Interrupting power tube cathodes as a form of standby

[mresistor]

Hi   is there any harm in interrupting the K bias as a form of standby?

[pdf64]

Probably not.
It’s not great to have the cathode floating (eg h-k voltage limit) but realistically the cathode probably just tend to rise to the voltage required to cut off anode current.

[mresistor]

thanks   

[HotBluePlates]

How about disconnecting the power tube grids from the output of the preceding stage?

[mresistor]

Interesting  HBP and I never thought of disconnecting the grid inputs.

[mresistor]

HBP   I don't think disconnecting the power tube grids is the way to go.  I wanted to know if there would be any harm in interrupting the K bias ckt from the power tubes while everything else remained operational.  For what it's worth I am going to interrupt the PT center tap for standby like Vox did on the conversion of Vox AC15CC1 to a Stout w/reverb, but that got me thinking about interrupting the bias connection.

[HotBluePlates]

HBP   I don't think disconnecting the power tube grids is the way to go.  I wanted to know if there would be any harm in interrupting ...

I'm not talking about changing/disrupting bias at all.  Whether the output tube grids connect to ground (cathode-bias) or a negative bias supply (fixed-bias) via resistors, all that should 100% stay in place.

Simply disconnect the signal-connection from phase inverter to the output tubes.  That might take the form of a switch actually disconnecting the coupling caps, or it might be the un-shorting of 1-10MΩ resistors between the coupling caps & the resistors already connected to the grids.  Or it could be a direct connection between the grid-inputs of a long-tail inverter (which then mutes its output).

The point is to think about "Standby" as "muting the signal feeding the output tubes" rather than "disconnect power."  The latter usually imposes some kind of noise or tube-life penalty. 

[mresistor]

Thanks HBP  for the explanation,  I have a limitation on the switch I have to use that probably prevents me from implementing the described mute switching. Sounds cool though and I'll llook at if I can implement it with this switch. I'm planning on using a PPIMV as well and it might get a little messy in that area.

[PRR]

On push-pull amps, some designers just short the two sides of the driver together. Resistance-loaded voltage amplifiers can be shorted without any strain. A single-pole switch does the job and 50mA (0.050A) rating is over-kill.

[mresistor]

HBP and PRR   is this what you're talking about?..  If so is this going to work with a Type 3 master volume?

[mresistor]

Ok would this work on the other side of the capacitors so as not to have the Master volume affecting it?


Oh Ok re-reading what HBP has said the inputs to the PI could be shorted together to be used as a mute switch too.

[sluckey]

If you want a Type 3 master volume then you don't need a mute switch. Just turn the MV to zero and it accomplishes the same as the mute switch PRR suggested.

There's really nothing wrong with interrupting power tube cathodes.

[mresistor]

Ahhh  true that..    thanks Sluckey   

[SoundmasterG]

HBP and PRR   is this what you're talking about?..  If so is this going to work with a Type 3 master volume?

Take a look at the Silvertone 1484/1485 amps. Schematic is here on the schematic section of the site. They short the power tube grids together for standby. It mostly works. Since the two sides of the phase inverter output aren't exactly balanced with each other, if you jam the strings hard you can get a little strangled noise out but it mostly works as a standby.

Greg

[mresistor]

so it works with a cathodyne PI too..