Stand By and Slow Heater Turnon

[Strelok]

Hello all you happy people,

What's the deal with Stand By nowadays?

I read many things in Kevin's excellent books.
If "cathode stripping" is an issue than I should have those switches in all my preamps too!
Or does that only come into play with amps of "high voltage", which I learned is originally 1000V+ ?

I'm building a Jim Kelly amp like in the project, and for geographical reasons (the layout of my PS that is) I woul dlike to have that switch after the first resistor.
That should be no problem I think?

Is the strain on the switch not too much?


Also, I saw an excellent schematic for building a slow power-up for the heaters.
Is anyone doing this?
Is this schematic only meant for DC operation?
I'm asking this because I remember a BJT there.


Kind regards,

Strelok

[makinrose]

You really don't need a standby on most MI tube gear. The Kelly Amp certainly won't need one. While Fender used to put them after the first filter cap this shortens the switch life. If you must have one it's safer to simply make it an audio mute that disconnects some part of the audio circuit.

[K O'Connor]

Hi Guys

Cathode stripping does not happen in any typical guitar or hifi circuit ever built, or which hobbyists are likely to build.

To be a concern, with tetrodes or pentodes Vs has to be over 700V; in a triode amp Va has to be over 700V.

have fun

[Strelok]

Thanks a lot people!

How about the heater surge protection.
What would be best to use, the surge limiting in PoP, or the soft-start in TUT?

I see that the soft-start should not have the center tap connected.
Can I use the two resistors to create a faux-center-tap as usual, to create a stand-off?


Eric-Jan

[K O'Connor]

Hi Strelok

The TUT method for heater surge protection (Fig.2-18) is universal and easy to adjust for whatever the heater load is just by changing the limit resistor value. One relay control circuit is shown in Fig.2-19, and can be any type of timer circuit you wish to deploy. Fig.2-20 shows an idea from Wireless World that provides reduced net voltage output over a set time constant, but does not provide specific current limiting you can set.

Figs. 3-18, 3-19 of POP have active current limiting that requires some excess voltage be available, which is often the case for DC heaters. Same with TUT2 Fig.2-74.

Limiting the heater turn-on surge will increase tube life, but remember that if a tube is not mechanically upset, it will generally last beyond fifty years even without such measures.

[Strelok]

Hi Kevin!


Thank you very much again!
I was also taking into account the heater winding and the stress on the switch.
But... it was that I wanted to have something special in my amp as well.
Now I know that for electronic reasons there is not a need for it.

Thanks so much!


Eric-Jan

[Amptroll]

Hello, Strelok and Kevin,
   If the Fig.2-20 you are referencing to (I am not close to my TUT library) is the one that limits the AC voltage until the RC network charges and uses the IRFZ40 MOSFET; I can testify that this circuit works great.  As far as STD BY switches I have been using some of the methods of cathode STD BY switching discussed in TUT with great success also.  Again there is one that uses a MOSFET, 22uF eCap, 220k resistor, 10k resistor, 12V zener and a SPST switch which I have implemented with great success.  I try to stay away from mechanical switches with the voltages that are present in valve amps, because of the contact arcing deterioration of the contacts. 
Take care and peace always,
Rob


Hi Strelok

The TUT method for heater surge protection (Fig.2-18) is universal and easy to adjust for whatever the heater load is just by changing the limit resistor value. One relay control circuit is shown in Fig.2-19, and can be any type of timer circuit you wish to deploy. Fig.2-20 shows an idea from Wireless World that provides  reduced net voltage output over a set time constant, but does not provide specific current limiting you can set.

Figs. 3-18, 3-19 of POP have active current limiting that requires some excess voltage be available, which is often the case for DC heaters. Same with TUT2 Fig.2-74.

Limiting the heater turn-on surge will increase tube life, but remember that if a tube is not mechanically upset, it will generally last beyond fifty years even without such measures.