Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pullshocks on July 09, 2025, 01:02:10 pm

Title: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: pullshocks on July 09, 2025, 01:02:10 pm

Recently in another thread, Merlin cautioned  "EEK! Don't put a standby switch directly in series with a choke! The flyback will kill the choke."


I have to admit, I dont know anything about flyback.  But I don't want to kill the choke I am adding to my 6V6 SE project amp, so I thought I should check the wiring I plan to use. 


I could just eliminate the standby switch, but that would leave a big hole in my front panel


Attached below is my planned schematic, and the "Least Bad" standby switch from Merlin's web page. Which seems to have a standby switch going directly into the choke.....


Thanks in advance....

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: tubeswell on July 09, 2025, 04:13:53 pm

Merlin's switch has a resistor in parallel with it that partially powers up the choke when power is on - avoiding a sudden full 'flyback' voltage spike (that can otherwise occur from the choke's self inductance creating an instantaneous spike puncturing through the choke's winding insulation (and the stdby switch merely shorts this resistor to bring the HT up to full DC voltage in a 'soft' way - so there is hardly any spike)

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: stratomaster on July 09, 2025, 09:33:45 pm

You can use the hole to house a switch that grounds a grid in the preamp. It'll mute the amp without affecting the HT or causing inrush.

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: Merlin on July 10, 2025, 03:22:36 am

Your scheme will indeed generate sparks, because there's nowhere for the flyback energy to dissipate. You could implement a standby by switching off the heater supply to the rectifier tube instead?

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: kagliostro on July 10, 2025, 07:41:17 am

What about an old scool standby ?

Disconnect G2 from HT PS

(or disconnect G2 from the HT PS and connect it to the Bias negative voltage if present in your amp)

Franco

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: rumpus on July 10, 2025, 09:33:52 am

Quote from: pullshocks on July 09, 2025, 01:02:10 pm

I have to admit, I dont know anything about flyback.

It's simple to understand, especially if you compare an inductor to a capacitor. An inductor stores energy in a magnetic field; the amount of energy stored is proportional to the current flowing through it. A capacitor stores energy in an electric field; the amount of energy stored is proportional to the voltage across it.

If the voltage across a capacitor is constant, no current flows; changing the voltage requires current to flow proportional to how fast the voltage changes. If the current through an inductor is constant, there is no voltage across it; changing the current requires a voltage proportional to how fast the current changes.

Shorting across a capacitor changes the voltage suddenly - the energy stored in the capacitor is released as a very large current flow. This is why you use a resistor, not a screwdriver, to discharge the power supply capacitors before you work on your amp.

Opening a switch in series with an inductor changes the current suddenly - the energy stored in the inductor is released as a very large voltage spike. This is why you don't put a switch directly in series with your choke.

This leads me to ponder... why couldn't the choke be protected with a diode to ground? It would be reverse-biased during operation; but when the standby switch is opened, causing an abrupt negative change in the current flow, it would be forward-biased by the resulting negative voltage spike to provide a path for the current to continue to flow. (I've seen protection diodes at the plates of the output tubes in a push-pull power amp that do this.)

But it's simpler just to not have a standby switch at all in a single-ended amp.....

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: passaloutre on July 10, 2025, 10:37:26 am

Quote from: rumpus on July 10, 2025, 09:33:52 am

This is why you use a resistor, not a screwdriver, to discharge the power supply capacitors

I feel targeted

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: Merlin on July 10, 2025, 10:55:31 am

Quote from: rumpus on July 10, 2025, 09:33:52 am

This leads me to ponder... why couldn't the choke be protected with a diode to ground?

Hmm I guess that would work 🤔  Still gives me the willies though!

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: pullshocks on July 10, 2025, 11:33:39 am

Thanks everyone.  Glad I found this out during the assembly/wiring phase.  I love fireworks in the sky, but not in my amp.


It is a sobering reminder that I am in the category of "knows just enough to be dangerous."  If the issue had not come up in Jennings' thread

Twin Preamp channel switching and other head scratchers (https://el34world.com/Forum/index.php?topic=32859.0) , I would not have been aware of this. I previously had a smaller choke between Node B and C as in the attached.  If I understand Merlin's comment correctly, this  is like the "normal Fender arrangement" and can dissipate throught the power tube (single ended). Indeed, I used it that way (standby switch and all) for 20+ years with no known issues. At some point I got the idea that upgrading the choke and moving it between nodes A and B would have some advantages.  But I was ignorant of the standby switch / choke / flyback issue. that comes with that change.

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: rumpus on July 10, 2025, 01:00:00 pm

Quote from: pullshocks on July 10, 2025, 11:33:39 am

It is a sobering reminder that I am in the category of "knows just enough to be dangerous."

Haha, isn't that why we're all here to learn from each other? I've come to realize that the more you know, the more subtly dangerous the things that you don't know yet are... maybe sometime I'll tell the shocking story about how I learned in my early years that not all tube amps have bleeder resistors on the power supply... (spoiler: I lived to tell the tale).

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: HotBluePlates on July 10, 2025, 03:15:37 pm

Quote from: pullshocks on July 09, 2025, 01:02:10 pm

I could just eliminate the standby switch, but that would leave a big hole in my front panel

Quote from: pullshocks on July 10, 2025, 11:33:39 am

... I was ignorant of the standby switch / choke / flyback issue. that comes with that change.

The schematic in your most-recent post shows a Master Volume control right before the 6V6 grid.

Wire your "Standby Switch" as:  Master Volume Wiper --> Switch --> Ground.  Put a 5-10MΩ resistor across the switch if you like.

You will still mute the amp with the switch when desired, and not dink with the power supply.

Title: Re: "Least Bad" standby switch for SE amp with Choke and extra filter node
Post by: pullshocks on July 10, 2025, 05:47:26 pm

Quote from: HotBluePlates on July 10, 2025, 03:15:37 pm

Quote from: pullshocks on July 09, 2025, 01:02:10 pm

I could just eliminate the standby switch, but that would leave a big hole in my front panel

Quote from: pullshocks on July 10, 2025, 11:33:39 am

... I was ignorant of the standby switch / choke / flyback issue. that comes with that change.

The schematic in your most-recent post shows a Master Volume control right before the 6V6 grid.

Wire your "Standby Switch" as:  Master Volume Wiper --> Switch --> Ground.  Put a 5-10MΩ resistor across the switch if you like.

You will still mute the amp with the switch when desired, and not dink with the power supply.

Whoa.... :worthy1:
If there was ever any doubt that HBP is the man,  this settles it.