Standby on screen or cathode

[Guus]

I have a technical question. (I'm Dutch, sorry for my bad englisch)

I build a two stroke guitar amp, ie a single end with 2 EL84 parallel. Works fine, till now (half a year) no problems.
I switch between : one EL84 or two EL84. (the right speaker imp goes along) by switching off the cathode of one EL84 from ground.

By repairing an old Faylon amp I found out that you can make a stand by modus by switching the screen of the end tube off power.
So I quess, is there the possibility to switch off the screen of the El84 tube I don't want to use.

What are the pro's and contra's?

Guus

[Structo]

High Guss,

I sure someone more knowledgeable will chime in but a good way to switch off a power tube is to switch in a 10K or higher resistor from the cathode to ground.
Effectively lifting the ground to the tube.

That is how a half power switch is usually wired on a four output tube power amp.

Not sure if it would work on a single ended amp.

[Guus]

Hi Tom,

Thanks for your reply.

Lifting the cathode from ground works realy very well, but I doubt, can it hurt the tube? This because all the voltages stay on. And the cathode stays on emitting electrons. Is this a problem? I don't know.

Maybe the methode 'switching off the screenvoltage' is a much safer/better way.
Like I said, I discovered this methode on an old Faylon tube amp made in end sixties/early seventies. (Faylon Hurricane 200). The screens of the 4 EL503 tubes were, when in 'standby', lifted form the power supply.
So this methode works too.

What I don't understand is: if switching the screen off volage for 'stand by' works so well, why don't I see this on more tube amps?

Guss

[pdf64]

I've started implementing standby switching on the screens. It seems really neat not to have any surge currents from charging caps.
I know a signal mute function would probably do the same job but it's nice save power tube life at the same time.
A downside is that switch ratings don't tend to include use at ~450Vdc but that's the case for some other standby methods too.
Pete

[Firestorm]

The screen standby circuits I've seen don't just disconnect B+2, but connect the screens to ground, which will completely shut off the tube. Otherwise the tube just thinks it's a poorly designed triode. That's where I'd be concerned about switch rating; toggling between 300-400 volts and zero is "arc ready." Cathode standby switches don't have to be so robust; the tube's impedance is pretty high.

Anyone want to weigh in on the risk of cathode poisoning from leaving the cathode hot with no current flowing? I suspect it takes a long time to do damage.

[JazzGuitarGimp]

The screen standby circuits I've seen don't just disconnect B+2, but connect the screens to ground, which will completely shut off the tube. Otherwise the tube just thinks it's a poorly designed triode. That's where I'd be concerned about switch rating; toggling between 300-400 volts and zero is "arc ready." Cathode standby switches don't have to be so robust; the tube's impedance is pretty high.

Anyone want to weigh in on the risk of cathode poisoning from leaving the cathode hot with no current flowing? I suspect it takes a long time to do damage.

If I'm not mistaken, Merlin refers to this as cathode depletion in his power supply book. He says receiving tubes (what we use in audio amps) don't suffer the effects of cathode depletion. Only transmitting tubes have an issue with this. His statements are in reference to the myth that allowing the tubes to warm up in standby extends their life. He says standby switches are totally unnecessary in tube amps.

[Firestorm]

The screen standby circuits I've seen don't just disconnect B+2, but connect the screens to ground, which will completely shut off the tube. Otherwise the tube just thinks it's a poorly designed triode. That's where I'd be concerned about switch rating; toggling between 300-400 volts and zero is "arc ready." Cathode standby switches don't have to be so robust; the tube's impedance is pretty high.

Anyone want to weigh in on the risk of cathode poisoning from leaving the cathode hot with no current flowing? I suspect it takes a long time to do damage.

If I'm not mistaken, Merlin refers to this as cathode depletion in his power supply book. He says receiving tubes (what we use in audio amps) don't suffer the effects of cathode depletion. Only transmitting tubes have an issue with this. His statements are in reference to the myth that allowing the tubes to warm up in standby extends their life. He says standby switches are totally unnecessary in tube amps.

I think cathode depletion is also called cathode stripping and, yes, it is high-voltage phenomenon associated with transmitting tubes.

The poisoning thing is more a matter of screwing up the cathode's emission characteristics by ion bombardment or electrochemical changes between cathode layers. I mentioned it because I see it written about without much solid information.

[Guus]

Hi guys, realy thanks for the 'reply's about 'STAND BY' modus.

Still I have not everything clear. So please help.
Looking at the given information I understand:

* DISCONNECTING THE SCREEN:
- in that case you have to use a connection to ground;
- you switch a high voltage, so the risk of arcing comes up.

