Truth or Myth?

[John C]

...And its all in his book too, for anyone wondering.

[Merlinb]

An amp with a rectifier tube doesn't need a standby switch. Truth or Myth?

The amplifying valves in a typical guitar amp do not benefit from a standby switch, except possibly when they are DC coupled and have no arc protection. On the whole though, you can forget about the amplify valves- they don't care how fast or often the high voltages are applied.

However, the one valve in an amp which might benefit from a standby is a rectifier valve (assuming the standby switch is not placed after the reservoir cap). However, there is not a definitive answer to this either.

If you let the rectifier warm up before applying the AC anode voltage then the rectifier must supply the full instantaneous inrush current to the reservoir capacitor. In an old, tired rectifier this can cause it to momentarily saturate and arc, since the cap is not yet charged meaning the anode-to-cathode voltage is abnormally high for a few milliseconds.

If you don't let it warm up first then the inrush is reduced by the slowly falling resistance of the rectifier valve, but the rectifier may have to work at saturation until it warms up some, but the saturation level is less than above, and rises with the cathode temp. This too can lead to arcing, although intuitively I think it would be less likely than above. Indeed, I have heard of GZ34s arcing in new Vox AC30s (which have no limiting resistors AFAIK) when the standby is used.

An inrush limiting circuit seems to be the best solution all round, rather than a pure, all-or-nothing switch. A resistor in parallel with a switch can do this, as shown on my site. And if you use a DPDT then the second switch could provide a complete mute function.

[Structo]

Thanks Merlin,
I believe I have seen such a resistor before but can't recall where.
I think it was a 100K across the standby switch, is this what you are talking about?
So it allows the first caps to charge a bit before switching to operate?

[ampdoc1]

Check this out for an explanation of the standby switch. If you look at this guy's web page it seems like he knows what he's talking about!

a'doc1

[Cliff Schecht]

Good post. This made me laugh:

In deference to prevailing wisdom, though, I use the standby switch when
cycling power on other people's amps; otherwise they get a bit pissy...

[Merlinb]

Indeed, I have heard of GZ34s arcing in new Vox AC30s (which have no limiting resistors AFAIK) when the standby is used.

In fact, here's someone on the Weber forum complaining about exactly this problem!
http://www.weberorders.com/forum/index.php?topic=3248.0

[mlp-mx6]

Having a tube rectifier means the rectifier(s) will "warm up" slowly easing the voltages nice and slow for about 10 seconds until the amp is ready to play.

In my experience this is only true for indirectly-heated rectifiers (like a GZ34). I think you would be surprised how quickly a 5U4 starts conducting, for example.

[Merlinb]

In my experience this is only true for indirectly-heated rectifiers (like a GZ34). I think you would be surprised how quickly a 5U4 starts conducting, for example.

The warm up is still an infinity compared with the milliseconds you get from an SS recto, or a pre-heated valve.

[Structo]

Indeed, I have heard of GZ34s arcing in new Vox AC30s (which have no limiting resistors AFAIK) when the standby is used.

In fact, here's someone on the Weber forum complaining about exactly this problem!
http://www.weberorders.com/forum/index.php?topic=3248.0

I'm getting a login screen when I try that link.

Got another one?

[ChrisM]

Indeed, I have heard of GZ34s arcing in new Vox AC30s (which have no limiting resistors AFAIK) when the standby is used.

In fact, here's someone on the Weber forum complaining about exactly this problem!
http://www.weberorders.com/forum/index.php?topic=3248.0

I'm getting a login screen when I try that link.

Got another one?

Unfortunately you have to be a member of their forum.

[Merlinb]

I'm getting a login screen when I try that link.
Got another one?

Don't worry, it's not really worth reading, I was just reinforcing my point since a convenient example happened to turn up right away!

[Structo]

Merlin,
I really value your opinion and have read a lot of your tech stuff on your website.

Is a 100K resistor across a standby switch all that really needs to be done to slowly charge the caps to avoid the in rush and arcing or is the resistor and diode necessary in your view?

I'm afraid I don't really understand which stage to use the diode on or if it is even applicable in my amp.

I have a D'lite which I believe is a DC coupled amp, no follower.
I wired it so the first cap is before the standby switch.

my other amp, is a 6V6 Rocket that I just finished.
I has a follower tone stack with a pre tube (two stage) before that.
This amp has a 5U4 rect, right now all the caps are after the standby switch.

Is the standby resistor only good on a PI type filter?

[Merlinb]

Is a 100K resistor across a standby switch all that really needs to be done to slowly charge the caps to avoid the in rush

Yes, but only if someone actually uses the standby switch! If they don't then... maybe use the diode as well to be sure. Alternatively you can use a vanilla neon bulb instead of a diode (no resistor required with the neon) and this may be easier to attach to the valve socket.

I'm afraid I don't really understand which stage to use the diode on or if it is even applicable in my amp.

It's only required if the grid is connected directly to the anode of the previous stage. In the case of level shifting it's rarely necessary. Although I say it's "necessary", grid-cathode arcing is a fairly rare failure mode in ECC83/12AX7s (it becomes more common with higher gm types), but I'm a belt-and-braces man myself. Probably should be used in Messy Boogers though, which use abnormally high voltages on their DC-coupled stages.

[Andy Le Blanc]

The one common statement I've heard from most older players is about just
how much hum, buzz, and noise they used to have to put up with on a stage
with their guitar and amps, the standby silences the hum when your not playing.

With amps without a ground wire, the standby and ground switches were
worth while features.

Its not so much the case now.

I don't mind a standby, a lot of players still expect one.

I like the old cathode bias trick of using the pre amp tubes heaters to bias
a pair of 6L6's, takes advantage of the control'd heater warm up time.
Everything has a relatively soft start as the filaments through the circuit heat.

But you have to wait, and that's really what a standby is for.
Set up your crap, turn it on, leave it on, but still have a way to turn it on and off
without out having to wait on the tube's warm up time.

[Ken Moon]

As far as muting goes, a good way to do that in a modern amp is to use a switch (footswitch is even nicer) that grounds the PI input.

Me, I just use a stomp box tuner for a mute switch