Standby Switch Quicky Query

[rp]

I read through some old standby switch TAG posts and the modern thought on it is that it should go away, but alas I have holes and labels in my amps so the standby must stay. I'm going to be lending my amps out more so I'm thinking of going with Merlinb's recommendation to put a ~100K resistor across the switch but some of my amps have the standby btwn the first cap and the choke BF style to protect the rectifier, and not btwn the rectifier and the first cap Tweed style.

Merlin recommends the switch and resistor before the reservoir cap though technically my amps don't have a reservoir cap as the first cap is the plate. So can I avoid rewiring and just put a resistor on the switch as is - BF style, or is it really necessary to rewire with the switch btwn the rectifier and plate cap? In other words is the benefit of protecting the choke lost if I don't rewire?

[Firestorm]

Merlin's right, of course, but frankly, I'd keep it simple and leave the amps alone. Most Blackface Fender's have worked fine for 50 years even though wired "wrong."

[Milkmansound]

you can minimize popping by putting a .047 cap (at least 600V) across the switch - perhaps it serves the same purpose as the resistor to soften the spike when the switch is thrown with a choke on one end of it

read that here somewhere, actually. It works very well.

A diode across the choke really makes the Fender style standby switch quiet (Merlin's website) however I have not had great results with it when the amp has onboard reverb. The reverb transformer usually feeds from that first B+ node and can pick up noise.

[Structo]

I have taken to putting a .01uF ceramic across my standby switches to kill any popping.

Also the Dumble way of having the standby after the large filter caps.

[pdf64]

my amps don't have a reservoir cap as the first cap is the plate

The cap after the rectifier is called the reservoir cap http://en.wikipedia.org/wiki/Reservoir_capacitor

[rp]

I've got the flu and am bored so I changed one of the amps around and put the switch before the first cap and added a 150K 2W R using Merlin's formula from an old post. Seems awright. Fades out as you'd think it would, amp seems totally quite though I'm soldering outside and there's some noise around. I was worried it's be an extra part that could fail but if it does the amp just goes back to a time-tested '50s Fender STBY set up.

Tom, there's no pop, so this might negate the need of a cap. Looks like I'll be using this set up from now on.

I thought this was a reservoir cap (image)? But I guess the first cap is the reservoir cap:

BTW I though Merlin was involved in the AX84-P1? There's no resistor on the switch.

[Milkmansound]

nothing like breathing in solder fumes when you're sick

Last time I had a cold I built an entire amp!

[Thermionic_Emission]

Have you guys ever considered a DPDT switch for the standby switch. You could put a 50K-100K or so between the tube rectifier and first filter cap. The resistor could then be shorted out when you switch the standby to play. You could configure NO/NC contacts so that when plate voltage is OFF, resistor is in circuit and when HV is ON, resistor is shorted by switch. It's an idea I haven't played around with yet. Would allow you to use a larger 1st cap, because you'd be current limiting the initial charge via resistor.

Cheers,

rob

[Colossal]

Have you guys ever considered a DPDT switch for the standby switch. You could put a 50K-100K or so between the tube rectifier and first filter cap. The resistor could then be shorted out when you switch the standby to play.

Hi Rob,

This is the method outlined by Merlin. I've been using it for years. You just need a simple ON-OFF switch. The inrush current limiting resistor goes between the two poles. When it's on the OFF (standby) position, rectified raw B+ is moving through the resistor. Flip the switch to ON and it's bypassed. I use a double throw, single pole switch and double up on the contacts (jumper each pole). I also use an 0.01uF 1kV ceramic cap in parallel with the resistor to avoid switch pop.

I like this approach as it allows the caps to charge slowly and then to full when the standby is thrown. This is nice for tube rectifiers although a GZ34 is a slow start anyway. The standby switch may be technically obsolete but there is some utility gained by the player...

[Structo]

If you use a resistor across the standby, it won't mute all the way when in standby will it?

[Colossal]

If you use a resistor across the standby, it won't mute all the way when in standby will it?

The current flow is so low that there is no audible sound. Try 100-220k 3W. When you flip the standby to play, the time to full volume is virtually instant as the caps charge the rest of the way.

[rp]

I heard no sound, but I was soldering outside and there was some light traffic going by. On stage you'd hear nothing, can't say in a studio yet. I had no pop, but I let the 5AR4 do the warm up with the stby on play so the caps were charged already. Maybe on the first use from a cold start it might pop, but it shouldn't after that.

When you hit the standby the sound fades out like a power-down, soft and nice. I like it.