4 tube power amp

[jeff]

 If you're using an amp with 4 power tubes is there a way of shuting off 2 tubes instead of pulling them out and changing the speaker load?

Do you just ground the grids of 2 power tubes?

If so what's going on when 2 tubes are running and 2 are idling? As opposed to only having 2 tubes.

[sluckey]

Use a switch to open the cathodes of one push/pull pair.

Grounding the grids is not a good idea with a fixed bias amp because you'll destroy the tubes when you kill the grid bias. Grounding the grids is not a good idea with a cathode biased amp because the grids are usually only separated by a couple of low value grid stopper resistors. Grounding one grid will also kill most of the signal to the other grid.

[jeff]

Use a switch to open the cathodes of one push/pull pair.

Grounding the grids is not a good idea with a fixed bias amp because you'll destroy the tubes when you kill the grid bias. Grounding the grids is not a good idea with a cathode biased amp because the grids are usually only separated by a couple of low value grid stopper resistors. Grounding one grid will also kill most of the signal to the other grid.

Good point about the fixed bias.

As far as the cathode biased I was thinking of a switch that puts the grids in parallel with the "on" tubes or to ground.

What happens to a tube that has a signal on the grid, screen voltage and a plate voltage that's swinging(connected to "on" plates) but the cathode isn't grounded?

[sluckey]

Nothing. When you open the cathode of a tube, no current flows from cathode to plate or screen. The tube is effectively dead, open.

[PRR]

If cathode-biased: The cathode will rise to a high voltage, roughly twice the normal cathode voltage, or something more than Vg2/Mu. Could be over 50V. If you break the resistor but leave the cap engaged, check where it goes to and compare to the cap rating.

For practical cases, you could use a cathode resistor 20X bigger than normal. The tube won't be OFF but its smal flow will be overwhelmed by the normal flow in the other tubes.

You can also break the heater connection. It will take several seconds to fade-out and 11-20 seconds to come back to full voice. Makes much sense as a per-gig or per-set choice, no darn good as a solo boost or lick-lifter.

[eleventeen]

I have an old BF Pro Reverb (#00138!) which has had a Twin Rev power and output transformer & 2 extra 6L6 sockets (and 470 ohm screen R's & 1500 ohm grid R's) installed. This was a popular mod back in the 70's (when watts were worshipped) and a zillion of them were done by Stars Guitars here in the SF Bay Area. Change 5U4/GZ34 to SS rectifier; usually have to remove 1 screw & rotate the choke some to make room for the bigger OT. In place of the power switch, I installed a DPDT center-off switch (works side-to-side, not up-down) which effectively gives 40 watt-power off-80 watt settings.

This amp still has the original 1965 electrolytics and I'm a tad hesitant to turn it on! (Just kidding) At least, without solid plans to power it down to straight dual 6L6 configuration. Geez, I gigged with that amp for 15+ years and probably haven't turned it on for 5.

All the DPDT switch does is to lift the cathodes of 2 of the 6L6's (uhh...ones on the *opposing* sides of the push-pull output circuit)