Yep, another standby / mute switch musing

[Jennings]

I know, I know...don't shoot me...


My amps are valve rectified and don't have either a stand by or mute switch, and that suit me perfectly.  A friend has asked me to build an amp for him, and I'm having a hard time convincing him to not include a "standby" switch...furthermore he wants his on/off switch and standby switch in one switch. 


I'm pretty sure he just uses his standby switch as a mute for the amp really, giving silence and an instant start when it's time to play.  That's simple enough to do with a signal shunt to ground at the input, or power tube grids perhaps...or even a switch on the power tube cathodes (they're cathode bias) to switch in a big resistor and bias the tubes into silence.  And so on.


But since he wants a combined AC power on/off and his standby/mute in one switch, I'm thinking putting the above parts of the circuit close to the AC on/off would be a recipe for unwanted hum issues. 


I guess I could resort to using an On-Off-On 3 position switch to give one of the outer settings as 6.3VAC heater power on, and the other outer setting to give on for both the heaters and the B+.  But then I'm just pandering to his love of the traditional standby function, and introducing an annoying / confusing switching motion between those settings.


What are people's thoughts for the "best" way to appease my friend's whim in a way that works "best" in practice?

[Merlin]

Does he also want a tube recto, or SS?

I'm thinking putting the above parts of the circuit close to the AC on/off would be a recipe for unwanted hum issues

Definitely. Tbh I would just give the baby his bottle. If it's an SS recto you could use a dual 3-way switch in this classic implementation. He'll be happy and you can shrug it off.

[Jennings]

He does also want the tube rectifier...it's an EZ81 (6CA4).  JJ, or other modern equivalent.  And it's an 18W based amp, so I was planning to just feed all valves from the same 6.3VAC filament supply.  Would it be best to switch the HT after the EZ81, rather than before?  Possibly after the first filter cap?  I'm not using a choke, just a resistor, between the first two filter caps.

[Merlin]

Would it be best to switch the HT after the EZ81, rather than before?  Possibly after the first filter cap?  I'm not using a choke, just a resistor, between the first two filter caps.

Yes you can do that -definitely AFTER the first filter cap. The switch will suffer more by switching DC than AC, so make sure it's chunky and decent quality.

[Jennings]

Thanks Merlin...I'll go with that approach, and better make it a nice beefy Carling type as you advise.

[SEL49]

Edit:

Go here and look at the DPST switch and the wiring diagram
Switches-7

[Willabe]

Our host Doug sells those switches in his on line store;

https://hoffmanamps.com/MyStore/catalog/parts7.htm

Doug's switch wiring info;

http://www.el34world.com/charts/SwitchDPST.htm

[Merlin]

Doug's switch wiring info;

http://www.el34world.com/charts/SwitchDPST.htm

Oof, he really needs to update his diagram. Switch before cap is how you kill rectifiers.

[EL34]

Doug's switch wiring info;

http://www.el34world.com/charts/SwitchDPST.htm

Oof, he really needs to update his diagram. Switch before cap is how you kill rectifiers.

It's not my diagram
Sluckey suggested I stock that switch and he may have done the diagram also?

If you want to modify it, I will update it.

[stratomaster]

I asked the question a while back but answers seemed to not directly address the question, so I'll ask it here.

Is there benefit in an AB763 style power supply arrangement to moving the standby switch to between the screen node and the first dropping resistor? 

In my mind this limits the steady state DC current across the switch to <10mA whereas inclusive of the screen it's up near 30mA.  Assuming the inrush is 5-10X steady state, then this could mean as much as 200mA or so difference in inrush at the switch.  Plus this reduces any effect of the choke. 

[Merlin]

It's not my diagram
Sluckey suggested I stock that switch and he may have done the diagram also?
If you want to modify it, I will update it.

Here you go:

[Merlin]

Is there benefit in an AB763 style power supply arrangement to moving the standby switch to between the screen node and the first dropping resistor? 
Assuming the inrush is 5-10X steady state, then this could mean as much as 200mA or so difference in inrush at the switch.  Plus this reduces any effect of the choke.

It would certainly reduce the current in the switch, but it probably doesn't make a lot of difference in the long run, if you're using a heavy-duty switch. You could also put the switch here, which would still put the output tubes into standby (important: the cap stays connected to the choke to prevent nasty flyback voltage).

[EL34]

Thanks Merlin
I updated the drawing courtesy of Merlin 

https://el34world.com/charts/SwitchDPST.htm