Fender Twin switching off 2 power tubes

[jodie]

Hi guys,

I´m looking for a nice way to switch off two of the four 6L6s instead of just pulling them.
I´m thinking of leaving the tubes idling with everything connected, just disconnecting the input grids from the PI with the bias voltage still connected.
Maybe I´m overlooking something here, because I´ve never heard anything about this way of reducing the number of output tubes?

Thanks

Jodie

[Structo]

Something to consider is that even with two tubes pulled and cutting the power in half, only translates into a 3 db drop in volume.

So for a Twin, you either have to use an attenuator or pedals to hit the sweet spot at lower volumes.

Those amps are a good example of a super clean amp that gets incredibly loud!

[Firestorm]

What Structo said ...

But if you still want to do it you probably don't want to leave the input grids just hanging there unconnected -- all sorts of stray noise can find it's way into the power amp. You could ground out the grids on two tubes, but that is sort of a switching nightmare.

The usual way to shut two tubes off is to lift the cathode grounds via a switch -- no ground, no current, tube "off." This will double the impedance the tubes expect to "see" (in fact, this "two tube mod" is what jazzers sometimes do when they want to run a single 8-ohm 15" speaker in a Twin -- of course now, you can get a 4-ohm 15.

It'll still work if you don't change the speaker load (same thing as hooking up an extension speaker after all), but the amp will sound different since it's running into half the appropriate load. You might even lose a little output power. There's an analysis of the effect of impedance in a Twin in TUT 1.

[Bob-I]

Hi guys,

I´m looking for a nice way to switch off two of the four 6L6s instead of just pulling them.
I´m thinking of leaving the tubes idling with everything connected, just disconnecting the input grids from the PI with the bias voltage still connected.
Maybe I´m overlooking something here, because I´ve never heard anything about this way of reducing the number of output tubes?

Thanks

Jodie

No... do not disconnect the grids. They're supplying the bias voltage C-. Without that the tubes will go into full dissipation and burn up in a few minutes.

Disconnect the cathodes using a simple DPST toggle.

[solderstain]

Disconnect the cathodes using a simple DPST toggle.

Amen. People have been doing that trick on Twins, and MANY other four output tube amps, for DECADES. Works great. I have a switch like that on my old metal-panel Super Lead.

The tone will change, but you already know that if you've been pulling two of the tubes. It's not just the different reflected impedance. When you flip the switch, the B+ for the rest of the amp goes up. The bias setting you so carefully set with four tubes active is now different. No biggie. There are ways around that. On my SL, I just set the bias so it's roughly half-way between the 'ideal' settings - a bit 'hot' for one setting, a bit cold for the other. There's a more complicated method that will still allow the two-tube and four-tube bias settings to be correct regardless of switch position, but it's more work than MOST people would want to go through. But it works.

Even if you don't have an OT with switchable secondaries, you won't really hurt the OT unless you play with the amp DIMED for EXTENDED periods of time. Facts is facts: Most of us DON'T play with the amp dimed for extended periods, so OT overheating isn't a REAL problem for MOST of us... and Fender Twin OTs are pretty robust.

[jodie]

Thanks for all the answers, I think you saved me from doing lots of useless things!

[paulster]

Disconnecting the input grids and tying them to bias voltage should work just fine and your output impedance would be unchanged.

The downsides are that, at the switching point there'll be a moment when you don't have a bias voltage unless you have your bias splitter after the switch and independent grid wires to all four tubes, and that the net effect will be a volume reduction of only 3dB.

[jodie]

I have to get the output power down, because I want to use Dana´s VVR3, which can only handle up to 50 watts.