Why is output transformer CT switched by standby?

[stratomaster]

Typical AB763 type implementations of standby will switch the B+/OT center tap after the reservoir cap.  What is the harm in having the CT to the upstream side of the standby switch?  It seems like this would reduce the amount of current across the switch with no real downside. Am I missing something?

If the plates are at voltage, there's bias on the grids, and a path to ground in the cathodes, I don't see a way to harm the tubes by having no/low voltage on the screens. 

[PRR]

Ampeg did that in some models. It does not change the stress on the switch. It makes my head hurt.

Or maybe I misunderstand. Draw it.

[stratomaster]

The usual practice is to wire B+ to the choke side of the switch.  I'm asking why we don't normally wire to the reservoir cap side.

[kagliostro]

If the standby is connected between the rectifier and the first cap there will be more stress because of the inrush current charging the first cap when you close the circuit (usually there isn't a resistor between the first cap and rectifier)

If the standby switch is placed after the first e-cap, the charge is maintained and inrush current limited and life of the rectifier is longer

Often things are done following old schematics, not the best tech

(Why we go on using PT with CT and vacuum tube rectifiers ?  They are unnecessary if you add a SAG resistor to a SS FWB rectifier)

Someone say that to put the standby switch between CT and ground is possible way of electrocution

Franco

[stratomaster]

Thanks for the response.  I'm not asking about moving the switch before the reservoir cap.  I'm asking about moving the B+ node (output transformer center tap) to before the switch.  This will switch out the choke and everything down stream of it. 

The only difference between normal AB763 standby and what I'm asking about is whether or not the B+ is switched by the standby.

[Ronquest]

If you want to be easy on your standby switch, stop using it.

[kagliostro]

I'm not able to understand what you want to do

Please post a simple pencil drawn schematic, also a phone photo of the drawing will be enough, don't require a scanner if you haven't

Franco

[dwinstonwood]

I don't fully understand all of this, but the Valve Wizard explicitly warns against putting a standby switch in series before a choke (as in your drawing in Rely #2).
He states that when the switch is opened an "almighty flyback voltage"* will pass through the choke, and could damage the choke insulation and the switch.

But, I don't know if I'm clear on what you are asking about.

* Designing Power Supplies for Valve Amplifiers, Blencowe, pg. 121

[pdf64]

To minimise stress on the switch contacts (and everything else), mod the standby switch such that both wires go to the same lug 

Some Peaveys leave the OT CT/anode node unswitched  https://el34world.com/charts/Schematics/files/Peavey/Peavey_classic50.pdf

If there’s a choke, it’s the voltage (back emf) that stresses the switch.

One way around that is to have the feed to the screens as the only thing that gets switched off. That’s not a total mute though.

I still prefer my first idea, or use the switch for a sag resistor, cathode bias, or pretty much anything other than a HT standby.

[kagliostro]

I still haven't understood what you mean, however, try reading here, maybe you'll find the answer to your question

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


Franco

[stratomaster]

The Peavey Classic 50 schematic linked above is very similar to that I'm talking about.  The output transformer is always energized when the power is switched on.

The main difference is the use of a resistor vs a choke. 

I understand the rationale for not using standby as it is usually implemented.  I also understand the info on valve wizard's standby page.  My question is specifically about the AB763 type of implementation with a rectifier tube and a choke. 

The standard implementation energizes just the reservoir cap in standby mode.  When switched to play all the power supply nodes become energized simultaneously (B+, B+1, etc). The steady state current draw after the switch is on the order of hundreds of mA.

If the B+ node is moved before the switch then the power tubes have voltage everywhere at all times except the screens.  The PI and preamp/reverb/trem have no voltage at their nodes.  The steady state current draw is on the order of 10s of mA after the switch. 

I would think the power tubes would be stable with no screen voltage, so no harm there.  I'd also think the inrush when switching to play mode would be limited to an order of magnitude below the usual wiring.  The wild card is the choke. 

It seems like the choke shouldn't pose any worse problems since the voltage being switched at it's input will be slightly lower with the OT and plates energized.  But I'm not sure of that.

[kagliostro]

Now I've understand better what you are asking for (don't mean I know an answer)

I can say only that I know a different approach to the Standby Switch used at Chambonino Amps (seems that now the pages are out of service)

However here an example of Standby Switch that acts interrupting the supply to G2


This is the PS of a 500W Guitar Amp that uses 12 x EL36 Tubes




This is the PS of a 1000W Guitar Amp that uses 4 x 813 Tubes



Franco

[mresistor]

To minimise stress on the switch contacts (and everything else), mod the standby switch such that both wires go to the same lug 


Thats kind of like pulling the death cap and leaving the ground switch in there..   

[mresistor]

I'm with PDF64  either don't use a HV interrupt standby switch at all,  or implement a screen turn-off switch or a post PI cross-line switch or MV.


Also one could use a power tube cathode interrupt switch.

[stratomaster]

I understand all the reasons not to use the standby. I also understand the reliability and thermal benefits of solid state rectifiers over tube based ones. 

Again, I'm specifically asking if there is a problem with/good reason not to wire the B+ to input lug of the standby switch in the presence of a choke and tube rectifier.

Merlin's page outlines the ab763 style implementation of the standby switch as the least bad way to do it. Is wiring as I've proposed it at least as good as that or even marginally less bad?

[sluckey]

From a technical standpoint, I prefer to not have any B+ floating around inside an amp when the standby switch is open.

[mresistor]

Oh ok   the terminolgy B+ is what was confusing me. So we are stating that B+ is only the lead going to the OT center tap. 
And you're asking if it is worse not as bad or maybe even better to place the OT ct or B+ lead to the reservoir cap side of the standby switch in the AB763 configuration.
I think the only down side is like Sluckey says other than that there isn't much difference other than what you have stated.
Is it worse or better, I have no idea. Why not try it.


