Tube Rectified Amps: Standby via Switch on Rectifier Heater - Good Idea?

[pdf64]

Title say it all really; even on directly heated cathodes it should provide a slower start up than a switch somewhere directly in the B+.
Lower surge currents and thereby facilitates quick blow B+ fuses
Maybe the >30 second warm up of a GZ34 is it's biggest downside, as the user would expect it to be operational quicker than that and may as well just turn the whole amp on/off?
Anyone tried this?
It's come to the fore due to a service on my friend's DrZ EZG50, which puts the standby switch between the GZ34 and reservoir cap; I would prefer it to be between the reservoir and rest of the amp but that would entail a fair bit of rewiring and additional turret. For the moment I've dissuaded him from using standby to reduce stress on the rectifier but it would be nice to have a better solution.
Thanks - Pete

[R.G.]

Not a bad idea. It ensures a slow start up, in a way that a switch between rectifier and filter cap cannot.

A real question lurking under this is: why is there a standby switch at all, and what is it supposed to do?

If it's to mute the amp, there are many other ways.

If it's to save hours on the tubes, it can also be done other ways.

There are also other ways to reduce stress on the rectifiers with a hard standby switch. One way is with a current limiting clamp; I've designed one of these and its schematic is available at geofex.com. Another is to use the standby switch to switch a large-value resistor into or out of the path from the filter caps to the circuits. Another is to use the standby to switch in a large-value resistor in the path from the power tube cathodes to ground. Or to pull the power tube screens to a much lower, or even negative voltage, or to switch the power tube bias to turn off the output tubes.

Which of these is best depends on what "standby" is supposed to do, and whether that objective is relevant to today's amplifiers and how they're used, as opposed to how they were used in the Golden Age.

[nickb]

Hi pdf64,

I have an idea that you might want to proceed with caution. If you draw excess current before the the rectifier is properly warmed up can can get an arc and strip the coating off the cathode. I make no claim to expertise here, it's just my reading of Robert B Tomer's "Getting the Most of Vacuum Tubes", see page 17 under "Arcing" and page 35 paragraph 3.

Nick

[eschertron]

I agree with Nick. From what I've read, turning off the heater circuit would have the opposite effect from that intended by the standby switch - which is to protect the cathode from a high potential when cold.

[Steve Conner]

That's true, but on the other hand, delayed warmup rectifiers like the GZ34 were supposed to warm up after all the other tubes. According to that argument, they would destroy themselves.

[nickb]

Good point Steve!

There's a backgrounder here: http://www.peavey.com/support/techno.../chapter_6.pdf

Nick

[Steve Conner]

I've seen that, but it doesn't answer my question: Why doesn't the GZ34 strip its own cathodes during the delayed warmup?

I've never seen a convincing answer. I suspect that either it does (so Mullard could sell more of them ) or cathode stripping itself is a myth.

[nickb]

I think the answer is that
(1) It's a sinusoidal signal i.e rising slowly (limits load cap charging current)
(2) The tube data specifies a minimum series resistance and maximum load capacitance
So as long as you stay within those you would be OK.



Nick

[Enzo]

Or it doesn;t really occur at the voltages and currents we face in audio.

There are amps with power kill standby switches in about any configuartion you can think of, and there are amps that don;t have them at all, and in my memory, I cannot think of so much as a single amp that has a reputation for killing rectifier tubes. I think discussions like this one are solutions in search of a problem. We rationalize that such and such a circuit will prevent... something that never happens anyway. This approach will be better because it dramatically reduces the likelihood of Bengal Tigers attacking your amp. And I hang garlic on my amp to prevent vampires from sucking the blood from my amp - so far it has worked flawlessly, haven't lost a drop. People hear about cathode stripping, then wory about preventing it. Never mind that it just doesn;t happen on your Fender Twin anyway.

[JoeM]

It's been mentioned that at the voltages typically found in guitar amps, cathode stripping isn't an issue.


[edit] Oops I see Enzo has already explained this.

[jpfamps]

The use of the standby switch in guitar amps is in many respects an anomaly; it doesn't seem to be a feature on the vast majority of consumer electronics tat also feature valves.

It seems that Fender first introduced the standby switch on amps such as the Tweed Pro and the Bassman just after the introduction on the DC coupled cathode follower stage. Fender weren't known for lavishing money unnecessarily on there products, so is it possible it was introduced to reduced failure of the CF?

Merlin in his power supply book also speculates that it might have been introduced to protect the filter caps from the unloaded HT.

Possibly it was then seen as a "nice" feature to have on an amp.

Other manufacturers such as Silvertone and later Selmers implement the standby by shorting out the audio signal.

[eschertron]

I agree with Nick. From what I've read, turning off the heater circuit would have the opposite effect from that intended by the standby switch - which is to protect the cathode from a high potential when cold.

:headsmack: d'oh! when the rectifier heater's not energized, there's a low likelhood of it generating any voltage...

so as long as the filter caps discharge quickly enough, then the mythical cathode stripper never appears. I understand now.

[R.G.]

That's the discussion I was wanting to promote.

If what you're trying to do is mute the amp, mute it. If what you're trying to do is increase tube life by reducing power-on hours, it may be more destructive to toss in more thermal heat/cool cycles than to mute.

Cathode stripping is supposed to be not a problem below B+ of about 700V, according to every text I've been able to find it in from the Golden Age.

[Merlinb]

Tube rectifiers (at least the kind we use) do NOT like hot switching. It even says so on the data sheets, if you find the right ones. In other words, they don't like being pre-warmed before connecting them to the reservoir cap. Apart from that you can pretty much do what you want, standby wise. Not that it will have any significant effect on tube life in a valve amp- with the possible exception of grid-cathode arcing in cathode followers...

[jpfamps]

That's the discussion I was wanting to promote.

If what you're trying to do is mute the amp, mute it. If what you're trying to do is increase tube life by reducing power-on hours, it may be more destructive to toss in more thermal heat/cool cycles than to mute.

Cathode stripping is supposed to be not a problem below B+ of about 700V, according to every text I've been able to find it in from the Golden Age.

The "problem" is now that it I think it would be hard to market a valve guitar amp without a standby switch due to "conventional wisdom" that it is needed to extend valve live.

The muting function is now largely redundant as most players have a muting tuner pedal in their rigs.