the standby switch

[bob p]

IME that Silvertone method doesn't seem to work as well as real standby switch, as it relies upon perfect balance between the push-pull segments to null-out the entire signal. Although the Silvertone method dramatically reduces volume, I can still hear a faint signal if I play my Twin Twelve with the standby engaged. Of course, the Silvertone method isn't doing anything to help to prolong tube life, or to turn-off B+ on the amp, or to reduce heat -- it's just a mute switch.

Solid state thumps can be a real problem. I have had many an annoying thump problem with SS gear, and I've had to add speaker relays to the amps to deal with it. I can imagine that anyone who's playing out wouldn't want embarrassing turn-off thumps, and if they're already familiar with Fender-type standby switches they would probably appreciate a simple Vox-Thomas type standby to get rid of the thump problem. The funny thing is that today there are still plenty of SS amps that thump and most people seem willing to just accept it.

[Steve A.]

Solid state thumps can be a real problem. I have had many an annoying thump problem with SS gear, and I've had to add speaker relays to the amps to deal with it. I can imagine that anyone who's playing out wouldn't want embarrassing turn-off thumps, and if they're already familiar with Fender-type standby switches they would probably appreciate a simple Vox-Thomas type standby to get rid of the thump problem. The funny thing is that today there are still plenty of SS amps that thump and most people seem willing to just accept it.

Tell me more about the Vox-Thomas type standby switch!

My latest project is developing mods for the 1990's series of Fender solid state amps like the Princeton 112/+ and 65, the Champion 110 and 30 and my most recent acquisition, the 90+ watt Deluxe 112. The older amps (with the # referring to the speakers) produce some pretty loud pops when turning off; the newer amps added a 0.1uF/250v poly cap between the PT secondary going to the rectifier bridge which helps some. However what works even better is turning down the MV first, which is incredibly easy with these amps: I replace the NFB resistor on the last op amp before the FX loop or power amp section with a pot. These amps are LOUD and a master volume tames the beast.

Steve Ahola

[loudthud]

The Vox-Thomas amps used a three position rotary switch. Off-Standby-Play. In the middle position, the power was switched on but the speaker was not connected until the Play position was selected. The actual switch was kind of odd. The AC was switched with what looks like the back of a Tweed Champ volume control. The speaker part looked like a ceramic wafer switch. They couldn't handle the current needed for 120W at 2 ohms.

[Steve A.]

The Vox-Thomas amps used a three position rotary switch. Off-Standby-Play. In the middle position, the power was switched on but the speaker was not connected until the Play position was selected.

We could do that with a progressive DPDT toggle switch- right? I got 5 or 6 of them many years ago and never did use them. Regular On-Off-On DPDT switches work fine on tube amps. In the middle is Off. Flip it one way for Standby and flip it the other way for On. But that would not solve the problem with ss amps popping loudly on shutdown.

The Fender amps I've been working on use a plastic rocker switch for power so a separate switch could be added to disconnect the speaker.

Thanks!

Steve Ahola

[J M Fahey]

FWIW I have added "standby" switches to some of mySS amps in special situations.
They actually cut the speaker out "hot" lead.

My amps pop as much as many commercial ones and people *usually* don't complain; a few worry "is something wrong?" but when told not to, they soon forget it.

But in some cases this is not acceptable, I have made amps for: Theaters/conference rooms/churches/yoga/meditation centers where (specially in the latter) any extraneous noise is *absolutely* forbidden.
Imagine a place with subdued lighting, incense smoke in the air, people with closed eyes meditating and "becoming one with the Universe" (no kidding) ... and then you hear a nasty POP!!!

So I tell them to turn "power" ON, count to 30 mentally and only then turn "standby" ON ; repeat in the reverse order when powering OFF .

Works like a charm.

Much better than a relay, by the way.

[R.G.]

I believe that the Thomas Vox amps' off/standby/on switches were a solution to "thump" as well, and perhaps equally marketing features; they emulated the function of tube amps of the time.

They were, as noted, marginal for the Beatles, as the current rating for the switches on the wafers was more like 6-7A instead of the 10-12A needed by the Beatle for 120W into 2 ohms. And they break over time, the phenolic wafers becoming brittle and breaking.

Being perhaps the world's premier Thomas Vox nut ( ) , I designed a drop-in replacement to do the same job with a low-power 3-position rotary switch rated for well under 1A. It's a small PCB with a common Lorlin CK2371 4P3T rotary switch ($4.90 each, ones) and two 120Vac-coil, 16A contact PCB mount relays (Schrack RT314615, $2.80 each), and it controls the 120V indicators for "On" and "Standby" like the original, as well as having enough current capability to stand full-power operation. I threw in a position for a MOV on the main incoming AC just for grins.

