Screen Kill Standby

[Wilder Amplification]

I've seen a few dual rail amps use this setup and I've never seen any adverse effects from using one, but was wondering if any of you here have seen any problems caused by killing the screens to put the amp into standby mode?

[R.G.]

I've seen a few dual rail amps use this setup and I've never seen any adverse effects from using one, but was wondering if any of you here have seen any problems caused by killing the screens to put the amp into standby mode?

Hadn't thought about this. Wouldn't that have the same issues positive and negative as biasing the grids fully off to kill tube current entirely?

The issue with grid cutoff schemes is that when tubes stay this way a long time, they develop "sleeping sickness", a high resistance condition of the cathode caused by chemical changes in the oxides that take place when tube current stays at zero with hot cathodes.

Just thinking about it, I'm not sure that's any different from turning off the B+, either.

Hmmm...

[Wilder Amplification]

Hadn't thought about this. Wouldn't that have the same issues positive and negative as biasing the grids fully off to kill tube current entirely?

That's what I was thinking. Since lowering screen voltage shifts the grid curves down the graph, once you kill the screen voltage entirely that would shift your normal "operating point curve" way down past cutoff and the tube wouldn't pass any current, so in a way it does satisfy that requirement. But not sure if you just shut the screen power off and leave the screen floating or if you wanna ground the screen, or if it's a bad idea altogether.

Of course then there's the question of "Does it sound better than killing both the plate and the screen" or does it just help to induce the placebo effect.

[Enzo]

Peavey has been turning off the screens for a standby on MANY of their tube amp models for a long long time. One might or might not like their amps, but I certainly have seen no ill effects from it. ANd I have been running a Peavey authorized repair shop for the last 24 years, so it is not like I don;t see their amps.

[soundmasterg]

KOC has screen standbys in a few of his books. I used it on one amp that is basically 3 amps in one head, and it works great there. I say try it out.

Greg

[Gregg]

Peavey has been turning off the screens for a standby on MANY of their tube amp models for a long long time.

Marshall does it as well:

[Sanfton]

I've seen a few dual rail amps use this setup and I've never seen any adverse effects from using one, but was wondering if any of you here have seen any problems caused by killing the screens to put the amp into standby mode?

Screen (or Screen + Preamp) supply switching is perfectly safe, and a good idea -PROVIDED you don't have a choke in the amp, which applies to the Marshall circuit below, and most Peaveys (that I've had in).

With the screen unpowered, the whole of the voltage gradient within a Pentode/Beam Tube exists across the Anode-to-Screen separation. The Electric field (E) around G1 and cathode is essentially only due to the bias voltage in this condition.

It is true that the preamp valves are going to develop sleeping sickness if you leave it on standby for hours - but that's true for any standby scheme that takes the power off the preamp.

If you have a choke, and you use it to smooth the screen supply (not sure what other ways there are) you are in trouble. This is the likely reason it was never applied to the vintage amps.

The problem is that the choke sets up a mighty magnetic field (B) when some screen/preamp current is flowing in it. If you then go and open a switch in series with this choke, and the current has nowhere to go. This forces the magnetic field to collapse, precipitating an increase in voltage across the choke that keeps on rising until the energy stored in the choke can go somewhere! Usually the energy finds something to break down, like your switch contacts or the choke insulation. Boom!

If you want to switch the screens, use a resistor in place of a choke (yuk) or, better still, a MOSFET gyrator. Keep the screen voltage capacitor size modest, too (32 ..160uF) or you'll suffer from surge current spikes when the switch turns ON.

[ThermionicScott]

I don't have any links handy, but I've seen one standby scheme where the PI outputs were connected together to prevent any AC signal from going to the output tubes. It muted the amp and left the tubes drawing the usual idle current. Seems like a good way to avoid most of the potential issues.

- Scott

[Sanfton]

I don't have any links handy, but I've seen one standby scheme where the PI outputs were connected together to prevent any AC signal from going to the output tubes. It muted the amp and left the tubes drawing the usual idle current. Seems like a good way to avoid most of the potential issues.