To me B+ starts off the rectifier.  But that might not be convention.

[PRR]

...the terminolgy B+ is what was confusing me. So we are stating that B+ is only the lead going to the OT center tap. ..... ..... To me B+ starts off the rectifier.  But that might not be convention.

Which is why I was asking for a drawing. But the one in Reply #2 has no screens or OT? There is no cut-in-stone convention. We can't read minds.

[sluckey]

This is what strato has in mind...

[stratomaster]

This is what strato has in mind...

I need to get familiar with the electronic schematic tools everyone here uses. Way clearer than my chicken scratch.

But yes, this is exactly it.  Assuming the standby switch has to be wired at the choke, is there any reason not to wire the OTCT as indicated?

[tubeswell]

This is what strato has in mind...

I need to get familiar with the electronic schematic tools everyone here uses. Way clearer than my chicken scratch.

But yes, this is exactly it.  Assuming the standby switch has to be wired at the choke, is there any reason not to wire the OTCT as indicated?

No reason.

[WimWalther]

(Why we go on using PT with CT and vacuum tube rectifiers ?  They are unnecessary if you add a SAG resistor to a SS FWB rectifier)

Adding a simple resistor after a SS rectifier doesn't properly simulate a tube rectifier. The only thing it does is add a voltage drop that is directly proportional to current per Ohm's law.

Vacuum diodes have a purveyence characteristic that can be plotted as a curve. Much like SS diodes, vacuum diodes are current-dependent nonlinear resistances. But unlike SS diodes, vacuum diodes do not maintain a very small, tight voltage drop (0.7-1.2V) across a broad range of currents.

Also, typical SS diodes don't offer the fast / soft / quiet operation you get from vacuum diodes. If you want those characteristics in SS, you need to use a device like a HexFRED rather than a simple 1N4007 etc.

[PRR]

Breaking DC current in a choke makes a BIG spark. In a 400V supply and iron-core choke, 4,000 volts is likely. This is a classic switch killer.

But hey, it will work until it fails. Try it.

[stratomaster]

Breaking DC current in a choke makes a BIG spark. In a 400V supply and iron-core choke, 4,000 volts is likely. This is a classic switch killer.

But hey, it will work until it fails. Try it.

I understand that.  However, is this exacerbated by the B+ line being always on? Is it partially mitigated by the reduced current draw?

For the purposes of this question/thought experiment the goal isn't "perfect".  It's "not any worse than stock".

[WimWalther]

Breaking DC current in a choke makes a BIG spark. In a 400V supply and iron-core choke, 4,000 volts is likely. This is a classic switch killer.

But hey, it will work until it fails. Try it.

And a 2Kv 0.01uF cap across the switch won't cope?

[kagliostro]

Adding a simple resistor after a SS rectifier doesn't properly simulate a tube rectifier. The only thing it does is add a voltage drop that is directly proportional to current per Ohm's law

But it is very very close as result on the sound

When you use a SS rectify the principal thing that is missing respect to a vacuum rectifier is that the vacuum rectifier has a resistance (value depending on which tube you use) that SS rectify don't has

If you do a search you can find that also Tube Gurus (KOC and others) share this opinion, use your ears to verify

---

Also, typical SS diodes don't offer the fast / soft / quiet operation you get from vacuum diodes. If you want those characteristics in SS, you need to use a device like a HexFRED rather than a simple 1N4007 etc

This is the reason to simply use UF diodes instead of 1N diodes as many people do

Also only one UF diode in series to the 1N diodes rectifier can be used to avoid the related problems

---

To protect the switch contacts there are specific components formed by a cap with an in series resistor to be connected to the contact of the switch

https://it.rs-online.com/web/p/condensatori-per-reti-rc/0209213

The more, you can add an in series diode, see Merlin's pages


In doubt, as more people suggested, use a mute

Franco

[pdf64]

… And a 2Kv 0.01uF cap across the switch won't cope?

I think the cap acts to reduce the magnitude of the back emf spike. So a super high voltage rating isn’t necessary.
My understanding is that it’s better to use quick make / break Carling type switches for such an application. A slow break switch acts to help prolong the arc, hence more damage to switch contacts.

Ditto to pdf64.  FWIW I use an inductive power resistor  rather than non-inductive - on the theory that some type of reactance will better emulate a tube rectifier.

Given the context of mains frequency rectification, the reactance of a wirewound resistor will be miniscule. Consider that it’s a series circuit also incorporating the transformer winding.
This application requires the resistor to have a sufficient voltage rating, as the instantaneous power up surge voltage will be pretty much the full HT.

… For the purposes of this question/thought experiment the goal isn't "perfect".  It's "not any worse than stock".

By the same token it’s no better than stock, so it seems something of a change for change’s sake.
Whereas several ‘better than stock’ alternatives have been mentioned.

(Why we go on using PT with CT and vacuum tube rectifiers ?  They are unnecessary if you add a SAG resistor to a SS FWB rectifier)

Adding a simple resistor after a SS rectifier doesn't properly simulate a tube rectifier. The only thing it does is add a voltage drop that is directly proportional to current per Ohm's law…

I think the idea is to achieve a similar effect to a valve rectified power supply.
In normal operation, the most significant characteristic of that is sag.
That seems principally due to -
1/ protecting anode resistance (this can be significant despite being often overlooked).
2/ the differential in voltage dropped across the valve between idle and full load current draws.
3/ low value reservoir cap

I agree that the 2nd point isn’t straight line resistive, but it’s only part of the picture, and to all intents and purposes, a suitable value resistor as proposed by kagliostro can provide a reasonable approximation of that.