• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Lets settle the b+ on cold tubes issue!

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So far no one has presented a shred of evidence to suggest that the delayed application of the HT gives any extension of the working life, or that a shock start has any deleterious effects, in receiving valves.
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No, but sudden application of HT on already hot tubes ALWAYS results in monstruous cathode peak currents before the link caps (and -worst- the large cathode bypass ones) be fully charged.

My Faith:
- ALWAYS use grid bias for power tubes applied as soon as possible.
- For penthodes, consider delaying the application of the screen voltage (easy, since screen voltage of power pentodes MUST ALWAYS be regulated) :)
- Unless you use IST :mad:, lo power tubes have sufficient resistance in their anodes to protect'em.

Yves. :cool:
 
Salut Yves,

For sure, HT must always be applied slowly.

Your delay in applying screen voltage is the same as delay in applying HT, so far as the cathode can see. The E field is nearly zero at the cathode for Vg2=0V. No danger to the cathode or the grid performance if you do this.

And since you are regulating and delaying VG2, why not use the regulator on the low-power tubes too. MOSFETs are so much cheaper than tubes.

The effect described by van de Weijer does not cause high ANODE current to flow - it causes contamination of the grid after many heating/cooling cycles - making for increased noise, and distortion.

The distortion is due to unwanted cathodes on the grid bouncing up and down with Vgk.
 
Electrometer use is very different from normal use of course (5pA grid current is astonishingly low!) but I will do my best to hunt out the science behind the 6BS7 datasheet advice; it must exist, unless it was only a precautionary measure they added 'just in case'.


The Brimar advice is quite specific: If you want to maintain stability and low
grid current, you must delay application of HT by 1 minute.

Electrometer applications care about grid current. Everything else is second
importance. HT delay is therefore vital to keeping stable grid current. So what is going on?

Well, van de Weijer says that as the cathode is heated & cooled over long-term operation, particles of cathode slew away. When the cathode is cold, it doesn't conduct, and isolated charge may remain. HT applied to Cold cathodes may rip the charged particles toward the anode, and it may get stuck to the grid. Once on the grid, it acts as an emission source, degrading grid current, degrading noise, and raising distortion.

It's not a huge effect, but gets worse with every cold start.

This mechanism is the only explanation we have now for the grid current degradation.

Moving away from electrometers & noisy guitar amps, what other application cares about stable valve performance, low grid current and low noise?

well, what about OSCILLOSCOPES?

Starting at the top, can Tektronix teach us anything? You don't need to look far.

See the 1960 Tek manual for the 551 scope with SS rectifiers and Valve amplification.

It reminds us that the best instruments need low noise valves and high stability generally. We are reminded not to replace tubes without a good cause, or put them in different parts of the scope. Why? Because you'll have the tedious job of recalibrating the scope.

Since tube noise and stability is important, do Tek care about HT delay?

They care.

"A time delay relay K600 delays the application of dc voltage to the amplifier tubes in the instrument for almost 25 seconds. This delay allows the tube heaters to bring the cathodes up to emission temperature before operating potentials are applied."

I suppose someone is going to stand up and say that the Staff of Tektronix were prone to superstition, or didn't know as much as you do.

Once again, it's your valves, and your money. Design at a level below commercial guitar amps as your model, or up with the best electronic instruments. There's no thermionic Police to interfere, so far as I know.
 

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If you think that the Founder and original designer of Peavey amps is ignorantly making claims about the proper use of amps he designed, then the onus is on YOU to substantiate what you say.
Not true I'm afraid. If someone says something exists (like cathode stripping), the onus is on THEM to prove it. It is practically impossible to prove that something doesn't exist. It should be easy to prove that a standby switch improves the working life of receiving valves, but so far no one seems to have done it. The only conclusions I have ever read are that there is no significant improvement.

There are good reasons for a standby switch of course, but Peavey made a mistake when he added cathode stripping to the list. I'm not saying Weijer's idea is unreasonable. Intuitively it is believable, but that is no reason to assume it is true.

This also applies to Weijer's idea. We cannot start talking about it as if it is a real proven phenomenon. It remains speculation, educated guesswork, until someone proves it.