- Scott

Scott, The Selmers of the 1960s just pulled B+ from the PI -

[Steve Conner]

The problem is that the choke sets up a mighty magnetic field (B) when some screen/preamp current is flowing in it. If you then go and open a switch in series with this choke, and the current has nowhere to go. This forces the magnetic field to collapse, precipitating an increase in voltage across the choke that keeps on rising until the energy stored in the choke can go somewhere! Usually the energy finds something to break down, like your switch contacts or the choke insulation. Boom!

All you need to fix that is a freewheeling diode. (I've tried it and it works, but I also needed a RC snubber to remove the last trace of "pop" in the speaker.)

[Wilder Amplification]

Hi Sanfton...I am in fact choke filtering the screen supply, but the supply in question uses a seperate screen tranny. Was considering killing standby with a DPST on the AC side of the grounded CT rectifier. Marshalls do this on their single rail supplies and use choke filtering. Would this not work or be a good idea?



The bias would still be live since it's fed off of a 100VAC tap on the same tranny.

[Sanfton]

Hi Sanfton...I am in fact choke filtering the screen supply, but the supply in question uses a seperate screen tranny. Was considering killing standby with a DPST on the AC side of the grounded CT rectifier. Marshalls do this on their single rail supplies and use choke filtering. Would this not work or be a good idea?

The bias would still be live since it's fed off of a 100VAC tap on the same tranny.

Switching the ac-side is much less problematic, because the field in the choke will collapse slowly, emptying the first cap into the second (make the second at least as large as the first!)

But Steve Conner's idea is good. Put a diode across the choke (anode to output side) to absorb the turn-OFF pulse. Beware though, there's a lot of energy potentially available at switch-OFF, and I'd recommend the kind of switching power-supply diodes with 600V 10A rating in a TO220 package. This is especially important on EL34 amps, which take a lot of screen current. If the dc resistance of the choke is less than 50 ohms, I'd run two diodes in parallel, because low DCR = fast turn-OFF = high peak OFF-current spike.

The snubber is also useful, and I would try the LCR PC/HV/S high slew-rate durability capacitors (Farnell or Newark). 1500V 0.1uF and 100R 3W.

[Steve Conner]

Put a diode across the choke (anode to output side) to absorb the turn-OFF pulse.

The snubber is also useful, and I would try the LCR PC/HV/S high slew-rate durability capacitors (Farnell or Newark). 1500V 0.1uF and 100R 3W.

A 1N5408 worked fine for me. Don't put it across the choke, it'll screw up the ripple rejection. You have to switch the input end of the choke with your standby switch, and you put the diode from this end of the choke to ground. (anode to ground)

When you open the switch, the free end of the choke will shoot negative, and the diode will catch it at 0V.

If you switch the output end, the voltage will shoot upwards when you open the switch, and there's nowhere to clamp that to.

Those PC/HV/S capacitors are pretty hardcore and probably overkill for the job. I once bought 60 and assembled them into a 20kV bank to drive a Tesla coil. I couldn't kill them.

In my Ninja Deluxe amp, I used the above arrangement, with (IIRC) a 10nF 1kV ceramic disc cap and a 100 ohm 2W resistor forming the snubber. The choke in this amp carries the entire HT current, not just the screens. The standby switch is pop-free and has worked for about a year so far. I used a double-pole switch with the two sections in series, to help the DC breaking capacity.

[imaradiostar]

Hey Jon, why not use a dpdt switch to open the connection into the plate supply bridge and the input the the first screen supply filter cap on one switch?

jamie

[Wilder Amplification]

Hey Jon, why not use a dpdt switch to open the connection into the plate supply bridge and the input the the first screen supply filter cap on one switch?

jamie

My worry on that was having two different voltages from two different sources on the same switch. Probably not really a cause for worry though...just me being overly cautious and trying to over-engineer shit as usual.