5150 II power amp disengage

[Swampy]

Hey everyone,

I'm just pondering the best way to add a switch to a 5150 II to disengage the power amp so the preamp out can be used. I'm think a 3PDT after the preamp out jack that operates signal switching and PT plate voltages, so the amp can run without a speaker cabinet or dummy load attatched.

What's the general consensus?

[jmaf]

2 cents worth: SPST switch lifting all 4 power tube cathodes at once and you've got a dead power section.

[Swampy]

Do you think the system will experience the same scale surge with switching on the anodes and, I would presume, screens?

[jmaf]

Do you think the system will experience the same scale surge with switching on the anodes and, I would presume, screens?

The current surge should not be your biggest worry in a standby switch IMO. Voltage and safety should go first.

Having said that, there are probably some precautions you should take when switching off the power tube cathodes. See this: The Amp Garage :: View topic - cathode switching as a standby

[Gtr_tech]

I'd just stick a plug with no cable attached (or maybe shorted to gnd) into the FX return. That'll kill the signal to the output section and you can use the FX send as a line out. I wouldn't expect great tone tho. The ideal thing to do is mic the spkr since the output section and spkr has a lot to do with the end result sound wise.

[Swampy]

Jmaf, you've misread my original question - the switch I propose disengages plate voltage, not cathode voltage.

Chur

[Gtr_tech]

Forget about lifting plate voltage. That will cause B+ to rise across the board and alter the tone of the amp/preamp response. If all you want to do is use the pre out, keep a spkr load on the amp (or a dummy load, just to keep the output section happy) and use the dummy plug in the fx return and use the send as a pre out. It don't get any easier than that.

[Gtr_tech]

And FWIW, lifting output tube cathodes will do the same thing. If the tubes are out of the circuit, they draw no current and b+ rises as a result.

[jmaf]

And FWIW, lifting output tube cathodes will do the same thing. If the tubes are out of the circuit, they draw no current and b+ rises as a result.

There are a few details here to take note. If he uses a spdt switch, on the standby side instead of opening the circuit he can add a large value resistor to lift the power tube cathodes way above zero, which should drive them into cutoff and still keep a current path for +B through the cathode. I've never tried this, but it may work.

Having said that, yes, inevitably, by cutting off current through the power tubes, +B will rise. I don't remember the 5150 voltages and ratings, I only recall it's a nightmare to work on the L joined printed circuit boards, so I'm not sure that running it without tubes would extrapolate the cap ratings.

[nashvillebill]

I've never had a 5150, but don't these have the typical Peavey standby switch which kills the screen grid voltage?? Meaning the preamp section would still be operational.

[Gtr_tech]

Mmmm....forgot about that. Thats just crazy enough to work. And K poisoning wouldn't be an issue 'cause there's still a bit of I through the output tubes. B+ would still be a bit higher tho I think.