Question:
- why connecting screen voltage to ground and not just lifting from screen? What is the problem when the tube thinks it's a bad triode? There is absolutely no sound coming out when lifting screen. (tested on Faylon)
Also, I think when just lifting the screen, that's the same as the situation that a screenresister is burned? I that case you put in a new resistor and the tube and amp works again.

- about the arcing problem. What is the difference with the normal 'stand by' switching of the DC powerline. In that place in the powerline there is the same or even a higher voltage and current you switch?

* DISCONNECTING CATHODE FROM GROUND:
- there would be, in this kind of amp (guitar amp), no problem with cathode poisening or damageing the cathode. So no problems when using this way of 'stand by' switching. Right ?

Question:
Mentioned is too, when disconnecting the cathode from ground you better put in a big resister to ground. So switching the cathode resister (cathode biased amp) by a resister iwth a much higher value (more ohms).
Why? What happens if not? And so, what value shoud I use?


regards
Guss.

[darryl_h]

When I was building large ( 200+ watt ) amplifiers in the 1970's, the standby switch disconnected the output valve screens. This was necessary, as the 6CA7 output valves were operating with 800 volts on the plates and 400 volts on the screens.

There was no problem with arcing on the standby switch contacts, probably because the current being handled was minimal. I don't think this standby method caused any problems with output valve longevity, but their life was short and brutal anyway...

With the screens disconnected, a massive input signal could produce a very low level, distorted output signal in the speakers.

[pdf64]

With the screens disconnected, a massive input signal could produce a very low level, distorted output signal in the speakers

Yes, I've found that too.
My hunch is that is because I've left the screens floating, rather than switched to ground, in standby; if grounded, or even just ground referenced via a resistor, I think there would be silence.
The tiny signal is quite useful actually, more so that a total mute, as it enable confirmation (when up close) that everything is rigged up basically correct, without faffing with any other controls.

To use a dual throw switch with maybe 480Vdc at one end and 0V at the other would probably be outside any normal switch's voltage rating. However, I don't envisage it causing a problem for a decent switch; obviously not a make before break or a sub miniature type.
Pete

[Firestorm]

I think what daryl_h and pdf64 is true; because the bias conditions of the tube assume the presence of screen voltage, simply removing B+ from the screen cuts off the tube so only a very large signal can bring it out of cutoff. That kind of switch would be subject to relatively little stress.

But you already have a cathode lift as "standby" installed, so why change? Putting a large R from cathode to ground and shorting it with the switch might reduce switching noise, but the switch is already referenced to ground and the tube's impedance is high so it might not matter.

I read what Merlin had to say on the cathode poisoning business and it seems that any condition where the cathode is hot with no current flowing contributes to it. But he does say that it takes many hours and that improved cathode chemistry helped solve it. Whether current production tubes use the same improved chemistry is anybody's guess.

If you were planning to play a whole gig with one tube shut off, you may as well remove it entirely, since leaving it heated wears it as fast as if you were using it.

[Structo]

If you were planning to play a whole gig with one tube shut off, you may as well remove it entirely, since leaving it heated wears it as fast as if you were using it.

I agree with this.

One time I turned on my D'lite 40 (2 x 6L6) amp and started playing.
It sounded kind of weak but not too bad.

I looked into the back and there was only one 6L6!
Not sure why I had pulled it but there is was.
I was surprised that it made sound but someone here explained it to me.

And, I didn't leave it on for very long anyway.

[Guus]

Realy thanks for the reply's.

Firestorm 'advised, when playing a gig on low volume, to pull the tube I don't use. Of course that is possible, but....

- You know, it is more simple just to swap a switch then to pull / mount a tube on stage. I had gigs where I start with one EL tube and later on in the evening I switched to 2 EL tubes.
The past half year I played serveral (low volume) gigs/rehearsels where 1 tube was on 'stand by' the whole gig (4- 5) hours. So I think from start using the amp the tube has been in total at least ~24 hours on 'stand by' with cathode lifted.
I didn't notice anything wrong on the gigs where both tubes were operating.
So far I don't have problems with the tube.

- Reading the repy's tells me that it will be just a matter of time that the tube cathode get's worn out. For me there is no reason not to believe this,

So,.. that brings me back to a safer methode.
The idee to lift the screen works but reply's tell me that there will be disadvantages too.
I could make a lift on plate and screen of the tube. Now I use one DPDT switch that lifts the cathode and at the same time switches the speaker to the right OT impedance.
I would have to use 2 switches

Are there other options?

Sorry, I hope my messages don't get you tired/bored

regards
Guss

[Firestorm]

I think you already have a perfectly acceptable solution. No need to reinvent it. If switching the tube back on is noisy, try the added resistor trick. Tubes wear out. When they do, you get new ones.

[Guus]

OK Firestorm,
I do that.

Thanks for the time answering my questions.

Guss