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Valve amp standby

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Gareth Magennis
Guest

Mon Dec 02, 2019 11:55 pm   



Hi,

in a previous post of mine about a faulty Engl amp I had that destroyed all its valves, I believe it was suggested by Phil Allison that Standby mode can be inmplemented in a push/pull configuration simply by removing the Screen Grid HT.

Since this grid does not pull much current, switching it off is fairly trivial, as opposed to the trauma of switching the whole HT in and out.


I posted that I had never come accross such an implementation and wondered why, but yesterday I finally did.

It is in this thing:
https://www.fryette.com/power-station-integrated-reactance-amplifier/


Interestingly, this HT switching is done by a solid state device - this one:
http://www.cosmo-ic.com/object/products/KAQV216.pdf


The switching is done via the switched speaker jack.
No speaker lead, no Screen Grid HT.





Gareth.

Phil Allison
Guest

Fri Feb 07, 2020 6:45 am   



Gareth Magennis wrote:

--------------------------
Quote:


in a previous post of mine about a faulty Engl amp I had that destroyed all its valves, I believe it was suggested by Phil Allison that Standby mode can be inmplemented in a push/pull configuration simply by removing the Screen Grid HT.

Since this grid does not pull much current, switching it off is fairly trivial, as opposed to the trauma of switching the whole HT in and out.


I posted that I had never come accross such an implementation and wondered why, but yesterday I finally did.

It is in this thing:
https://www.fryette.com/power-station-integrated-reactance-amplifier/


Interestingly, this HT switching is done by a solid state device - this one:
http://www.cosmo-ic.com/object/products/KAQV216.pdf


The switching is done via the switched speaker jack.
No speaker lead, no Screen Grid HT.



** Hmmmmm........

Wot if a power tube develops a screen to cathode short ?

Bye-bye nice little SS switch ?

Tube amp output stages are regularly subject to some appalling accidental abuse plus some disturbing deliberate ones.

Damage needs to be limited to fuses and maybe the odd resistor.

Some designers get too cute.


..... Phil

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