This mechanism is the only explanation we have now for the grid current degradation.
Let's not get ahead of ourselves. I'm sure we (or people wiser than us) can come up with some other possible causes.

And I'm sure you can find innumerate examples of equipment which advices delayed application of the HT, but still it get's us no closer to understanding why it might be necessary. All you are doing is finding examples which suggest it might be a good idea. And if it really has a beneficial effect in those electrometers and oscilloscopes (presumably it does), does that still apply to audio amp, or have we gone way past the point of diminishing returns? Finding out why is immeasurably better than taking someone else's word for granted.
 
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And I'm sure you can find innumerate examples of equipment which advices delayed application of the HT, but still it get's us no closer to understanding why it might be necessary. All you are doing is finding examples which suggest it might be a good idea. And if it really has a beneficial effect in those electrometers and oscilloscopes (presumably it does), does that still apply to audio amp, or have we gone way past the point of diminishing returns? Finding out why is immeasurably better than taking someone else's word for granted.

Actually, I agree with that, mostly. I would much rather someone stepped up to teach us something, based on their experience in designing a thermionic cathode. It's a question for a professional who has actually been involved in finding why some cathode materials work reliably for long periods, while others are maybe prone to crumbling, or storage of charge for longer periods. etc etc.

Maybe that will happen.

Until then, you have to decide what to believe, based on what you think of the experience of others.

The point about the electrometer and scope applications, is that degraded tubes really screw their performance up badly.

On the other hand, a non-critical audio design will not be able to tell the difference. Where do YOUR design aims lay?

I am only saying that my designs aim to eliminate any degradations I can find, and fix.

It's not a certainty that anything is happening, but their is enough evidence, among the actions of Designers whom I respect, that I will take simple steps, like using damper diode rectifiers for triode amps, and FET regulation (with soft-start) for Pentodes.

You may choose to do something different, safe from any disturbance from me.



.
 
Fender have an article to the same effect:

Fender News:The Standby Switch

Rubbish. Then why don't the Fender owner's manuals suggest the same? The owner's manuals say put the amp on standby while you slug a pint during the break. That's it. Power up sequence is power up. Both switches on.

And who wrote that article? Engineer or guitar player or you? Can't tell. No pride in authorship there.

And if a 75 cent switch saves the power tubes, why don't all Fender tube amps come with one?

Prolong the life by what metric? Minutes, hours, days?
 
I looked at my Tektronix 545 schematic as I knew it had a time delay relay in it. The manual has the same claim that all tubes remain High Voltage off for 25 seconds.

The circuit is interesting in that they use the time delay tube to turn on a latching relay which de-energizes the time delay tube. This insures the 25second trun on delay if power momentairly drops out, and promotes the full 25 second delay. Without the latching relay de-energizing the time delay tube, it would be hot when power dropped out and the time delay would be considerably less.

That said, it appears to me that only portions of the High Voltage are delayed for 25 seconds.

-150V is delayed.
+100V is not delayed, and it's regulator is bypassed by K700 t horugh a 750 Ohm resistor.
+225 is delayed
+350 is delayed
+500 is not delayed, but it is stacked on top of +350, and it has part of it's regulator biased by -150, so it may come uppart way, but not to full potential.
 
That he has personally channeled the late engineers who designed those circuits. Or his belief that, since it is beyond his ability to figure out the logic of earlier designs, it is beyond that of competent designers.

It is you who made assumption on behalf of those late engineers. Indeed, there is logic behind your assumption, however there is also logic behind other assumptions, different than yours.

The point is that probably no one of the participants here has proved facts concerning the issues discussed here. However, some of the disbelievers, like you, claim that those phenomena doesn't exist – unless some hard proof wile be shown. Since to your view, which is actually your belief, those phenomena doesn't exist, anyone who claims otherwise must be deluded. Such an approach is typical of fanatics in various fields.

As for myself, I don't know. However, when in doubt I prefer to take precautions. They may be unnecessary, but better be safe than sorry. Especially when it concerns tubes that cost me a fortune and I don't have the money to buy other such tubes when they will be gone.
 
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