to continue.  So Mod1 preferably needs some more components, not only for turn-off, but also the stress during turn-on from switch bounce and capacitor charging. 

Mod 2 may also not be too good for output transformer over-voltage due to wanting to stop current instantly.

Disconnecting cathodes runs the risk of stressing heater-cathode voltage interface.

It should not be assumed that a switch rated at say 250VAC can be used at a higher voltage level, no matter what the operating current - that is pure speculation.  Perhaps if you went through all the applied tests done on the switch to gain certain accreditation marks, then that could provide some confidence, but remember that the switch itself is operated by a human. who could get a shock if the switch failed.

 

[Whoops]

Thanks,
you can see the full schematic here:

http://bolt60.no.sapo.pt/documents/bolt60/Bolt60 - Schematic.jpg

The schematic doesn't show where the power rails go? Does the original switch remove anode supply, but not screen supply?

Yes, it was a design mistake , that was addressed some years later by Roland.
It's a common mod to perform on these amplifiers.

I've seen it done with the position 1 or position 2, always using the same switch. I guess people didnt realized the switch itself could present a problem.

 

[DerEber]

so 250V AC at 3A

should be approximately:

500VAC at 1.5 amps
and
500VDC at 300mA max

does this seems right?

Well I am not a switch designer, but the Volt rating and the Ampere rating are adressing different aspects of the switch design, isn't it?

For example Voltage rating could relate to how far the contacts stay away from each other when switched off, whereas maximum ampere could relate to how thick the contacts and the relating conductors are.
So using the switch at half the Volts does in no way mean that it can stand double the Ampere and other way round.
This does not mean that the switch you mentioned will not work of a lifetime in your circuit, but I think you cannot do calculations like this for a switch.

best,
Stephan

 

[PRR]

> the Volt rating and the Ampere rating are adressing different aspects of the switch design, isn't it? For example Voltage rating could relate to how far the contacts stay away from each other when switched off, whereas maximum ampere could relate to how thick the contacts and the relating conductors are. So using the switch at half the Volts does in no way mean that it can stand double the Ampere and other way round.

+1.

The 250V and 3A never happen at the same time.

When ON, the contact drop *must* be below 0.2V at test current. If it is any higher the contacts will melt. (Slightly different melting-voltages for the several base metals.)

It is fairly unlikely, inside a building, to have persistent current through an "open" switch. In the cases where you "can", like turn-off of horsepower motor with a microswitch, you really must buy the switch by HorsePower, not V and A.

Turns out the Euro code requires 3mm air gap for 250V rating -OR- a micron symbol saying the gap is less and you must depend on a cord/plug for positive disconnection if the switch can't break the voltage. I'd be wary of that rating in higher voltage use.

I am studying a different switch problem at MUCH higher power. In a field where switches may be designed for 110V or 20,000V, and 6A or 6,000A. Rather different details needed to stand Voltage and Current.

> does not mean that the switch you mentioned will not work of a lifetime

The old Carling appliance switches are known to often last a lifetime as higher-volt standby switches. I would not trust them as emergency crusher shut-offs, but fine for a stage amp. I think they are 250VAC 3A. I think they are a fairly crude contact system, which may take odd abuse better than some newer "optimized" switches.

 

[Audio1Man]

I hope that the switch is mounted to a GROUNDED METAL CHASSIS.

The HV DC switches as several others have pointed out need to have a DC ratings (large gaps and or blow out magnets to break the current path). I suggest that you look @ a solid state solution. I show some figures that can solve the problem. Several HV 600 volt P FETS or PNP  and NPN parts are easy to get.

Duke

 

[CJ]

the switch is the most common component that fails in electronics because it is electro-mechaniacal,

bypassing the standby gets rid of this problem, bypassing the pwr switch is another way to bullet proof an amp,
but is not commercially viable, and may violate certain codes,

Ampeg uses the cathode standby switch in the V4 if you want to see how that is done,

 

[Whoops]

the switch is the most common component that fails in electronics because it is electro-mechaniacal,

bypassing the standby gets rid of this problem, bypassing the pwr switch is another way to bullet proof an amp,
but is not commercially viable, and may violate certain codes,

Ampeg uses the cathode standby switch in the V4 if you want to see how that is done,

Thanks CJ, I will check that and learn it.

But for this situation here I really just want to make it simple. So it would or position 1 or position 2 of the first post.

I've seen people doing it both ways, using the stock switch so I was just curious about what the differences could be and what of those positions should I choose to relocate the standby switch in my amp.

Thank you

 

[Whoops]

Any switching of an inductive energy supply should really be assisted in minimising over-voltage stressing that could allow an arc to continue.  So Mod1 preferably needs some more components, not only for turn-off, but also the stress during turn-on from switch bounce and capacitor charging. 

Can you elaborate a bit more please?

thanks

 

[Whoops]

Mod 2 may also not be too good for output transformer over-voltage due to wanting to stop current instantly.

It's the way it's done in Fender Amplifiers,
you can check these schematics:

http://www.davidsonamp.com/sf/images/twn_rev_ab763.jpeghttp://www.freeinfosociety.com/media/images/1683.gif