Switching off or muting Power Amp Section Only

[lonote]

I have started drawing up plans for a mid-power 6L6 bass amp that will include a side-chain pre-out with a dedicated cathode follower driver for a balanced out.

As I read an older thread today about Martin Manning's Dumble-ish SE amp & he mentions options to use the speaker or not, it occurred to me that with my own project there may be times that the power amp/speaker might not be needed.

I am wondering what the simplest &/or most effective way of basically muting the power section would be, without throwing the B+ rail off too much?

[sluckey]

Just disconnect the input signal.

[LOUDthud]

I hate to see a power section just sitting idle when it's not being used. I'd try to rig up some kind of standby switch that would just switch off the power amp.

[Stevem]

Just do what Danelectro did many times and use a switch to short across the PI outputs after the coupling caps.

Just be sure the tube and switch are close to each other because you do not want long leads heading back up towards the up stream preamp sections .

[lonote]

Thanks for the suggestions.

It seems like a simple thing to do, but then maybe not...as I am inclined to have the power section unpowered myself.

Years & years ago I had installed a pre-out in a Princeton Reverb that I used as a pre-amp for bass, running it into a SS power amp. I would just pull the power tubes & call it good. Guess I am sort of coming from that direction.

I knew just enough to be dangerous, though it seemed to work fine. Still trying to move past that level...

[sluckey]

I am wondering what the simplest &/or most effective way of basically muting the power section would be, without throwing the B+ rail off too much?

If you just disconnect the power amp input signal with a simple switch, the B+ rail will not be affected at all.

[lonote]

I am wondering what the simplest &/or most effective way of basically muting the power section would be, without throwing the B+ rail off too much?

If you just disconnect the power amp input signal with a simple switch, the B+ rail will not be affected at all.

Thanks Steve, that IS the easiest.

No other caveats/considerations about the idling power section?

[R.G.]

I've been through this as part of a power amp add on I designed back in the 1990-2005 era.

The simplest way to un-power it is to un-power it. I used a single MOSFET in the B+ line and turned the MOSFET off and later a MOSFET in each output tube cathode. It's fairly easy to design transistors and so on to enable/disable a MOSFET gate for this. Worked fine.

There are multiple ways to design the gate drive. For a high-side MOSFET, you can get a photovoltaic gate driver with 500V isolation between an LED input and an 8V (or so) gate voltage output for about $2. A logic signal to the LED turns the MOSFET on and/or OFF. If the MOSFET(s) are in the cathodes, you can just put everything referenced to ground, easier.

Some people were wary of turning off the cathodes of a tube without also turning off the B+ and/or heaters. Easy enough - turn off the heaters as well. There are solid state relays, but a mechanical relay works just as well here. Turning the tubes back on will need a few seconds of delay after the heaters are back on before the B+ is turned back on.

[martin manning]

The down side of turning off the power tubes and their heaters is that the preamp will then operate on higher than usual voltages without corrective measures.

[lonote]

I've been through this as part of a power amp add on I designed back in the 1990-2005 era.
The simplest way to un-power it is to un-power it.

Thanks RG, but I'm afraid that would be way beyond my very limited ability with SS circuitry. I only started dabbling with MOSFETs last year, referencing your MOSFET Follies, in fact.


The donor transformer set for my project is from a basket-case Ultralinear 70W Fender & I planned to keep the power supply/UL circuit design in my bass amp project, as drawn by Fender.

I also have a working version of the exact same amp, so I went hands on, measured voltages on the functioning amp, then pulled the power tubes & checked them all again. As Martin mentions, I expected a big shift in voltage but I was surprised to find that the preamp plates were only 2V-3V higher.

Unless I am missing something, it would seem that I will be able switch off the B+ to the OT without creating to much of an issue for the downstream B+ nodes.

Also thinking that a power amp IN might be useful, so I will probably include a pot on the input of the PI, adding another way to mute the power amp.

[B Ingram]

I have started drawing up plans for a mid-power 6L6 bass amp ...

I am wondering what the simplest &/or most effective way of basically muting the power section would be, without throwing the B+ rail off too much?

I assume a "bass amp" won't be cathode biased.

I'd personally use one large coupling cap (like 0.2µF or more) to connect the grid of each side of the push-pull output to "Ground" or the "Bias Filter Cap" (which is the same as saying "Ground for AC").
Alternately, a very large coupling cap from one output of the phase inverter to the other output.

Futzing with killing voltage- or current-supply to the output tubes is not worth it in my opinion; I'd just kill the audio driving the power tubes.

Unless I am missing something, it would seem that I will be able switch off the B+ to the OT without creating to much of an issue for the downstream B+ nodes.

Switching high voltage tends to kill the switch over time, whether it's a typical Standby or a switch to disconnect the OT center-tap.

Unless you throw your amp on a load, drive the power section with a known-voltage, and Measure Power Output, you probably won't even notice that your output tubes are down to 40% strength compared to a new set. However, an arcing Standby switch will catch your attention early, even if it takes quite a while before you figure out the switch is going bad.