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Tube amp high-voltage delay
Tube amp high-voltage delay
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Old 9th October 2019, 09:37 AM   #1
jan.didden is offline jan.didden  Europe
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Default Tube amp high-voltage delay

Some of you might remember my article in AudioXpress of a few years ago for a tube amp high voltage delay. Many people build it, but it was time for a New & Improved version (really!).

AudioXpress published it and agreed to allow free download for diyaudio members (nice people there!).

Jason agreed to make a kit available in the diyaudio store.

Let me know what you think and we can disuss details of the design here.

Jan
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Old 9th October 2019, 10:09 AM   #2
jan.didden is offline jan.didden  Europe
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Some visuals.

Jan
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File Type: png concept HVdel.PNG (13.9 KB, 722 views)
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Old 9th October 2019, 10:49 AM   #3
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jan.didden View Post
Let me know what you think and we can disuss details of the design here.
Well, you asked for it
Mona
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Old 9th October 2019, 11:47 AM   #4
jan.didden is offline jan.didden  Europe
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What? You can use a red pencil?

The depletion mode FET should have a different symbol?
What do you mean by the other red stuff?

Jan
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Last edited by jan.didden; 9th October 2019 at 11:54 AM.
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Old 9th October 2019, 12:18 PM   #5
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jan.didden View Post
What? You can use a red pencil?
Like that it's more easy to see the changes.
Quote:
The depletion mode FET should have a different symbol?
Yes.
Quote:
What do you mean by the other red stuff?
A zener for safety, if the fet fails and replaced by another one without internal zener
The devider R8-R9 can produce a voltage much higher then Vd, no good for the PIC.
With R10=680Ω the current is limited to less then 1mA.In your article you stated 1,8mA. Still rather low for PIC+Q2drive+LED+zener
Mona

Last edited by Ketje; 9th October 2019 at 12:21 PM.
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Old 9th October 2019, 12:27 PM   #6
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Ketje View Post
Yes. A zener for safety, if the fet fails and replaced by another one without internal zener
As you noticed, the FET has an internal zener. But yes, if you want to replace it with one without a build-in zener, it is a good idea to add one.

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Originally Posted by Ketje View Post
The divider R8-R9 can produce a voltage much higher then Vd, no good for the PIC.
The PIC has internal clamp diodes.

Quote:
Originally Posted by Ketje View Post
With R10=680Ω the current is limited to less then 1mA. In your article you stated 1,8mA. Still rather low for PIC+Q2drive+LED+zener
Mona
Part of that 1.8mA does not flow through R10. R10 is only there to assure some minimal headroom. The kit has a LED specifically selected for low current-high brightness, as described in the article.

The whole idea of this is to have a reliable delay switch that can be retro-fitted to pretty much any tube amp, without upsetting the supply and grounding, that uses minimal current and doesn´t need heatsinking.

Also maximum flexibility to set the delay from 20 secs to 254 secs in a simple way.

Jan
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Last edited by jan.didden; 9th October 2019 at 12:32 PM.
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Old 9th October 2019, 12:42 PM   #7
Bigun is offline Bigun  Canada
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Tube amp high-voltage delay
I keep reading that a delay in the high-voltage supply for a tube amp is unnecessary and can even be counterproductive if not well implemented, so is there generally not a consensus, it’s every body decides for them selves ? The article says the jury is out but my perception is at least on this form the people with a solid understanding of the science say it is not required. My post is probably taking this thread off in the wrong direction but I’m about to consider building a high voltage 845 amp I keep my eye out for important topics like this
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Last edited by Bigun; 9th October 2019 at 12:45 PM.
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Old 9th October 2019, 01:16 PM   #8
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jan.didden View Post
The PIC has internal clamp diodes.
I looked it up, you are right, clamp current 20mA no problem.
More comfortable the the old Cmos and TTL-logic, one had to be carefull not to activate the parasitic PNP transistor.
Quote:
Part of that 1.8mA does not flow through R10. R10 is only there to assure some minimal headroom.
Yes, i see now, only the current for the PIC and zener.
That's also the reason for D2 AND D6 !
Mona
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Old 9th October 2019, 01:42 PM   #9
jan.didden is offline jan.didden  Europe
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Exactly!
I really thought a lot about keeping the current consumption low.
The earlier unit used a separate heater winding that had to be floating which meant often a separate small transformer. That's a nuisance.
But supplying it from the HV means it must be mean and lean ;-)
Even at 600V, 1.8mA is just over one watt, which is manageable.


Jan
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Old 9th October 2019, 01:46 PM   #10
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Bigun View Post
I keep reading that a delay in the high-voltage supply for a tube amp is unnecessary and can even be counterproductive if not well implemented, so is there generally not a consensus, it’s every body decides for them selves ? The article says the jury is out but my perception is at least on this form the people with a solid understanding of the science say it is not required. My post is probably taking this thread off in the wrong direction but I’m about to consider building a high voltage 845 amp I keep my eye out for important topics like this
It depends on who you read, but there are people that feel it is absolutely necessary to extend tube life. The other thing is that at start-up, when no current is drawn, the HV can rise quite a lot higher than the operating design value. A 420V supply with 450V caps can easily go over 450V at no-load. That is also avoided with this unit.

I have also noticed is that it often prevents a switch-on thump because of the gentle HV rise.

Jan
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