The old multideck rotary switches have become incredibly pricey. The only 3-deck rotary Mouser has is $55.00.

I think I've forgotten to mention that one of the issues with standby switching schemes that just interrupt the high voltage is the inrush current. This can be solved with a current clamp on the incoming high voltage to the filter cap. Article here: Tube Amp Current Clamp

[loudthud]

We could do that with a progressive DPDT toggle switch- right? I got 5 or 6 of them many years ago and never did use them. Regular On-Off-On DPDT switches work fine on tube amps. In the middle is Off. Flip it one way for Standby and flip it the other way for On. But that would not solve the problem with ss amps popping loudly on shutdown.

The On-On-On switch would work, except for something RG just reminded me of. On those Vox amps the are red and green pilot lights that tell you what position the switch is in. There is a separate pole on the rotary switch just to control the lights.

The center off switch would eliminate the turn-off thump because the speaker would be disconnected when power is disconnected.

[52 Bill]

I think that the least disruptive method would be to ground out the input grids of the power tubes. Or would it be better to disconnect them?

Won't grounding the input grids redplate the power tubes?

The early Vox tube versions of the Berkeley and Cambridge Reverb had ON/OFF/ON toggle switches wired as ON/OFF/STANDBY.

[R.G.]

The On-On-On switch would work, except for something RG just reminded me of. On those Vox amps the are red and green pilot lights that tell you what position the switch is in. There is a separate pole on the rotary switch just to control the lights.

Yeah. One whole SP3T section was used for just the indicators. One SP3T section was used for pass-throughs for the PCB traces for the other two. The other two controlled the 120Vac to the relay coils, turning the AC on in two positions for the 120V to the power supply and one position for connecting the speaker to the output jack.

[Steve A.]

Won't grounding the input grids redplate the power tubes?

I hadn't thought of that so I guess it would be better to ground the signal ahead of the PI if you were going to take that route. Of course anything along those lines would not do anything to prolong the life of the power tubes.

Thanks!

Steve

[Steve A.]

The center off switch would eliminate the turn-off thump because the speaker would be disconnected when power is disconnected.

So you don't need to disconnect the speaker before you disconnect the power? In that case a two position DPDT switch should work- right?

BTW the progressive DPDT switches are Off-On (1 pole)-On (both poles)

Thanks!

Steve

[bob p]

So you don't need to disconnect the speaker before you disconnect the power? In that case a two position DPDT switch should work- right?

BTW the progressive DPDT switches are Off-On (1 pole)-On (both poles)

Thanks!

Steve

Steve, I use a 120 VAC DPDT relay to do the switching for me on stereo amps. The relay energizes when the amp's AC power is switched on, and in the energized position it connects the speaker load to the amp. When the AC power is switched off, the de-energized position connects a small load resistor to the amp. The switching of the AC relay is instantaneous with the AC power being turned on or off. Because it takes some time for the amp's DC power supply to rise when the amp is switched-on, or to collapse after the AC power is switched-off, the annoying power-off thumps will happen several seconds after the speaker disconnect occurs. Doing it this way with a switched AC relay completely eliminates the need for a standby switch, or for a user to learn a two-switch power-on and power-off procedure. I like the relays because they're idiot proof, but you could do the same thing with a switch on the speaker output of the amp.

You don't even really need the little load resistor.

[bob p]

Regular On-Off-On DPDT switches work fine on tube amps. In the middle is Off. Flip it one way for Standby and flip it the other way for On. But that would not solve the problem with ss amps popping loudly on shutdown.

I like to cut down on the number of switches, so I've also been doing it that way for longer than I can remember.

I just bought an OTT15 that uses that kind of standby. Standby is in the middle, high power and low power PT taps are in the other positions.

But that would not solve the problem with ss amps popping loudly on shutdown.

I've heard people say that the thumps in SS amps are caused by asymmetric collapse of the PSU rails. If that's the case then fixing the asymmetry should cure the problem. I have some amps that have never thumped for over a decade, but have only begun to thump now that they have gotten older. with these amps I've also noticed that the thumps seem to be worse when the amp is connected to a lower resistance load.

[trobbins]

When turning off a valve output stage amp, the speaker relay may end up open circuiting the output transformer's output winding for a brief time, which could cause plate voltage stress on the OT especially if the signal level was at max at the time.

[Merlinb]

So I tell them to turn "power" ON, count to 30 mentally and only then turn "standby" ON ; repeat in the reverse order when powering OFF .

Works like a charm.

Much better than a relay, by the way.

In what way is this better than a relay?!? You're relying on the user (= newb) to remember what to do, and to be wise/reliable/competent enough to do it. This is a problem that should be solved automatically or not at all.